Acinetobacter Baumannii – Virulence Factors and Epidemiology of Infections

Gram-negative rod Acinetobacter baumannii is an opportunistic pathogen with epidemic potential and contributing to outbreaks of infection worldwide. A. baumannii rods are etiological agents, mainly of lower respiratory tract infections in mechanically ventilated patients, bloodstream infections and wound infections in field hospitals. Increasing drug resistance, hight mortality among patients infected with this pathogen and the difficulty in treating the infections led the World Health Organisation (WHO) to include this pathogen in the list of the most challenging pathogens of the current decade.

In 1911, the microbiologist Martinus Beijerinck, using a medium enriched with calcium acetate, isolated a microorganism from the soil which he named Micrococcus calcoaceticus. The lack of precise characterisation led to a new name being given to the species already known as Bacterium anitratum and Moraxella lwoffii var. glucidolytica – later M. glucidolytica. In 1953 Jean Brisou proposed to include M. glucidolytica in the genus Achromobacter, but a year later together with Andre-Romain Prevot he came to the conclusion that in order to distinguish them from the mobile representatives of Achromobacter it is worth creating a new taxon, proposing the name Acinetobacter (akinetos – unable to move, bactrum – rod). In 1968 Paul Baumann’s team finally determined that the above taxa (then considered separable) belonged to a single genus – Acinetobacter, which was formalised three years later by a decision of the Subcommittee on Taxonomy of Moraxella and Related Bacteria [38, 52]. The final name of the microorganism, Acinetobacter baumannii, was given to honour two American bacteriologists: Paul and Linda Baumann [39]. In the 1970s, A. baumannii was recognized as an opportunistic pathogen. Initially, two species were included in the genus Acinetobacter: A. lwoffii and A. calcoaceticus [72]. In 2015, the genus Acinetobacter included 33 species [21]. It currently includes more than 50 species, most of which are environmental [100]. The genus Acinetobacter belongs to the gammaproteobacteria (this name refers to Proteus, a Greek god capable of changing form) [62]. The current taxonomic position of the species A. baumannii is shown in Fig. 1.

Fig. 1

These bacteria are oval, almost spherical Gram-negative rods, occasionally appear in diplococcal form. Despite its name indicating lack of motility, Acinetobacter has the ability to move. These organisms are oxidase-negative absolute aerobes, within which a distinction can be made between species capable of oxidising glucose (A. baumannii) and those incapable of this process (A. lwoffii, A. haemolyticus) [64, 88].

Due to the high phenotypic similarity and thus difficulty in distinguishing between six species of the genus Acinetobacter: A. calcoaceticus, A. nosocomialis, A. pittii, A. seifertii, A. lactucae (A. dijkshoorniae) and A. baumannii, the group A. calcoaceticus-baumannii (Acb) complex was formed for them [24, 68]. The species of the genus Acinetobacter most frequently causing infections in humans is A. baumannii, A. calcoaceticus comes second and A. lwoffii third. Other Acinetobacter species can also be etiological agents of infections, but much less frequently than the above three. An exception is A. seifertii (genetically closely related to A. baumannii), which is the most frequent cause of infections in Asia [100].

Within the genus Acinetobacter, especially A. baumannii, there are naturally competent strains, i.e. capable under natural conditions of extracting DNA from the surrounding environment in order to use free nucleic acids as substrates for repair of their own genetic material or incorporation of new fragments into the genome. The process of acquiring new characteristics and modifying those already possessed is genetically determined and requires the participation of the products of several genes. Bacteria may also apply a kind of predation, consisting in causing the death of nearby cells belonging to unrelated species in order to enrich the gene pool [91, 93]. For example, A. baumannii is able to lyse K. pneumoniae or S. aureus cells and then integrate DNA fragments from them into its own genome, which enabled the tested strains to acquire resistance to antimicrobials (β-lactams, including carbapenems) [91]. Natural transformation significantly affects the plasticity of the A. baumannii genome, and as a result favours the emergence of multi-drug resistant strains. It is worth noting that in A. baumannii there is a predominant tendency to obtain non-coding DNA fragments instead of coding sequences, which increases the pool of mobile genetic elements, and it has been found that the preferred source of genetic material for this rod is DNA from other Gram-negative bacteria [91, 93]. In addition, chemical compounds that pollute the environment can increase the potential of bacterial cells to take up foreign DNA. The molecular mechanism of this phenomenon is based on the enhancement of recA gene expression (coding homologous recombination factor) by compounds with mutagenic potential, which increases (up to twofold) the frequency of integration of the uptaken DNA into the chromosome, and thus the efficiency of genomic DNA transformation. Thus, for example, commonly used water chlorination, aimed at increasing the safety of drinking water, may contribute to an increase in transformation efficiency and thus to the emergence of a new, potentially dangerous strain [59]. This mechanism may further increase the efficiency of coselection, a phenomenon that significantly contributes to the increase of antibiotic resistance in strains living in environments contaminated with heavy metals in particular [36].

