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School of Molecular & Biomedical Science |
Microsporum canisTeleomorph: Arthroderma otaeColonies (SDA) are flat, spreading, white to cream-coloured, with a dense cottony surface which may show some radial grooves. Colonies usually have a bright golden yellow to brownish yellow reverse pigment, but non-pigmented strains may also occur. Macroconidia are typically spindle-shaped with 5-15 cells, verrucose, thick-walled and often have a terminal knob, 35-110 x 12-25 µm. A few pyriform to clavate microconidia are also present. Macroconidia and/or microconidia are often not produced on primary isolation media and it is recommended that sub-cultures be made onto Lactritmel Agar and/or boiled polished rice grains to stimulate sporulation. RG-2 organism.
Growth on Rice Grains: good growth of white aerial mycelium with production of yellow pigment. Microscopy reveals numerous macroconidia and microconidia similar to those described above. Lactritmel Agar: Flat, white suede-like to cottony colony with a bright yellow reverse. Microscopy reveals moderate numbers of thick-walled, multiseptate, long, spindle-shaped macroconidia, some of which show a terminal knob. Walls of macroconidia are slightly rough or echinulate especially at terminal knobs. Numerous clavate to pyriform microconidia are also present in this strain. Lactritmel agar with sterile soil added is also an excellent medium for the stimulation of macroconidial development in M. canis. Reverse Pigment on Potato Dextrose Agar: Bright yellow (both M. audouinii and M. canis var. equinum are salmon to pinkish-brown). Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special nutritional requirements. Cultures are flat, white, suede-like to downy, with a yellow to pale yellow-brown reverse. Hair Perforation Test: Positive at 14 days. Key Features: distinctive macroconidia and culture characteristics. Abundant growth and sporulation on polished rice grains and in vitro perforation of hair.
Dysgonic strains of M. canis are rare but may also occur. Cultures are typically heaped and folded and yellow-brown in colour. Macroconidia are usually absent in these strains. However, typical colonies and macroconidia of M. canis are usually produced by this variant when subcultured onto polished rice grains. Note the dysgonic type colony of M. canis is similar to that of Microsporum ferrugineum. Clinical significance:Microsporum canis is a zoophilic dermatophyte of world-wide distribution which is a frequent cause of ringworm in humans, especially children. Invades hair, skin and rarely nails. Cats and dogs are the main sources of infection. Invaded hairs show an ectothrix infection and fluoresce a bright greenish-yellow under Wood's ultra-violet light. Mycosis: DermatophytosisFurther reading: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.
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