ABSTRACT
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.