The occurrence of fungi in drinking water has received increased attention in the last decades, and fungi are now generally accepted as drinking water contaminants. The knowledge about the occurrence and diversity of fungi in water has increased considerably from a low knowledge base. However, the relevance of waterborne fungi for water quality and human health is poorly understood and still conflicting. Scientific reports on effective treatment against fungi in water are also few. This article presents a review of the literature on fungal water studies, including some general results, and considerations of significance, limits, contradictions, precautions, and practical consequences.

In order to determine the occurrence, distribution, and significance of mold species in groundwater-and surface water-derived drinking water in Norway, molds isolated from 273 water samples were identified. Samples of raw water, treated water, and water from private homes and hospital installations were analyzed by incubation of 100-ml membrane-filtered samples on dichloran-18% glycerol agar. The total count (number of CFU per 100 ml) of fungal species and the species diversity within each sample were determined. The identification of mold species was based on morphological and molecular methods. In total, 94 mold species belonging to 30 genera were identified. The mycobiota was dominated by species of Penicillium, Trichoderma, and Aspergillus, with some of them occurring throughout the drinking water system. Several of the same species as isolated from water may have the potential to cause allergic reactions or disease in humans. Other species are common contaminants of food and beverages, and some may cause unwanted changes in the taste or smell of water. The present results indicate that the mycobiota of water should be considered when the microbiological safety and quality of drinking water are assessed. In fact, molds in drinking water should possibly be included in the Norwegian water supply and drinking water regulations.

Aims:  In order to determine the occurrence of filamentous fungi in public drinking water systems in Norway, water from 14 water supply networks from all over the country was sampled and analysed. Networks with both ground and surface water sources were included in this study. Methods and Results:  During a one‐year period, 273 water samples were collected. Frequencies of fungi in samples from raw water, treated water and from home and hospital installations were determined on the basis of incubation of 100 ml membrane‐filtered samples on dichloran 18% glycerol agar media. Filamentous fungi were recovered from 62% of all samples. In ground water 42·3% of the samples were positive for mould growth, while surface waters yielded 69·7% positive samples. Conclusions:  The risk to recover moulds from surface water is three times higher compared with ground water. It is more likely to detect moulds in cold waters and showers than in hot waters. Significance and Impact of the Study:  By analysing the water reaching the consumers, the results reported in present study indicate that filamentous fungi in drinking water is not negligible, and that moulds should be considered as part of the microbiological analysis parameters in drinking water.

Control strategies against waterborne fungi have been studied only to a small degree. In order to increase knowledge of the effect of water treatment on waterborne fungi, the dose–response effect of four commonly used drinking water disinfection methods was tested on selected fungal species: Aspergillus calidoustus, Penicillium spinulosum, Trichoderma viride and Fusarium solani. These species are all common in Norwegian drinking water, and are regarded as emerging pathogens, toxigenic and/or food contaminants. Spore suspensions were treated with ultraviolet (UV) irradiation, ozone, chlorine and chloramine. Three different doses were tested for each disinfectant and all doses were repeated three times. Bacillus subtilis was included as a control for the disinfection processes and to compare the resistance. P. spinulosum and T. viride were resistant to chlorine and chloramine. They were also very tolerant to UV-irradiation and ozone, although the highest doses of ozone had a small inactivation effect, especially on T. viride. Ozone was found to be most effective against fungi in general. It is apparent that fungal species may have very different properties when it comes to tolerance to water disinfection, and the inactivation of B. subtilis spores cannot be used as an indicator of fungal spore inactivation after water treatment.

A total of 123 Trichoderma strains were isolated from Norwegian surface-sourced drinking water. The water samples included raw water, treated water, and water from private homes and hospital installations. Trichoderma species are difficult to differentiate morphologically, but recent molecular identification tools, including DNA barcoding, successfully distinguish between closely related species. The diversity of Trichoderma spp. was explored by DNA sequencing of internal transcribed spacer (ITS) and translation elongation factor 1 alpha (TEF-1α). Sequence identification was performed in the TrichOKEY version 2.0 barcode program and in the multilocus similarity search database TrichoBLAST, combined with traditional blast searches in the EMBL/GenBank. A total of 11 known Trichoderma/Hypocrea species were identified. In addition, one group of unidentified Trichoderma strains was found to represent a separate, strongly supported subclade within the Pachybasium'A'/Hamatum clade, based on their TEF-1α haplotypes. Trichoderma viride comprised 49% of the identified strains, and was represented by four and eight slightly different ITS and TEF-1α haplotypes, respectively. Approximately 22% of the surface-derived water samples were positive for T. viride, and the species was frequently isolated throughout the surface-sourced drinking water distribution system. The results indicate that a broad range of Trichoderma species are present in Norwegian surface-sourced drinking. Water treatment has minor effect in removing Trichoderma from raw water, and active growth in the water distribution system is likely to occur.

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Recent studies have changed the taxonomy of Aspergillus section Usti, and a novel species, Aspergillus calidoustus, has been erected. It was also demonstrated that clinical isolates previously identified as A. ustus actually belong to the emerging pathogen A. calidoustus. Aspergillus ustus were frequently isolated from Norwegian water systems, and due to the taxonomical progress, these waterborne strains could be identified more precisely. A MLST study including ITS, calmodulin, β-tubulin and actin sequences was conducted on 32 strains previously identified as A. ustus. All strains were identified as A. calidoustus, which was verified by physiological, biochemical and phylogenetic analyses. This is the first report of that A. calidoustus is able to colonize water distribution systems. In respect to the potential role of water systems as a source of nosocomial infections in patients with immunodeficiency, attention should be given to water systems in hospitals and other healthcare units, especially the heated-water installations.

Microalgae comprise a phylogenetically very diverse group of photosynthetic unicellular pro-and eukaryotic organisms growing in marine and other aquatic environments. While they are well explored for the generation of biofuels, their potential as a source of antimicrobial and prebiotic substances have recently received increasing interest. Within this framework, microalgae may offer solutions to the societal challenge we face, concerning the lack of antibiotics treating the growing level of antimicrobial resistant bacteria and fungi in clinical settings. While the vast majority of microalgae and their associated microbiota remain unstudied, they may be a fascinating and rewarding source for novel and more sustainable antimicrobials and alternative molecules and compounds. In this review, we present an overview of the current knowledge on health benefits of microalgae and their associated microbiota. Finally, we describe remaining issues and limitation, and suggest several promising research potentials that should be given attention.