The European Union currently has no specific regulations on fungi in water. The only country where fungi are listed as the parameter is Sweden, with the maximal number of 100 CFU per 100 mL. The present study thus compared culturable mycobiota from Swedish drinking water with Slovenian, which has no specific requirements for fungi. Fungi were isolated with up to 38 CFU/L from 75% of Swedish samples. The most common were the genera Varicosporellopsis (27.3%), Paracremonium (14.5%), and black yeasts Cadophora, Cyphellophora, and Exophiala (18.2%). Using the same sampling and isolation methods, 90% of tap water samples in Slovenia were positive for fungi, with Aspergillus spp. (46%), Aureobasidium melanogenum (36%), and Exophiala spp. (24%) being the most common. The observed differences between countries are likely the consequence of geographical location, the use of different raw water sources, and water treatment methods. However, the core species and emerging fungi Aspergillus fumigatus, Candida parapsilosis sensu stricto, Exophiala phaeomuriformis, Bisifusarium dimerum, and Rhodotorula mucilaginosa were isolated in both studies. These findings point out the relevance of tracking the presence of emerging fungi with known effects on health in drinking water and encourage further studies on their transmission from raw water sources to the end-users.

  • Fungal biota significantly differs between different geographical locations.

  • Groundwater more likely harbours Aureobasidium and Exophiala.

  • Surface water is associated with fungi colonising soil and plant material.

  • A. fumigatus, C. parapsilosis, E. phaeomuriformis, B. dimerum, and R. mucilaginosa present core-genera of drinking water.

  • Drinking water carries fungal species that are listed by WHO as emerging pathogens.

Over the centuries with raising urbanisation, followed by environmental pollution and outbreaks of mainly gastrointestinal illnesses, people became aware of the health risk posed by contaminated water (Gray 2014). Today, the United Nations (UN) declares access to drinking water as one of the fundamental human rights (UN 2010). To ensure the quality of water, methods for water processing became more sophisticated with time. Still, the choice of proper methods greatly depends on the type of main water source, and the concentrations of organic material and metals (Gray 2014). The process of acquiring drinkable water is controlled and evaluated with regular monitoring. Microbiological parameters included in monitoring differ among continents and countries and are adapting to the local or global changing presence of pathogenic microorganisms (WHO 2011). While the number of pathogenic bacteria and protozoa are regularly monitored worldwide, fungi are usually neglected. According to the national legislation, their presence is checked by microscopic observation of filtered water samples in the Czech Republic and Hungary (Novak Babič et al. 2017). The only country where fungi are listed as one of the parameters is Sweden, requiring fungal detection and enumeration by culture with a parametric value of 100 CFU/100 mL (NFA 2001). However, over the last 30 years, many countries reported fungal presence in drinking water and usually encountered diverse species of fungi assigned to the genera Acremonium, Alternaria, Arthrinium, Aspergillus, Aureobasidium, Beauveria, Botrytis, Candida, Chaetomium, Cladosporium, Epicoccum, Exophiala, Fusarium, Geotrichum, Gliocladium, Mucor, Naganishia, Paecilomyces, Penicillium, Phialophora, Phoma, Phomopsis, Rhizopus, Rhodotorula, Sporothrix, Trichoderma, and Verticillium (Kinsey et al. 1999; Göttlich et al. 2002; Gonçalves et al. 2006; Hageskal et al. 2006; Grabińska-Łoniewska et al. 2007; Kanzler et al. 2008; Pereira et al. 2009; Sammon et al. 2010; Defra 2011; Heinrichs et al. 2013a, 2013b; Novak Babič et al. 2016). Yet, the number and species of fungi may significantly differ among the countries, due to diverse environmental and anthropogenic influences (Novak Babič et al. 2017). The present study was carried out in Göteborg, Sweden, and followed the study previously conducted in Ljubljana, Slovenia, by Novak Babič et al. (2016) in order to compare fungal diversity in drinking water between the two geographically distinct European cities, which produce drinking water according to different parameters for fungal enumeration, different raw water sources and treatment procedures.

