Chapter 2

Yeasts pathogenic for humans

D.G. Ahearn

Contents

1. Description 9 2. Key to selected human- and animal-associated yeasts 11

1. Description

The principal yeasts pathogenic for humans are Candida albicans and Cryptococcus neoformans. Candida albicans is an asexual, diploid (possibly aneuploid), plemorphic fungus with an ascomycetous-type cell wall. This species is endogenous in the oral-, gastrointestinal-, or urinogen-ital tracts of humans and other warm blooded animals. Candida albicans may be categorized as an obligate commensal because its host-free occurrence is relatively rare. Probably 40 to 60% of the adult population harbors C albicans. Infections are usually of endogenous origin, but patient to patient spread can occur (Ahearn 1988, Jones 1990).

The syndromes produced by Candida albicans are highly diversified including vulvovaginitis, dermatitis, cystitis, fever, myositis, hepatic disfunction, and mental confusion. These may occur singly or in combinations dependent upon the superficial, locally invasive, or deep nature of the infection. Candida albicans is involved in millions of superficial infections per year (mainly vulvovaginitis) and in over 90% of the systemic or deep yeast infections (estimated in the USA for the 1980's to range from 5000 to 8000 per year). Strains of C albicans vary in their virulence, but as with all yeast infections, the balance between transient or commensal colonization or parasitism is dependent upon the physiological status of the host. Chronic superficial candidiasis is often associated with malnutrition, pregnancy, and diseases with impaired immune systems such as diabetes and malignant neo­plasms. Oral candidiasis and localized esophagitis show a particularly high frequency among patients infected with the human immunodeficiency virus (Phelan et al. 1987). Recently, a yeast phenotypically similar to C albicans, but apparently genetically distinct, C dubliniensis, has been associated with AIDS (Sullivan et al. 1995). Candidiema may be indicative of deep or systemic yeast infections involving two or more organs. Such grave infections are frequently iatrogenic resulting from hyperalimentation, broad spectrum antibiotics, and immunosuppressive or antineoplastic therapies.

In contrast to C albicans, Cryptococcus neoformans,

the anamorph of Filobasidiella neoformans, is not usually a commensal of humans. Cryptococcus neoformans, a basidiomycetous yeast, includes two varieties that are often haploid when isolated from nature. Cryptococcus neoformans var. neoformans includes serotypes A, D, and AD, and C neoformans var. gatti includes serotypes B and C. Serotypes A and D are usually associated with soil enriched with pigeon droppings. Serotype A is most common in the U.S.A. and serotype D is reported mostly from Europe. Serotypes B and C seem confined to warm regions (Kwon-Chung and Bennett 1984); in particular serotype B has been associated with the flowering of Eucalyptus camaldulensis (Ellis and Pfeiffer 1990). Cryptococcus neoformans most often primarily infects the lungs with mild symptoms. In stressed patients, a pneumococcal-type pneumonia is not uncommon. This species has a predilection for the central nervous system and may produce, if un­treated, a fatal meningoencephalitis, particularly among the immunocompromised. Reporting of cryptococcosis is not mandatory, therefore its incidence is unknown. In Georgia, U.S.A. in 1978, Bowman and Ahearn noted 76 cases of cryptococcosis. Kaufman and Blumer (1978) estimated that there were about 300 cases of cryptococcal meningitis throughout the U.S.A. for that same year. In 1987, in Georgia, U.S.A. at least 146 infections occurred (Ahearn, unpublished). Throughout the U.S.A., there has been a notable increase in reported cryp­tococcosis. The Centers for Disease Control, Atlanta, Georgia, U.S.A., has documented cryptococcosis as the primary presenting syndrome for the acquired immune deficiency syndrome (AIDS) in 135 cases for 1983, 279 cases for 1984, 514 cases for 1985, and 704 cases for 1986 (R.M. Selik, personal communication). This steep increase involves fatal cryptococcal meningitis in 7-17%) of patients with AIDS (Gal et al. 1986, Zuger et al. 1986). By 1990, cryptoccoccosis had become the fourth to fifth most important secondary infection complicating AIDS in the U.S.A. (R.M. Selik, personal communication). At least 2000 cases of extrapulmonary cryptococcosis (an incidence of about 7%) occurred among AIDS patients in the U.S.A. in 1990 (Centers for Disease Control 1991). Probably 450 more cases of cryptococcosis occurred among non-AIDS patients for this same year. The actual rate of increase of cryptococcosis over the past decade cannot be determined because not only is

