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Virulence and drug susceptibility of Mycobacterium celatum

Laboratory of Bacteriology and Medical Mycology¹ and Laboratory of Ultrastructures², Istituto Superiore di Sanitá, Viale Regina Elena 299, 00161 Rome, Italy

Author for correspondence: Graziella Orefici. Tel: +39 06 49902333. Fax: +39 06 49387112. e-mail: marella{at}iss.it

The virulence and drug susceptibility of a clinical isolate of Mycobacterium celatum which showed smooth transparent (ST) and smooth opaque (SO) colonies were studied. While ST cells multiplied intracellularly and maintained their coccobacillary form in a human macrophage model of infection, SO cells formed long filaments and completely destroyed the phagocytes. In BALB/c mice, the ST variant, but not the SO variant, grew efficiently in the spleen, liver and lung. The ST variant was usually more resistant in vitro than the SO variant to drugs, with MIC values for clarithromycin (CLA), azithromycin (AZI), ciprofloxacin, sparfloxacin, amikacin, clofazimine, ethambutol and isoniazid being higher than those of the SO variant. In beige mice infected with the more highly virulent variant ST, CLA and AZI were the most active drugs in terms of viable count reduction in organs and mutant selection. Together, these observations indicate that the ST variant of M. celatum is a virulent form that can be efficiently inhibited in vivo by CLA and AZI.

Keywords: Mycobacterium celatum, macrophage infection, mice infection model, drug susceptibility

Abbreviations: AMI, amikacin; AZI, azithromycin; CIP, ciprofloxacin; CLA, clarithromycin; CLO, clofazimine; EMB, ethambutol; INH, isoniazid; i.p., intraperitoneally; MAC, Mycobacterium avium–intracellulare complex; RFB, rifabutin; RMP, rifampicin; SO, smooth opaque; SPA, sparfloxacin; ST, smooth transparent.

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