Purification and some properties of beta-lactamase from Mycobacterium smegmatis

Characterization of broad-spectrum Mycobacterium abscessus class A β-lactamase

OBJECTIVES

Imipenem and cefoxitin are used to treat Mycobacterium abscessus infections and have moderate activity against this fast-growing mycobacterium (MIC₅₀ of 16 and 32 mg/L, respectively). M. abscessus is highly resistant to most other β-lactams, although the underlying mechanisms have not been explored. Here, we characterized M. abscessus class A β-lactamase (Bla(Mab)) and investigated its role in β-lactam resistance.

METHODS

Hydrolysis kinetic parameters of purified Bla(Mab) were determined by spectrophotometry for various β-lactams and compared with those of related BlaC from Mycobacterium tuberculosis. MICs of β-lactams were determined for M. abscessus CIP104536 and for Escherichia coli producing Bla(Mab) and BlaC.

RESULTS

Bla(Mab) had a broad hydrolysis spectrum, similar to that of BlaC, but with overall higher catalytic efficiencies, except for cefoxitin. As expected from its in vivo efficacy, cefoxitin was very slowly hydrolysed by Bla(Mab) (k(cat)/K(m) = 6.7 M(-1) s(-1)). Bla(Mab) hydrolysed imipenem more efficiently (k(cat)/K(m) = 3.0 × 10(4) M(-1) s(-1)), indicating that the in vivo activity of this drug might be improved by combination with a β-lactamase inhibitor. β-Lactamase inhibitors clavulanate, tazobactam and sulbactam did not inhibit Bla(Mab). This enzyme efficiently hydrolysed clavulanate, in contrast to BlaC, which is irreversibly acylated by this inhibitor. Bla(Mab) and BlaC were functional in E. coli and the resistance profiles mediated by these enzymes were in agreement with the kinetic parameters.

CONCLUSIONS

M. abscessus produces a clavulanate-insensitive broad-spectrum β-lactamase that limits the in vivo efficacy of β-lactams.

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.

Altered permeability and beta-lactam resistance in a mutant of Mycobacterium smegmatis

Beta-lactam resistance in mycobacteria results from an interplay between the following: (i) beta-lactamase production, (ii) affinity of the penicillin-binding proteins (PBPs) for the drugs, and (iii) permeation of the drugs. A laboratory mutant of Mycobacterium smegmatis was studied in order to evaluate the roles of these factors in beta-lactam resistance. Mutant M13 was between 7- and 78-fold more resistant than the wild type to cephaloridine, cefoxitin, cefazolin, cefamandole, and cephalothin. Increased beta-lactamase activity toward these antibiotics was not observed in the mutant. The PBP profiles of the wild type and M13 were comparable. However, the affinities of PBP 1 for the beta-lactams tested were lower for the mutant than for the wild type. The permeation of the drugs measured in intact cells was lower for M13 than for the parent strain. The liposome swelling technique, which could be used for cephaloridine, also supported this view. Reduced permeation was not restricted to the beta-lactams alone. Glycine uptake was also lower in M13. Taken together, the results suggest that decreased affinities of PBP 1 for beta-lactams, combined with the decreased permeability of the cell wall of the mutant, lead to the development of high-level acquired beta-lactam resistance.

Distribution and characterization of beta-lactamases of mycobacteria and related organisms

SETTING

The detailed distribution and precise features of mycobacterial beta-lactamases urgently need to be elucidated.

OBJECTIVE

To study the distribution pattern of beta-lactamases among mycobacteria, their enzymatic profiles and degree of contribution to the expression of drug resistance of some mycobacteria to beta-lactam antibiotics.

DESIGN

Cell-associated beta-lactamase was measured by nitrocefin disc method. beta-lactamases obtained from some mycobacteria were studied for their substrate specificity, metal ion-dependency and isoelectric focusing (IEF) patterns. Changes in the minimum inhibitory concentrations (MICs) of beta-lactams for rapidly growing mycobacteria due to the combined use of tazobactam were measured.

