Aquatic yeasts: diversity, characteristics and potential health implications

There has been a rising interest in the levels, diversity and potential impacts of yeasts in aquatic environments. Some of the species isolated from such niches are known pathogens or have pathogenic and antifungal resistance features. This deems it necessary to understand the characteristics and potential health implications of such environmental yeasts species. Studies on these subjects are limited. Most studies on aquatic yeasts have linked them to water pollution. However, the current gold standards to determine microbial pollution of water use bacteria as the main indicator organisms. Including yeasts in water quality standards may provide a different dimension on the quality of water when determining its fit-for-use properties. Pathogenic yeasts cause superficial infections or life-threatening infections, especially in immunocompromised people. Some of the yeast species isolated in recent studies were resistant to commonly used antifungal agents of clinical and veterinary relevance. With the high prevalence rate of HIV in sub-Saharan Africa, particularly in South Africa, antifungal resistance is a public concern as it poses serious medical and economic challenges. Most available studies are concerned with clinical environments only. There is, thus, a need to review the literature that also focuses on aquatic environments. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wh.2020.270 ://iwaponline.com/jwh/article-pdf/18/2/91/709053/jwh0180091.pdf Mzimkhulu Ephraim Monapathi (corresponding author) Carlos Cornelius Bezuidenhout Unit for Environmental Science and Management – Microbiology, North-West University, Potchefstroom, South Africa E-mail: monapathimz@gmail.com Mzimkhulu Ephraim Monapathi Faculty of Applied and Computer Science – Chemistry, Vaal University of Technology, Vanderbijlpark Park, South Africa Owen Howard James Rhode Agricultural Research Council-Grain Crops, Potchefstroom, South Africa This article has been made Open Access thanks to the generous support of a global network of libraries as part of the Knowledge Unlatched Select initiative.


INTRODUCTION
Dabrowski ). However, identifications made using this approach is laborious, inconclusive and often inaccurate (Kurtzman & Robnett ). Various studies showed that molecular analyses are more reliable when identifying yeasts to species level (Brandão et  Although yeasts constitute the aquatic environments microbial community, their biodiversity and distribution have been ignored (Yurkov & Pozo ). The present study explores the occurrence of yeasts in natural water resources with emphasis on freshwater systems. A structured review was conducted to determine the extent of current knowledge of yeasts in freshwater systems using the literature relevant to the characteristics, diversity and health implications of aquatic yeasts. The following databases were used during this research: EBSCOhost, Google scholar, Sabinet and Science Direct. The literature that included one or more keywords, such as yeasts, identification, uses, aquatic environments, microbial pollution, yeast infections, antifungal resistance and resistance mechanisms, were used as references.

YEAST DIVERSITY IN AQUATIC ENVIRONMENTS
Compared with other environments such as soils and indoors, the presence of yeasts in aquatic environments has received little attention. Furthermore, the occurrence of yeasts in water as compared with other microorganisms such as bacteria and protozoans has not been largely studied (Pereira et al. ). With limited studies on aquatic yeasts, most of them are concentrating on polluted water (Nagahama ). Few yeast species exclusively associated with aquatic environments (Libkind et al. ). The section below addresses the diversity of yeasts in different aquatic environments.

Freshwater
The diversity and ecology of yeasts in freshwater environments (temperate and tropical rivers, lakes and lagoons) has been reviewed in a study by Libkind et al. (). The review conforms to the present review with respect to yeast identification. Identification of yeasts was primarily based on morphological and physiological characteristics.
However, the identification was strenuous and in many cases, inconclusive (Kurtzman & Robnett ). As stipulated in Table 1

).
There is a lack of studies that link these isolates/ strains from the environment to those from clinical settings.

Virulence factors in pathogenic yeasts
The

PUBLIC HEALTH CONCERN OF FINDING PATHOGENIC YEASTS IN ENVIRONMENTAL WATER
The occurrence of opportunistic yeast species in environmental water suggests a potential risk to direct water users. This public health threat is worsened by poor susceptibility to commonly used antifungal drugs (Maciel et al. ). People at peril are communities that use water for domestic and agricultural purposes as well as activities where direct exposure is common such as recreation and religious cleansing or baptism (Zenani & Mistri ).
Direct contact with water polluted with pathogenic yeasts could cause diseases/infections in healthy and immunocompromised individuals (Monapathi et al. ; Maciel et al. ). This is a public and health concern and needs more research to highlight this aspect but also to generate sufficient data to evaluate if policy changes are required for including yeasts in water quality guidelines.

POSSIBLE ROUTES OF YEAST INFECTIONS: AQUATIC INTERVENTION
Some of the yeast species in water resources are pathogenic and infectious diseases may be transmitted through contaminated water (Ayanbimpe et al. ). Finding similar species in environmental water is thus cumbersome. For aquatic yeasts, contact transmission is

CONCLUSION
The number of peer-reviewed articles about yeast diversity in water has increased and some have originated in South Africa. The present review largely focused on freshwater environments. Most of the studies on yeasts in aquatic environments address water pollution aspects. Yet, bacterial indicator species are mainly the microbes that are used in water quality assessments. Declining water quality is a global concern, and in these systems, the chemical and physical conditions are such that yeast species could survive.
Some of these are known as pathogenic or opportunistic pathogens. Future studies are needed to generate data to determine whether it is necessary to include yeasts in water quality guidelines. This may be necessary if one con-