Detection and Clinical Implications of Biofilm Formation Among Clinical Isolates of Sphingomonas paucimobilis in Turkey

Sphingomonas paucimobilis (formerly Pseudomonas paucimobilis) is an aerobic, non-fermentative, Gram-negative bacillus characterised by production of a yellow pigment known as nostoxanthin, slow motility with a single polar flagellum, and positive oxidase and catalase reactions (1). Sphingomonas paucimobilis is distributed widely in nature, especially in water and soil (2). It is an oligotrophic bacterium and can be isolated from low-nutrient environments, such as drinking water distribution systems, water treatment plants, tap water, and water demineralisation filters and biofilms collected from water system (3-6). This bacterium was even isolated from water supply and humidity condensate samples gathered from the international space station (7).

In hospital settings, S. paucimobilis is seen in sporadic cases of various infections, such as catheter-related sepsis, primary bacteraemia, pneumonia, meningitis, peritonitis, septic arthritis, and urinary tract infections, and small outbreaks caused by contaminated hospital devices and contaminated intravenous fluids (8-17). The presence of indwelling devices, an impaired immune system, and co-morbidities such as malignancy and diabetes mellitus are reported risk factors for S. paucimobilis infection (14, 15, 18, 19). Biofilm production is a well-known bacterial virulence factor and causes treatment failure, particularly in patients with indwelling devices (20). Biofilm production by S. paucimobilis has been investigated extensively in non-clinical, but not clinical, isolates (4, 6, 21, 22).

This is the first study to assess biofilm production by S. paucimobilis clinical isolates, together with patient demographics, clinical aspects, risk factors, and outcomes.

Sphingomonas paucimobilis infection was identified in 33 patients (17 females, 16 males). The mean age of the patients was 61.2 years, ranging from a 1-year-old male to a 78-year-old female. The infections included 27 nosocomial and six community-acquired infections. Sphingomonas paucimobilis was isolated most commonly from blood samples (n = 16) as primary bacteraemia. In addition, seven cases of pneumonia, six cases of wound infection, and four cases of urinary tract infection were detected. Twenty-four of the patients were followed in the intensive care unit. Twenty-two patients had indwelling catheters. Malignancy (n = 11) and diabetes mellitus (n = 10) were the most common concomitant diseases. Biofilm formation by all isolates was determined and the degree of biofilm formation and other data are summarised in Table 1.

Table 2 shows the antimicrobial susceptibility test results. Tigecycline (84.85%) was the antimicrobial agent to which the isolates were most frequently susceptible, together with carbapenems and aminoglycosides, whereas fewer isolates were susceptible to ampicillin (21.21%), ampicillin/sulbactam (24.24%), cefuroxime (24.24%), and cefuroxime axetil (24.24%).

All isolates produced biofilms. Table 3 shows the correlations of biofilm formation intensity with demographic data and clinical variables. The intensity of biofilm formation was correlated only with blood samples (P = 0.003) in the sample types group and a stay in the intensive care unit (P = 0.002) in the risk factors group. No other variable was correlated with biofilm formation intensity (P > 0.005).

Sphingomonas paucimobilis was first isolated from the leg ulcer of a Japanese seaman in 1979 (26). Since then, many case reports, case series, and outbreaks have been presented. Clinical infections related to S. paucimobilis include bacteraemia, peritonitis, wound infections, adenitis, diarrhoeal disease, sepsis arthritis, osteomyelitis, and meningitis (8, 13, 18). Outbreaks have also been attributed to contaminated water sources or contaminated intravenous fluids (10, 16, 17, 27). Bacteraemia is the most common clinical infection caused by S. paucimobilis (8, 15, 18). In our series, 48.48% of the cases were identified as bacteraemia, followed by pneumonia, wound infections, and urinary tract infections. Most of the cases (78.79%) were nosocomial in origin. According to the reviews of Lin et al., Huesh et al., and Cheong et al. (8, 15, 19), most S. paucimobilis infections are healthcare associated. However, Toh et al. investigated 55 cases of S. paucimobilis infection and Bayram et al. evaluated 24 paediatric patients; both reported that the incidence of community-acquired infections was higher than that of nosocomial cases (14, 28). Since these studies assessed relatively few cases, it is impossible to reach a reliable conclusion regarding the source of such infections.

In many case reports, indwelling catheters and an immunosuppressed host were identified as risk factors for S. paucimobilis infections (8, 9, 18, 19, 28-30). Community-acquired infection, diabetes mellitus, and alcoholism were determined as risk factors for primary bacteraemia in the multivariate logistic regression presented by Toh et al. (14). Comorbidities such as malignancy and diabetes mellitus were also reported to be risk factors (18, 19). In our study, the most common concomitant diseases were malignancy (33.33%) and diabetes mellitus (30.30%). The presence of an indwelling catheter (72.73%) and hospitalisation in the intensive care unit (66.67%) were the most common risk factors in our study.

The antimicrobial susceptibility patterns of S. paucimobilis differ among studies. Tigecycline (84.85%) was the agent to which S. paucimobilis was most frequently susceptible in this study, but there are no data in the literature for comparison. Sphingomonas paucimobilis was also found to be sensitive to imipenem (75.76%) and meropenem (75.76%), netilmicin (78.79%), and amikacin (69.70%). Toh et al. and Bayram et al. (14, 28) reported that fluoroquinolones, carbapenems, and trimethoprim/sulfamethoxazole were the most effective antibiotics. Lin et al. stated that S. paucimobilis was frequently sensitive to fluoroquinolones, carbapenems, and beta-lactam/beta-lactamase combinations (19). Conversely, in our study, cephalosporins and beta-lactam/beta-lactamase were found to be less effective. In our study, the antimicrobials most commonly used for S. paucimobilis infections were carbapenems and third-generation cephalosporin/aminoglycoside combinations. With the exception of two patients who died of clinical syndromes other than infections, the remaining 31 patients were treated successfully. Some researchers suggest third-generation cephalosporin/aminoglycoside combinations for the treatment of this type of infection (8, 18, 19). However, standardised therapies cannot be established at present, because of the different antimicrobial susceptibility patterns among studies. Relying on antimicrobial susceptibility testing results is the most logical and appropriate approach, as always.

Demographic data showed no correlation with the degree of biofilm formation. However, biofilm formation intensity was significantly higher in primary bacteraemia (P < 0.005). Intensive care unit stay was also positively correlated with a high degree of biofilm production. Higher biofilm production is expected to correlate with the presence of an indwelling catheter, but that was not the case in our study (P = 0.346). This situation might be due to the relatively small number of isolates. There was no correlation between the degree of biofilm formation and presence of concomitant diseases. Studies including more patients and isolates are needed to explain the correlation of biofilm production with such variables.

Previously, this bacterium was regarded as being of low virulence (18, 19, 28, 31). In a recent study, however, the S. paucimobilis virulence factors were reported to resemble those of Pseudomonas spp. These factors included proteases (alkaline protease, LasA, LasB), adherence factors (lipopolysaccharide, type IV pili), iron uptake (pyoverdin, pyochelin), a biosurfactant (rhamnolipid), and antiphagocytosis factors (alginate) (32). Type IV pili and alginate production are also involved in biofilm formation. We determined biofilm production in all clinical isolates of S. paucimobilis included in this study. Therefore, biofilm production by S. paucimobilis can be accepted as an important virulence factor, especially in patients in intensive care units and those with suspected primary bacteraemia.