Stress-Tolerant Yeasts: Opportunistic Pathogenicity Versus Biocontrol Potential

Abstract

Stress-tolerant fungi that can thrive under various environmental extremes are highly desirable for their application to biological control, as an alternative to chemicals for pest management. However, in fungi, the mechanisms of stress tolerance might also have roles in mammal opportunism. We tested five species with high biocontrol potential in agriculture (Aureobasidiumpullulans, Debayomyceshansenii, Meyerozymaguilliermondii, Metschnikowiafructicola, Rhodotorulamucilaginosa) and two species recognized as emerging opportunistic human pathogens (Exophialadermatitidis, Aureobasidiummelanogenum) for growth under oligotrophic conditions and at 37 °C, and for tolerance to oxidative stress, formation of biofilms, production of hydrolytic enzymes and siderophores, and use of hydrocarbons as sole carbon source. The results show large overlap between traits desirable for biocontrol and traits linked to opportunism (growth under oligotrophic conditions, production of siderophores, high oxidative stress tolerance, and specific enzyme activities). Based on existing knowledge and these data, we suggest that oligotrophism and thermotolerance together with siderophore production at 37 °C, urease activity, melanization, and biofilm production are the main traits that increase the potential for fungi to cause opportunistic infections in mammals. These traits should be carefully considered when assessing safety of potential biocontrol agents.

Keywords: CAZy; biocontrol agent; biofilm; melanin; oligotrophism; opportunistic pathogen; protease; secretome; siderophore; stress tolerance; thermotolerance; virulence.

Figure 1

Quantification of biofilm formation by the selected yeast strains using the crystal violet assay.

Figure 2

(A). Spot assays for the selected yeast strains under oxidative stress due to 30 min and 60 min exposure to 20 mM H₂O₂compared to the control. (B). Predicted catalases, superoxide dismutases and peroxiredoxins in the representative proteomes. The size and color intensity of dots corresponds to the number of homologues.

Figure 3

(A). Production of siderophores by the selected yeast strains on chrome azurol S agar (CAS) (yellow to orange halos around colonies indicate siderophore production). (B). Prediction of the non-ribosomal peptide synthetases (NRPSs) in the proteomes of the selected yeast strains, together with the phylogenetic tree of their adenylation domains. The size and color intensity of dots corresponds to the number of homologues.

Figure 4

Predicted numbers of carbohydrate active enzymes (CAZy) in the proteomes of the selected yeast species, according to the dbCAN server. The size and color intensity of dots corresponds to the number of homologues.

Figure 5

Predicted numbers of enzymes involved in the toluene degradation pathway in the proteomes of the selected yeast species. The size and color intensity of dots corresponds to the number of homologues.

Figure 6

Overlap between the traits involved in pathogenesis and that are desirable for antagonism in biocontrol.