In 2014, sequences identified as moderate phages were found to be more abundant than conjugation elements in the genome of this rod, highlighting their important role as horizontal gene transfer vectors. Additionally, the presence of complex variable CRISPRCas (clustered regularly interspaced short palindromic repeats) systems in the Acinetobacter genome was observed. The collected data led to the hypothesis that the current population of A. baumannii may have evolved from the original small population of this species by negative selection [90].

Bacteria of the genus Acinetobacter are widespread saprophytes, found in wetlands, moist soil, ponds, sewage, water treatment plants, fish rearing tanks, and sea-water. As they are commonly found in soil and water in nature, their isolation is relatively easy. In the laboratory, pure cultures can be obtained with the use of media enriched with up to 0.2% acetate at pH 5.5–6.0. The bacteria are one of the predominant bacteria in tundra soils and also belong to the aerobic microbiota of saline lakes and brines. In addition, they can also be found in the air (detected, e.g., in dust during sand-storms). Importantly, in aerosols of the air near landfills, composting plants or sewage treatment plants, as well as in office spaces, Acinetobacter spp. dominates quantitatively over other genera [8].

It was shown that the addition of ethanol to the medium enhances the growth of A. baumannii and also increases the tolerance of this microorganism to salinity. In the presence of ethanol (acting as a signalling molecule) the bacterium can survive the salt concentrations inhibiting its growth under normal conditions. For this reason, co-culture with the yeast Saccharomyces cerevisiae, which secretes ethanol into the medium, also enhances the growth of this bacterium. The presence of this compound affects the induction of A. baumannii proteins, including those involved in lipid and carbohydrate anabolism, which enhances biofilm formation and reduces bacterial motility. The presence of ethanol also induces the production of indolyl-3-acetic acid (IAA), which is a plant hormone that enhances plant tolerance to the presence of bacteria. A. baumannii treated with this alcohol show increased pathogenicity in a nematode infection model. Noteworthy, in humans, ethanol predisposes the organism to A. baumannii infection, promoting adaptation and survival of this pathogen [69, 84].

Bacteria of the species A. baumannii are highly adaptable, which enables them to inhabit hospital environments. They inhabit there not only moist surfaces (e.g. respiratory systems of ventilators) but also dry ones (e.g. medical equipment), which is an unusual feature for Gram-negative rods [1]. They are part of the physiological permanent or temporary microbiota (of the skin, respiratory and genitourinary tract mucous membranes or colon) of both patients and medical staff [58].

Interestingly, several hydrogen peroxide-resistant Acinetobacter strains were isolated during assembly of the Mars Phoenix lander. Proteomic analysis revealed the presence of catalase and alkyl hydroperoxide reductase. Due to such high resistance, it will probably be necessary to include representatives of this genus in the biological conatmination of spacecraft during missions to detect traces of life beyond our planet [20].

A. baumannii is among the most common pathogens that are multidrug resistant (MDR), extensively drug resistant (XDR) and even pandrug-resistant (PDR). It is extremely important from a medical point of view, since above mentioned strains are increasingly the causes of nosocomial infections [70]. A 2007 study of etiological agents in intensive care units on five continents showed that A. baumannii was the fifth most common pathogen [94]. In Europe, it is the third most common pathogen causing mechanical ventilation-associated pneumonia (VAP), just after S. aureus and P. aeruginosa [48]. Bacterial pneumonia is a particular threat to patients in intensive care units (ICUs) [33].

The development of VAP occurs on average in every fifth patient (8–28%) mechanically ventilated. Mortality in this group of patients is particularly high and ranges from 24% to as much as 50%, and sometimes may even reach 76% [16, 41]. It is estimated that each year A. baumannii infections cause about 15,000 deaths [85]. The main etiological agents of nosocomial pulmonary infections are S. aureus, Enterobacteriaceae, P. aeruginosa and A. baumannii, and the type of agent depends both on the duration of the patient’s hospital stay and the antimicrobial treatment administered earlier. It should be emphasized that VAP caused by P. aeruginosa and Acinetobacter spp. developed in up to 65% of patients who received therapy with a broad-spectrum antibiotic in the two weeks preceding infection and in only 19% of those who did not receive such therapy [16, 41]. The risk of acquiring Acinetobacter spp. related VAP also increases in the presence of hypertension (40%), chronic obstructive pulmonary disease (28%), diabetes mellitus (23%), and invasive procedures: urinary catheters (99%), central vascular catheters (mainly carotid and subclavian) (83%), or nasogastric probes (74%) [29].

