Staphylococcus pseudintermedius: Is it a real threat to human health?

Staphylococci are ubiquitous, Gram-positive bacteria. Most of them are classified as opportunistic bacteria which inhabit the skin and the mucous membranes of various mammalian species. The composition of the natural microbiota differs between species and even between individuals, but several species of staphylococci colonize humans and animals more frequently. These include S. aureus, S. epidermidis, S. haemolyticus, S. lugdunensis, S. saprophyticus, and S. warneri in humans [1]; S. felis, S. epidermidis, S. haemolyticus, S. xylosus, and S. aureus in cats [2, 3]; and S. pseudintermedius, S. aureus, S. haemolyticus and S. lentus in dogs [4, 5, 6, 7]. Carriage of coagulase-positive Staphylococcus aureus is thought to present the greatest risk to humans. However, in dogs, and less so in cats, a similar risk is associated with colonization by S. pseudintermedius [5, 8]. Although it is primarily a Staphylococcus isolated from animals, cases of colonization and infections caused by S. pseudintermedius in humans have been increasingly reported in recent years, especially in people who have close contact with their animals. This is especially true for people who own dogs, which are natural carriers of S. pseudintermedius [8, 9, 10, 11, 12, 13].

In taxonomic terms, this species falls into the Staphylococcus Intermedius Group (SIG), which includes three other coagulase-positive species: S. delphini and S. intermedius and the recently described S. cornubiensis [14, 15]. All staphylococci that belong to the SIG share many common features, such as the appearance of colonies (depending on the culture medium, they are usually round, smooth, shiny, opaque, white or grey colonies up to 1-2 mm in diameter after 24 hours of incubation), the ability to induce hemolysis on blood agar (presence of α- and β- hemolysins), rapid growth on nonselective media (e.g., nutrient agar – incubation at 37°C in 18–24 hours) and selective media (e.g., Chapman’s medium – growth at 37°C in 24–48 hours) and are virtually indistinguishable in standard bacteriological examination [8, 9, 14, 16]. Therefore, for many years only one species, S. intermedius, was identified. Only in 2005, after analysis of genetic differences between individual isolates, separate species, including S. pseudintermedius, were distinguished [17]. Biochemical properties that differentiate S. intermedius strains from other SIG species include the ability to produce arginine dihydrolase and acid from β-gentiobiose under aerobic conditions and from D-mannitol under anaerobic conditions [9, 18]. Unfortunately, biochemical reactions do not unambiguously distinguish between S. pseudintermedius and S. delphini strains [9, 18, 19]. Therefore, molecular biology methods such as polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP) using the restriction enzyme MboI [20] or multiplex-PCR detecting the gene encoding the thermonuclease (nuc) of coagulase-positive staphylococci [21] are necessary to identify SIG species. The use of mass spectrophotometry (matrix-assisted laser desorption ionization – time of flight mass spectrometry, MALDI-TOF MS) for assaying S. pseudintermedius from animals is also described, although some authors have demonstrated that the sensitivity of this method is still low [22].

The species in question produce various virulence factors, some of which are similar to those produced by S. aureus. These factors favor the colonization of different body regions and promote infections [8, 23]. The determinants of S. pseudintermedius pathogenicity include adhesion factors - surface proteins involved in adhesion to host cells, as well as binding proteins (protein A, clumping factor - CF) - exoenzymes, exotoxins and superantigens. The following superantigens deserve special attention: TSST-1 (toxic shock syndrome toxine), exfoliative toxins, coagulase, staphylokinase, hyaluronidase, haemolysins, proteases and nucleases. The leukotoxin (Luk-I) produced by S. pseudintermedius, which resembles the Panton-Valentine leukocidin (PVL) produced by S. aureus, shows toxicity against multinucleated cells and macrophages, as well as CXCR2 receptor-presenting myeloid cells [8, 23, 24, 25]. Luk-I consists of the proteins LukS-I and LukF-I, which induce cell lysis [25]. Maali et al. [25] also indicate the production of δ-toxin and PSM, which exhibit potent cytotoxic activity against “non-professional phagocytic cells” (NPPc), e.g. dog keratinocytes, human epithelial cells and osteoblasts.

S. pseudintermedius can grow as a biofilm. Initial adhesion to the surface is mediated by MSCRAMM (microbial surface components recognizing adhesive matrix molecules) components such as spsE, ebpS and atl. Once adhered to the surface, an extracellular matrix is formed [26] [polysaccharide intercellular adhesion molecule – PIA (polysaccharide intercellular adhesin) or PNAG (polymeric N-acetyl-glucosamine)], which is encoded by the ica operon containing 4 distinct genes (A, B, C and D) [27, 28]. This is currently the best-known mechanism of biofilm formation in staphylococci.

