methylobacterium mesophilicum methylobacterium mesophilicum methylobacterium mesophilicum is a gram-negative, soil-dwelling bacterium, reported to be an opportunistic pathogen in immunocomprimised patients. methylobacterium mesophilicum scientific classification domain: bacteria phylum: pseudomonadota class: alphaproteobacteria order: hyphomicrobiales family: methylobacteriaceae genus: methylobacterium binomial name methylobacterium mesophilicum(austin and goodfellow 1979) green and bousfield 1983 synonyms pseudomonas mesophilica austin and goodfellow 1979 methylobacteria are typically slow-growing and produce pink-pigmented colonies. pink pigmentation is the primary diagnostic characteristic used in the initial isolation of methylobacteria. with extended incubation, pink colonies often take on a coral hue. m. mesophilicum and m. zatmanii are the two most commonly reported species isolated in clinical samples. 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 purification units. tap water is a suspected mode of transmission. the presence of these organisms in tap water prompted suggestions that it may be helpful to monitor these organisms in water distribution systems in hospital units for immunocompromised patients.