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South Africa’s rivers are frequently used by communities lacking proper sanitation infrastructure for domestic purposes; however, these surface waters may pose a health risk to immunocompromised individuals due to the presence of opportunistic pathogenic fungi in the polluted water. Although only a few studies have focused on the presence of clinically relevant fungal species in South African rivers, many known opportunistic pathogenic species were found to be predominant in these waters. Furthermore, strong evidence exists that increased numbers of clinically relevant species may be observed in future due to fungi acquiring thermotolerance in response to the global increase in temperature. Thermotolerance is a major factor contributing to pathogenesis in fungi, due to the generally low tolerance of most fungi toward mammalian body temperatures. It is therefore contended that combinatorial effects of water pollution and rising environmental temperatures could lead to an increase in the incidence of mycoses in South Africa. This is especially concerning since a relatively large population of immunocompromised individuals, represente mostly by HIV-infected people, resides in the country.

Increasing cases of equine infertility and early embryonic loss in the Western Cape, South Africa, were documented in recent years. These appeared to be associated with Corynebacterium uterequi isolated from the uteri of infected mares. The purpose of this study was, therefore, to investigate the physiology and potential pathogenicity of this bacterium. Histopathological analyses were conducted on five mares suffering from reproductive complications, and from which Corynebacterium strains were detected on culture of uterine swabs. The histopathology revealed that the mares suffered from various forms of endometritis, suggesting a potential role of Corynebacterium strains in the disease. An isolate from one of the biopsies, and 11 other tentatively identified C. uterequi isolates from the urogenital tracts of other mares, which all had a history of pregnancy complications, were subsequently identified using molecular techniques and characterised based on environmental stress tolerance, enzyme profiles, antibiotic susceptibility and ability to form biofilms. It was found that representatives of C. uterequi possessed several virulence-associated characteristics, including trypsin and urease activity, as well as the ability to form weakly adherent monoculture biofilms. Several isolates displayed resistance to trimethoprim-sulfamethoxazole. In conclusion, this study provided some insight into the general physiology and pathogenic potential of C. uterequi, and points to the possible role of C. uterequi in the onset of equine pregnancy complications. Moreover, the ability to form biofilms suggests the potential for chronic infection, which was observed in 60% of the mares. Further research, however, is needed to implicate C. uterequi as an equine pathogen.

Nitrogen availability is vital for the growth and survival of Cryptococcus neoformans in the natural environment. Two major ecological reservoirs were previously described for C. neoformans, namely, pigeon guano and the woody debris of various tree species. In contrast to the abundance of available nitrogen in guano, C. neoformans must adapt to severely limited nitrogen conditions within arboreal ecological niches. Previously, we demonstrated the role of nitrogen limitation in the production of cryptococcal virulence factors and drug tolerance. The genetic response underlying this adaptation to nitrogen deficiency, however, remains to be determined. Therefore, in the present study we investigated the transcriptomic response of C. neoformans to ecologically relevant nitrogen concentrations using RNA-sequencing. Our data revealed that low nitrogen conditions modulate the expression of numerous virulence genes in C. neoformans. Among these were, CTR4 and CGP1, which showed highly significant modulation under low nitrogen conditions. Furthermore, data analysis revealed the upregulation of antifungal tolerance-related genes in low nitrogen conditions, including genes involved in ergosterol biosynthetic processes and cell wall integrity. Overall, our findings provide insight into the survival of C. neoformans in nitrogen-poor ecological niches and suggest that pre-adaptation to these conditions may influence the pathobiology of this yeast.

The urease enzyme of Cryptococcus neoformans is linked to different metabolic pathways within the yeast cell, several of which are involved in polyamine metabolism. Cryptococcal biogenic amine production is, however, largely unexplored and is yet to be investigated in relation to urease. The aim of this study was therefore to explore and compare polyamine metabolism in wild-type, urease-negative and urease-reconstituted strains of C. neoformans. Mass spectrometry analysis showed that agmatine and spermidine were the major extra- and intracellular polyamines of C. neoformans and significant differences were observed between 26 and 37 °C. In addition, compared to the wild-type, the relative percentages of extracellular putrescine and spermidine were found to be lower and agmatine higher in cultures of the urease-deficient mutant. The inverse was true for intracellular spermidine and agmatine. Cyclohexylamine was a more potent polyamine inhibitor compared to DL-α-difluoromethylornithine and inhibitory effects were more pronounced at 37 °C than at 26 °C. At both temperatures, the urease-deficient mutant was less susceptible to cyclohexylamine treatment compared to the wild-type. For both inhibitors, growth inhibition was alleviated with polyamine supplementation. This study has provided novel insight into the polyamine metabolism of C. neoformans, highlighting the involvement of urease in biogenic amine production.

Environmental stress often causes phenotypic changes among pathogenic cryptococci, such as altered antifungal susceptibility, changes in capsule and melanin formation, as well as altered levels of the membrane sterol and antifungal target, ergosterol. We therefore hypothesised that nitrogen limitation, a prevalent environmental stress in the natural habitat of these yeasts, might affect virulence and antifungal susceptibility. We tested the effect of different nitrogen concentrations on capsule, melanin and ergosterol biosynthesis, as well as amphotericin B (AmB) and fluconazole (FLU) susceptibility. This was achieved by culturing cryptococcal strains representing Cryptococcus neoformans and Cryptococcus gattii in media with high (0.53 g/l), control (0.42 g/l) and low (0.21 g/l) NH4Cl concentrations. India ink staining was used to determine capsule thickness microscopically, while melanin and ergosterol content were determined spectrophotometrically. We found that lower nitrogen concentrations enhanced both ergosterol and capsule biosynthesis, while a variable effect was observed on melanisation. Evaluation of drug tolerance using time-kill methodology, as well as tests for FLU heteroresistance, revealed that the low nitrogen cultures had the highest survival percentages in the presence of both AmB and FLU, and showed the highest frequency of FLU heteroresistance, suggesting that nitrogen concentration may indeed influence drug tolerance.