Ecology, Epidemiology & Pathogenicity;
Trichophyton rubrum is a cosmopolitan, anthropophilic fungus meaning its primary reservoir is man. It is the most widely disseminated dermatophyte of man and most common cause of athlete’s foot (Tinea pedis), jock itch (Tinea cruris), ringworm (Tinea corpis), onychomycosis (Tinea unguium), and less commonly hair and scalp infections. Invasive infections in immunocompromised patients have been reported.
Trichophyton rubrum infections are of concern because nail infections caused by this dermatophyte are extremely difficult to cure.
Presented below is an isolate responsible for a case of onychomycosis, also referred to as Tinea unguium or simply stated a nail infection.
Rate of Growth;
Trichphyton rubrum is a moderately slow grower, reaching maturity within 14 days at 25o to 30oC.
Trichophyton rubrum species exhibits widely variable colonial morphology and the appearance is further influenced by the media on which it is isolated. Most typical strains are downy to cottony in texture with fine white aerial mycelium at the surface. The overall surface is white, sometimes becoming rose on ageing. The reverse is typically wine-red; however brown & yellow to olive-green hues may be present. SAB & Mycosel media tend to bring out the reddish-brown to yellow colours. Trichphyton rubrum often appears in literature as having two major forms of surface texture; a downy type and a granular type. Overlap between the two may be evident and even taxonomic relatedness is questionable. One source1 states that “deep red isolates with many macroconidia, once referred to as the ‘granular’ colony surface type, are now mostly recognized as representatives of the urease-positive T.raubitshekii.”
Trichophyton rubrum surface and reverse on SAB media after 10 days incubation at 30oC
Trichophyton rubrum produces hyaline septate hyphae. The downy type, described in this post, is characterized by the production of moderate numbers of clavate (club shaped) or pyriform (tear-drop shaped) microconidia (3-5.5 X 3-3.5 µm) with rare if any macroconidia. (The granular form is characterized by the production of moderate to abundant numbers of microconida as well as moderate to abundant numbers of long, narrow, thin-walled cigar or pencil-shaped macroconidia (40-55 X 6-7.5 µm) with parallel sides.) Macroconidia (4 – 10 cells in length) may form directly on the ends of thick hyphae singly or in groups. It may also produce small chains of barrel-shaped arthroconida from both the hyphae and macroconidia and are similar in size to the macroconidia.
Photos below were taken with a Digital Nikon Coolpix 8400 Camera or the Leica DMD-108 microscope camera as indicated. The DMD-108 has a digital magnification factor of X10 which can be added to any optical magnification.
Structure size may vary between photographs of identical magnification due to selective cropping of the photograph.
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(Click on any photo to enlarge for better viewing)
Toenail from which this Trichophyton rubrum was isolated. Portions of the nail specimen were placed on SAB and Mycocel media for isolation. Portion was also flooded with 10% KOH which both softens the nail and clarifies for better viewing of any fungal structure which may be responsible for the infection.
Photomicrograph of nail segment in KOH. Trichphyton rubrum fungal hyphae seen invading between nail cells. (dotted structure weaving through from upper left through to lower right) (DMD-108 Microscope X400)
Photomicrograph of T.rubrum fungal element (arrow) in toenail tissue (Magnification not noted)
Trichophyton rubrum microconidia on hyphae. This arrangement of microconida has been described as "birds on a wire" where the pyriform (teardrop shaped) microconidia are attached to the hyphae at the narrow end (tail).
Another view of the microconidia attached to the hyphae (DMD-108 LPCB X400)
Inset: isolated view of the microconidia on hyphae ( LPCB X250 Nikon)
Closer view of pyriform shaped microconidia on hyphae (DMD-108 LPCB X400)
This structure looks like I would expect the pencil-like macroconidia would appear, that is except for the size. Though I neglected to take measurements on this structure, it was clearly less than the 45-55 µm usually noted as the average length. Clearly visible are 4 cells within the structure. Perhaps this is still an immature macroconidium.
(LPCB X400 Nikon Camera - I suspect circular lines are from spherical aberrations on the Leica microscope lens at this particular magnification.)
Arrows; indicating presence of Arthroconidia, either free or forming at ends of hyphae,
(LPCB X400 Nikon)
T.rubrum can be differentiated from T.mentagrophytes by its typical microconidaia which are clavate (club shaped) to pyriform (tear-drop shaped), solitary, sessile alongside undifferentiated hyphae. T.mentagrophytes produces microconidia in clusters that are spherical.
Other Physiological Tests for T.rubrum;
- Urease Negative
- Hair perforation test negative.(1)
- BCP-milk solids-glucose: no change in pH, restricted growth.
- Potato glucose agar or cornmeal glucose agar enhance the production of red pigment on the reverse if the strain is unpigmented on Sabouraud Glucose (Dextrose) agar.
- Requires no additional growth factors for growth. Unlike T.megninii, T.rubrum does not require histadine for growth.
Molecular Testing; has been used to reassess the classification of closely related Trichphyton species but remains beyond the scope of the average clinical laboratory and this post.
(1) Hair perforation test; sterile hair can be inoculated with Trichphyton species and examined after a period of growth to see if the fungus has physically invaded/perforated the hair.