Sampling of water and cultivation of fungi

Tap water samples were collected from water pipes in 100 private homes in Ljubljana, Slovenia and 55 private homes in Göteborg and Mölndal, Sweden. One litre of cold, running tap water was collected in sterile containers and filtrated using 0.45-μm membrane filters (Merck, Millipore). Fungi from filters were recovered on Dichloran Rose Bengal Agar (DRBC; Oxoid Ltd, England) after incubation at 30 °C for 3–5 days. Pure cultures of fungi were obtained on malt extract agar (MEA) and deposited in the Ex Culture Collection of the Infrastructural Centre Mycosmo, MRIC UL, Slovenia: http://www.ex-genebank.com/, at the Department of Biology, Biotechnical Faculty, University of Ljubljana.

Extraction of genomic DNA from pure cultures

Following the same procedure as described in the Slovenian study (Novak Babič et al. 2016), DNA from 3-day-old pure yeast cultures was extracted using PrepMan Ultra reagent (Applied Biosystems) following the manufacturer's instructions. DNA extraction of filamentous fungi grown for 5 days on MEA was carried out with mechanical lysis of 1 cm2 of mycelium, described by Van den Ende & de Hoog (1999). All DNA samples ready for further applications were stored at −20 °C.

Taxonomical identification of fungi

Identification of fungal strains was carried out with the use of the standard rDNA identification barcodes for fungi. In both studies, we amplified nucleotide sequences of internal transcribed spacer region 1 (ITS1), 5.8S rDNA, and ITS 2 (ITS) with primer pair ITS5 and ITS4 (White et al. 1990) to identify filamentous fungi. Yeasts were identified by sequencing D1/D2 domains of 28S rDNA using primer set NL1 and NL4 (Boekhout & Kurtzman 1996). All sequences were obtained at Microsynth AG, Switzerland, and assembled with FinchTV 1.4 (Geospiza, PerkinElmer, Inc.). Phylogenetic and molecular evolutionary analyses were conducted using the software Molecular Evolutionary Genetics Analysis (MEGA), version 7 (Kumar et al. 2016). Fungi were identified with the use of the BLAST algorithm at the NCBI web page and compared to the other taxonomically important databases (e.g. Index Fungorum and Westerdijk Fungal Biodiversity Institute, Utrecht, The Netherlands).

The genera Paracremonium and Varicosporellopsis prevail in tap water in Göteborg

Fifty-five samples of tap water from private homes were analysed for the fungal presence and 75% of them were positive for fungi under tested conditions. Isolated and identified fungal species, their pathogenic potential, and their frequency in analysed tap water samples are listed in Table 1. The number of grown fungi differed among the samples from 1 to 38 CFU/L, which was within the limits set in Swedish legislation for drinking water quality (100 CFU/100 mL). Almost one-third of the isolates belonged to the genus Varicosporellopsis (27.3%), followed by Paracremonium (14.5%); together they were present in 47.3% of tested water samples. Black yeasts and yeast-like fungi from the genera Cadophora, Cyphellophora, and Exophiala were altogether isolated from 18.2% of the samples. Aspergillus fumigatus was isolated from 7.3% of the samples. The most commonly isolated yeast was identified as Candida parapsilosis sensu stricto (7.3%), followed by Rhodotorula mucilaginosa (3.6%).

Table 1

Frequency of strains isolated from tap water samples in Göteborg and Mölndal (Sweden), with representative EXF- and GenBank numbers

