abscessus (4–6). One of them, M. abscessus Group II strains, was reported as M. massiliense and M. bolletii (7). As a genetic identification method to differentiate M. massiliense from M. abscessus and other species recently became available, human infections caused by M. massiliense have been continuously

reported (8–12). Nearly half of the RGM isolates initially identified as M. abscessus, which is the species of RGM that is most frequently drug discovery isolated in Korea, are actually M. massiliense (7). So far, differentiation between M. abscessus and M. massiliense depended on sequence analysis of housekeeping genes (e.g. rpoB and hsp65) (7, 9). However, additional housekeeping genes were analyzed because of the discordant results between rpoB and hsp65 gene analysis (7, 13). Clarithromycin is a 14-membered ring macrolide that binds

to the large ribosomal subunit in the vicinity of the peptidyltransferase center and inhibits protein synthesis, which results in the arrest of bacterial growth (14). Clarithromycin is given orally, and is highly active against many species of NTM. Although M. massiliense shares many traits with M. abscessus and M. bolletii, M. massiliense can be differentiated by marked susceptibility to clarithromycin (2, 7, 11). Moreover, patterns of clarithromycin resistance differed between M. massiliense and M. abscessus (7), which led us to investigate another mechanism, involvement of erm. This is because the erm gene is frequently involved in macrolide resistance in human pathogens as with the 23 rRNA gene mutation. PLX4032 The erm gene encodes N6-mono or N6, N6-dimethyltransferases that cause specific methylation of nucleotide A2058 and/or neighboring nucleotides (A2057 and A2059; based on Escherichia coli numbering) in the 23S rRNA, which Carnitine palmitoyltransferase II results in resistance to macrolide. Because Mycobacterium species possess only one or two rrn operons, alteration of this specific site is critical to the development of resistance (25). Among the 33 erm genes that have

been reported and numbered to date, five innate erm genes [erm(37), erm(38), erm(39), erm(40) and erm(41)] have been identified within the genus Mycobacterium (15). Recently, three types of erm(41) of M. abscessus were reported. One M. massiliense clinical isolate was confirmed to have short erm(41) by PCR and was reported as one of the three erm(41) types without sequence analysis (16). Because quite different responses of M. massiliense compared to M. abscessus against clarithromycin were observed in our previous report (7), exact information on erm(41) of more clinical M. massiliense isolates, and their relevance to the susceptibility pattern of clarithromycin was needed. In the present study, the erm(41) sequences of M. massiliense, M. abscessus and M. bolletii isolates were investigated in relation with MIC to clarithromycin, and a simple erm(41) PCR to differentiate M. massiliense from closely related M. abscessus and M.