The fungus Candida albicans is found in the human microbiota and is usually non-pathogenic in healthy individuals, but it can cause life threatening infections. Nosocomial Candida infections are on the rise in the United States, primarily due to an increased number of invasive procedures, transplants, and use of broad range antibiotics and immunosuppressive agents. One important virulence factor in Candida species is its ability to transition between two morphologies: yeast and filamentous cells. Filamentous formation is controlled by several transcription factors that induce filamentation and several negative regulators that repress filamentation. Rfg1 is one of several partner proteins thought to function in combination with Tup1 to repress genes associated with filamentation and potentially influence the virulence of Candida. In fact, a Candida albicans mutant strain lacking Rfg1 was found avirulent in a mouse model, but overexpression of RFG1 has no effect on virulence. Here we investigate the negative regulators Tup1 and Rfg1 in Candida and their effect on interactions between Candida albicans and various bacteria.
In their natural environment, bacteria and unicellular eukaryotes are found together exhibiting both synergistic and antagonistic interactions. Our previous studies have documented decreases in the levels of both RFG1 and TUP1 when wild-type Candida was grown near Acinetobacter baumannii. Interestingly, during such bacterial-fungal coexistence the levels of Rfg1 and Tup1 are not consistent with the action of a repressor of filamentation. RFG1 and TUP1 transcription is usually elevated in yeast cells, but in the presence of Acinetobacter baumannii, their levels do not increase, instead actually decrease. These results suggest TUP1 and RFG1 are impacted by cellular signals that form part of the interactions between Candida and other commensals. Here, we looked at over-riding such decreases and how this influences the effects of bacteria on the morphology and hyphal specific gene transcription of Candida when grown as a biofilm.
Previously constructed tet-RFG1 and tet-TUP1 strains were used to investigate the effects of regulating the levels of RFG1 and TUP1 on the interactions with Acinetobacter baumannii. RFG1 overexpression affects biofilm formation by altering the morphology. Overexpression of TUP1 altered the population of cells present in the biofilm. Further exploration of modulated levels of RFG1 and TUP1 grown near A. baumannii showed somewhat alleviated inhibition of filamentation caused by the higher levels of RFG1 or TUP1.
These results not only confirm that Acinetobacter baumannii acts as a repressor of filamentation, but that RFG1 and TUP1 play a key role in changes observed during fungal-bacterial interactions and that role may be something other than filamentous repression. This will help uncover the multifactorial nature of filamentous repression in Candida albicans and the role of regulators in microbial communities. Our findings may have an impact on discovering new therapies for preventing Candida infections.
*This scholar and faculty mentor have requested that only an abstract be published.