Understanding candidal effect on antimicrobial resistance of clinical bacterial species: an evolutionary approach

Abstract

Ventilator-associated pneumonia (VAP) is a major healthcare-associated infection in critically ill patients and is frequently associated with biofilm formation on endotracheal tubes. These biofilms consist of diverse microbial communities, including bacteria and fungi, which enhance antimicrobial resistance and complicate treatment. Candida species are opportunistic fungal pathogens commonly implicated in nosocomial infections. Their strong biofilm-forming capacity and interkingdom interactions with bacteria may influence antimicrobial resistance phenotypes. This thesis investigates interactions between Candida species and Klebsiella pneumoniae isolated from polymicrobial biofilms in VAP, focusing on antimicrobial resistance and spontaneous mutation to resistance. We hypothesised that the presence of Candida in biofilms enhances Klebsiella resistance and mutation rates. The first experimental chapter examined antimicrobial susceptibility in mixed biofilm models treated with meropenem and fluconazole. Mixed biofilms of K. pneumoniae and Candida albicans demonstrated increased resistance compared to mono-species biofilms, indicating that polymicrobial interactions significantly affect treatment efficacy. The second chapter assessed bacterial mutation rates using fluctuation assays. Co-culture with Candida species resulted in a measurable reduction in K. pneumoniae mutation rates. Environmental factors such as ethanol, a potential Candida metabolite, may contribute to this effect and warrant further investigation. The final chapter analysed mutational spectra under mono- and co-culture conditions to characterise resistance to rifampicin. Genomic sequencing identified resistance-associated genes, highlighting the genetic adaptability of K. pneumoniae within polymicrobial communities. Overall, this study demonstrates that fungal–bacterial interactions within biofilms influence antimicrobial resistance in VAP. The presence of Candida modifies both resistance phenotypes and mutational dynamics in K. pneumoniae, underscoring the importance of considering fungal components in diagnostic and therapeutic strategies. Clinically, mixed-species biofilms may reduce the effectiveness of standard antimicrobial regimens, supporting consideration of combination or antifungal co-therapy in selected cases. Future work should refine in vitro models to better replicate lung conditions and identify fungal factors that modulate bacterial adaptation.