Comparison of two inoculation methods for Microsporum canis culture using the toothbrush sampling technique

The fungal culture toothbrush method is a common method for obtaining material for fungal cultures to diagnose dermatophytosis. The optimal technique for inoculation onto the agar surface has not been studied.

To compare two inoculation techniques; the first involved pressing the toothbrush onto the plate surface (Procedure A) and the second involved pressing the toothbrush onto the agar, as well as transferring hairs and scales entrapped in the bristles. (Procedure B). The hypothesis was that transferring hairs onto the plate would increase the likelihood of obtaining positive cultures.

Twenty-six cattery-housed cats were sampled using the toothbrush technique. Two toothbrush samples were obtained from each cat.

The two toothbrush samples from each cat were randomized to Procedure A or B, and the investigator was blinded to inoculation technique. Cultures were performed on a medium specific for dermatophytes. The number of positive plates, and the presence and abundance of colonies of dermatophytes and contaminant moulds were compared between the two techniques.

Twenty-one cats were culture-positive for Microsporum canis. Procedure A resulted in a significantly higher number (P < 0.01) of positive plates (20 of 21; 95%) compared with Procedure B (seven of 21; 33%). These results were due mainly to higher plate invasion by contaminant moulds, using Procedure B.

Based upon the findings of this study, the optimum inoculation technique is to press toothbrush bristles onto agar plates to maximize growth of M. canis and minimize introduction of contaminant inoculation.