Methylobacterium mesophilicum is a Gram-negative soil bacterium.
Methylobacteria are slow-growing, pink-pigmented organisms that have been reported to be opportunistic pathogens in immunocompromised patients. Methylobacterium mesophilicum and Methylobacterium zatmanii have been the two most commonly reported species isolated in clinical samples. Tap water has been implicated as a possible mode of transmission for these bacteria in the hospital environment. Methylobacteria have been reported to exhibit resistance to chlorination and have been isolated from tap water in various clinical settings, including an investigation of a pseudo-outbreak, water from dental units, and blood bank purification units. The association of these organisms with tap water recently prompted Hornei et al. to suggest that it may be helpful to monitor for these organisms in water distribution systems in hospital units for immunocompromised patients.
Pink pigmentation is the primary diagnostic characteristic used in the initial isolation of methylobacteria. With extended incubation, pink colonies often become coral in appearance. Based upon these results, it would appear that a laboratory wishing to monitor water for methylobacteria would be advised to use an agar medium such as NA, Sab, PCA, or R2A incubated at 30°C for 5 to 7 days to allow for the recovery of these slow-growing, pink-pigmented organisms. Depending on the type of medium, both the SP and MF methods were capable of recovering the target organisms. The MF procedure allows the analyst to examine larger volumes of water, and the white membrane provides a good background for distinguishing pigment.
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