Permeability of the mycobacterial cell wall

Drug resistance in tuberculosis

Drug-resistant tuberculosis remains a worldwide problem. New laboratory methods have improved our ability to more rapidly identify resistant strains, but the most effective approach is to prevent the appearance of resistance by appropriate choice of antibiotics and directly-observed therapy. Mycobacterium tuberculosis is treated with familiar and unique drugs; consequently, mechanisms of resistance have some unique features. All drug resistance thus far identified develops by mutational events rather than acquisition of resistance genes from other bacteria. An agenda is presented for countering the appearance of further drug resistance in mycobacteria.

Biosynthesis of diaminopimelate, the precursor of lysine and a component of peptidoglycan, is an essential function of Mycobacterium smegmatis

Diaminopimelate (DAP) is a unique metabolite used for both the biosynthesis of lysine in bacteria and the construction of the peptidoglycan of many species of bacteria, including mycobacteria. DAP is synthesized by bacteria as part of the aspartate amino acid family, which includes methionine, threonine, isoleucine, and lysine. Aspartokinase, the first enzyme in this pathway, is encoded by the ask gene in mycobacteria. Previous attempts to disrupt this gene in Mycobacterium smegmatis were unsuccessful, even when the cells were supplied with all the members of the aspartate family, suggesting that unlike other bacteria, mycobacteria may have an absolute requirement for this pathway even when growing in rich medium containing DAP. The purpose of this study was to determine if the ask gene and the aspartate pathway are essential to M. smegmatis. This study describes a test for gene essentiality in mycobacteria, utilizing a counterselectable marker (streptomycin resistance) in conjunction with a specially constructed merodiploid strain. We have used this system to show that the ask gene could not be disrupted in wild-type M. smegmatis, using selective rich medium supplemented with DAP unless there was an extra copy of ask provided elsewhere in the chromosome. Disruption of ask was also possible in a lysine auxotroph incapable of converting DAP to lysine. The ask mutant, mc21278 (ask1::aph), exhibits multiple auxotrophy (Met-, Thr-, DAP-, and Lys-) and is complemented by the ask gene. This is the first description of DAP auxotrophy in mycobacteria. The ask mutant lyses when deprived of DAP in culture, a characteristic which can be exploited for the reproducible preparation of protoplasts and mycobacterial extracts. The evidence presented here indicates that the aspartate pathway is essential to M. smegmatis and that DAP is the essential product of this pathway.

Intracellular accumulation of norfloxacin in Mycobacterium smegmatis

To evaluate the intracellular accumulation of norfloxacin in mycobacteria, two methods were used with Mycobacterium smegmatis. A radiometric method (K. V. Cundy, C. E. Fasching, K. E. Willard, and L. R. Peterson, J. Antimicrob. Chemother. 28:491-497, 1991) was used without great modification, but the fluorometric method (P. G. S. Mortimer and L. J. V. Piddock, J. Antimicrob. Chemother. 28:639-653, 1991) was changed considerably. Indeed, adsorption of the quinolone to the bacterial surface was characterized by measuring the level of accumulation of 0 degree C. Taking into account the adsorption, the pH of the washing buffer was increased from 7.0 to 9.0 to improve the desorption of norfloxacin from the cell surface. Both the fluorometric method, with the technical improvement, and the radiometric method could be used to estimate the intracellular accumulation of norfloxacin, which resulted from the difference between the whole uptake measured at 37 degrees C and the adsorption measured at 0 degrees C. A total of 35 ng of norfloxacin per mg of cells (dry weight) penetrated into the M. smegmatis cell, and the steady state was achieved in 5 min. Use of inhibitors of the proton motive force revealed that transport of norfloxacin was energy independent. Thus, the same mechanisms of quinolone accumulation that occur in eubacteria seem to occur in mycobacteria, at least in M. smegmatis.

Characterization of mutations in Mycobacterium smegmatis involved in resistance to fluoroquinolones

Fluoroquinolone-resistant mutants of Mycobacterium smegmatis have been obtained in vitro by using ofloxacin as a selecting agent. Two types of mutants were identified according to their quinolone resistance patterns. Type 1 showed a low level of resistance to ofloxacin (MIC of 8 micrograms/ml), whereas a high level of resistance to this drug (MICs of 32 to 64 micrograms/ml) characterized type 2. By using two oligonucleotide primers homologous to DNA sequences flanking the quinolone resistance-determining region (QRDR) in the gyrA gene of Escherichia coli and Staphylococcus aureus, a 150-bp DNA fragment was obtained by PCR amplification from total DNA of two wild-type and five mutant strains of M. smegmatis. The nucleotide sequences of the amplified fragments were determined. The deduced amino acid sequence from the wild-type strains showed ca. 79% similarity with the QRDR in the gyrase A subunit from other gram-positive and gram-negative bacteria. The DNA sequences obtained from the fluoroquinolone-resistant mutants of M. smegmatis exhibited nucleotide modifications compared with the wild-type QRDR. The QRDR from type 1 mutants had a C-T or an A-G transition leading to a change from Ala-83 to Val or Asp-87 to Gly, respectively. The QRDR from type 2 mutants had a Val-83 mutation or both Val-83 and Gly-87 mutations detected in the type 1 mutants. These results suggest that point mutations in the QRDR of the mycobacterial gyrA gene are responsible for acquired quinolone resistance in M. smegmatis.

