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Journal of Clinical Microbiology, November 1998, p. 3389-3391, Vol. 36, No. 11
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
First Isolation of Trichophyton fischeri
in the United States
S. A.
Rosenthal,1
J. S.
Scott,2
R. C.
Summerbell,3,4 and
J.
Kane4,5,*
Department of Botany2
and
Department of Laboratory Medicine and
Pathobiology,4 University of Toronto,
Ontario Ministry of Health,3 and
Sunnybrook-Dynacare Medical
Laboratories,5 Toronto, Ontario, Canada, and
New York University Medical Center, New York, New
York1
Received 2 April 1998/Returned for modification 22 April
1998/Accepted 27 July 1998
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ABSTRACT |
A 49-year-old male with Pneumocystis carinii pneumonia
was seen at Bellevue Hospital in New York, N.Y. Sputum samples yielded cultures of Candida lusitaniae, Mycobacterium
avium, and a filamentous fungus, Trichophyton
fischeri. T. fischeri is a nonpathogenic fungus which resembles
the dermatophyte Trichophyton rubrum. This is the first
record of the species from U.S. sources. This case exemplifies the
ecological differences between T. fischeri and T. rubrum and illustrates how correct identification of the former species can minimize diagnostic confusion. The two species are distinguished from each other by the type of growth on Casamino Acids-erythritol-albumin agar and by micromorphological
differences.
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TEXT |
Trichophyton
fischeri was described by Kane (3) as a
nonpathogenic species resembling the common dermatophyte
Trichophyton rubrum. The original isolations
were airborne contaminants. Later isolates obtained by one of us
(J.K.) in the Ontario Ministry of Health diagnostic reference mycology
laboratory were often from clinical specimens; however, in each case,
investigation revealed that T. fischeri was medically
insignificant and apparently represented only a chance contamination of
the clinical material. A cluster of some recent cases of this
kind, which were apparently related to an environmental
source of T. fischeri contamination in a
commercial diagnostic laboratory, are documented in Table 1.
Since T. fischeri is relatively uncommon and since its
distinction from T. rubrum is facilitated by the
use of Casamino Acids-erythritol-albumin agar (CEA),
which has not been readily available, recognized isolations have
almost entirely been from Canada, where CEA originated. This study
reports the first isolation of T. fischeri from the
United States. It was found as a contaminant in the sputum of a patient with pneumonia caused by Pneumocystis carinii. Its
unequivocal status as a nonpathogen in this case helps reinforces the
evidence for its status as a potential airborne contaminant in indoor
situations. This in turn helps to clarify its significance when
isolated from skin, a situation in which confusion with the pathogenic
T. rubrum may be especially problematic.
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 yielded
Mycobacterium 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 Candida
Inhibitory 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 of
T. 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 of
T. 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. rubrum
produces strictly monopodial branching with elongated internodes
and regularly spaced, clearly subdominant lateral branches (Fig.
2) (5).
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FIG. 1.
Broom-like, verticillate, or dichotomous branching in
the marginal mycelium of T. fischeri on BHI for 7 days.
Magnification, ×1,000.
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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).
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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 between
T. fischeri and
T. rubrum in
general
is given in Table
2.
The close morphological similarity between
T. fischeri
and
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. fischeri
isolations, 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 macroconidial
Trichophyton raubitschekii.
In practice, the number
of tests needed per annum for prospective
T. fischeri
isolates is very small. For example, the approximately
1,580
T. 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.
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FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Microbiology, B-121, Mycology Laboratory, Sunnybrook Dynacare
Laboratory Services, 2075 Bayview Ave., Toronto, Ontario M4N 3M5,
Canada. Phone: (416) 480-4242. Fax: (416) 480-6845.
| |
REFERENCES |
| 1.
|
Fischer, J. B., and J. Kane.
1971.
The detection of contamination in Trichophyton rubrum and Trichophyton mentagrophytes.
Mycopathol. Mycol. Appl.
43:169-180[Medline].
|
| 2.
| Gupta, A. K., C. B. Horgan-Bell, and R. C. Summerbell. Onychomycosis associated with Onychocola
canadensis: 10 case reports and a review of the literature.
J. Am. Acad. Dermatol., in press.
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| 3.
|
Kane, J.
1977.
Trichophyton fischeri sp. nov.: a saprophyte resembling Trichophyton rubrum.
Sabouraudia
15:231-241[Medline].
|
| 4.
|
Kane, J., and C. M. Smitka.
1980.
A practical approach to the isolation and identification of members of the Trichophyton rubrum group.
Pan Am. Health Organ. Sci. Publ.
396:121-134.
|
| 5.
|
Kane, J.,
R. C. Summerbell,
L. Sigler,
S. Krajden, and G. Land.
1997.
Laboratory handbook of dermatophytes.
Star Publishing, Belmont, Calif.
|
| 6.
|
Sigler, L., and H. Congly.
1990.
Toenail infection caused by Onychocola canadensis gen. et sp. nov.
J. Med. Vet. Mycol.
28:405-417[Medline].
|
Journal of Clinical Microbiology, November 1998, p. 3389-3391, Vol. 36, No. 11
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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