A study of bacterial bloodstream infections (BSI) in 16 hospitals in southern Poland between 2016 and 2019 found that Gram-negative bacteria were responsible for 27.8% of BSI cases, of which carbapenem-resistant A. baumannii was responsible for 70.6% of ICU infections. This rod was present in surgical wards in all 16 hospitals [17]. Epidemiological studies in Poland and Ukraine on the causation of VAP have shown a different frequency of occurrence of individual species. However, it was found that Gram-negative bacteria predominate as etiological factors of this disease. Importantly, MDR strains of A. baumannii were much more frequently isolated in Poland (26.9%) than in Ukraine (14.6%), which may result from differences in the strategy of antibiotic use in both countries [37]. Severe MDR A. baumannii infections are associated with high mortality rates, up to 70% in VAP and 43% in bloodstream infections. In response to the growing problem of antibiotic resistance, the UK government announced in 2014 that failure to curb the overuse of antibiotics would result in 10 million deaths per year from bacterial infections by 2050, more than from cancer, which accounts for 8.2 million deaths per year [70].

The A. baumannii bacterium, along with Enterobacter spp., Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterococcus faecium and Escherichia coli, is one of the multidrug-resistant pathogens that the Infectious Diseases Society of America has designated with the acronym ESKAPEE. This acronym is derived from word escape, which refers to the particular resistance of these bacteria to antibiotics currently available against them [77, 78]. In 2017, WHO included A. baumannii on the list of antibiotic-resistant pathogens for which finding effective antimicrobial therapies is considered a priority – carbapenem-resistant A. baumannii was identified as one of three pathogens of critical importance [56, 89].

A. baumannii is one of the most frequently isolated pathogens among ICU patients. This is due to the specific features of this microorganism, such as its high ability to accumulate various resistance mechanisms (as a result of mutations and acquisition of plasmids, transposons, integrons, resistance islands) and the ability to survive in adverse environmental conditions (dryness, contact with disinfectants), as well as the production of biofilm.

It is also estimated that approximately one million A. baumannii infections occur annually worldwide, including approximately 45,000 in the USA, half of which are caused by carbapenem-resistant strains, causing a mortality rate of nearly 20% [85]. The problem of increasing drug resistance of A. baumannii strains is enormous; one report indicates that overall worldwide in 2004, 23% of A. baumannii clinical isolates showed multidrug resistance, while in 2014 it was already 63% [35]. Infections caused by XDR strains have a very high mortality rate of up to 70%, and up to 88% for XDR strains showing resistance to carbapenems [6]. Carbapenem-resistant A. baumannii (CRAB) strains are currently the cause of really serious nosocomial infections, especially in the ICUs. The increase in the number of strains resistant to colistin (ABCR, A. baumannii colistin-resistant), the so-called antibiotic of last resort, is considered to be an even greater problem. Mortality among patients infected with ABCR strains is about 85% and is more than twice as high as in the case of infection with strains susceptible to this antibiotic [60].

WHO in 2017 reported that 45–65% of CRAb strains and 40–70% of MDR strains were responsible for healthcare-associated infections in the US between 2011 and 2014 [85]. Among the few countries that conduct surveillance for the prevalence of A. baumannii infection in the community is the United States. The US Centers for Disease Control and Prevention (CDC) stated in a report in 2013 that MDR strains of A. baumannii cause about 7,000 infections annually in the US, accounting for about 10% of all nosocomial infections in the country, 500 of which end in deaths [14]. Such surveillance is also carried out in England by Public Health England, but on a narrower scope – only for bloodstream infections.

A. baumannii rods are often isolated from urine, blood, cerebrospinal fluid or surgical wound samples of patients with nosocomial infections. In some patients, A. baumannii is part of the natural microbiota of mucous membranes and, under appropriate conditions, can cause infections of the wounds, respiratory tract, urinary tract (catheter-related infections), peritoneum or meninges. Patients who are critically ill, mechanically ventilated, after surgical procedures, weakened by prolonged hospitalization and related treatment with broad-spectrum antibiotics belong to the special risk group [2, 28]. The main hospital reservoir of A. baumannii are humid environments: bathrooms and kitchens [43]. In contrast to the aforementioned hospital-acquired A. baumannii (HA-Ab) risk factors for colonization and multidrug-resistant A. baumannii infection, community-acquired A. baumannii (CA-Ab) risk factors include alcohol and cigarette abuse, sub-tropical and tropical climates, but also general health, including certain comorbidities such as diabetes or chronic lung disease. Exposure of macrophages to ethanol (in physiological doses supplied to the body by alcohol abusers) has been shown to significantly reduce the ability of cells to phagocytose (6.25 mmol/l dose by 23.4%, and 12.5 mmol/l dose by 51.7%). This effect is most likely due to a significant reduction in the expression of a regulator of the actin polymerisation signalling cascade (GTPase-RhoA). Additionally, ethanol inactivates NO synthase, thereby increasing the survival of A. baumannii in macrophages. Furthermore, alcohol modifies the production of cytokines (produced mainly in lung epithelial cells), thus significantly exacerbating the course of infections [4]. Among patients with CA-Ab infections, a mortality rate as high as 64% was reported in 2015. At the same time, it should be emphasised that it is difficult to state unequivocally whether the reason for such a high mortality rate is solely due to the virulence characteristics of the strains or to the weakened host organism [21, 101].

