Introduction

The ascomycetous black yeast Exophiala dermatitidis (Kano) de Hoog is one of cca. 40 species within the genus Exophiala (Herpotrichiellaceae, Chaetothyriales). Molecular studies connected the anamorph species with the teleomorph genus Capronia [1]. Numerous species of the genus Exophiala are opportunistic pathogens of human and warm-blood animals [2,3,4,5,6,7], while some are causative disease agents of cold-blooded animals [8]. New species of Exophiala continue to be described, in particular linked to polluted environment [9], human homes and human mycoses [10].

Exophiala dermatitidis is a phenotypically plastic, physiologically versatile, extremotolerant fungus. It is polymorphic since it grows as yeasts, hyphae or meristematic clumps [11], colonies range from olivaceous brown to black [12]. The fungus is able to degrade many substrates, it is frequently present in environments rich in aromatic hydrocarbons [13] suggesting its ability to assimilate them, which still needs to be demonstrated. It grows in a temperature range from 4 to 47 °C, survives a wide pH range and tolerates up to 3 M NaCl [12, 14, 15]. In spite of its adaptability, isolations of E. dermatitidis from nature were only rare and sporadic. It is known as an opportunistic pathogen of humans, currently classified under the Biological safety level 2 [16]. Exophiala dermatitidis was regularly reported as a causative agent of skin and subcutaneous infections [17], systemic infections [18], catheter-related [19] and respiratory tract infections [20], especially in immuno-compromised patients. It is regularly observed as a pulmonary colonizer of patients with cystic fibrosis [21], a disorder characterized by exceptionally mucous and salty lungs. Rare cases of brain infection, located primarily in East Asia, were also reported [22].

Exophiala dermatitidis has been occasionally retrieved from nature, from tropical rainforests [23] to glaciers [24]. Its occurrence increased considerably in human-made environments rich in aromatic hydrocarbons [13] and with high humidity, such as steam baths [25], bathrooms [26, 27], kitchens [28] and dishwashers [14, 29]. After its recent detection in drinking water [30] and hot dishwasher aerosols [28], it was speculated that water serves as a transmission vector from nature indoors, while high temperatures, plastic materials and oxidative agents enrich its presence both indoors and in household appliances, increasing the risk of human exposure. The isolation techniques are reviewed in Table 1.

Table 1 Review of publications reporting the presence of Exophiala dermatitidis from the environment
Full size table

The present study is thus a review of published findings on the occurrence of E. dermatitidis in habitats outdoors and indoors, supplemented with yet unpublished data. The summarized ecological data might unravel infection routes of this fungus and enable a better estimate of the potential risk of infections in predisposed people.

Exophiala dermatitidis as a Causative Agent of Human Infections

The genus Exophiala contains cca. 40 species, 17 of which were recognized as opportunistic human pathogens. The most common is E. dermatitidis [31], representing 30% of all Exophiala infections in the USA [17]. It caused mainly deep infections of lungs and abdomen (19%) and rarely of brain, ears or eyes. Sporadically it caused cutaneous (6%) and subcutaneous (2%) phaeohyphomycosis [17]. The species was involved globally in similar infections [32, 33], subcutaneous phaeohyphomycosis prevailing in (sub)tropical climate zones [34]. Other localized infections are keratitis [35, 36], otitis [37], peritonitis [38, 39], pneumonia [20, 40] and other pulmonary infections [41,42,43], in particular in cystic fibrosis patients [21, 44,45,46]. Localized systemic infections are usually related to predisposing factors such as cystic fibrosis, bronchiectasis, diabetes mellitus, catheterization, rheumatoid arthritis and leukaemia [19, 22, 47, 48].

