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Characterization of Novel Mycobacterium tuberculosis and Mycobacterium smegmatis Mutants Hypersusceptible to ß-Lactam Antibiotics

Department of Microbiology and Immunology, University of Rochester School of Medicine and Dentistry, Rochester,¹ The Wadsworth Center, New York State Department of Health, Albany, New York²

Received 28 September 2004/ Accepted 10 December 2004

Our laboratory previously constructed mutants of Mycobacterium tuberculosis and Mycobacterium smegmatis with deletions in the genes for their major ß-lactamases, BlaC and BlaS, respectively, and showed that the mutants have increased susceptibilities to most ß-lactam antibiotics, particularly the penicillins. However, there is still a basal level of resistance in the mutants to certain penicillins, and the susceptibilities of the mutants to some cephalosporin-based ß-lactams are essentially the same as those of the wild types. We hypothesized that characterizing additional mutants (derived from ß-lactamase deletion mutants) that are hypersusceptible to ß-lactam antibiotics might reveal novel genes involved with other mechanisms of ß-lactam resistance, peptidoglycan assembly, and cell envelope physiology. We report here the isolation and characterization of nine ß-lactam antibiotic-hypersusceptible transposon mutants, two of which have insertions in genes known to be involved with peptidoglycan biosynthesis (ponA2 and dapB); the other seven mutants have insertions which affect novel genes. These genes can be classified into three groups: those involved with peptidoglycan biosynthesis, cell division, and other cell envelope processes. Two of the peptidoglycan-biosynthetic genes (ponA2 and pbpX) may encode ß-lactam antibiotic-resistant enzymes proposed to be involved with the synthesis of the unusual diaminopimelyl linkages within the mycobacterial peptidoglycan.

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