Also, environmental Acinetobacter strains very often produce enzymes that give antibiotic resistance (e.g. carbapenemases, broad-spectrum beta-lactamases), making them an important reservoir of antibiotic resistance in the out-of-hospital environment [1]. Unfortunately, A. baumannii is becoming a significant etiological agent of animal diseases. Most infections with this pathogen occur in veterinary clinics, causing diseases such as pyoderma in dogs, necrotizing fasciitis in cats, urinary tract infection, equine thrombophlebitis and lower respiratory tract infection, foal sepsis, pneumonia in mink or skin lesions in falcons. Animal isolates show high genetic diversity. In addition, they have different sequence types from humans. With the transmission of the bacteria, animals can contribute to the spread of new carbapenemases and the risk of such transmission increases in companion animals. It should be noted that the bacterium is identified not only in sick but also in healthy individuals, particularly on the skin of dogs. Therefore, it cannot be excluded that animals may act as reservoirs of A. baumannii [80, 92].

In 2017, the potential reservoir of A. baumannii in birds was checked. The bacteria were isolated from different bird species (hens – from 3%, geese – 8% and white stork chicks – 25%). The virulence of the obtained strains proved to be comparable to that observed in clinical strains. Bacterial sequence analysis revealed a close relationship between the chicken isolate from Germany and the human clinical isolate from China, as well as links between the farm animal isolates and the human clinical isolates associated with international clonal lines. Stork isolates showed similarity to the human clinical from the USA. The study suggests that A. baumannii may be considered a zoonotic pathogen that can additionally transmit to livestock [98].

A comprehensive assessment of the epidemiological situation in Poland still requires further extensive research. In 2018, a report by the National Institute of Medicines, developed as part of the National Programme for the Protection of Antibiotics (NPOA), reported that almost 32% of hospitalisations had acquired infections in Polish ICUs. The incidence of pneumonia was 50.7% of infected patients (17% of all hospitalisations), vascular bed infections (including catheter-related infections) 35.4%, A. baumannii proved to be the third most common aetiological agent of pneumonia (13%, just after P. aeruginosa – 25.5% and Klebsiella spp. – 22.8%) and sixth for vascular bed infections (3.6%). As many as 61.8% of A. baumannii strains isolated showed resistance to carbapenems [23].

A 6-year observation (2011–2016) in the ICU of the University Hospital in Wrocław among 2549 patients, showed that A. baumannii was responsible for 31% of infections, with 73.8% related to pneumonia associated with artificial ventilation. There was also an increase in nosocomial infections caused by A. baumannii over the years, with 16.5% in 2011 and 41% in 2016. The strains studied were susceptible to colistin, amikacin, imipenem, meropenem and ciprofloxacin in 100%, 10.7%, 12.3%, 11.5% and 2.4% respectively, and multi-drug-resistant strains were 98.36% [25]. The prevalence of A. baumannii infections was lower in this centre than in the ICU of the Clinical Hospital of the Silesian Medical University in the first 12 months of its operation, where it was 38.8%.

The study of 234 patients hospitalized in the Department of Intensive Care Medicine of the Pomeranian Medical Academy of Independent Public Clinical Hospital No. 1 in Szczecin showed that the most common etiological agents were A. baumannii (18.6%) and P. aeruginosa (16.9%). Pneumonia was most frequently caused by A. baumannii – in 23.09%, while peritonitis was caused by E. coli (20.3%); A. baumannii caused 17.4% of infections.

A retrospective study of adult ICU patients (2547 patients) in southern Poland participating in a multicentre standardised infection control programme between 2013 and 2015, within the European Health-care-associated Infections Surveillance Network (HAI-Net), showed that A. baumannii was also the predominant aetiological agent of secondary bloodstream infections in the ICU (34.5%). Of these, 78.8% of strains showed resistance to imipenem, 72.7% to meropenem and doripenem, and 57.6% to sulbactam [95].

