Rare Case of Psychrobacter Sanguinis Bacteremia in a Homeless Patient with Thigh Phlegmon

Microorganisms have an incredible ability to thrive in diverse environments and under extreme, rare, and challenging conditions and cause infection in humans. The genus Psychrobacter comprises a variety of Gram-negative chemoheterotrophic, non-motile strict aerobes, which are osmotolerant, spherical or cylindrical in shape, and adapted to life at low temperatures. Psychrobacter spp. are primarily isolated from various sources, including ecosystems with wide variations in temperature and salinity, and from glacial ice, chilled meat and fish, and human clinical materials, such as blood or cerebrospinal fluid. Psychrobacter spp. may also be a component of the human microbiota – studies have shown the presence of P. arenosus, P. faecalis, P. phenylpyruvicus and P. pulmonis in the human intestine (Lager et al. 2016).

The genus Psychrobacter has only recently become better known after intensive 16S rRNA sequencing studies on their presence in various environments. The findings indicate that Psychrobacter spp. is a common and evolutionarily successful taxon whose biology can provide important information about environmental adaptation and survival. In the laboratory, it can be cultured at 30–37°C on brain heart infusion (BHI) medium or nutrient agar enriched with 5% blood (Lager et al. 2016).

The genus Psychrobacter was described initially by Juni and Heym in 1986 (Juni and Heym 1986), and at that time included only the species Psychrobacter immobilis. Since then, the genus has rapidly acquired further species, mainly due to the increasing exploration of marine and polar ecosystems (Brenner et al. 2005). Currently, The International Code of Nomenclature of Prokaryotes (ICNP) recognizes 44 valid species within the genus that belong to the family Moraxellaceae (LPSN 2024).

Based on the limited number of published case reports, Psychrobacter spp. are considered opportunistic pathogens (Lager et al. 2016; Deschaght et al. 2012). Clinical manifestations include bacteremia (Caspar et al. 2013; Leung et al. 2006; Guttigoli and Zaman 2000), meningitis (Ortiz-Alcántara et al. 2016; Le Guern et al. 2014), surgical wound infection (Stepanovic et al. 2007), and eye infection (Gini 1990), depending on the site of infection. Of these cases, only one was related to exposure to a marine environment; in this case, the patient developed bacteremia caused by P. phenylpyruvicus after eating raw mussels (Leung et al. 2006).

Psychrobacter sanguinis is an opportunistic pathogen with limited documented cases of human infection. While Psychrobacter spp. are commonly isolated from cold environments, their role in human bacteremia remains poorly understood. This case report highlights the challenges associated with identifying and treating Psychrobacter sanguinis infections, particularly in vulnerable populations such as homeless individuals with compromised immunity.

The present case study describes the presence of P. sanguinis, first described by Wirth et al. (2012) as the etiological factor of bacteremia in an 84-year-old man in the USA, in bacteremia in a 69-year-old homeless man. The bacterium was cultured from a positive blood sample in a BacT/ALERT 3D system (bioMerieux, France). A microscope preparation from the blood confirmed the presence of Gram-negative cocci (Fig. 2), and the growth of P. sanguinis colonies was noted on Columbia Agar w 5% Sheep Blood after incubation at 35°C for 24 hours (Fig. 3). An isolate was primarily identified as P. phenylpyruvicus using a VITEK MS matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometer (bioMerieux, France). Only the analysis of the 16S rRNA sequence, ordered for this case report, confirmed that the bacterium was P. sanguinis, or the new species Psychrobacterraelei proposed in 2024 (isolated only from a dog with peritonitis) (Manzulli et al. 2024).

Fig. 2.

Microscope view of gram-stained Psychrobacter sp. described in the report.

Fig. 3.

The growth of Psychrobacter sp. described in the report on Columbia Agar with 5% sheep blood.

The present case highlights a rare instance of P. sanguinis bacteremia, with only a limited number of previously documented cases in the literature (Wirth et al. 2012; Le Guern et al. 2014). A comparison with these reports underscores the unique aspects of this case, particularly the presence of a severe soft tissue infection in a homeless patient with multiple risk factors, including trauma, poor hygiene, and likely chronic immunosuppression due to lifestyle-related factors. In contrast to previously reported cases, which have often involved nosocomial settings, this case suggests that Psychrobacter spp. may be capable of causing severe infections outside of hospital environments, possibly through environmental exposure.

