Underdiagnosis of urinary tract infection caused by Methylobacterium species with current standard processing of urine culture and its clinical implications

Methylobacterium species are environmental opportunistic bacteria, and urinary tract infection (UTI) caused by these pathogens has not yet been documented. Four cases of UTI with Methylobacterium bacteraemia in immunocompetent female patients are reported. Their urine cultures, processed according to standard procedures (i.e. incubation at 35°C in ambient air for 24 h before incubation at room temperature for a further 24 h), were either negative or positive for Escherichia coli. Specially designed experiments indicated that colonies of Methylobacterium species were visualized on blood agar only after incubation at 35°C for at least 40 h, and growth was completely suppressed when concurrently incubated with much smaller inocula of E. coli. The isolates were variably susceptible to cephalosporins, but 100% susceptible to aminoglycosides. This study suggests an underdiagnosis of UTI caused by Methylobacterium species when the standard procedure of processing urine cultures is used, and implies that administration of aminoglycosides is important when treatment of UTIs with cephalosporin fails.