First Isolation of Trichophyton fischeriin the United States

Case report.A 49-year-old male patient with AIDS suffering from pneumonia was seen at Bellevue Hospital. Sputum samples yielded cultures of Candida lusitaniae and a filamentous fungus which resembled T. rubrum. The latter was provisionally identified as T. fischeri Kane by one of us (S.A.R.) and was sent to J.K. for confirmation. The cause of the pneumonia was determined to be P. carinii. Three months following the initial examination, additional sputum samples yieldedMycobacterium avium. Fungal infection other than pneumocystosis was ruled out in the clinical diagnosis.

Laboratory analysis.The New York isolate was subcultured on a variety of media (4, 5) in order to identify it. These methods and their applications are outlined below.

(i) CEA.CEA (sold commercially as CandidaInhibitory Agar; Biomedia Unlimited, Toronto, Canada). CEA was introduced by Fischer and Kane (1) to selectively inhibit the growth of Candida albicans by depleting the medium of biotin for which C. albicans has an absolute, exogenous need. This allows dermatophytes to be isolated from clinical specimens when there is a dermatophyte-Candida mixture (5). Subsequent to its introduction in the clinical laboratory for this purpose, CEA was discovered to facilitate the distinction ofT. rubrum from T. fischeri; the former produces a typical, red undersurface pigment on CEA, while the latter does not. It should be noted that some variant isolates ofT. rubrum fail to produce red pigment entirely; these cannot be confused either with typical isolates of the species or with T. fischeri, which specifically fails to form red pigment on CEA but forms it on Sabouraud-peptone-glucose agar (SAB) and many other media.

(ii) BHI agar.Microscopic examination of T. fischeri growing on brain-heart infusion agar (BHI) reveals a pattern of clustered branching points resembling verticillate, dichotomous, or broom-like branching in the mycelium of the colony margin (Fig. 1). There are many atypically short branch internodes intermixed with internodes of normal length (Fig. 1). T. rubrumproduces strictly monopodial branching with elongated internodes and regularly spaced, clearly subdominant lateral branches (Fig.2) (5).

• Open in new tab
• Download powerpoint

Fig. 1.

Broom-like, verticillate, or dichotomous branching in the marginal mycelium of T. fischeri on BHI for 7 days. Magnification, ×1,000.

• Open in new tab
• Download powerpoint

Fig. 2.

Strictly monopodial branching in the marginal mycelium of T. rubrum on BHI for 7 days. Magnification, ×392 (magnification less than that in Fig. 1 because of longer internode lengths in T. rubrum).

Other tests employed in the study of the New York isolate included those for urease production in Christensen’s urea broth (5), vitamin requirements, growth on bromcresol purple-milk solids-glucose agar (BCPMSG) (5), and hair perforation.

The isolate proved to be T. fischeri in every morphologic and physiologic characteristic. In brief, it was a deeply velvety colony, heavily sporulating with teardrop-shaped microconidia and a few thin macroconidia. It produced a red colony reverse on SAB but not on CEA. It was urease negative and negative for both alkalinity and clearing of suspended solids on BCPMSG at 7 days and at 25°C. It was negative for hair perforation. A comparison of the differences betweenT. fischeri and T. rubrum in general is given in Table 2.

The close morphological similarity between T. fischeriand T. rubrum may lead to the unnecessary treatment of patients for dermatophyte infection when the identification of the fungus is in error. The presence of the saprophyte T. fischeri should be considered when an unusually heavily sporulating T. rubrum-like colony grows from a KOH-negative skin scraping or when a similar colony is isolated from a source unlikely to yield T. rubrum, as in the present case. Investigators studying environmental materials such as room dust and public aquatic facilities for the presence of dermatophytes should be aware of the possibility of isolating this nonpathogenic organism. The isolate in the present case was almost certainly derived from airborne particulate material. Although the geographical distribution of T. fischeri is only beginning to be mapped, both the original finding of the organism as a contaminant of a medium preparation room (3) and the cluster of isolations noted in Table 1 suggest that growth indoors is likely. This species is, therefore, likely to be widely distributed; the present case aids in confirming this probability.

The small number of records to date of T. fischeriisolations, in comparison with the extreme abundance of T. rubrum, presents a practical problem. Clearly it is not practical to screen all T. rubrum-like isolates from KOH-negative skin and nail specimens to rule out this organism. Nor does a single isolation of T. fischeri from the United States justify such an addition to laboratory costs. It is possible that T. fischeri is indeed very rare or that, as with Onychocola canadensis, a regularly seen, cosmopolitan (2), onychomycotic pathogen that went unnoticed for decades until described by Sigler and Congly in 1990 (6), its apparent rarity may be an artifact of inadequate detection procedures. In laboratories already using CEA medium for other applications, the most practical approach is to screen only isolates deriving from KOH-negative specimens and matching the description of T. fischeri. Its heavy microconidial production alone separates it from the great majority of T. rubrum isolates, and its relative paucity of macroconidia separates it from granular-type T. rubrum isolates as well as the heavily macroconidialTrichophyton raubitschekii. In practice, the number of tests needed per annum for prospective T. fischeriisolates is very small. For example, the approximately 1,580T. rubrum-like isolates obtained in one of our laboratories (that of J.K.) entailed an estimated 104 screening tests for T. fischeri (estimates are given because a proportion of the records are inaccessible due to changes resulting from laboratory mergers). That is, even with a high index of suspicion, only approximately 6.6% of T. rubrum-like isolates were tested. A total of seven T. fischeri isolates were confirmed.

Laboratories not using CEA may be justified in monitoring the situation until the abundance of T. fischeri in their geographic area is clarified by further CEA or other (e.g., molecular) studies. However, they run a risk that a small number of patients may be treated with antifungal drugs unnecessarily.