Identification of the strainsFrequency of isolationRepresentative strain – EXF No.aGenBank Accession No.bPathogenic potentialReferences
Aspergillus fumigatus 7.3% EXF-10126
EXF-10137
EXF-10156
EXF-10157 
OP508521
OP508522
OP508523
OP508524 
Disseminated infections
Respiratory infections
Subcutaneous infections
Rhinocerebral infections
Skin and nail infections,
Ear infections, keratitis 
(Warris et al. 2003; de Hoog et al. 2014
Bisifusarium dimerum 1.8% EXF-10141 OP508525 Rare, eye infection Vismer et al. (2002)  
Cadophora luteo-olivacea 1.8% EXF-10167 OP508526 No data, plant pathogen Travadon et al. (2015)  
Cadophora malorum 7.3% EXF-10150
EXF-10159
EXF-10166
EXF-10177 
OP508527
OP508528
OP508529
OP508530 
Candida parapsilosis sensu stricto 7.3% EXF-10133
EXF-10144
EXF-10174
EXF-10179 
OP494178
OP494179
OP494180
OP494181 
Disseminated infections,
Mucosal infections,
Respiratory infections,
Sepsis 
de Hoog et al. (2014)  
Coniochaeta ligniaria 3.6% EXF-10129
EXF-10178 
OP508539
OP508540 
Subcutaneous infections,
Keratitis, sinusitis, peritonitis 
Perdomo et al. (2011)  
Cyphellophora olivacea 1.8% EXF-10146 OP508534 No data, plant pathogen Gao et al. (2015)  
Cosmospora sp. 5.5% EXF-10125
EXF-10143
EXF-10184 
OP508531
OP508532
OP508533 
No data 
Debaryomyces hansenii 1.8% EXF-10182 OP494182 Rare, sepsis Desnos-Ollivier et al. (2008)  
Exophiala lecanii-corni 1.8% EXF-10147 OP508535 Disseminated infections,
Respiratory infections,
Skin and nail infections,
Phaeohyphomycosis 
de Hoog et al. (2014)  
Exophiala phaeomuriformis genotype 1 5.5% EXF-10145
EXF-10173
EXF-10180 
OP508536
OP508537
OP508538 
Inopinatum lactosum 1.8% EXF-10135 OP508553 No data 
Paracremonium binnewijzendii 14.5% EXF-10136
EXF-10148
EXF-10152
EXF-10153
EXF-10155
EXF-10161
EXF-10163
EXF-10172 
OP508541
OP508542
OP508543
OP508544
OP508545
OP508546
OP508547
OP508548 
No data 
Penicillium sp. 1.8% EXF-10149 OP508549 Disseminated infections Ramírez et al. (2018)  
Rhodotorula mucilaginosa 3.6% EXF-10140
EXF-10181 
OP494183
OP494184 
Catheter-related infections,
Keratitis, sepsis 
de Hoog et al. (2014) 
Sagenomella griseoviridis 1.8% EXF-10132 OP508550 Systemic illness in animals Gené et al. (2003)  
Saprolegnia sp. 3.6% EXF-10124
EXF-10175 
OP508551
OP508552 
No data 
Varicosporellopsis Americana 27.3% EXF-10127
EXF-10130
EXF-10131
EXF-10134
EXF-10138
EXF-10151
EXF-10154
EXF-10160
EXF-10162
EXF-10164
EXF-10168
EXF-10169
EXF-10170
EXF-10171
EXF-10183 
OP508554
OP508555
OP508556
OP508557
OP508558
OP508559
OP508560
OP508561
OP508562
OP508563
OP508564
OP508565
OP508566
OP508567
OP508568 
No data 
Yarrowia lipolytica 1.8% EXF-10142 OP494185 Rare, cutaneous infections, sepsis Boyd et al. (2017)  
Identification of the strainsFrequency of isolationRepresentative strain – EXF No.aGenBank Accession No.bPathogenic potentialReferences
Aspergillus fumigatus 7.3% EXF-10126
EXF-10137
EXF-10156
EXF-10157 
OP508521
OP508522
OP508523
OP508524 
Disseminated infections
Respiratory infections
Subcutaneous infections
Rhinocerebral infections
Skin and nail infections,
Ear infections, keratitis 
(Warris et al. 2003; de Hoog et al. 2014
Bisifusarium dimerum 1.8% EXF-10141 OP508525 Rare, eye infection Vismer et al. (2002)  
Cadophora luteo-olivacea 1.8% EXF-10167 OP508526 No data, plant pathogen Travadon et al. (2015)  
Cadophora malorum 7.3% EXF-10150
EXF-10159
EXF-10166
EXF-10177 
OP508527
OP508528
OP508529
OP508530 
Candida parapsilosis sensu stricto 7.3% EXF-10133
EXF-10144
EXF-10174
EXF-10179 
OP494178
OP494179
OP494180
OP494181 
Disseminated infections,
Mucosal infections,
Respiratory infections,
Sepsis 
de Hoog et al. (2014)  
Coniochaeta ligniaria 3.6% EXF-10129
EXF-10178 
OP508539
OP508540 
Subcutaneous infections,
Keratitis, sinusitis, peritonitis 
Perdomo et al. (2011)  
Cyphellophora olivacea 1.8% EXF-10146 OP508534 No data, plant pathogen Gao et al. (2015)  
Cosmospora sp. 5.5% EXF-10125
EXF-10143
EXF-10184 
OP508531
OP508532
OP508533 
No data 
Debaryomyces hansenii 1.8% EXF-10182 OP494182 Rare, sepsis Desnos-Ollivier et al. (2008)  
Exophiala lecanii-corni 1.8% EXF-10147 OP508535 Disseminated infections,
Respiratory infections,
Skin and nail infections,
Phaeohyphomycosis 
de Hoog et al. (2014)  
Exophiala phaeomuriformis genotype 1 5.5% EXF-10145
EXF-10173
EXF-10180 
OP508536
OP508537
OP508538 
Inopinatum lactosum 1.8% EXF-10135 OP508553 No data 
Paracremonium binnewijzendii 14.5% EXF-10136
EXF-10148
EXF-10152
EXF-10153
EXF-10155
EXF-10161
EXF-10163
EXF-10172 
OP508541
OP508542
OP508543
OP508544
OP508545
OP508546
OP508547
OP508548 
No data 
Penicillium sp. 1.8% EXF-10149 OP508549 Disseminated infections Ramírez et al. (2018)  
Rhodotorula mucilaginosa 3.6% EXF-10140
EXF-10181 
OP494183
OP494184 
Catheter-related infections,
Keratitis, sepsis 
de Hoog et al. (2014) 
Sagenomella griseoviridis 1.8% EXF-10132 OP508550 Systemic illness in animals Gené et al. (2003)  
Saprolegnia sp. 3.6% EXF-10124
EXF-10175 
OP508551
OP508552 
No data 
Varicosporellopsis Americana 27.3% EXF-10127
EXF-10130
EXF-10131
EXF-10134
EXF-10138
EXF-10151
EXF-10154
EXF-10160
EXF-10162
EXF-10164
EXF-10168
EXF-10169
EXF-10170
EXF-10171
EXF-10183 
OP508554
OP508555
OP508556
OP508557
OP508558
OP508559
OP508560
OP508561
OP508562
OP508563
OP508564
OP508565
OP508566
OP508567
OP508568 
No data 
Yarrowia lipolytica 1.8% EXF-10142 OP494185 Rare, cutaneous infections, sepsis Boyd et al. (2017)  

