Application of triphenyltetrazolium chloride in microbial limit test of pharmaceuticals and cosmetics

An alternative method was developed for counting viable microbial contaminants in pharmaceuticals and cosmetics with insoluble materials and low bioburden. The method uses triphenyltetrazolium chloride (TTC). The inhibitory effect of TTC on microbial growth observed by others was eliminated by adding it as an overlay, after incubation of sample in culture medium for 48 h. Six samples of pharmaceutical suspensions and 6 samples of cosmetics were evaluated by most-probable number (MPN) and pour-plate techniques with TTC in comparison with MPN with subculturing. The equivalence between MPN technique with TTC and subculturing was demonstrated for all samples and between pour-plate and subculturing for 10 samples. The differences were probably due to some error inherent in both techniques and not from application of TTC. Comparison of coefficients of variation showed that plate countings were more precise than the MPN method, as expected.