The main mechanism of resistance of clinical strains of this rod to carbapenems are CHDL-type carbapenemases, the expression of which is affected by the presence of insertion sequences above blaCHDL genes. A study of strains isolated in a Warsaw hospital between 2009 and 2014 confirmed the prevalence of the above resistance mechanisms in Polish clinical isolates [83]. In a recent study, all strains from patients hospitalised in the vascular surgery department in Kraków (SSI – surgical site infections, from surgical site infections and from wounds) were found to be XDR, resistant to carbapenems. All of them possessed blaOXA-23, blaOXA-24 and blaOXA-51 genes and all of them produced less abundant biofilm in comparison to the reference strain ATCC19606 [87].

In Poland, about 22% of patients are hospitalized in the ICU for sepsis and septic shock, and the mortality rate in this group reaches as high as 46%. In 2015, it was found that Gram-negative bacteria were responsible for 58% of sepsis cases, among which the fourth most commonly isolated group were rods of the genus Acinetobacter [49]. In a similar study conducted in Bosnia and Herzegovina, A. baumannii was found to cause 4.5% of sepsis cases [82]. In addition, recent observations in ICUs in Greece showed that nearly 42% of patients with XDR-resistant A. baumannii infection developed sepsis caused by strains resistant to colistin (an antibiotic of last resort), leading to septic shock; mortality in this group was 100%. In contrast, among patients with bloodstream infection with colistin-susceptible strains, 50% survived [71]. Furthermore, it has been reported that A. baumannii can coexist unhindered in the same niches with other bacteria (e.g. with S. aureus), which may worsen the course of infection [13].

The ability to move, adhere to the colonized surface and create a biofilm are The most important factors in the pathogenesis of A. baumannii. This bacterium is motile in response to iron chelation. Iron deficiency is a universal signal to the bacteria that they are inside the host, which they react to with increased expression of virulence factors. Iron concentration influences the expression of the pil and com genes, which enable cell movement [26]. Quorum sensing is also a factor that controls mobility, as evidenced by reduced mobility as a result of inactivation of the abaI gene encoding the inducer synthetase of this system [18]. Light, especially blue light, is another inducer of A. baumannii motion, but this mechanism has not yet been fully elucidated [65].

The ability to colonize the surface of the skin or mucous membranes of the host organism is key to the invasiveness of this pathogen. It have been described in vitro two types of adhesion of A. baumannii to bronchial epithelial cells: adhesion to the host cell in diffuse form and adhesion of bacterial aggregates in localized areas of the cell. Bacteria interacted with epithelial cells through fimbriae. The particular strains differed in terms of quantity, but no correlation was found between the number of bacteria colonizing the epithelium and the origin of the strain. However, it was observed in the context of a clonal line – clone II (European clonal type) to be much more adherent than clone I [51].

Adherence to biotic and abiotic surfaces enables the development of biofilms. The production of biofilm is also an important factor increasing the tolerance of bacteria to antimicrobial agents [77]. For the formation of biofilms on abiotic surfaces, it is necessary to synthesize pilus by genes of the CsuA/BABCDE system, conditioning the synthesis of fimbriae and OmpA protein. In addition, the conserved Bap protein is also important, as it seems to be essential for communication between biofilm-forming bacterial cells, and mutations of the genes encoding them result in a weakening of adherent abilities, inhibiting the growth of the structure. The development of biofilm also depends on the ability of clinical strains to produce and secrete exopolysaccha-ride poly-β-1-6-N-acetylglucosamine (PNAG) [54, 67]. It was found that A. baumannii grows faster under the influence of the stress factor of nutrient deficiency. A significant reduction in the expression of genes active during biofilm formation (ompA, bfmR, csuAB) was observed. This means that the pressure in the form of nutrient deficiency reduces adhesion to solid surfaces, but also reduces biofilm formation, and thus the colonization of abiotic surfaces [9].

A capsule gives A. baumannii the ability to avoid immune reactions (the complement system and phagocytosis). It can modify the structure of phosphoethanolamine of the capsular lipopolysaccharide (LPS). It is the main component of the outer membrane of gram-negative bacteria. This lipid-polysaccharide heteropolymer is composed of endotoxic lipid A, an oligosaccharide core and an O antigen. Lipopolysaccharide is an immunoreactive molecule because it induces the release of tumor necrosis factor (TNF) and interleukin 8 (IL-8) in a TLR-4 (Toll-like receptor 4) dependent manner [50]. LPS therefore plays an important role in the virulence of A. baumannii cells. It has been described that the deletion mutation of LpsB glycosyltransferase, which plays a role in LPS biosynthesis, reduces the pathogenicity of strains during soft tissue infection [55, 61]. Also, inhibition of the enzyme UDP-3-O-acyl-N-acetylglucosamine deacylase by the LpxC gene encoded, resulted in decreased TLR-4 activation, decreased inflammation and increased phagocyte activity and most importantly, increased survival of infected mice [53]. Stimulation of host TLR-4 receptors by LPS is of key importance in development of sepsis. During septic shock, LPS also impairs the host’s immune system and its ability to reduce infection by a mechanism known as immunoparalysis or reprogramming of the immune system. The occurrence of immunoparalysis during sepsis impairs the production of pro-inflammatory cytokines in monocytes, disturbs phagocytic potential of neutrophils, causes T-lymphocyte anergy and even their apoptosis. Therefore, it may be potentially beneficial to implement a therapy that strengthens the host’s immune system and allows it to inhibit the excessive multiplication of bacteria at the very beginning of the infection [50, 100].