The Psychrobacter sp. antibiogram was assessed based on the disc diffusion method. Due to the lack of appropriate European Committee on Antimicrobial Susceptibility Testing (EUCAST) recommendations, the results of P. sanguinis were interpreted using the breakpoint values recommended for non-fermenting Gram-negative bacilli (Table I). Previous studies have shown that Psychrobacter spp. clinical isolates were susceptible to the following antibiotics: amoxicillin, amoxicillin with clavulanate, ticarcillin with clavulanate, piperacillin, piperacillin with tazobactam, cefalotin, cefotaxime, ceftazidime, cefpiroa, cefepime, imipenem, gentamicin, trimethoprim/sulfamethoxazole, nalidixic acid, ofloxacin, ciprofloxacin and fosfomycin (Caspar et al. 2013; Leung et al. 2006; Guttigoli and Zaman 2000).

The molecular identification was performed by analysis of the 16S rRNA gene to identify the bacterium. In the PCR reaction, the 16S rRNA gene was amplified using the following primer pair: forward, 27F: 5′-AGA GTT TGA TCM TGG CTC AG-3′ and reverse, 1492R: 5′-TAC GGY TAC CTT GTT ACG ACT T-3′. The obtained sequences were quality-checked and trimmed using FastQC and Chromas v. 2.6.6 software. A contig sequence of 1101 base pairs was obtained using the DNA Base Assembler v. 5.15.0 tool and deposited in GenBank under accession number PP892960.

The sequence was analyzed using the GenBank Basic Local Alignment Search Tool (BLAST). The sequence shared 99.91% homology with a reference sequence of P. sanguinis (GenBank: KR232928.1) and P. raelei (Gen-Bank: MK771149.1). Our sequence showed a point mutation at T1074A compared to P. raelei (GenBank: MK771149.1) and at G514A compared to P. sanguinis (GenBank: KR232928.1).

The evolutionary history was inferred using the Neighbor-Joining method (Saitou and Nei 1987). The optimal tree is shown in Fig. 4. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (500 replicates) are shown below the branches (Felsenstein 1985). The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Maximum Composite Likelihood method (Tamura et al. 2004) and given as the base substitutions per site. This analysis involved 22 nucleotide sequences. All ambiguous positions were removed for each sequence pair (pairwise deletion option). A total of 1109 positions were included in the final dataset. Evolutionary analyses were conducted in MEGA11 (Tamura et al. 2021).

Fig. 4.

Neighbor-joining phylogenetic tree based on 16S rRNA gene sequences from Psychrobacter spp. type strains, Psychrobacter sp. isolate PP892960 from our study, and two extra Psychrobacter sanguinis strains. Bootstrap values (>50%) are indicated at branch nodes.

The identification of S. pyogenes in the wound culture highlights the complexity of wound infections. S. pyogenes is a well-known pathogen responsible for various soft tissue infections, including cellulitis, necrotizing fasciitis, and phlegmon (Brouwer et al. 2020). Given the patient’s poor hygiene and prolonged exposure to unsanitary conditions, the risk of polymicrobial contamination was significantly increased (Lavigne et al. 2019). The presence of S. pyogenes may have weakened the immune response, creating an environment conducive to secondary infections, such as Psychrobacter sanguinis bacteremia. Opportunistic pathogens like Psychrobacter spp. typically exploit immune system dysregulation, which can arise due to prior infections, trauma, or prolonged antibiotic therapy (Brouwer et al. 2020). This underscores the importance of comprehensive microbiological evaluation and targeted antimicrobial treatment in complex, polymicrobial infections.

From a clinical perspective, the detection of P. sanguinis in a bloodstream infection raises the question of whether this species should be considered a true pathogen or an incidental environmental contaminant. While previous reports have suggested that Psychrobacter spp. are opportunistic rather than primary pathogens, their repeated isolation from human infections, including bacteremia and meningitis, supports their potential role as emerging infectious agents (Caspar et al. 2013; Ortiz-Alcántara et al. 2016). This case suggests that P. sanguinis should be considered in differential diagnoses in laboratories equipped with modern bacterial identification technologies, such as MALDITOF mass spectrometry or next-generation sequencing (NGS). Further epidemiological studies are needed to determine the prevalence and pathogenic potential of Psychrobacter spp. in human infections.

In conclusion, this case report emphasizes the need for targeted antibiotic therapy and surgical management in complex infections. Our findings contribute to the limited but growing body of literature on Psychrobacter spp. as human pathogens and illustrate the challenges in diagnosing and treating infections caused by rare bacteria. Given the increasing detection of Psychrobacter spp. in human infections, further research is warranted to define better its clinical relevance, pathogenic potential, and optimal therapeutic approaches.