10 //. Importance of yeasts

reporting of the disease not mandatory, but the basis for defining AIDS, which is receiving intensive monitoring, has become less stringent during the past few years. Nevertheless, the worldwide increase in the incidence of life threatening cryptococcosis is alarming. Although oral candidiasis, and to some degree esophagitis, are prognostic of HIV infection (Phelan et al. 1987), systemic candidiasis has not shown the same relative increase among AIDS patients as has cryptococcosis.

Candida tropicalis is probably the third most impor­tant yeast pathogen of humans. This species does not normally produce germ tubes or chlamydoconidia, but elongated pseudohyphal cells may develop commonly in some strains in serum at 37°C. Candida tropicalis, like C. albicans, is a diploid ascomycetous-type yeast, but it is commonly isolated from host-free habitats. Both species have overlapping antigenic properties; C tropicalis and C. albicans serotype A bear the mannan specific epitotes 1,4,5,6, whereas C. albicans serotype B has 1,4,5,13b (Tsuchiya et al. 1965). Hamajima et al. (1988) detected a cell wall antigen (t) in C tropicalis that permits its serological distinction from C. albicans ser A. DNAs from the holotype strains of these species show insignificant DNA relatedness, but rare strains with phenotypic properties intermediate between those of the holotypes show intermediate (40-50%) relatedness with the separate species (Hamajima et al. 1988; Ahearn and Meyer, unpublished). Deep or systemic infections by C tropicalis appear to be increasing and there is some suggestion that the prognosis for a disseminated infection with C tropicalis is more grave than that for C albicans (Ahearn and Lawrence 1984).

Because host factors in general are preeminent in yeast infections, it is difficult to determine specific virulence factors for many yeasts. Nevertheless, there appears to be an association of specific species with certain syndromes. In part, this may reflect a natural habitat association. For example, the usually commensal or mildly pathogenic, lipophilic Malassezia spp. may cause fungemia and fatal pulmonary infections of neonates receiving lipid supplementation via broviac catheters (Gueho et al. 1987b, Danker et al. 1987, Larocco et al. 1988). Even the important industrial yeast, Saccharomyces cerevisiae, which can be expected to be ingested in normal diets, may produce fatal complications in immune deficient patients (Ossama et al. 1989; Aucott et al. 1990). Funguria or fimgemia by skin mycota or commensals of mucus membranes (e.g., Candidaparapsilosis, Candida glabrata, respectively) frequently follow combinations of extensive antibacterial therapy and catheterization (Solomon et al. 1984, Van Cutsem et al. 1986). In other instances, such as with the emerging pathogens Trichosporon spp., Candida lusitaniae and Candida noruegensis, the selective effect of antifimgal therapies may be an additional factor (Ahearn and McGlohn 1984; Gargeya et al. 1990, Nielsen et al. 1990, Walsh et al. 1990).

Selected yeasts implicated as pathogens of humans and

their common habitats are listed in Table 2. This list is restricted in comparison to general lists of medically important (or human- and animal-associated) yeasts be­cause it includes only those species that fit the criteria of McGinnis et al. (1980): - the fungus must be isolated; - the fungus must be properly identified; - the clinical disease must be compatible with a myco­

sis; - the fiangus must be seen in clinical and pathological

specimens. Species such as Rhodotorula rubra, Candida famata,

C haemulonii, C krusei, C lipolytica, and C. rugosa, which have been associated mainly with transient fung­emia, have not been included. Undoubtedly, specific case histories in the future will indicate these species and others can produce a broader syndrome. For example, C haemulonii, a genetically heterogenous form species complex, is more frequently becoming associated with lower limb ulcers and fiingemia (Gargeya et al. 1991, Lehmann et al. 1991).

