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Vol. 30, No. 10JOURNAL OF CLINICAL MICROBIOLOGY, OCt. 1992, p. 2748-27520095-1137/92/102748-05$02.00/0

Colony Morphotype on Sabouraud-Triphenyltetrazolium Agar:a Simple and Inexpensive Method for Candida



Departamento de Microbiologia e Inmunologia, Facultad de Medicina y Odontologia,Universidad del Pais Vasco, Bilbao, SpainReceived 27 January 1992/Accepted 27 June 1992

A new method of Candida subspecies discrimination on Sabouraud-triphenyltetrazolium agar is reported.Eive hundred sixty-two strains of Candida and Torulopsis glabrata, previously identified by conventionalmycological methods, were studied. Each strain received a three-letter code and a number based on its colonialmorphology. Sixteen morphotypes were found for Candida albicans, 6 were found for Candida parapsilosis, 4were found for both Candida guilliermondii and Candida krusei, and 12 were found for Candida tropicalis. Noneof the 56 T. glabrata strains studied grew on this agar. A reproducibility of 95% was found for C. albicans. Thesimplicity and low cost could make this method useful for typing Candida spp.

Candida albicans and other Candida spp. are opportunis-tic pathogens involved in an increasing number of infectionsof patients with immunodeficiency. Different typing meth-ods, including biotyping (11), enzyme typing (15), morpho-typing (13), serotyping (1), resistotyping (8), killer typing(14), protein typing (2, 7), and karyotyping (10), have beendescribed previously, and attempts have been made todetect a possible correlation of some C. albicans types withthe virulence of the isolates (5, 9). However, it is still notpossible to type C. albicans with a degree of certainty, andthe lack of a suitable typing scheme has undoubtedly hin-dered studies of the epidemiology and virulence of thispathogenic yeast (3). These typing methods have usually notbeen applied to other Candida spp. (9), and most of theminvolve a relatively complicated methodology. In this article,we report the development of a new and inexpensive typingmethod and its application to evaluation of the morphotypespresent in 562 strains of Candida spp. from different clinicalspecimens.Data were obtained from 562 unrelated strains of C.

albicans, Torpulopsis glabrata, and other medically impor-tant Candida spp., previously identified by conventionalmycological methods, from a variety of clinical samples.Most of them were from skin and mucosae, with 384 isolatesfrom oral, vaginal, and respiratory samples and 178 fromblood, peritoneal fluid, or organ biopsies. C. albicans 10261and 3153 and their mutants IR2h (proteinase deficient) andCA2 (germ tube deficient), respectively, sent by A. Cassone(Istituto Superiore di Sanita, Rome, Italy), were also evalu-ated. Fungal identification was based on morphology onSabouraud dextrose agar (Difco, Detroit, Mich.) and cornmeal extract-Tween 80 agar (Oxoid, Basingstoke, UnitedKingdom), germ tube formation, as well as biochemicalcharacterization with the use of the API 20C Aux (APISystem S.A., Montalieu Vercieux, France) and the 16-disccarbon auxanogram (Diagnostics Pasteur, Paris, France)systems.The Sabouraud-triphenyltetrazolium (STTZ) agar, con-

* Corresponding author.

taining 2,3,5-triphenyltetrazolium chloride (Sigma ChemicalCo., St. Louis, Mo.) (0.1 g), peptone (10 g), glucose (20 g),agar (20 g), and distilled water (1 liter), was distributed in 40-to 50-ml quantities into 15-cm petri dishes. Plates were eitherinoculated immediately or stored at 4C for no longer than 8weeks. Each isolate was maintained by monthly transfers inSabouraud dextrose agar slants and storage at 4C. Beforepreparation of inocula, each isolate was grown on a freshSabouraud dextrose agar plate at 37C for 24 h. Three or fourcolonies were resuspended in 1 ml of distilled water andadjusted to a MacFarland standard of 5 to prepare eachinoculum. Before inoculation, all plates were incubatedinverted at 37C for 1 h to dry agar surfaces. Plates wereinoculated in triplicate with 10 ,ul from the cell suspension byusing a micropipette and allowing the inoculum drop to dryon the agar surface. Cultures were incubated at 37C for 6days. After incubation, a three-letter code and a numberwere given to each strain, according to its colonial morphol-ogy (Fig. 1): texture, S = smooth and R = rough; color, P =pink (P1 = pale pink = Pantone 176C, P2 = dark pink =Pantone 1785C), V = violet (Vi = pale violet = Pantone251C, V2 = dark violet = Pantone 262C) (Pantone Inc.), 0 =orange, W = White, etc.; and presence of mycelial halo(hyphae or pseudohyphae), Y = yes and N = no. The codeNG was used if no growth was observed. Pantone is aregistered trademark for color specification that is orientedto facilitation and color standardization of publishing andgraphic design work.We were not able to make a direct comparison between

the morphotype method with STTZ agar and referencemethods, which are not well established for Candida typing;therefore, we attempted to subdivide the strains within C.albicans species on the basis of their likelihood of diversity.We compared strains from various geographical and epide-miological situations with those from infected and uninfectedpersons. With this purpose, 277 additional C. albicansstrains from Barcelona (J. M. Torres Rodriguez, InstitutoMunicipal de Investigaciones Medicas-Hospital del Mar,Barcelona, Spain), London (G. C. White, Candida Unit,Division of Hospital Infection, Central Public Health Labo-ratory, Colindale, London, United Kingdom), and Madrid


