Investigation of antibiotic resistance genes, biofilm formation and clonal relationship of Stenotrophomonas maltophilia strains isolated from clinical specimens

Stenotrophomonas maltophilia is an obligate aerobic, non-fermentative bacterium and one of the important Gram-negative pathogens. It has emerged as an increasingly reported opportunistic pathogen, particularly in hospitalized, immunocompromised patients, cancer patients, and individuals with cystic fibrosis. The ability to adhere to both biotic (epithelial mucosal tissues) and abiotic (medical device) surfaces and to form biofilms is a key virulence factor of S. maltophilia. Due to its broad intrinsic resistance to antibiotics and chemotherapeutic agents, it is difficult to treat, and increasing resistance rates to the commonly used treatment agent trimethoprim-sulfamethoxazole (TMP-SMX) have been reported. The emergence of new antimicrobial resistance mechanisms in S. maltophilia necessitates the monitoring and reporting of antimicrobial susceptibility testing of its isolates. In this study, it was aimed to determine the antibiotic susceptibilities, biofilm formation, the presence of biofilm-associated rmlA and rpfF genes, TMP/SMX resistance-related sul1 and sul2 genes, and to investigate the molecular epidemiology of S. maltophilia isolates obtained from various clinical specimens sent to the Medical Microbiology Laboratory of Pamukkale University Faculty of Medicine Research and Training Hospital. Isolates were identified using the Phoenix ID/MIC (Becton Dickinson, USA) automated identification system or the MALDI-TOF MS Biotyper system. Among the isolates included in the study, 39 (39%) were from sputum, 26 (26%) from respiratory tract samples, 16 (16%) from bloodstream infections, 14 (14%) from wound samples, 4 (4%) from urine, and 1 (1%) from cerebrospinal fluid (CSF). In terms of hospital departments, 53 (53%) of the isolates were from various intensive care units, 40 (40%) from internal medicine departments, and 7 (7%) from surgical units. Antibiotic susceptibility testing was performed using the Kirby-Bauer disk diffusion method, and zone diameters were interpreted according to the breakpoints of the Clinical and Laboratory Standards Institute (CLSI). According to the antibiotic susceptibility test results, 90% of the 100 S. maltophilia isolates were susceptible to TMP/SMX, 92% to levofloxacin, and 53% to minocycline. Among all isolates, 95 (95%) were negative for the sul1 gene, while 5 (5%) were positive. All isolates were negative for the sul2 gene. Biofilm formation was evaluated using the microtiter plate method. Biofilm formation was observed in all isolates: 40 (40%) showed strong biofilm formation, 32 (32%) moderate, and 28 (28%) weak. The biofilm-related rmlA and rpfF genes were also investigated. Of all isolates, 91 (91%) were positive for rmlA, while 9 (9%) were negative. Regarding the rpfF gene, 44 (44%) isolates were positive, and 56 (56%) were negative. According to PFGE results, 23 different clusters and 95 distinct PFGE types were identified. The largest cluster was cluster Q, consisting of 17 isolates. Other clusters were S (14 isolates), G (9 isolates), T (7 isolates), H (6 isolates), A (5 isolates); D, I, P, V (4 isolates each); K, N, R (3 isolates each); and B, C, E, F, J, L, M (2 isolates each). Only clusters O, U, and W had a single isolate. Among the 17 isolates in cluster Q, 10 were from ICUs, 6 from internal medicine departments, and 1 from a surgical unit. In terms of specimen type, the cluster Q isolates consisted of 7 from sputum, 7 from tracheal aspirates, 2 from wound samples, and 1 from a blood sample. All sul1 positive isolates were distributed among different clusters. The clustering rate based on PFGE types was found to be 8%. Keywords: Stenotrophomonas maltophilia, TMP-SMX resistance, biofilm, clonal relationship