Iron uptake systems, including siderophores also contribute to virulence. Iron, although it is quite ubiquitous in the environment, is inaccessible to cells mainly due to heme chelation and iron-binding proteins such as lactoferrin. Most aerobic bacteria, including A. baumannii, obtain irons using siderophores in an environment with limited access to iron. The best characterized siderophore is acinetobactin [50]. The acinetobactin-mediated system plays a key role in the ability of the ATCC 19606T reference strain to colonize, damage epithelial cells and kill infected laboratory animals. This suggests that the ability to extract iron from the environment plays a key role in virulence [31]. With the limited availability of iron, the development of bacterial cells, which do not express genes encoding acinetobactin, is possible thanks to siderophores – baumannoferrins A and B [73]. In addition, A. baumannii also captures and uses heme, which is a product of host metabolism, especially present in damaged cells and tissues with infection (such as necrotizing fasciitis) [19, 102]. Moreover, this bacterium can bind zinc [66].

Among the virulence factors there are porins, mainly the OmpA protein – a transmembrane protein of the outer membrane. This protein is associated with the outer membrane vesicles system (OMV), which includes the outer membrane, periplasmic proteins, phospholipids and LPS. OMVs transfer virulence factors (including OmpA) into host cells [50]. Lee et al. reported that the mechanism by which A. baumannii causes damage to human respiratory cells during infection is the induction of apoptosis [51]. In addition, OMVs also take part in the horizontal transfer of the OXA-24 carbapenemase gene, which proves the possibility of OMV’s participation in the spread of anti-biotic resistance among strains [50]. OmpA, apart from its cytotoxic properties and the ability to bind factor H of the complement system, plays a significant role in the colonization of the lung epithelium – It promotes adhesion to proteins of the extracellular matrix, including fibronectin [63].

Among the protein secretion systems that contribute to the pathogenesis of A. baumannii, in addition to OMV, type II (T2SS), V (auto-transporters) and VI (T6SS) secretion systems should be mentioned [63, 73, 81, 96]. The type II secretion system is involved in the transfer of proteins from the periplasmic space to the extracellular environment. This process is a two-step process. First, target proteins are transferred to the periplasm by the Sec or Tat system, from where proteins are sequentially secreted out of the cell by T2SS [96]. It has been observed that deletion of the gspD or gspE genes of the T2SS system results in an inability to secrete LipA lipase, which is a substrate for T2SS. This in turn results in the inability of strains to grow in the presence of long-chain fatty acids as the sole carbon source. Inactivation of the T2SS system and its substrate, LipA, also has a negative effect on bacterial survival in vivo in a neutropenic mouse model of bacteraemia [45].

Another virulence factor is the type VI secretion system (T6SS), which is used by many bacteria to introduce effector proteins during infection of eukaryotic cells or to eliminate competing bacteria [7, 96]. Although T6SS appears to contribute significantly to the virulence of A. baumannii in a strain-specific manner, its role has not been determined in all strains tested. This system mediates the secretion of various effector proteins, including proteins that are toxic to other bacteria, enabling the killing of competing bacteria [12, 76]. Interestingly, some strains have found an association between T6SS and antibiotic resistance. The large, conjugating plasmids (pAB04-1 or pAB3) confer resistance to many antibiotics but also encode a T6SS inhibitor. Therefore, there is great pressure leading to the loss of these plasmids in some cells [97].

The V-type secretion system (T5SS) also called autotransporter uses the Sec system to transport proteins out of the inner cell membrane. This system is known to mediate biofilm formation and adhesion to extracellular matrix components. Moreover, it participates in obtaining virulence, which was tested in a murine model of systemic A. baumannii infection [27].

Another important factor in A. baumannii virulence is phospholipase. This enzyme, by metabolizing phospholipids present in mucous membranes, reduces the stability of the host cell membranes and lyses eukaryotic membrane cells thus facilitating invasion [86]. There are three classes of phospholipases: phospholipase A, phospholipase C and phospholipase D, differing in the phospholipid hydrolysis site, where phospholipase A is not mentioned as the virulence factor of A. baumannii [11, 44, 86]. Active phospholipase D contributes to the increase in the ability of A. baumannii infection to spread from the lungs to other organs, which has been demonstrated in a mouse model of pneumonia [3].