aEXF Number indicates the number designated to fungi in the EX Culture Collection of the Infrastructural Centre Mycosmo, Biotechnical Faculty, University of Ljubljana, Slovenia.

bGenBank Accession Number indicates the number designated to a single sequence in the GenBank database of National Center for Biotechnology Information (NCBI), USA.

Aspergillus spp. and black yeasts are predominant in tap water in Ljubljana

One hundred samples of tap water from private homes were analysed for the fungal presence and 90% of them were positive for fungi. Isolated and identified fungal species, in analysed tap water samples, are described in detail in the study by Novak Babič et al. (2016). The numbers of fungi per sample were within the limits set in Swedish legislation for drinking water quality (100 CFU/100 mL). Almost half of the samples (46%) were positive for Aspergillus spp., followed by black yeast-like fungi Aureobasidium melanogenum (36%) and Exophiala spp. (24%). The most commonly isolated yeasts were Meyerozyma guilliermondii (20%), R. mucilaginosa (20%), and C. parapsilosis sensu stricto (18%) (Novak Babič et al. 2016).

Availability and quality of safe drinking water are two of the main future goals set by the World Health Organisation (WHO), particularly due to the ongoing global warming, pollution, and raising human population (WHO 2011). Besides commonly known agents of, mainly gastrointestinal, illnesses, in the last years also less conventional microorganisms have been taken into consideration as possible parameters for water quality assessment. One of them is fungi, which have been observed and monitored in drinking water in many countries worldwide, yet there are still many unknowns regarding their presence, diversity, and ecology in water systems (Novak Babič et al. 2017). In comparison to the conventional microbiological parameters, only a few fungi, such as Candida albicans and Candida auris, are believed to be solely related to humans, while the rest are in essence saprophytes and only occasionally involved in so-called opportunistic infections (de Hoog et al. 2014). Infections caused by fungi in general occur in people with serious immune impairment, after transplantations or other surgical procedures with the highest risk in hospitals and facilities for a long-term recovery, although the infections may occur also in a home environment. At high risk are also elderly, neonates, people with AIDS, cancer, or autoimmune illnesses, such as cystic fibrosis (de Hoog et al. 2014). Possible risk for fungal infections related to water occurs through drinking, dermal contact, and inhalation of aqueous aerosols (Novak Babič et al. 2017). The long-term health effect of fungi through drinking was previously monitored in USA, Spain, and Italy, and associated with mycotoxins intoxication, caused by Aspergillus originating from wells. Intoxication led to sporadic amyotrophic lateral sclerosis (ALS) in people and animals, with symptoms improving after chlorination of water (Vinceti et al. 2010; Alonso et al. 2017; French et al. 2019). Infections of skin and nails more often occur after trauma, or as an occupational risk in people doing work and sports related to constant water exposure. The genera Aspergillus, Candida, Cryptococcus, Fusarium, Mucor, and black yeasts like Exophiala and Cladophialophora are often associated with occupational exposure (Novak Babič et al. 2020). The third route is the inhalation of aerosols which may be associated with sinusitis and pulmonary infections. Fungi like Exophiala, Scedosporium, Cladosporium, Fusarium, and Aspergillus have been associated with aerosolised water, particularly in cystic fibrosis patients. Water sources for these genera have been thus extensively studied in hospitals and dental units (Anaissie et al. 2003).