The increasing incidence of yeast infections has stimu­lated the development of commercial yeast identification systems (Land et al. 1991). Some of these systems provide for accurate identification of most strains of the common medically important yeasts, but generally the data bases for these systems include information for less than 50 species. Also, given strains may deviate from the norm in key characters. For example, C. albicans may not readily produce germ tubes, chlamydoconidia, or even pseudohyphae (Ahearn and Schlitzer 1984). Some strains of Cryptococcus neoformans, particularly a mating types, may produce melanin only under certain conditions (Nurudeen and Ahearn 1979); other rare strains may not demonstrate urease activity or assimilate inositol. Rhamnose may not be assimilated by rare strains of C. lusitaniae (Gargeya et al. 1990). An increasing number of auxotrophs of C albicans and C glabrata have been observed among yeasts submitted for identification to the Centers for Disease Control, Atlanta, GA. Such isolates, probably induced by the administration of antineoplastic drugs, do not grow on the generally used synthetic media, but often grow in the complex fermentation broths.

A selective phenotypic key for the identification of the more common adventitious yeast pathogens and some of the common animal-associated yeasts is presented below Steps in the key include multiple characters, so that the key permits a presumptive species identification, i.e., a distinction of the adventitious species from most species presented as animal-associated in this monograph. For definitive identification, however, additional diagnostic tests must be performed. The key does not distinguish the genotypic species present in the form species C al­bicans, C haemulonii, C parapsilosis, C tropicalis and Malassezia furfur

2. Yeasts pathogenic for humans / D.G. Ahearn

Table 2

Syndromes and habitats of yeasts causing infections in humans

Species

Candida albicans

Candida glabrata

Candida guilliermondii

Candida kefyr {= Kluyueromyces

marxianus)

Candida krusei

Candida lusitaniae

Candida noruegensis ("^Pichia

norvegensis)

Candida parapsilosis

Candida tropicalis

Candida uiswanathii

Cryptococcus neoformans

Malassezia furfur (M. pachydermatis)

Pichia angusta

Saccharomyces cereuisiae

Trichosporon spp.

Representative infections

Superficial

vaginitis

vaginitis (rare)

otitis (rare)

unknown

unknown

unknown

unknown

onchomycosis

(rare)

vaginitis (rare)

unknown

skin (rare)^

dermatitis

unknown

unknown

white piedra

Deep

disseminated

pyleonephritis, osteomyelitis

endocarditis, joint infection

disseminated

disseminated

disseminated

disseminated

endocarditis, endophthalmitis

peritonitis, endocarditis

meningitis

pulmonary, CNS

pulmonary

lymph nodes

disseminated

disseminated

Association

with humans

mucus

membranes

urine, sputa,

feces

sputa, feces

sputa

sputa

sputa, feces

sputa, urine

skin, sputa

sputa, urine,

feces

unknown

sputa (rare)

skin

unknown

sputa

skin, hair

Habitats

Host-free

soil, air, water, plants (rare)

soil, water (rare)

soil, water

dairy products

water, soil

animal manure

water, soil

soil, water, plants

organically enriched soil and

aquatic habitats

unknown

aged pigeon droppings,

eucalyptus flowers

unknown

insects, soil

food (fruits), soil

soil, water

Secondary manifestation of deep infection.

2. Key to selected human- and animal-associated yeasts 1. a Growth on Sabouraud's, malt agars, or yeast nitrogen base with glucose agar -^ 2

b No growth on the above media without addition of olive oil Malassezia furfur: M. sympodialis:

c No growth on yeast nitrogen base with glucose agar without olive oil, growth present on Sabouraud's or malt agars Malassezia pachydermatis:

2(la). a Germ tubes produced Candida albicans: C. dubliniensis:

b No germ tubes produced -^ 3 3(2b). a Hyphae or pseudohyphae absent or scarce -^ 4

b Hyphae or pseudohyphae well developed ^ 1 6 4(3a). a Inositol assimilated (rarely negative), starch synthesized -^ 5

b Inositol not assimilated, starch not synthesized -^ 9 5 (4a). a Nitrate assimilated -^ 6

b Nitrate not assimilated -^ 7 6(5a). a Sucrose assimilated Cryptococcus albidus:

b Sucrose not assimilated Cryptococcus terreus: 7(5b). a Lactose assimilated Cryptococcus laurentii:

b Lactose not assimilated -^ 8 8(7b). a Galactitol assimilated; melanin synthesized Cryptococcus neoformans:

b Galactitol not assimilated; melanin not synthesized Cryptococcus uniguttulatus:

9(4b). a Cultures red or orange; nitrate not assimilated Rhodotorula mucilaginosa: b Cultures not red or orange; nitrate assimilated or not assimilated -^ 10

10(9b). a Nitrate assimilated -^ 11

b Nitrate not assimilated —^ 12 1 l(lOa). a Methanol assimilated Pichia angusta:

b Methanol not assimilated Pichia jadinii: 12(10b). a Lactose assimilated Candida famata:

b Lactose not assimilated -^ 13

p. 782 p. 783

p. 782 p. 476 p. 479

p. 748 p. 764 p. 759

p. 762 p. 765 p. 820

p. 286 p. 314 p. 503

12 //. Importance of yeasts

13(12b).

14(13b).

15(14b).

16(3b).

17(16a).

18(17a).

19(17b).

20(19a).

21 (20b).

22(16b).

23(22b).

24(23b).

25(24a).

26(25a).

27(26b).

28(25b).

29(28a).

30(29b).

D-Xylose and cellobiose assimilated (see also C. famata and C. norvegensis) Candida guilliermondii: p. 511 D-Xylose assimilated; cellobiose not assimilated —^14 Galactitol assimilated Candida haemulonii: p. 511 Galactitol not assimilated -^ 15 Raffinose assimilated and fermented Saccharomyces cerevisiae: p. 361 Raffinose neither assimilated nor fermented Candida glabrata: p. 508

Candida norvegensis (Pichia norvegensis): p. 328 Arthroconidia produced ^ 1 7 Arthroconidia not produced -^ 22 Diazonium Blue B (DBB) reaction positive -^ 18 DBB reaction negative -^ 19

Nitrate assimilated Trichosporon pullulans: p. 869 Nitrate not assimilated Trichosporon ovoides complex: p. 869 D-Xylose assimilated -^ 20 D-Xylose not assimilated Geotrichum capitatum: p. 575 Cellobiose assimilated Geotrichum fermentans: p. 576 Cellobiose not assimilated -^ 21 Arthroconidia in chains within primary filaments Galactomyces geotrichum: p. 210 Arthroconidia in branched chains arising on branches from primary filaments Geotrichum klebahnii: p. 578 Lactose fermented Candida kejyr (Kluyveromyces marxianus): p. 236 Lactose not fermented -^ 23 Raffinose and melibiose assimilated Candida guilliermondii: p. 511 Raffinose and melibiose not assimilated -^ 24 Trehalose assimilated -^ 25

Trehalose not assimilated Candida krusei: p. 519 Cellobiose assimilated -^ 26 Cellobiose not assimilated -^ 28 L-Rhamnose assimilated (rarely negative) Clavispora {Candida) lusitaniae: p. 148 L-Rhamnose not assimilated -^ 27 Triphenyl Tetrazolium Chloride (TTC) reaction positive Candida tropicalis: p. 563 TTC reaction negative to weak Candida viswanathii: p. 570 Maltose fermented, TTC reaction positive or negative —̂ 29 Maltose not fermented, TTC reaction positive (rose to red color) Candida parapsilosis: p. 536 Sucrose fermented, TTC reaction positive (deep maroon color) Candida tropicalis: p. 563 Sucrose not fermented, TTC reaction negative —> 30 Maltose assimilated (repeat germ tube test) Candida albicans: p. 476 Maltose not assimilated Candida zeylanoides: p. 571