Although penicillin binding proteins (PBPs) in microorganisms are mainly involved in the final steps of peptidoglycan biosynthesis, the PBP6/8 protein, encoded by the pbpG gene, is the virulence factor of A. baumannii. The strain which mutated the pbpG gene was characterized by lower survival in the rat model of soft tissue infection and pneumonia [50].

Sex-related differences in susceptibility to certain strains of A. baumannii have been described in a mouse model. In the blood infection model, female mice were found to be approximately twice as susceptible to the hypervirulent colistin-resistant strain (LAC-4 ColR obtained by spontaneous mutation) than male mice, but more than 10 times more resistant in the lung infection model to another strain (VA-AB41, isolated from lung and skin) [56]. The female hormone 17β-estradiol has been shown to be responsible for changes in pulmonary macrophage and neutrophil populations, resulting in an enhanced inflammatory response and impaired eradication of the pathogen [74]. Studies conducted in an insect model (larvae of Galleria mellonella) revealed the activity of 300 genes active during in vivo infection – genes that are both well understood and those that have not been characterized so far [34].

The host defense mechanisms against A. baumannii are being elucidated more and more thoroughly. The recently described role of the inflammasome in inducing an early response against A. baumannii infection is worth mentioning. Inflamasome is a large intracellular complex of proteins involved in the activation of the pro-inflammatory cascade. It has been shown that although the NLRP3 inflammasome is essential for effective infection control, the clinical strain of A. baumannii 8879 with the XDR phenotype, belonging to clone II (which causes bacteremia in ICU burn wound patients), induces increased production of IL-1β and IL-18 cytokines via the NLRP3 inflammasome and induces lung damage. Undoubtedly, further research on the significance of the activation of this pathway by A. baumannii is necessary [22]. A graphic summary of the most important factors of pathogenesis and their mechanisms of action is presented in Figure 2.

Fig. 2

The increase of resistance of A. baumannii to antimicrobials leaves few therapeutic options. Unfortunately, there are no effective treatment regimens for infection caused by multidrug resistant strains of A. baumannii. The lack of large prospective clinical trials makes it difficult to evaluate combination therapy in infections with MDR strains. Most of the available data on the efficacy of individual treatments are derived from case series (out of control), clinical observations, in vitro studies and animal models. Various studies show contradicting results for the same combinations of antimicrobial agents [10, 28].

Carbapenems are usually the first-line drugs in the treatment of infections caused by MDR strains. However, nearly half of the strains isolated from people with healthcare-associated infections reported to the CDC National Healthcare Safety Network in 2014 turned out to be CRAB strains [10]. Other β-lactam antibiotics, including broad-spectrum cephalosporins (ceftazidime or cefepime) are also used to combat infections caused by A. baumannii. The β-lactamase inhibitor sulbactam shows high bactericidal activity against A. baumannii isolates. However, as reported by Kanafani et al., even resistant to carbapenems strains may be in vitro susceptibility to this compound [46]. Aminoglycosides such as tobramycin and amikacin are also used in the treatment of MDR A. baumannii infection. These antibiotics are mainly used in combination with antibiotics belonging to other groups. However, numerous multidrug-resistant isolates remain indirectly sensitive to this group of drugs [28].

Therapeutic options are limited in the case of resistance to the above antimicrobial agents. In such situations, polymyxins (colistin and polymyxin B – not used clinically) and tetracyclines (especially minocycline and tigecycline) are used as the last-line drugs. There are no randomized studies on their effectiveness, mainly because they are reserved for use against microorganisms with high resistance. However, it is known that colistin exhibits significant nephro- and neurotoxicity. Observational studies have reported 57–77% of cure or improvement in health following the use of colistin in seriously ill patients with MDR infections (including pneumonia, bacteremia, intra-abdominal infection, and central nervous system (CNS) infection). During in vitro studies, both polymyxins exhibit the highest antimicrobial activity and, although they are toxic to the kidneys, are often used as rescue drugs [28, 33].

Also APIC (Association for Professionals in Infection Control and Epidemiology) emphasizes the role of controlling Ab-MDR outbreak infection and limiting strain transmission. The methods of control include communication between health care facilities where resides an infected patient. Each medical center should document the Ab-MDR development in a patient. However, the basic principle of epidemic control is the identification of the reservoir of strains and its elimination (decontamination of medical equipment, including a ventilator). In addition, APIC also mentions educating not only medical staff about how strains are transmitted, but also visitors. It is also important to cohort people colonized or infected with Ab-MDR and their medical caregivers [5]. In the case of outbreaks infection, admitting new patients to a given medical facility should be limited. In the process of extinguishing an epidemic, staff education, scrupulous hand hygiene and decontamination of the hospital environment are extremely important [99].