According to the research conducted so far, the presence and diversity of fungi in drinking water may significantly differ among the countries. Those differences were often linked to locations of primary water sources, exposure to UV irradiation, ion composition of water and presence of organic material. In addition, concentrations of dissolved oxygen, choice of water treatment procedures, temperatures in storage tanks, the use of materials for water distribution systems, and consequently biofilm formation were among the factors influencing fungal distribution in water (Novak Babič et al. 2017). The present study was conducted as a follow-up of the study carried out by Novak Babič et al. (2016) in order to compare culturable fungi in drinking water in two geographically distant cities, Göteborg (Sweden) and Ljubljana (Slovenia). Sweden was selected as it has implemented international legislation on fungi in drinking water. Due to the geographical distance, selected cities have different climates, and different types of aquifers and consequently use a different source of raw water for the production of drinking water (Table 2).

Table 2

Environmental and anthropogenic factors affecting the presence of fungi in drinking water

Environmental and anthropogenic factorsCity, County
Ljubljana, SloveniaGöteborg, Sweden
Climate Mild continental Marine west coast 
Continentality type Continental, subtype subcontinental Oceanic, subtype semicontinental 
Average max. temperature [°C] 26 °C, July 21 °C, July 
Average min. temperature [°C] −4 °C, January −4 °C, February 
Average annual precipitation [mm] 1,393 670 
Average annual daylight [h/day] 12 h 00′ 12 h 00′ 
Type of aquifer Unconfined (sand-gravel, partly karstic) Unconfined-confined (separated by clay) 
Origin of raw water Groundwater Surface water (river) 
Cleaning procedures None Limestone treatment, flocculation, sedimentation, ultrafiltration 
Disinfection Occasionally chlorinated Continuously chlorinated 
Building materials used for water transport Steel, cast iron, polyethylene, polyvinyl chloride, ductile iron, asbestos-concrete Cast iron, polyethylene, polyvinyl chloride 
Number of citizens 330.000 550.000 (urban) – 1.000.000 (suburban) 
Legislation including fungi None specifically, just general heterotrophic count at 22 and 37 °C. Fungal count (100 CFU/100 mL) and general heterotrophic count at 22 and 37 °C. 
The most common fungi Aspergillus spp. (46%),
Aureobasidium melanogenum (36%),
Exophiala spp. (24%),
Meyerozyma guilliermondii (20%),
Rhodotorula mucilaginosa (20%),
Candida parapsilosis sensu stricto (18%). 
Varicosporellopsis americana (27%),
Paracremonium binnewijzendii (14%),
Cadophora spp. (9%),
Aspergillus spp. (7%),
Candida parapsilosis sensu stricto (7%),
Exophiala spp. (7%). 
Environmental and anthropogenic factorsCity, County
Ljubljana, SloveniaGöteborg, Sweden
Climate Mild continental Marine west coast 
Continentality type Continental, subtype subcontinental Oceanic, subtype semicontinental 
Average max. temperature [°C] 26 °C, July 21 °C, July 
Average min. temperature [°C] −4 °C, January −4 °C, February 
Average annual precipitation [mm] 1,393 670 
Average annual daylight [h/day] 12 h 00′ 12 h 00′ 
Type of aquifer Unconfined (sand-gravel, partly karstic) Unconfined-confined (separated by clay) 
Origin of raw water Groundwater Surface water (river) 
Cleaning procedures None Limestone treatment, flocculation, sedimentation, ultrafiltration 
Disinfection Occasionally chlorinated Continuously chlorinated 
Building materials used for water transport Steel, cast iron, polyethylene, polyvinyl chloride, ductile iron, asbestos-concrete Cast iron, polyethylene, polyvinyl chloride 
Number of citizens 330.000 550.000 (urban) – 1.000.000 (suburban) 
Legislation including fungi None specifically, just general heterotrophic count at 22 and 37 °C. Fungal count (100 CFU/100 mL) and general heterotrophic count at 22 and 37 °C. 
The most common fungi Aspergillus spp. (46%),
Aureobasidium melanogenum (36%),
Exophiala spp. (24%),
Meyerozyma guilliermondii (20%),
Rhodotorula mucilaginosa (20%),
Candida parapsilosis sensu stricto (18%). 
Varicosporellopsis americana (27%),
Paracremonium binnewijzendii (14%),
Cadophora spp. (9%),
Aspergillus spp. (7%),
Candida parapsilosis sensu stricto (7%),
Exophiala spp. (7%). 

