DESCRIPTIONS OF MEDICAL FUNGI
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Descriptions of Medical Fungi 215<br />
Trichophyton verrucosum is a zoophilic fungus causing ringworm in cattle. Infections in<br />
humans result from direct contact with cattle or infected fomites and are usually highly<br />
inflammatory involving the scalp, beard or exposed areas of the body. Invaded hairs<br />
show an ectothrix infection and fluorescence under Wood’s ultra-violet light has been<br />
noted in cattle but not in humans. Distribution is worldwide. RG-2 organism.<br />
Morphological Description: Colonies are slow growing, small, button or disk-shaped,<br />
white to cream-coloured, with a suede-like to velvety surface, a raised centre, and<br />
flat periphery with some submerged growth. Reverse pigment may vary from nonpigmented<br />
to yellow. Broad, irregular hyphae with many terminal and intercalary<br />
chlamydospores. Chlamydospores are often in chains. The tips of some hyphae are<br />
broad and club-shaped, and occasionally divided, giving the so-called “antler” effect.<br />
When grown on thiamine-enriched media, occasional strains produce clavate to<br />
pyriform microconidia borne singly along the hyphae. Macroconidia are only rarely<br />
produced, but when present have a characteristic tail or string bean shape.<br />
Confirmatory Tests:<br />
Trichophyton verrucosum Bodin<br />
Growth at 37 O C: Unlike other dermatophytes, growth is enhanced at 37 O C.<br />
Nutritional Requirements: all strains require thiamine and approximately 80% require<br />
thiamine and inositol. There is no growth on casein vitamin free agar (T1), minimal<br />
submerged growth on T1 + inositol (T2), good growth on T1 + inositol and thiamine<br />
(T3) and good growth on T1 + thiamine only (T4).<br />
All strains produce typical chains of chlamydospores, often referred to as “chains of<br />
pearls”, especially when grown on BCP milk solids glucose agar at 37 O C. When grown<br />
at 25 O C on milk solids glucose agar a “halo”-like zone of peripheral clearing of milk<br />
solids occurs within 7 days.<br />
Microscopic examination of young colonies (4 to 5 day old), grown from a very small<br />
inoculum, on Sabouraud’s dextrose agar containing 0.5% yeast extract and incubated<br />
at 30 O C, show characteristic terminal vesicles (not chlamydospores) at the tips of<br />
hyphae. The number of vesicles produced is greater from primary inoculations of skin<br />
scrapings or hairs than from subcultures.<br />
Key Features: Culture characteristics and requirements for thiamine and inositol, large<br />
ectothrix invasion of hair, clinical lesions and history.<br />
a<br />
b<br />
Trichophyton verrucosum (a) young button-shaped colonies and (b) mature culture.