Stability of tandemly repetitive subelement PCR patterns in Trichophyton rubrum over serial passaging and with respect to drug pressure.

Trichophyton rubrum is the most significant agent of dermatomycoses worldwide, primarily causing tinea pedis and tinea unguium. PCR analysis of tandemly repetitive subelements (TRS) within the rDNA nontranscribed spacer region is a major tool for molecular typing of T. rubrum. The aim of this study was to investigate the stability of TRS PCR patterns by analyzing isogenic strains of T. rubrum. Twenty-seven groups of isogenic T. rubrum strains were examined, each composed of an original clinical isolate and its 3 subcultures, maintained on a drug-free medium, a medium containing fluconazole and itraconazole. TRS typing was performed for the original strains and their subcultures grown after 12 passages, at 4-week intervals, on respective media. To add more objectivity to the results, TRS typing for each of the isogenic strain was performed three times, using DNA isolated from three different colonies. Among 27 groups of isogenic strains, all but one were exclusively composed of strains with identical TRS-1 and TRS-2 PCR patterns. In one group, 3 isolates from the last, twelfth passage had identical TRS-1 PCR profiles (type 1), yet different TRS-2 PCR profiles, as compared with the original strain (type I vs. type II). The mechanism underlying the genotype switch was a deletion of a single repeat unit in the TRS-2 locus, as evidenced by sequence analysis. In the interpretation of TRS typing results, microevolutionary events need to be taken into account, urging drawing epidemiological conclusions with caution and in conjunction with other genotyping data and traditional contact tracing information.