RESULTS

In slow growers, Mycobacterium tuberculosis complex possessed strong and M. kansasii showed strong to intermediate beta-lactamase activity, while M. avium complex lacked such an activity. All the rapid growers possessed strong to intermediate activity. The beta-lactamases of test organisms including M. tuberculosis, M. kansasii, M. fortuitum etc, exerted both penicillinase and cephalosporinase activities and were not metalloenzymes. M. tuberculosis, M. kansasii, and M. smegmatis exhibited the species-specific IEF patterns of beta-lactamases. Tazobactam potentiated the in vitro antimicrobial activities of some beta-lactams against M. fortuitum and M. chelonae.

CONCLUSION

Many mycobacteria possessed peculiar beta-lactamases and the enzymes were partly attributable to their drug resistance to certain beta-lactam antibiotics.

Cloning and assessment of mycobacterial promoters by using a plasmid shuttle vector

We have constructed a promoter selection vector for mycobacteria to analyze the sequences involved in mycobacterial transcriptional regulation. The vector pSD7 contains extrachromosomal origins of replication from Escherichia coli as well as from Mycobacterium fortuitum and a kanamycin resistance gene for positive selection in mycobacteria. The promoterless chloramphenicol acetyltransferase (CAT) reporter gene has been used to detect mycobacterial promoter elements in a homologous environment and to quantify their relative strengths. Using pSD7, we have isolated 125 promoter clones from the slowly growing pathogen Mycobacterium tuberculosis H37Rv and 350 clones from the fast-growing saprophyte Mycobacterium smegmatis. The promoters exhibited a wide range of strengths, as indicated by their corresponding CAT reporter activities (5 to 2,500 nmol/min/mg of protein). However, while most of the M. smegmatis promoters supported relatively higher CAT activities ranging from 100 to 2,500 nmol/min/mg of protein, a majority of those from M. tuberculosis supported CAT activities ranging from 5 to only about 100 nmol/min/mg of protein. Our results indicate that stronger promoters occur less frequently in the case of M. tuberculosis compared with M. smegmatis. To assess the extent of divergence of mycobacterial promoters vis-à-vis those of E. coli, the CAT activities supported by the promoters in E. coli were measured and compared with their corresponding activities in mycobacteria. Most of the mycobacterial promoter elements functioned poorly in E. coli. The homologous selection system that we have developed has thus enabled the identification of mycobacterial promoters that apparently function optimally only in a native environment.

Beta-lactamase of Mycobacterium fortuitum: kinetics of production and relationship with resistance to beta-lactam antibiotics

The kinetics of both intracellular and extracellular beta-lactamase production and the relationship between extracellular enzyme and in vitro susceptibility of Mycobacterium fortuitum to beta-lactam antibiotics have been studied. To this end we used a panel of stable nitrosoguanidine-induced mutants of M. fortuitum derived from the parental strain ATCC 19542 and differing in beta-lactamase production from 0.0001 to 278 U/liter in Mueller-Hinton broth. For overproducers of beta-lactamase (mutants A188, B180, C207, D316, and E31), MICs of benzylpenicillin, amoxicillin, ampicillin, and cephaloridine progressively increased with the amount of enzyme released into the medium, whereas MICs of imipenem and cefoxitin did not. The resistance of the mutants to amoxicillin was reduced up to 32-fold by clavulanic acid, whereas that to ampicillin was reduced 8-fold by sulbactam. These data suggest that the enzyme participated in the mechanisms of resistance to the beta-lactam antibiotics. However, for a mutant of M. fortuitum (gamma 27) with virtually nonexistent beta-lactamase production, the antibiotics still had relatively high MICs (for instance, benzylpenicillin and cephaloridine had MICs of 64 and 32 micrograms/ml, respectively). This suggests that, aside from beta-lactamase production, other mechanisms such as cell wall permeability and/or affinity for penicillin-binding proteins could coexist in M. fortuitum and explain its natural resistance to beta-lactam antibiotics.

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.

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

Penicillin-susceptible mutants and beta-lactamase-negative mutants were isolated from Mycobacterium smegmatis after nitrosoguanidine mutagenesis. Both the mutants were found to be susceptible to low levels of penicillin and cephalosporins by twofold dilution testing. Clavulanic acid reduced the minimal inhibitory concentrations of beta-lactamase-labile beta-lactams for the penicillin-susceptible mutants and the parent strain, but had no effect on the susceptibility of the beta-lactamase-negative mutants. Comparison of the beta-lactamase activities found in these mutants and the parent strain indicated that there was a rough correlation between the beta-lactamase level in these organisms and their susceptibility to beta-lactams.