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NOTES 2749

FIG. 1. Candida morphotypes on S1TZ agar. (A) Inoculated 15-cm petri plate with different morphotypes; (B) RP2N (rough, dark pink,without mycelial halo) morphotype; (C) SPlN (smooth, pale pink, without mycelial halo) morphotype; (D) SPlY (smooth, pale pink, withmycelial halo) morphotype; (E) SV2N (smooth, dark violet, without mycelial halo) morphotype.

(M. C. Cabronero, Departamento de Microbiologia, Facul-tad de Medicina, Universidad Complutense, Madrid, Spain)were studied. Twenty-three isolates from oral, rectal, andskin samples from eight children at 15 days and 3 and 6months of age (three patients with oral thrush and fivehealthy children with colonization) were also studied (I.Contreras, Unidad Pediatrica, Centro de Salud, Algorta-Getxo, Spain).The intralaboratory reproducibility of the test was studied

with 78 strains. Each strain was tested in triplicate twice,first at the beginning of the experiment and then after thestrain was subcultured on Sabouraud dextrose agar for 60days. This second culture was read blindly with respect tothe first test results. The test reproducibility is the percent-age of times that the code of a strain remained the same onrepeated testing. This was calculated from the binary stan-dard deviation. The variance of a binary distribution is givenby the equation s2 = p(l - p)In, where p is the proportionpositive and n is the total number (3). The discriminatoryindex (D) used here is derived from that of Hunter andGaston (6). The discriminatory index is the probability thattwo randomly chosen strains, sampled consecutively, wouldbe distinguished by the test, and it is based on Simpson'sdiversity index.

Sixteen morphotypes were found among the 394 C. albi-

cans strains, the predominant (88.33%) morphotypes beingSPiN (162 strains), SPlY (68 strains), SP2Y (60 strains), andSP2N (58 strains) (Table 1). Strain 10261 and its mutant IR2h(proteinase deficient) showed morphotypes SPiN andRP1N, respectively. Strain 3153 and its mutant CA2 (germtube deficient) had the same morphotype (SP1N). None ofthe 56 T. glabrata strains studied grew on STTZ agar. Theabsence of growth was a characteristic feature for thisspecies. However, this method has no value for identifica-tion of Candida species because the same morphotypes areobserved with different species of the genus. The 13 C.guilliermondii strains studied were also divided into differentmorphotypes. As shown in Table 1, the distribution wasmore homogeneous than in other species, the strains belong-ing to four morphotypes: SV1N (38.46%), SPiN (30.78%),SV2N (15.38%), and SPlY (15.38%). Candida krusei showedthe least homogeneous distribution, since 14 of 20 strainsbelonged to biotype SPlY (70%). Although rare, morpho-types SOY and SON were unique for this species. Eightmorphotypes were observed for 34 Candida parapsilosisstrains, 85.29% of the strains being grouped into morpho-types SV2N, SV1N, RV1N, and SP1N. Twelve morpho-types were found among 45 C. tropicalis strains, withmorphotype SV2Y found for 45% of the strains. The distri-bution of the morphotypes according to the strains' geo-

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TABLE 1. Morphotypes of 562 strains of Candida on STTZ agarSpecies Morphotype' No. of(no. of isolates) Mrhteastrains (%)

C. albicans (394) SPlN 162 (41.12)SPlY 68 (17.26)SP2Y 60 (15.23)SP2N 58 (14.72)NG 11 (2.79)RP1N 6 (1.52)RP2Y 6 (1.52)RP1Y 6 (1.52)SV1Y 4 (1.03)RP2N 3 (0.76)SV2Y 3 (0.76)RV2Y 2 (0.51)RV2N 2 (0.51)RV1Y 1 (0.25)SV1N 1 (0.25)SV2N 1 (0.25)

T. glabrata (56) NG 56 (100)

C. guilliernondii (13) SV1N 5 (38.46)SPlN 4 (30.78)SV2N 2 (15.38)SPlY 2 (15.38)

C. krusei (20) SPlY 14 (70)SPlN 4 (20)SOY 1 (5)SON 1 (5)

C. parapsilosis (34) SV2N 11 (32.35)SV1N 8 (23.53)RV1N 5 (14.71)SPlN 5 (14.71)RP1N 2 (5.88)RV2N 1 (2.94)SPlY 1 (2.94)NG 1 (2.94)

C. tropicalis (45) SV2Y 18 (45)SV1Y 5 (12.5)RV2Y 5 (12.5)SV2N 4 (10)NG 4 (10)SP2Y 3 (7.5)