In some cases E. dermatitidis showed remarkable neurotropism, acting as a true pathogen, infecting mainly healthy individuals [34]. It colonized the brain tissues as cerebral metastases, with a high mortality rate [18, 22, 49, 50], with sometimes severe secondary cutaneous mutilation [51,52,53,54]. Brain infections were reported mainly from Asia [55, 56]. Today, it seems probable that at least part of these cases concerned humans with rare inherited innate immune disorders, such as CARD9 or STAT1, previously unknown [57], although it remains puzzling why so many such cases are observed in East Asia. Cases of meningitis in the USA were due to epidural injection of steroids into cerebrospinal fluid which proved to be contaminated with E. dermatitidis [58]. Its presence was reported also in stool samples of otherwise healthy individuals in very low frequencies (0.5–1.2%) during periods of severe diarrhoea [25, 59].

Two probable routes of infection are noted in black fungi: via traumatic implantation in the skin, or via inhalation. Disseminated infections may have a pulmonary onset, but the severe (sub)cutaneous are secondary, i.e. emerging via the blood. Conversely, cutaneous infection with cerebral metastasis has been reported [60]. A possible connection between clinical and environmental isolates via trauma was confirmed in an AFLP analysis of 285 strains from global sampling study [61]. Infection routes for neurotropic infections remain unknown [22].

Exophiala dermatitidis in Terrestrial and Water-Related Natural Environments

Although E. dermatitidis was relatively frequently isolated from clinical specimens around the globe, its main natural habitat(s) remained unknown. Sporadically it has been isolated from plant material in Brazil [62], from tropical fruits in Thailand [23], from faeces of frugivorous birds, hornbills and flying foxes in Thailand [23], and from the cuticle of leaf-cutting ants in Brazil [63]. Based on these data it was for long speculated that tropical rainforests are its main natural source.

Its presence has later been noted in sea water and in fresh water-related environments, such as deep-sea sediments in India [64], deep-sea hydrothermal vents in Portugal [65], beach sand in Greece [66], natural hot springs in Thailand [23] and in glaciers in Italy and on Antarctica [24, 67] (Table 1).

All environments listed above have a strong connection with low-nutrient water. To confirm this hypothesis we additionally sampled different water sources in Slovenia: melt water from the Triglav glacier, 5 natural springs of mineral water, 10 sea water samples from the Adriatic Sea and 18 samples of brine from the salterns. We also analysed 24 samples of saltern sediments in contact with brine (mineral mud from salterns or fango), used for therapeutic skin treatments. Exophiala dermatitidis was recovered with a low frequency from glacial melt water, once from mineral water and once from fango, while samples of Adriatic Sea water and brine were negative (Table 2). Although frequencies are low, positive sampling sites were invariably oligotrophic.

Table 2 Presence of Exophiala dermatitidis in natural and man-made environment analysed in the present study
Full size table

Exophiala dermatitidis is Enriched in Environments Polluted with Aromatic Hydrocarbons

Order Chaetothyriales harbours numerous etiologic agents of chromoblastomycosis, phaeohyphomycosis and other diseases of vertebrate hosts, which range from mild cutaneous to fatal cerebral or disseminated infections and affect humans and cold-blooded animals. Chaetothyriales also comprise polyextremotolerant species with aquatic, rock-inhabiting, ant-associated and mycoparasitic lifestyles. Members of nearly all Chaetothyriales clades can degrade toxic monoaromates and tolerate or sometimes even prefer tannin-rich material, aromates and etheric oils, gasoline and mine waste rich in heavy metals, suggesting a high degree of versatile extremotolerance and tendency to pathogenicity. A recent study that compared genomes of 23 species within the Chaetothyriales [68] revealed a reduction of genes for carbohydrate degrading enzymes, expansion in certain protein degrading enzyme families, in alcohol dehydrogenase domains and in the trichothecene efflux pump. Analysis of cytochrome p450 genes (CYPs) which play a fundamental role in primary, secondary and xenobiotic metabolism and in a large number of detoxification reactions as well as in the metabolism of specific xenobiotics revealed an extraordinary p450 expansion, indicating that some black yeasts are among the Ascomycota species with the highest number of CYPs. PHA (2-hydroxy phenylacetate) and HGD (homogentisate 1,2-dioxygenase) genes involved in aromatic compound metabolism were organized in a syntenic cluster with additional conserved genes coding for hypothetical proteins, an MFS transporter, a trehalose-6-phosphate hydrolase (T6P-hydrolase) and a fumarylacetoacetase, which have been linked to protection against heat, freezing, starvation, dehydration and desiccation stress and pathogenicity [68].

