OBJECTIVES: In polymicrobial infections the presence of one microorganism may contribute to increase the pathogenesis of another. Candida albicans and the bacterial pathogen Staphylococcus aureus are the most frequent combination of microorganisms isolated from the human host. Our preliminary studies had described the complex and dynamic interactions between these two pathogens with important clinical and therapeutic implications. In this study, we aimed to elucidate the mechanisms orchestrating inter-species interactions with pathogenic implications, namely those mediated by quorum sensing molecules. Specifically, we investigated the impact of the C. albicans secreted quorum sensing molecule farnesol on S. aureus on a molecular level.
RESULTS: Fluorescence microscopy demonstrated accumulation of intracellular ROS in the farnesol treated cells (Fig. 1). Comparative transcriptional analysis of sensitized and desensitized cells relative to each other and the respective controls demonstrated major transcriptome alterations involving key regulatory pathways. A total of 769 genes were found to be differentially regulated in the farnesol-sensitized cells compared to control cells and 492 in the de-sensitized compared to sensitized cells. Most notable among the up-regulated genes in the farnesol sensitized cells are redox sensors and global regulators involved in ROS detoxification via the thiol-specific redox system (Fig. 2, 3), specifically, the oxidant sensors SarA and Spx that induce transcription of antioxidant enzymes such superoxide dismutases (SODs), catalase (KatA) and alkyl hydroperoxide reductase (Ahp). In S. aureus, once ROS mediated cell damage occurs, cells express several enzymes including thioredoxin/thioredoxin reductase (Trx) and methionine sulfoxide reductases (Msr) to repair protein damage. Interestingly, in desensitized cells, expression of the global regulators SpxA and SarA remained high, even after the external pressure with farnesol is removed. Functional categorization of genes in sensitized cells indicated down-regulation of various metabolic and biosynthesis pathways, as well as pathogenesis associated genes. In contrast, in desensitized cells, genes involved in S. aureus virulence such as hemolysins, toxins, host tissue binding proteins and cell wall adhesins were significantly upregulated compared to the sensitized cells, as well as their control cells not previously exposed to farnesol.  
CONCLUSIONS AND FUTURE DIRECTIONS: The findings from this study demonstrate that farnesol induces a stress response by the bacterial cell via the thiol-based redox system under control of major global regulators of virulence. Collectively, the findings support the hypothesis that farnesol-induced transcriptional modulations of key regulatory networks and pathways may impact S. aureus fitness and virulence, and, in turn, the pathogenesis of mixed C. albicans-S. aureus infections. Functional analyses and in vivo studies to evaluate the pathogenic potential of the farnesol sensitized cells are underway using mouse models of systemic staphylococcal infection.

3 / Major transcriptional modulations in S. aureus induced by the C. albicans secreted quorum sensing molecule farnesol