Abstract
We compared the desoxycholate agar (DOA) media method to investigate the presence of coliforms at milk plants and its proposed substitute, the ISO Enterobacteriaceae test. In raw milk samples with only coliforms or Enterobacteriaceae, there was a good correlation (r≧0.98) between the number of bacteria on DOA and violet red bile glucose (VRBG) agar. In raw milk samples that contained Pseudomonas, which is a psychrotrophic bacteria, as well as coliforms or Enterobacteriaceae, the correlation between the number of bacteria on DOA at 30℃ and VRBG agar at 37℃ was weak. It was assumed that this was because the capability of supporting the growth of psychrotrophic bacteria by VRBG agar is inferior to that demonstrated by DOA; the difference in incubation temperatures greatly influenced this effect. The ratio of the bacteria grown on VRBG/DOA agar plates (VRBG/DOA) using the coliforms such as E. aerogenes NBRC 13534T and K. oxytoca JCM 1665 was 0.67–1.09 and 0.95–1.31, respectively. Because the bacterial count of Pseudomonas grown on VRBG agar was not suitably counted, the count differed depending on the strains used, incubation temperature, and layering conditions. For tests verifying the suitability of raw milk for shipment, if the purpose is to detect the presence of Enterobacteriaceae only, then the ISO Enterobacteriaceae test can be used. However, if psychrotrophic bacteria are also to be detected, then using the ISO Enterobacteriaceae test may not be suitable.
