Discordant carbapenem susceptibility in Methylobacterium species and its application as a method for phenotypic identification.

We encountered a meropenem-resistant, imipenem-susceptible clinical Methylobacterium isolate and sought to determine if discordant carbapenem susceptibility (meropenem resistance/ imipenem susceptibility) was common among different Methylobacterium species. We concurrently investigated whether this phenotype was expressed in the genus Roseomonas, the other recognized group of pink-pigmented nonfermenting gram-negative bacilli. The genus Methylobacterium is comprised of pink-pigmented nonfermenting gram-negative bacilli. These organisms are widely distributed in nature and have been isolated from chlorinated potable water supplies (6). Their presence and role as opportunistic pathogens and presence in the nosocomial setting are well documented (1, 4, 5, 7, 9, 10, 14). Isolates can be cultivated on a variety of solid media, but small colonies may only be detected after incubation for 4 to 5 days (15). Optimal growth occurs at 25 to 30°C; most strains fail to grow at 37°C. Biochemical characterization allows definitive identification and differentiation from other pink-pigmented nonfermenters (15). 16S rRNA-based molecular studies have allowed phylogenetic analysis and rapid identification of Methylobacterium species (6, 13). Current interpretive standards for antibiotic susceptibility testing do not apply to Methylobacterium species (12). Two large Methylobacterium studies assessing in vitro antibiotic susceptibility have demonstrated broad resistance to beta-lactam antibiotics (2, 6). To date, isolates tested have been imipenem susceptible (1, 7, 9). No data with regard to meropenem susceptibility has been published to our knowledge. Four clinical isolates from our laboratory were identified as Methylobacterium species, by morphologic and biochemical characteristics, at a reference laboratory (Laboratoire de Santé Publique du Québec). We also acquired three Methylobacterium species and three Roseomonas species from the American Type Culture Collection (ATCC; Table 1). Susceptibility testing was performed by E-test (imipenem and meropenem; AB BIODISK) and disk diffusion (imipenem and meropenem, 10-mg disks; OXOID) on Sabouraud dextrose agar (SDA) and nutrient agar (NA). E-test and disk diffusion methods were carried out as described in the manufacturer’s instructions and published protocol (11), respectively. Incubation of plates at 25°C for 72 h allowed all isolates to attain optimal growth for susceptibility determination. All Methylobacterium species isolates were extremely susceptible to imipenem but highly resistant to meropenem (Table 1). All Roseomonas isolates were extremely susceptible to both carbapenems. In vitro, meropenem is more active than imipenem against members of the family Enterobacteriaceae and most oxidasepositive and/or glucose-nonfermenting gram-negative bacilli (3, 8). The marked discordance between meropenem and imipenem susceptibility that we describe has not to our knowledge been observed in other gram-negative bacilli. We assessed three distinct Methylobacterium reference