Identification of Yellow-Pigmented Bacteria Isolated from Hospital Tap Water in Japan and Their Chlorine Resistance

Twenty-five yellow chromogenic strains isolated from hospital tap water samples collected nationwide were identified by partial 16S rDNA sequencing. In addition, the chlorine resistance of the isolates was experimentally investigated. The results showed that of the strains tested, 12 strains (48.0%) were Sphingomonas ursinco/ainatatoria, which was most frequently identified, followed by 2 strains (8.0%) of Mycobacterium frederiksbergense and 1 strain (4.0%) each of Sphingomonas adhaesiva, Sphingopyxis witfiariensis and Porphyrobacter donghaensis. The other strains were not identified clearly but they belonged to the order of Alphaproteobacteria. On the other hand, the identification results by sequencing and biochemical property testing were not consistent in any of the strains, showing that it was difficult to accurately identify the yellow chromogenic bacteria in tap water based on only their biochemical properties. When the 25 isolates were exposed to 0.1 mg/I residual free chlorine for 1 minute, 22 isolates (88.0%) survived. When the CT (Concentration Time) value killing 99.99% of the bacteria was investigated in 6 of these survivors, M. frederiksbergense (Y-1 strain) was most resistant to chlorine with the CT value of 32 mg·min/l, followed by S. ursincola/natatorla (Y-7 strain) with the CT value of 3.3 mg·min/l. The CT values of Y-5 (Sphingomonas sp.), Y-27 (S. ursincola/natatoria) and Y-21 (Asticacaulis sp.) were within the range of 0.9-0.1 mg·min/l. Of the 6 strains, S. adhaesiva (Y-10) showed the weakest resistance with the CT value of 0.03 mg·min/l. It was clarified that most yellow chromogenic bacteria isolated from hospital tap water were Sphingomonas spp., and these bacteria were experimentally resistant to chlorine.