Mycobacterium chelonae
Rachel Carter

Mycobacterium chelonae is a rare pathogen that causes infection among humans. It has been reported among both immunocompetent and immunosuppressed patients, with the immunosuppressed being at a higher risk. M. chelonae is reported mainly in people and in fish.

M. chelonae is gram - positive bacillus, opportunistic, non-tuberculosis, and fast growing Mycobacterium which is exceptionally pathogenic. These organisms are also ubiquitos in nature but have been found with increasing frequency in other environments that include medical offices and surgical suites. They are involved in cases of community-acquired infections of skin and soft tissue, the usual scenario being puncture wounds from stepping on a nail and open lacerations or fractures. Catheter-related infections are seen most often with long term central intravenous catheter, but they may occur with peritoneal or shunt catheters. M. chelonae is increasingly recognized agent of keratitis, especially after injury with a foreign body or following office ophthalmologic procedures.

The infection is more frequent in males and in the elderly and has also been reported among both immunocompetent and immunosuppressed patients. Underlying conditions with disseminated infection includes: organ transplantation, RA, autoimmune disorders with 92% involving corticosteroids. The risk factors for localized infection include trauma and medical procedures such as subcutaneous injections and the number of injections. More specifically, M. chelonae has been implicated in chronic granulomatous infections involving tendon sheaths, bursea, bones, and joints after direct inoculation of the pathogen through accidental trauma, surgical incisions, puncture wounds, or injections. Risk factors for catheter infections include corticosteroids and chronic renal failure.

Skin lesions are pigmented, erythematous, non-tender, subcutaneous nodules, distributed in a sporotrichoid pattern. They can occur in singly, or in groups on one or more extremities.

There are a few possible treatments that have been found to treat M. chelonae. Small lesions will improve under antibiotics and local wound care. The drug, Clarythromycin, has been found to be the choice drug for the localized disease, with adult dosage at 500mg a day and pediatric dose at 7.5mg a day for four to six months. Extensive lesions and antibiotic resistance require surgical debridement. In most patients, due to resistance, the use of two drugs may be required.

Systemic forms of M. chelonae are uncommon. The prognosis outlook is good for immunocompetent patients treated with antibiotics, with a 90% survival rate. Immunocompromised patients, on the other hand, the illness is so severe that the survival rate is only 10%.

In the lab, blood cultures using standard bacteriological medium were developed that yielded the gram-positive bacillus. This was subsequently processed by Zeihl-Neelsen striving to identify M. chelonae. The cultures of skin and soft tissue need to be plated at 28-30°C, because M. chelonae will only grow on low temperatures on primary isolation. Although it is classified as a fast growing mycobacterium, it does not grow in the normally allotted culture time, so false negative results are common. The medium incubation period is 30.5 days, the range being 15-59 days.

References:

http://ae.inno-vet.com/articles/2001/0501/56.html

http://archopht.ama-assn.org/issues/v118n8labs/ecp90043.html

http://jrs.slackinc.com/vol143/brief.pdf

Mycobacterium chelonae infections, BenAbid F. Stoecker, WV. Revised Oct 01.

*Disclaimer - This report was written by a student participating in a microbiology course at the University of Missouri - Rolla. The accuracy of the content of this reports is not guaranteed and it is recommended that you seek other sources of information to verify the content.
Return to Microbe of the Week 2003

Return to UMR Microbiology HomePage Go to DJW's HomePage