Resistance of A. baumannii strains to carbapenems in some parts of the world exceeds 90%, and the death rate for the most common CRAB infections, i.e. nosocomial pneumonia (HAP) and bloodstream infections, may approach 60%. The antimicrobial drugs currently used in the treatment of CRAB infections (ie polymyxins, tigecycline, and sometimes aminoglycosides) are far from perfect due to their pharmacokinetic properties and increasing rates of resistance [42]. Therefore, given the lack of good therapeutic options, it is essential to develop new therapies as well as conduct clinical trials to develop and apply an effective treatment regimen. Until then, it is essential to prevent the transfer of A. baumannii strains in healthcare settings.

New treatments for A. baumannii infection are based on new antibiotics with specific activity against the bacteria: new inhibitors of β-lactamases combined with an antibiotic or inhibitors of protein synthesis in ribosomes (pyrolocytosine antibiotic RX-P873 with great potential against clinical isolates of A. baumannii). Another approach is new modifications of existing antibiotics (e.g. siderophore-conjugated cephalosporins). An example of a siderophore cephalosporin is cefiderocolol. Antimicrobial peptides (AMP) may have a similar effect by disrupting the functioning of bacterial membranes. Proline-rich A3-APO as example of AMP is isolated from the skin of frogs and toads, showing high bactericidal potential in preclinical models of blood and wound infections. Another perspective is the phage therapies that are currently experiencing a renaissance. Since 2010, phages specific to A. baumannii (the first: AB1 and AB2) have been identified that lyse this species or representatives of this genus. Noteworthy is the AP22 phage isolated in 2012, showing the broadest activity against clinical strains of A. baumannii (fighting 68% of hospital isolates) [30, 32, 100]. Species specific phages can also be useful in decontaminating rooms where patients are present. In 2013, Taiwan tested for the first time the effect of incorporating an aerosol containing active bacteriophages (8 strains) against CRAB into the standard procedure for decontaminating hospital rooms at the ICU. During the 8-month period of the study, a decrease in the number of CRAB isolates by 41.69% was observed, as well as an increase in the number of carbapenem-sensitive strains (probably due to the targeting of the phages tested against CRAB strains) [32, 40]. The significant potential of phages to decolonize and interfere with the formation of biofilm by A. baumannii (phages AB7-IBBI, AB7-ABB2) was confirmed. An interesting possibility are also lytic peptides isolated from these phages – e.g. LysSS endolysin. In a study from 2020, inhibition of the development of MDR A. baumannii strains was shown, but it was not found to be cytotoxic to human cells (in the study on the human lung A549 cell line) [47]. Some researchers turn to more traditional methods, such as essential oils known since ancient times. They show very different effectiveness, depending on the bacterial strain and the plant from which they were isolated. These substances are unlikely to be effective enough to be used alone, but may be a useful adjunct to antibiotic therapy. A completely different approach is suggested by the work on phototherapy. This solution would be based on the local production of reactive oxygen species, but it would carry the risk of damaging nearby tissues. Vaccination (preventive and therapeutic) against Acinetobacter infections is another strategy. The difficulty is that there are around 40 serotypes among the A. baumannii strains, which would require vaccines to be multivalent. The most promising targets seem to be the OmpA protein and the conserved NucAb nuclease. However, in clinical practice, passive immunization, which consists in administering ready-made antibodies to the patient, might be more useful. It is therefore important to develop a monoclonal antibody (or antibody mixture) that can inactivate more than 90% of A. baumannii isolates. Another potential therapeutic option is to trap or mask metal ions (Fe, Zn), as limiting the availability of these ions inhibits bacterial growth. However, it should be borne in mind that this type of therapy could turn out to be a double-edged sword, due to the above-described effect of metal deficiency on virulence [42, 57, 75, 79, 100]. Improving the outlook for the control of A. baumannii is associated with a better understanding of the mechanisms of virulence and resistance of this species to antibiotics [6, 15].

A. baumannii is Gram-negative bacteria widely distributed in the environment. They are easily isolated from wet areas, but they are also able to survive on dry surfaces. As opportunistic pathogens, they are often a component of the physiological microbiota of animals and humans. Due to the special ability of A. baumannii to survive adverse conditions, exceptional durability, natural resistance to many groups of antibiotics, the ability to quickly acquire and develop a variety of resistance mechanisms, the narrowing range of drugs that can be used in therapy and the increasing number of isolates resistant to carbapenems these bacteria are a serious cause for concern and the intensified search for effective treatments.