On Sabouraud's dextrose agar, colonies are slow growing, small, button-or-disk-shaped, white to cream coloured, with a suede-like to velvety surface, a raised centre, and flat periphery with some submerged growth. Reverse pigment may vary from non-pigmented to yellow. Broad, irregular hyphae with many terminal and intercalary chlamydospores. Chlamydospores are often in chains. The tips of some hyphae are broad and club-shaped, and occasionally divided, giving the so-called "antler" effect. When grown on thiamine-enriched media, occasional strains produce clavate to pyriform microconidia borne singly along the hyphae. Macroconidia are only rarely produced, but when present have a characteristic tail or string bean shape. RG-2 organism.
Cattle infection and culture of Trichophyton verrucosum.
Clavate to pyriform microconidia of T. verrucosum.
Characteristic rat tail or string bean shaped macroconidia of T. verrucosum.
Growth at 37C: unlike other dermatophytes growth is enhanced at 37C
Nutritional requirements: all strains require thiamine and approximately 80% require thiamine and inositol. There is no growth on casein vitamin free agar (T1), minimal submerged growth on T1 + inositol (T2), good growth on T1 + inositol and thiamine (T3) and no growth on T1 + thiamine only (T4).
Growth on T1 vitamin free agar vs T3 with inositol and thiamine.
(1) Characteristic chains of chlamydoconidia and
(2) terminal vesicles at the tips of hyphae of T. verrucosum.
(1) All strains produce typical chains of chlamydoconidia, often referred to as "chains of pearls", when grown in brain heart infusion broth containing para-aminobenzoic acid (P.A.B.) and agar at 37C.
(2) Microscopic examination of young 4 to 5 day old colonies, grown from a very small inoculum, on Sabouraud's' dextrose agar containing 0.5% yeast extract and incubated at 30C, show characteristic terminal vesicles (not chlamydoconidia) at the tips of hyphae. The number of vesicles produced is greater from primary inoculations of skin scrapings or hairs.
Trichophyton verrucosum is a zoophilic fungus causing ringworm in cattle. Infections in humans result from direct contact with cattle or infected fomites and are usually highly inflammatory involving the scalp, beard or exposed areas of the body (ie. nails, skin). Invaded hairs show an ectothrix infection and fluorescence under Wood' ultra-violet light has been noted in cattle but not in humans. Geographic distribution is world-wide.
Rebell, G., and D. Taplin. 1970. The Dermatophytes. 2nd. revised edition. University of Miami Press, Coral Gables, Florida. USA.
Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.