Groundwater-derived raw water in Ljubljana is usually completely suitable for drinking according to the parameters listed in the Drinking Water Directive (EEC 1998), thus no cleaning procedures are applied, and chlorination takes place only occasionally in cases of faecal coliforms detection. On the contrary, drinking water in Göteborg is obtained after the application of the physico-chemical treatment on river-derived raw water. Drinking water is also continuously chlorinated. Both cities regularly monitor drinking water according to the Drinking Water Directive, while in Sweden additional monitoring for fungi is performed, with limits set to 100 CFU/100 mL (NFA 2001). After comparing the results from both studies, significant differences were observed in the numbers and diversity of isolated fungi. Although the fungal parameter in Swedish legislation was not exceeded in any city, as many as 90% of tap water samples from Ljubljana were positive for fungi while the reported percentage for tap water in Göteborg was 75%. The most common fungal species isolated from tap water in Ljubljana were Aspergillus spp. (46%), A. melanogenum (36%), Exophiala spp. (24%), M. guilliermondii (20%), R. mucilaginosa (20%) and C. parapsilosis sensu stricto (18%) (Novak Babič et al. 2016). Using the same method for sampling and isolation yielded significantly different results for drinking water in Göteborg with Varicosporellopsis and Paracremonium being mostly isolated genera. Varicosporellopsis americana was the predominant species (27%). Species of the genera Cadophora spp. (9%), Aspergillus spp. (7%), C. parapsilosis sensu stricto (7%) and Exophiala spp. (7%) were also encountered (Table 1). The three species, e.g. A. melanogenum, M. guilliermondii, and Rhinocladiella similis commonly detected in water from Ljubljana were completely absent in water from Göteborg.

The observed differences in the most frequently encountered fungal taxa from drinking water in both cities may be a consequence of geographical location, the use of different raw water sources and water treatment methods (Table 2). Fungi commonly isolated from Göteborg water have a rare or unknown pathogenic potential (Table 1) and were in literature so far associated with plants, soil, and surface water (Table 1) (DEFRA 2011; Novak Babič et al. 2017). On the other hand, many fungi detected in drinking water from Ljubljana were often isolated from groundwater in other European countries (DEFRA 2011; Novak Babič et al. 2017). Those findings suggest the relation between the presence of fungi in drinking water and their presence in raw water sources, pointing out the relevance of tracking the presence of fungi in raw water sources since they are transmitted to drinking water despite the applied cleaning procedures.

