[Isolation of Methylobacterium spp. from drinking tank-water and resistance of isolates to chlorine].

On bacteriological examination of 100 samples of drinking tank water, standard plate count bacteria (36 degrees C, 24 h) and coliforms were not detected, conforming to the water quality criteria under the Water Supply Law. Ninety-five percent of test samples had concentrations of residual chlorine exceeding 0.1 mg/l. However, one characteristic heterotroph was isolated from 70% of these water samples. The bacteria showed glucose non-fermentative Gram-negative rods and formed pink colonies when cultured on standard agar medium at 30 degrees C for 7 days. It was oxidase-positive, catalase-positive and motile. Furthermore, utilization of methyl alcohol was a characteristic. From these characteristics, it was identified as genus Methylobacterium. The isolated Methylobacterium of 118 strains were divided into 60 types using 20 biochemical tests. Thirty Methylobacterium strains were examined for tolerance to chloride by contact with free residual chlorine of 0.1 mg/l concentration for 5 min. Considerable resistance to residual chlorine was evident. The TW-7 strain showed especially high tolerance, even surviving contact at 1.0 mg/l concentration of free residual chlorine for 10 minutes.