In accordance with the observations above, aromatic hydrocarbons, such as oil, gasoline and components of different detergents which are toxic for most microorganisms, can serve as a substrate for enrichment of E. dermatitidis [13, 26, 69]. A number of related Exophiala species (particularly E. xenobiotica and E. oligosperma) can assimilate alkylbenzenes (i.e. toluene, ethylbenzene and styrene) as the sole source of carbon and energy. This uncommon metabolic feature among eukaryotes has been investigated for bioremediation purposes, e.g. the treatment of polluted air in biofilters, however, with concerns regarding the potential biohazard of enriching and aerosolizing black yeast cells [70].

Although a recent study [71] showed that only a single strain out of 9 tested E. dermatitidis assimilated toluene, while all 9 strains did not grow on hexadecane and polychlorinated biphenyl 126 (PCB126) as the sole carbon and energy sources, the reviewed ecological data show higher frequency of occurrence of E. dermatitidis in habitats with aromatic hydrocarbons. For example, creosote-treated railway sleepers revealed a higher frequency of E. dermatitidis (3–10%) in comparison with environments without aromatic hydrocarbons (Table 1) [23, 29, 62, 72], especially in humid and temperate climates [73]; Exophiala dermatitidis was also found in crude oil in Britain [74]. Additionally we sampled in Slovenia 8 handles of petrol pipes and 11 cement floors of gas stations polluted with oil or gasoline, 19 rubbers on car tank reservoirs, 21 sinks for waste water in carwashes and 3 railway sleepers, which were in contact with sea water. Only the railway sleepers were negative for E. dermatitidis, while we successfully recovered it with frequencies between 5 and 18% from all other listed habitats (Table 2).

Exophiala dermatitidis Mostly Invades Indoor Environments

In developed countries, tap water system represents the connection between the natural water environment and the indoor environment, where water is used for drinking, housework and hygiene. Presence of Exophiala species in biofilms at domestic water taps was thoroughly investigated in Germany [75]. Although this study recovered many different Exophiala species, primarily E. lecanii-corni, no strains of E. dermatitidis were obtained. However, a recent study revealed the presence of E. dermatitidis in 12.5% of sampled Slovenian groundwater samples and in 6% of tap water samples (Table 1) [30]. This study showed the importance of chemical characteristics of water samples exposing the concentration of calcium and magnesium ions, and nitrate as the crucial factors with positive influence on the fungal presence in water [30]. These results might explain the differences between the two studies, suggesting that areas with hard water are more likely colonized with black yeasts [30]. This is in accordance with previous study [11], showing that higher calcium concentrations favour polarized yeast/hyphal growth, whereas lower values induced multicellular and meristematic growth in E. dermatitidis [11].

In indoor environments E. dermatitidis was isolated primarily from niches in regular contact with water. Its first indoor isolation was from a humidifier [76]. Twenty years later it was detected in steam baths in Slovenia and in The Netherlands [25], later also in Austria and Thailand [23, 59], with a higher frequency of occurrence than in any natural sample. The study reported high temperature (40–42 °C) as the decisive parameter and this matches with prevalence of E. dermatitidis in (sub)tropical climates [61, 72]. Temperatures in bathrooms are lower than in saunas, and frequencies are concomitantly lower (Table 1): E. dermatitidis has been isolated from bathrooms in Japan [26] and Turkey [27].

Highest frequencies of E. dermatitidis in indoor environments are steam baths and dishwashers, where high temperatures, water and changing pH combine. Dishwashers are characterized by the presence of plastic elements, silicon or rubber seals, which might act as an enrichment factor or trapping surfaces. The enriching could be due to the presence of aromatic compounds present in the added surfactants (i.e. linear alkylbenzene sulphonate or LAS), related to alkylbenzenes, as well as to slowly released aromatics (e.g. bisphenols) from the rubber seals. Additionally, rugose and hydrophobic rubber and plastic surfaces offer favourable surface conditions for the promotion of biofilm formation.

