RESUMEN
Competition for nutrients in a polymicrobial biofilm may lead to susceptible species being subjected to nutritionalstress. The influence of bacterial growth rates and interspecies interactions on their susceptibility and response tonutritional stress is not well understood. Pseudomonas aeruginosa and Staphylococcus aureus are two prevalentcausative pathogens that coexist in biofilm-associated infections. Despite being the slower-growing species, P.aeruginosa dominates in a two-species biofilm by inducing phenotypic switching of S. aureusto a metabolicallychallenged small colony variant (SCV) via the release of 2-heptyl-4-hydroxyquinoline N-oxide (HQNO). Wehypothesize that P. aeruginosa experiences nutritional stress in competition with S. aureus, and that the release ofHQNO is an adaptive response to nutritional stress. We present an individual-based two-species biofilm model inwhich interactions between entities induce emergent properties. As the biofilm matured, the difference in growthrates of the two species caused a non-uniform distribution of nutrients leading to nutritional stress for P. aeruginosa and a concurrent increase in the proportion of S. aureus subpopulation. The latter resulted in increasedrelease of autoinducer, and subsequently the upregulation of P. aeruginosa cells via quorum sensing. UpregulatedP. aeruginosa cells released HQNO at enhanced rates, thereby inducing phenotypic switching of S. aureus toSCVs which consume nutrient at a reduced rate. This shifted the nutrient distribution back in favor of P.aeruginosa, thereby relieving nutritional stress. Increase in nutritional stress potentiated the transformation of S.aureus into SCVs. HQNO production decreased once nutritional stress was relieved, indicating that phenotypicswitching acts as a regulatory stress-adaptive response.
RESUMEN
Staphylococcal isolates from specimen submitted to the Medical Microbiology laboratory of Ahmadu Bello University Teaching Hospital, Zaria were collected over a period of 6 months (February-July 2012), characterized by microbiological standard procedures and the S. aureus small colony variant (SCV) isolates were isolated. The antibiotic susceptibility pattern of the isolates was determined by the Kirby-Bauer-CLSI modified disc agar diffusion (DAD) technique. The SCV isolates were assessed for the carriage of four virulence genes; sdrE (putative adhesin) icaA (intracellular adhesin) hlg (hemolysin), Cna (collagen adhesin). A total of 258 non-duplicate staphylococcal isolates made up of 219 (84%) S. aureus and 39 (15%) coagulase-negative staphylococci (coNS) where obtained. A total of 48 (22%) isolates were determined to be S. aureus SCV mainly from wound/abscess (31%). S. aureus SCV isolates were generally resistant to all the nine antibiotics tested with only minimal sensitivity to tigecyclin (10.4%) and ciprofloxacin (18.8%). Statistically, there was no significant difference between the microbial load and the different antibiotics that were used, (P ≥0.05). None of the S. aureus SCV isolates carried the four virulence genes which were tested in this study. The results have therefore proved that S. aureus small colony variant exist in our environment and they are more resistant to most antimicrobial agent than their wild type.