A significant phenomenon that occurs in the bacterial biofilm is the ability of microorganisms to communicate using a “quorum-sensing” mechanism. This system is associated with the activity of the agr gene (accessory gene regulator) - agrA, agrB - involved in the regulation of biofilm structure and subsequent detachment of biofilm fragments [26]. However, the influence of the agr gene on the expression of ica and production of PIA/PNAG has not been demonstrated [28]. In staphylococci, cases of ica-independent biofilm production have also been reported, which are associated with the presence of the biofilm-associated protein (bap) and the transcription regulator rbf (regulator of biofilm formation) [28]. Staphylococcal biofilm is particularly difficult to control because it has increased resistance to antimicrobials and host immune response mechanisms, thus making antibiotic therapy difficult [8, 10, 23, 29]. In addition, as biofilm forms, strains permanently colonize abiotic surfaces, and this complicates the use of materials such as catheters, stents, and implants in human and veterinary medicine [29]. Biofilm formation by S. pseudintermedius has been studied by various authors, but it is still not fully understood and described. It has been shown that the percentage of S. pseudintermedius strains that were capable of forming a biofilm was up to 100% among the bacteria isolated, for example, from skin infections, wounds, hospital infections and surgical wounds [10, 30, 31]. Strains of S. pseudintermedius belonging to the dominant European ST71 type show a high ability for biofilm production and are often resistant to all types of antimicrobial drugs used in veterinary medicine [32, 33].

As in the case of S. aureus, increasing resistance to commonly used chemotherapeutics has been observed among S. pseudintermedius strains in recent years. Methicillin-resistant S. pseudintermedius (MRSP) strains are isolated both from humans and animals [8, 13, 31, 34, 35, 36, 37]. Resistance to β-lactam antibiotics is most often associated with the presence of the mecA gene within the staphylococcal chromosomal cassette mec (SCC mec). As a mobile genetic element (MEG), it can be transferred horizontally between strains of the same or different Staphylococcal species [8, 9, 24, 36]. The SCCmec cassette is composed of the methicillin resistance determinant (mecA, mecB, or mecC) contained in the mec gene complex and includes site-specific ccr recombinase genes that are responsible for insertion of the cassette into the genome. It can also include a wide range of additional genes [38, 39, 40, 41]. SCCmec types were initially defined due to their combination of the mec gene complex class and the ccr recombinase gene. However, the classification of SCCmec elements and the use of the terminology have been complicated by the existence of composite cassettes and pseudo-SCCmec elements that do not contain ccr genes [41]. Currently, there are eleven SCCmec cassettes of MRSA strains in the International Working Group on the Staphylococcal Cassette Chromosome (IWG-SCC) database. They are numbered I to XI, according to the order in which they are described [41]; 13 major types of SCCmec cassette elements have been identified so far [40]. Although chromosome cassette sequences from strains other than S. aureus often vary from those identified in Staphylococcus aureus, all are classified as ccrA, ccrB, or ccrC [39]. In staphylococci other than Staphylococcus aureus, several additional ccr allotypes have been reported. For example, in S. pseudintermedius KM241, ccrA5 was identified as a counterpart to the ccrA gene [39]. Several SCCmec MRSP cassettes have been described, including SCCmec III (previously known as II-III), which is found in the globally dominant type ST71 as well as ST316 and ST25, variants of SCCmecVT, (ST496, ST64 and ST751) and the newly reported SCCmecNA45, which contains the mec class C1 gene and the ccrC recombinase gene.

Other SCCmec elements that are not recorded by IWG-SCC have also been described in S. pseudintermedius, for example SCCmec57395, which does not contain ccr genes, and SCCmecKM241 and SCCmecAI16 [38, 41]. As reported by Krapf et al. [40] in Central Europe, most MRSP isolates contain SCCmec I II elements, followed by S CCmec IV and least frequently by SCCmec V /VT. Furthermore, S CCmec elements found in S. pseudintermedius have been shown to have significant similarity to those identified in MRSA strains, which suggests that interspecies transfer of mobile genetic elements is involved [8, 41].

Up to 92% of healthy dogs are carriers of S. pseudintermedius [25, 32]. This Staphylococcus can be isolated from many anatomical sites in healthy animals, including the ears, conjunctival sacs, nostrils, oral cavity, skin of the groin, and perineum [8, 11, 12, 13]. MRSP strains were most commonly isolated from dogs from the nose and anus, but these were also the most common sampling sites [8]. Carriage of S. pseudintermedius can be permanent, intermittent, or short-term [42], though the types of carriage have not been specified and are only defined by researchers for specific studies. Hartman et al. [42] looked at the number of isolations of S. pseudintermedius strains over a 12-month period and the relatedness of the bacteria when assessing carriage in dogs. They concluded that a single isolation of S. pseudintermedius within a given time period indicates short-term carriage, that at least two isolations of the bacteria (but not in consecutive swabs) are defined as intermittent carriage, and that two or more isolations in consecutive swabs from the same body region are interpreted as permanent carriage. Importantly, these studies also show that animals colonized by S. pseudintermedius permanently or intermittently are colonized with higher numbers of S. pseudintermedius cells than those colonized on a short-term basis, which increases the risk of S. pseudintermedius infections in these dogs and the potential for transmission to other animals and humans [43].