Locally isolated species may often be isolated in high numbers, such as A. melanogenum in Ljubljana, or V. americana in Göteborg. Their presence could be related to the source of raw water, anthropogenic pollution or the degradation of organic matter (e.g. plant debris) (DEFRA 2011; Novak Babič et al. 2017). Their role in the environment, possible mycotoxin production, and their effect on human health are often scarce, thus attention should be paid in cases when their presence in water is related to seasonal changes or if they start to appear in elevated numbers.

Despite observed differences in fungal community structures, the species A. fumigatus, C. parapsilosis sensu stricto, Debaryomyces hansenii, Exophiala phaeomuriformis, Bisifusarium dimerum, R. mucilaginosa, and Yarrowia lipolytica have been isolated in both studies (Figure 1). These fungi were already previously isolated from raw and drinking water from other European countries, including Austria, Belgium, Finland, France, Germany, Greece, Hungary, Norway, Poland, Portugal, Serbia, The Netherlands, and the United Kingdom (Novak Babič et al. 2017), and very likely represent the core fungal biota of fresh waters around Europe.
Figure 1

Fungal diversity in drinking water from Ljubljana (Slovenia) and Göteborg (Sweden). Differences and similarities of fungi, isolated from drinking water from the two distant European cities with different geology, climate, raw water source, and the production of drinking water.

Figure 1

Fungal diversity in drinking water from Ljubljana (Slovenia) and Göteborg (Sweden). Differences and similarities of fungi, isolated from drinking water from the two distant European cities with different geology, climate, raw water source, and the production of drinking water.

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Out of these, species A. fumigatus, C. parapsilosis species complex, E. phaeomuriformis and R. mucilaginosa represent a particular worry in drinking water in healthcare facilities and other institutions nursing immune-susceptible people (de Hoog et al. 2014). Worldwide reports so far include data on mycotoxin production, respiratory infections, skin infections, infections via catheters, gastrointestinal infections, and systemic infections. In addition, their possible environmental reservoirs, transmission through water and aerosolisation, as well as resistance to diverse spectra of antimycotics have been described (de Hoog et al. 2014). Although the health effect of fungi carried by drinking water has been often overlooked in water legislation, this changed in 2022 when EMEG issued the Drinking Water Directive (2020/2184) State of play: Guidance note for the analysis of microbiological parameters stating: ‘…they should be regarded as a risk based and recommended for clear at end-point in hospitals and buildings where immune-compromised users may congregate.’ (Niegowska et al. 2022). Closely after, the WHO issued the first-ever fungal priority pathogens list where Aspergillus fumigatus holds third place in the Critical Priority Group, and C. parapsilosis is listed in seventh place in the High Priority Group (WHO 2022). The new WHO and EMEG guidelines provide many possibilities for future research on fungi in drinking water. These should include deeper environmental studies on groundwater in order to understand the transmission of fungi between raw water sources and drinking water, and the effect of applied water cleaning procedures on the presence of the most abundant genera and the establishment of core-species. Similar research should also be conducted in hospitals and other nursing facilities as suggested by EMEG in order to assess the possible risk of water-transmitted fungi to human health.

Drinking water, suitable for consumption, often contains microorganisms not followed by regulations. One such is fungi, listed as the parameter only in Sweden (100 CFU/100 mL). Culturable fungi from Swedish drinking water samples were recovered from 75% of samples and were mainly associated with plant material and surface water. On the other hand, 90% of Slovenian drinking water samples were positive for fungi, with higher numbers of Aspergillus spp. and black yeasts, previously described from groundwater sources and clinical studies. The observed differences in mycobiota are likely the consequence of geographical location, the use of different raw water sources and water treatment methods. However, the small group of fungi, designated as the ‘core-species’, was present at both locations. Among these, A. fumigatus, Candida spp., and Fusarium spp. represent emerging fungi with known effects on human health. Our findings suggest the importance of tracking the presence of emerging fungi in drinking water and call for future research on their ecology and transmission routes to the end-users.

The authors thank Prof. Dr Nahid Kondori for her assistance during the sampling of drinking water in Mölndal and Göteborg, Sweden.

The study by M. N. B. was supported by the Slovenian Research Agency (ARRS) through the postdoctoral research project (grant number Z7-2668) and the research programme, grant number P1-0198. The study by N. G. -C. was supported by the research programme, grant number P4-0432.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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