Transcription and expression analysis, using lacZ and phoA gene fusions, of Mycobacterium fortuitum beta-lactamase genes cloned from a natural isolate and a high-level beta-lactamase producer

The gene encoding a class A beta-lactamase was cloned from a natural isolate of Mycobacterium fortuitum (blaF) and from a high-level amoxicillin-resistant mutant that produces large amounts of beta-lactamase (blaF*). The nucleotide sequences of the two genes differ at 11 positions, including two in the region upstream from the coding sequence. Gene fusions to Escherichia coli lacZ and transcription and expression analysis of the cloned genes in Mycobacterium smegmatis indicated that high-level production of the beta-lactamase in the mutant is mainly or wholly due to a single base pair difference in the promoter. These analyses also showed that transcription and translation start at the same position. A comparison of the amino acid sequence of BlaF, as predicted from the nucleotide sequence, with the determined N-terminal amino acid sequence indicated the presence of a typical signal peptide. The fusion of blaF (or blaF*) to the E. coli gene phoA resulted in the production of BlaF-PhoA hybrid proteins that had alkaline phosphatase activity. These results demonstrate that phoA can be used as a reporter gene for studying protein export in mycobacteria.

Prevention of drug access to bacterial targets: permeability barriers and active efflux

Some species of bacteria have low-permeability membrane barriers and are thereby "intrinsically" resistant to many antibiotics; they are selected out in the multitude of antibiotics present in the hospital environment and thus cause many hospital-acquired infections. Some strains of originally antibiotic-susceptible species may also acquire resistance through decreases in the permeability of membrane barriers. Another mechanism for preventing access of drugs to targets is the membrane-associated energy-driven efflux, which plays a major role in drug resistance, especially in combination with the permeation barrier. Recent results indicate the existence of bacterial efflux systems of extremely broad substrate specificity, in many ways reminiscent of the multidrug resistance pump of mammalian cells. One such system seems to play a major role in the intrinsic resistance of Pseudomonas aeruginosa, a common opportunistic pathogen. As the pharmaceutical industry succeeds in producing agents that can overcome specific mechanisms of bacterial resistance, less specific resistance mechanisms such as permeability barriers and multidrug active efflux may become increasingly significant in the clinical setting.

Effect of inhibitors of cell envelope synthesis on beta-sitosterol side chain degradation by Mycobacterium sp. NRRL MB 3683

The role of the lipid bilayer and the peptidoglycan of the mycobacterial cell wall in the permeation of beta-sitosterol into the cell and its transformation to androst-1-ene-3,17-dione (AD) and androsta-1,4-diene-3,17-dione (ADD) was studied. Specific inhibitors were used at concentrations affecting the biosynthesis of the assumed target structures, but causing only partial cell growth inhibition or exerting no effect on growth. m-Fluorophenylalanine and DL-norleucine which are known to disorganize the biosynthesis of amphipatic components of the outer layer of the lipid bilayer, used at concentrations 250 micrograms/ml and 400 micrograms/ml, respectively, increased the formation rate of AD+ADD from 0.3 (control) to 0.7 and 0.8 mg products/g dry weight/h. The disorganization of the underlying mycolyl-arabinogalactan structure by the action of the ethambutol at the concentration 40 micrograms/ml, at which the cell growth was apparently not affected, caused the decrease of the product formation from 135 mg/l to 70 mg/l. In the presence of isoniazid (350 micrograms/ml) only trace amounts of AD accumulated during 48 hours of transformation indicating much lower activity than that of the intact cells. The most effective among the tested inhibitors of peptidoglycan synthesis were glycine (15 mg/ml) and vancomycin (150 micrograms/ml) which enhanced the transformation activity of the treated cells nearly three times. Increased transformation rate was also obtained by the action of colistin at concentrations ranging from 10 micrograms/ml to 15 micrograms/ml.

Physical organization of lipids in the cell wall of Mycobacterium chelonae

Mycobacterial cell wall functions as an effective permeability barrier, making these bacteria resistant to most antibacterial agents. It has been assumed that this low permeability was due to the presence of a large amount of unusual lipids in the cell wall, but it was not known how these lipids are able to produce such an exceptional barrier. We report here the first experimental evidence on the physical arrangement of these lipids based on X-ray diffraction studies of purified Mycobacterium chelonae cell wall, a result suggesting that the hydrocarbon chains of the cell-wall lipids are arranged predominantly in a direction perpendicular to the cell wall surface, probably producing an asymmetric bilayer structure.

Spectrum of drugs against atypical mycobacteria: how valid is the current practice of drug susceptibility testing and the choice of drugs?

The in vitro activity of 13 drugs against 552 clinical isolates of atypical mycobacteria representing 12 species was performed in 7H11 agar medium at the National Reference Laboratory for Mycobacteria, using the 1% proportion method. All the species tested were resistant to isoniazid and pyrazinamide. In general, clofazimine and D-cycloserine showed the widest spectrum of activity except in the case of Mycobactrium fortuitum and M. chelonei which were resistant to both drugs, and the M. szulgai and M. terrae complex which was resistant to D-cycloserine. The next broad-spectrum drug was ethionamide, followed by ansamycin, rifampin, capreomycin, kanamycin, streptomycin and ethambutol. Among the fluoroquinolones, both ciprofloxacin and ofloxacin were active against M. xenopi, M. gordonae and M. fortuitum whereas M. kansasii and M. gastri were sensitive to ofloxacin only. When the species were listed in respect of the number of drugs to which they were susceptible (less than 10% of resistant strains), they were classified as follows; 7/13 drugs for M. kansasii, M. gastri and M. xenopi; 6/13 for M. gordonae; 5/13 for M. marinum; 3/13 for M. szulgai; 2/13 for M. fortuitum; 1/13 for the M. avium, M. scrofulaceum, M. simiae, and M. terrae complex, and none of the 13 in the case of M. chelonei. These results are discussed in relation to the multiple drug resistance of atypical mycobacteria. We conclude that the critical concentrations of drugs established for M. tuberculosis are not appropriate for atypical mycobacteria.