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Antimicrobial Agents and Chemotherapy, November 2004, p. 4163-4170, Vol. 48, No. 11
0066-4804/04/$08.00+0 DOI: 10.1128/AAC.48.11.4163-4170.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Lehrstuhl für Mikrobiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany,1 Departement de Mecanismes Moleculaires des Infections Mycobacteriennes, Institut de Pharmacologie et Biologie Structurale du CNRS et de l'Université Paul Sabatier (UMR 5089), Toulouse, France2
Received 18 February 2004/ Returned for modification 5 May 2004/ Accepted 26 June 2004
Mycobacteria contain an outer membrane of unusually low permeability which contributes to their intrinsic resistance to many agents. It is assumed that small and hydrophilic antibiotics cross the outer membrane via porins, whereas hydrophobic antibiotics may diffuse through the membrane directly. A mutant of Mycobacterium smegmatis lacking the major porin MspA was used to examine the role of the porin pathway in antibiotic sensitivity. Deletion of the mspA gene caused high-level resistance of M. smegmatis to 256 µg of ampicillin/ml by increasing the MIC 16-fold. The permeation of cephaloridine in the mspA mutant was reduced ninefold, and the resistance increased eightfold. This established a clear relationship between the activity and the outer membrane permeation of cephaloridine. Surprisingly, the MICs of the large and/or hydrophobic antibiotics vancomycin, erythromycin, and rifampin for the mspA mutant were increased 2- to 10-fold. This is in contrast to those for Escherichia coli, whose sensitivity to these agents was not affected by deletion of porin genes. Uptake of the very hydrophobic steroid chenodeoxycholate by the mspA mutant was retarded threefold, which supports the hypothesis that loss of MspA indirectly reduces the permeability by the lipid pathway. The multidrug resistance of the mspA mutant highlights the prominent role of outer membrane permeability for the sensitivity of M. smegmatis to antibiotics. An understanding of the pathways across the outer membrane is essential to the successful design of chemotherapeutic agents with activities against mycobacteria.
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