Leprosy Mailing List – September 18, 2018
Ref.: (LML) Dormancy in M. leprae
From: Joel Almeida, Mumbai and London
Dr. Umerov's hypothesis about M. leprae being dormant is quite plausible.
Phenotypic (non-heritable) resistance of bacteria to all antimicrobials is well recognised (1). Many microbes (including M. smegmatis, for example) have been shown to enter a state of altered and minimal metabolism. These "persisters" remain unaffected by all anti-microbials until the persisters become metabolically active, when they again become susceptible to anti-microbials.
M. leprae could well be particularly good at entering this dormant state, because of its apparent dependence on catabolic products from host cells. Bacterial dormancy is linked to survival in adverse environments. M. leprae retain viability for several months in a shaded environment in India, regrowing when introduced into mouse foot pads.(2)
Persisters in treated patients are a recognised challenge in TB and leprosy. Dormant bacteria in contacts or in the environment could well be a challenge to control or eventual eradication of leprosy. Visitors to the homes of Indian leprosy patients already treated with MDT show an increased risk of developing the disease, compared to the risk in the local population. (3) This remains true even after the patients have apparently been "cured" by MDT. Dormant M. leprae in patients, contacts or the environment could explain this observation.
Thanks to Dr. Umerov for the insightful contribution, and to LML for serving as such a uniquely valuable meeting place for the global fraternity of people interested in fighting M. leprae and mitigating its adverse impacts. The more attention we pay to science, the better our prospects of protecting people against M. leprae and its impacts.
(1) Balaban NQ, Merrin J, Chait R, Kowalik L, Leibler S. Bacterial persistence as a phenotypic switch. Science. 2004 Sep 10;305(5690):1622-5.
(2) Desikan KV, Sreevatsa. Extended studies on the viability of Mycobacterium leprae outside the human body. Lepr Rev. 1995 Dec;66(4):287-95.
(3) P. Vijayakumaran et al. Does MDT Arrest Transmission of Leprosy to Household Contacts? Int. J. Lepr. (1998) 66(2): 125-130.
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