Mycobacterium chelonae is a nontuberculous mycobacterium (NTM), which is classified as rapidly growing mycobacterium (RGM), class IV in the Runyon classification. M. chelonae was first isolated from a turtle in 1903 by Freidmann, who referred to it as "turtle tubercle bacillus. RGMs account for 50% of known mycobacterial species and are divided into six groups, which are as follows: Mycobacterium fortuitum group, M. chelonae/Mycobacterium abscessus complex, Mycobacterium smegmatis, Mycobacterium mucogenicum group, Mycobacterium mageritense/Mycobacterium wolinskyi and the pigmented RGM.
M. chelonae and M. abscessus were considered identical until 1992 when M. chelonae was elevated to species status. Both M. chelonae and M. abscessus have an identical sequence in the 54-510 region but can be differentiated by their intergenic sequence (ITS), hsp65, or gene sequences. Susceptibility profiles can be helpful, but they are not ideal in differentiating between the M. chelonae group and M. abscessus group. M. chelonae group is characterized by high MICs of cefoxitin (> 64 mg/L) and susceptibility to tobramycin (MIC ≤ 4 mg/L), whereas M. abscessus shows lower MICs of cefoxitin (≤ 64 mg/L) and resistance to tobramycin (MIC of > 8 mg/L) (3).
M. chelonae is ubiquitous in the environment and has been found in soil, water, and aquatic animals. M. chelonae grows optimally at 30-32 °C and may have a long incubation period. It is commonly associated with skin and soft tissue infections, especially infections of the extremities (cellulitis, abscessus). M chelonae also causes catheter-related infections and post-surgical infections after implants, transplants, and injections such as sclerotherapy. The eye is second most frequent organ involved. Pulmonary infections are rare when compared to M. abscessus. Invasive infections like bacteremia, osteomyelitis, intraabdominal abscess, and disseminated cutaneous infections are common in immunosuppressed patients such as those on steroids, monoclonal antibodies, and post-transplant immunosuppression. Patients with cancer and or chronic kidney disease are also susceptible to disseminated and invasive disease due to M. chelonae.
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