Delayed recognition of a pseudo-outbreak of Mycobacterium terrae

Pseudo-outbreaks of mycobacteria are difficult to recognize because of long incubation periods for growth and species identification. We report our experience with one clinical microbiology laboratory that isolated a species of nontuberculous mycobacteria from 14 patient specimens. These specimens came from 12 patients at 2 hospitals over a 6-day period and included 6 different fluids or tissues. Because of the delay between mycobacterial specimen submission and growth in culture, the outbreak was not noted until more than a month later. Initial species determination by a reference laboratory indicated that these isolates were Mycobacterium fortuitum. One patient received treatment for presumed M fortuitum brain infection, and it was not effective in changing her clinical course. The isolates were sent to the Centers for Disease Control and Prevention (CDC) for identification and typing by pulsed-field gel electrophoresis. The CDC determined that the isolates were an identical strain of M terrae, thus confirming a pseudo-outbreak. Combining pseudo-outbreak isolates with those correctly identified initially as M terrae during the 6-day period in question, there were 22 samples from 20 patients with M terrae. Since the pseudo-outbreak, the number of cultures of M terrae in the clinical laboratory has returned to baseline levels without any specific intervention.