In a global study involving 189 dishwashers, 28.6% of the rubber seals harboured a characteristic fungal community dominated by E. dermatitidis [14]. Later studies confirmed these observations (Table 1) [27, 28, 77] and revealed that E. dermatitidis colonizes the dishwashers interior with rubbers, drains and side nozzles being the most contaminated sites. Exophiala dermatitidis was also detected in 6% of dishwasher waste water samples [28], in hot aerosols released from the dishwashers immediately after the washing cycle, and on surfaces of kitchens with dishwashers (Table 1), indicating possible transmission routes and cyclic environmental contamination [28].

We confirmed these results by additional sampling of dishwashers, kitchen sinks and dish drying racks (Table 2). Results were comparable with previous studies. Additionally, we sampled rubber seals in kitchen refrigerators of which one positive indicated a broad thermotolerance of E. dermatitidis, and its preference for rubber and humidity.

Physiology and Polyextremotolerant Nature of Exophiala dermatitidis

The environmental occurrence of E. dermatitidis indicates its versatility. It assimilates d-glucose, d-galactose, d-xylose, L-arabinose, sucrose, maltose, α,α-trehalose, cellobiose, salicin, raffinose and melezitose. It can utilize alcohols like glycerol, meso-erythritol, xylitol, d-glucitol and ethanol, as well as ethylamine, lysine and cadaverine. It differs from other Exophiala species, being incapable to utilize nitrate, nitrite, creatine and creatinine [12]. As mentioned earlier, it can also be enriched by toxic monoaromatic hydrocarbons [13, 69].

Exophiala dermatitidis strains show three main genotypes A, B and C with rDNA ITS [78]. Genotype A is prevalent in dishwashers and the preponderant genotype on humans, B was mainly connected to the natural habitats, and C is extremely rare [14, 59]. Melanized cell walls are a unifying characteristic for black yeasts. Melanin is essential in resistance to stressful conditions [79]. Polyextremotolerant E. dermatitidis is able to sustain high dosages of UV and radioactive radiation [80, 81], and it can grow at temperatures above 40 °C, and tolerates temperatures from 4 to 47 °C [12, 14]. The same holds true for pH (2.5–12.5) and NaCl (up to 17%) [14, 77]. Its chaotolerance was confirmed in media containing NaCl, KCl, MgCl2, CaCl2, NaBr and MgSO4 at concentrations of 2.5, 2.5, 0.75, 0.5, 0.75 and 3 M, respectively [15]. Polymorphism, melanization and production of extracellular polysaccharides (EPS) are considered as the most important factors explaining its polyextremotolerant nature [5, 79].

Possible Routes of Infection

Exophiala dermatitidis is an important opportunistic pathogen able to cause a gamut of human infections. Since its natural niche is unknown, it was only sporadically isolated from natural environments, and the risk of exposure may be low. Its occurrence is enhanced by high environmental temperatures, the presence of aromatic, aliphatic and aromatic hydrocarbons, and the presence of water. Oligotrophism was suggested from its presence in low-nutrient waters of extremely differing temperatures, and salinities. Its isolation from household tap water confirms water as a route of transmission, where it is enriched in indoor niches such as bathrooms and kitchens, and even in steam baths and dishwashers. Creosote, mono- and polyaromates, rubber seals, plastics and detergents may all be favourable in strengthening its competition with susceptible co-inhabitant microorganisms (Fig. 1).

Fig. 1
figure 1

Presence of Exophiala dermatitidis in natural and human-made environments. This figure illustrates possible transmission routes between different environments and potential transmission to humans

Full size image

In practice, exposition risks of dry habitats such as railway ties are judged to be low as well as infections via small skin trauma. Effective transmission is expected to take place via inhaled aerosols (steam baths, shower heads and household appliances). The CF-population is the main group of patients with regular pulmonary presence of E. dermatitidis. These patients spend a lot of time indoors and also use personal aerosolizers to relief their stuffiness. Further research is needed to establish a potential connection between the presence of E. dermatitidis in indoor environments and its occurrence in the patients.