Dartmouth Medical School professor Deborah Hogan outlined the chemical interactions between bacteria and fungi in a seminar hosted by the chemistry department last Thursday. Hogan studies the interactions between two species of microbes, the soil bacterium Pseudomonas aeruginosa and the dimorphic, unicellular fungi Candida albicans, in biofilms.
Although biofilms can be composed of a myriad of organisms, Hogan chose to study P. aeruginosa and C. albicans because they are both opportunistic pathogens that humans routinely encounter, they are often found in mixed infections, and there was some evidence that they interact with each other.
When co-cultures of the two organisms were inoculated on a lawn of fungi, a red spot appeared. This red pigment was associated with an increase in fungal cell death.
By screening mutants that do not produce the red coloring, Hogan discovered that the pigment is an intermediate in the biosynthesis of pyocyanin, a blue pigment characteristic of P. aeruginosa.. This intermediate (5MPCA) was enough to greatly reduce fungal viability, while products upstream and downstream in the pathway were either not sufficient or not necessary.
C. albicans concentrates the 5MPCA in its cytoplasm, Hogan found. The red pigment is heterogeneous, because the fungi modify it in a way that prevents it from diffusing out. As a result, 5MPCA has increased toxicity relative to similar compounds that are not modified. The redox activity of the pigment creates oxygen radicals that damage the cells. The red pigment is also fluorescent, making it a useful tool for localizing fungi.
Hogan also said that close contact is necessary for Pseudomonas to kill Candida. In liquid culture, only the hyphal and not the yeast forms of Candida were killed, because Pseudomonas can form biofilms on the filaments. Hogan also explained that they noticed a change in morphology from hyphae to yeast in the presence of Pseudomonas. This transformation makes it less susceptible to the bacteria, and less virulent to humans, because the hyphal form is associated with increased virulence.
Hogan explained that they screened Pseudomonas for a mutant that did not produce a white halo when grown on a strain of C. albicans with a hyphal-specific lacZ fusion protein. The change in morphology is due to a homoserine lactone molecule (3OCl2HSL) that serves as a quorum-sensing molecule for Pseudomonas. The molecule does not affect fungal growth rate, only fungal morphology, by inhibiting the Ras1 signaling pathway. Other 12-carbon backbone molecules show similar effects in C. albicans. Ras1 is highly homologous to Ras in humans, an oncogene that has been linked to many cancers.
Hogan said that future research directions might include exploring how 12-carbon compounds inhibit hyphal growth and whether these molecules can have an effect on the a human Ras homolog in treating cancer and 5MPCA derivatives for use as antifungal agents.