Relationship between beta-lactamase activity and resistance to beta-lactam antibiotics in Mycobacterium smegmatis

Genome scale portrait of cAMP-receptor protein (CRP) regulons in mycobacteria points to their role in pathogenesis

cAMP Receptor Protein (CRP)/Fumarate Nitrate Reductase Regulator (FNR) family proteins are ubiquitous regulators of cell stress in eubacteria. These proteins are commonly associated with maintenance of intracellular oxygen levels, redox-state, oxidative and nitrosative stresses, and extreme temperature conditions by regulating expression of target genes that contain regulatory cognate DNA elements. We describe the use of informatics enabled comparative genomics to identify novel genes under the control of CRP regulator in Mycobacterium tuberculosis (M.tb). An inventory of CRP regulated genes and their operon context in important mycobacterial species such as M. leprae, M. avium subsp. paratuberculosis and M. smegmatis and several common genes within this genus including the important cellular functions, mainly, cell-wall biogenesis, cAMP signaling and metabolism associated with such regulons were identified. Our results provide a possible theoretical framework for better understanding of the stress response in mycobacteria. The conservation of the CRP regulated genes in pathogenic mycobacteria, as opposed to non-pathogenic ones, highlights the importance of CRP-regulated genes in pathogenesis.

Genetic analysis of the beta-lactamases of Mycobacterium tuberculosis and Mycobacterium smegmatis and susceptibility to beta-lactam antibiotics

Mycobacteria produce beta-lactamases and are intrinsically resistant to beta-lactam antibiotics. In addition to the beta-lactamases, cell envelope permeability and variations in certain peptidoglycan biosynthetic enzymes are believed to contribute to beta-lactam resistance in these organisms. To allow the study of these additional mechanisms, mutants of the major beta-lactamases, BlaC and BlaS, were generated in the pathogenic Mycobacterium tuberculosis strain H37Rv and the model organism Mycobacterium smegmatis strain PM274. The mutants M. tuberculosis PM638 (DeltablaC1) and M. smegmatis PM759 (DeltablaS1) showed an increase in susceptibility to beta-lactam antibiotics, as determined by disc diffusion and minimal inhibitory concentration (MIC) assays. The susceptibility of the mutants, as assayed by disc diffusion tests, to penicillin-type beta-lactam antibiotics was affected most, compared to the cephalosporin-type beta-lactam antibiotics. The M. tuberculosis mutant had no detectable beta-lactamase activity, while the M. smegmatis mutant had a residual type 1 beta-lactamase activity. We identified a gene, blaE, encoding a putative cephalosporinase in M. smegmatis. A double beta-lactamase mutant of M. smegmatis, PM976 (DeltablaS1DeltablaE : : res), had no detectable beta-lactamase activity, but its susceptibility to beta-lactam antibiotics was not significantly different from that of the DeltablaS1 parental strain, PM759. The mutants generated in this study will help determine the contribution of other beta-lactam resistance mechanisms in addition to serving as tools to study the biology of peptidoglycan biosynthesis in these organisms.

Characterization of a beta-lactamase produced in Mycobacterium fortuitum D316

A beta-lactamase from Mycobacterium fortuitum D316 was purified and some physico-chemical properties and substrate profile determined. On the basis of its N-terminal sequence and of its sensitivity to beta-iodopenicillanate inactivation, the enzyme appeared to be a class A beta-lactamase, but its substrate profile was quite unexpected, since nine cephalosporins were among the eleven best substrates. The enzyme also hydrolysed ureidopenicillins and some so-called 'beta-lactamase-stable' cephalosporins.