Depending on the study and the pattern of specimen collection, the isolation rate of S. pseudintermedius in healthy dogs ranges from 24.4-92%, while the percentage of MRSP strains ranges from 0-36.84 [44, 45, 46, 47, 48]. This Staphylococcus is considered to be the most common cause of purulent dermatitis in dogs [8, 49], and is also isolated from clinical specimens in wound infections, surgical wounds, external ear, upper respiratory tract, urinary and reproductive tract, and bone and bone marrow [9, 23, 40, 50, 51, 52]. Studies show that certain conditions, such as atopic dermatitis in dogs, promote more frequent colonization by S. pseudintermedius not only of the skin but also of the external auditory canal and conjunctival sacs [9, 53, 54]. There have also been reported cases of puppy death due to sepsis from S. pseudintermedius infection in the first two weeks of life. It has also been proven that colostrum and milk from female dogs can be a potential source of these bacteria for puppies [55, 56].

Carriage of S. pseudintermedius in cats is described less frequently than in dogs [3, 46]. According to some authors, felids are not natural hosts of S. pseudintermedius [9]. Data on the frequency of isolation of this staphylococcal species in cats are still fragmentary. Hanselman et al. [46] described the isolation of S. pseudintermedius in 6.8% of cats studied, of which the percentage of animals carrying MRSP strains was 1.2%. In the study carried out by Bierowiec et al. [2], this Staphylococcus was isolated more frequently from cats with bacterial infections of the skin and mucous membranes than from clinically healthy cats. According to previous reports, S. pseudintermedius has been isolated in cats from infections of the upper respiratory tract, conjunctiva, urinary tract, external auditory canal, skin, and wounds [2, 46, 57, 58]. Isolation of this species from lung tissue has also been documented [25].

In both dogs and cats, certain risk factors for S. pseudintermedius infection have been reported, including infection with MRSP strains. In particular, a strong correlation between animal infection and previous hospitalizations (including for surgical procedures), frequent veterinary office visits, administration of glucocorticoids, and treatment with antimicrobial chemotherapeutics has been demonstrated [18, 40, 59]. There are also reports of possible staphylococcal contamination of veterinary facilities and their medical equipment. This suggests the need for special attention to hygiene and disinfection of surfaces and equipment in veterinary facilities to reduce the risk of pathogen transmission to patients and clinicians [32, 60, 61, 62].

There are increasing reports of S. pseudintermedius being isolated in humans; however, human infections caused by S. pseudintermedius are rarely reported [13, 63]. Colonization of people in contact with dogs or cats is more commonly described and affects 0.4-8.9% of owners or professional animal handlers [12, 35, 64, 65]. The prevailing opinion is that human colonization with this Staphylococcus is temporary [64] and affects mainly immunosuppressed or elderly people [11, 35, 66]. There are, however, some reports that stress the possibility of long-term colonization of human skin and mucous membranes by MRSP strains [65] and of infections appearing in people without immune deficits [64]. Most likely, though, the prevalence of this Staphylococcus in humans is underestimated due to the misidentification of S. pseudintermedius strains as S. aureus [11, 65, 66, 67].

The most commonly identified site of S. pseudintermedius isolation in humans has been the nasal cavity and soft tissues [11, 64, 66]. This may be because a significant proportion of infections caused by this pathogen presented with acute or chronic rhinosinusitis [11, 64, 65]. The bacteria were also isolated from cases of endocarditis, sepsis, deep abscesses, infection of wounds resulting from dog bites, pneumonia, arthritis, skin and soft tissue infections with ulceration and necrosis [11, 12, 13, 63, 64, 67, 68, 69, 70, 71]. Researchers point to the high pathogenic potential of S. pseudintermedius in humans, comparable to that described in S. aureus [69]. In addition to its ability to colonize human tissues, this species may also limit the growth of other bacteria that are components of the natural microbiota of the skin and mucous membranes [10]. There has also been a suggestion that MRSP may colonize humans more frequently than MSSP [65]. The significant resistance to antimicrobial chemotherapeutics and the ability to produce biofilm of S. pseudintermedius strains isolated from humans is also alarming [64]. It is also associated with the rapid spread of the dominant clone ST71 among humans in Europe [65, 66, 72].

Many authors note that people in contact with animals, especially dogs, are much more likely to be colonized [12, 32, 40, 64, 66, 70]. Nevertheless, cases of S. pseudintermedius infection have also been described in individuals with no previous contact with animals [67, 71]. An increased risk of colonization among veterinarians and veterinary assistants has also been cited [22, 35, 65]. It has also been shown that animal dust and dander may be involved in the transmission of MRSP to humans within the household [64].

The cases of infections caused by S. pseudintermedius, both in humans and animals, reported so far indicate that this Staphylococcus has a similar pathogenic potential to Staphylococcus aureus. Therefore, proper and judicious use of antimicrobial chemotherapeutics and adherence to hygienic standards seem essential to limit the spread of these bacteria between humans and animals. Because for years S. pseudintermedius (previously classified as S. intermedius) was considered a typically animal species of coagulase-positive staphylococci, laboratory procedures that favored identification of the more common human Staphylococcus aureus over S. pseudintermedius were widely used. Thus, it is now believed that the percentage of humans colonized by S. pseudintermedius, as well as the incidence of infection, may be higher than reported in the literature.