Addiction toxin Fst has unique effects on chromosome segregation and cell division in Enterococcus faecalis and Bacillus subtilis

The Fst toxin of the Enterococcus faecalis pAD1-encoded par addiction module functions intracellularly to kill plasmid-free segregants. Previous results had shown that Fst induction results in membrane permeabilization and cessation of macromolecular synthesis, but only after 45 min. Electron micrographs of toxin-induced cells showed no obvious membrane abnormalities but did reveal defects in nucleoid segregation and cell division, begging the question of which is the primary effect of Fst. To distinguish the possibilities, division septae and nucleoids were visualized simultaneously with fluorescent vancomycin and a variety of DNA stains. Results showed that division and segregation defects occurred in some cells within 15 min after induction. At these early time points, affected cells remained resistant to membrane-impermeant DNA stains, suggesting that loss of membrane integrity is a secondary effect caused by ongoing division and/or segregation defects. Fst-resistant mutants showed greater variability in cell length and formed multiple septal rings even in the absence of Fst. Fst induction was also toxic to Bacillus subtilis. In this species, Fst induction caused only minor division abnormalities, but all cells showed a condensation of the nucleoid, suggesting that effects on the structure of the chromosomal DNA might be paramount.     

Dissection and Modulation of the Four Distinct Activities of Nisin by Mutagenesis of Rings A and B and by C-Terminal Truncation

Nisin A is a pentacyclic antibiotic peptide produced by various Lactococcus lactis strains. Nisin displays four different activities: (i) it autoinduces its own synthesis; (ii) it inhibits the growth of target bacteria by membrane pore formation; (iii) it inhibits bacterial growth by interfering with cell wall synthesis; and, in addition, (iv) it inhibits the outgrowth of spores. Here we investigate the structural requirements and relevance of the N-terminal thioether rings of nisin by randomization of the ring A and B positions. The data demonstrate that: (i) mutation of ring A results in variants with enhanced activity and a modulated spectrum of target cells; (ii) for the cell growth-inhibiting activity of nisin, ring A is rather promiscuous with respect to its amino acid composition, whereas the bulky amino acid residues in ring B abolish antimicrobial activity; (iii) C-terminally truncated nisin A mutants lacking rings D and E retain significant antimicrobial activity but are unable to permeabilize the target membrane; (iv) the dehydroalanine in ring A is not essential for the inhibition of the outgrowth of Bacillus cells; (v) some ring A mutants have significant antimicrobial activities but have decreased autoinducing activities; (vi) the opening of ring B eliminates antimicrobial activity while retaining autoinducing activity; and (vii) some ring A mutants escape the nisin immune system(s) and are toxic to the nisin-producing strain NZ9700. These data demonstrate that the various activities of nisin can be engineered independently and provide a basis for the design and synthesis of tailor-made analogs with desired activities.     

Temperature- and surfactant-induced membrane modifications that alter Listeria monocytogenes nisin sensitivity by different mechanisms

Nisin interacts with target membranes in four sequential steps: binding, insertion, aggregation, and pore formation. Alterations in membrane composition might influence any of these steps. We hypothesized that cold temperatures (10 degrees C) and surfactant (0.1% Tween 20) in the growth medium would influence Listeria monocytogenes membrane lipid composition, membrane fluidity, and, as a result, sensitivity to nisin. Compared to the membranes of cells grown at 30 degrees C, those of L. monocytogenes grown at 10 degrees C had increased amounts of shorter, branched-chain fatty acids, increased fluidity (as measured by fluorescence anisotropy), and increased nisin sensitivity. When 0.1% Tween 20 was included in the medium and the cells were cultured at 30 degrees C, there were complex changes in lipid composition. They did not influence membrane fluidity but nonetheless increased nisin sensitivity. Further investigation found that these cells had an increased ability to bind radioactively labeled nisin. This suggests that the modification of the surfactant-adapted cell membrane increased nisin sensitivity at the binding step and demonstrates that each of the four steps can contribute to nisin sensitivity.     

Synergistic action of rapid chilling and nisin on the inactivation of Escherichia coli

The effect of rapid and slow chilling on survival and nisin sensitivity was investigated in Escherichia coli. Membrane permeabilization induced by cold shock was assessed by uptake of the fluorescent dye 1-N-phenylnapthylamine. Slow chilling (2°C min⁻¹) did not induce transient susceptibility to nisin. Combining rapid chilling (2,000°C min⁻¹) and nisin causes a dose-dependent reduction in the population of cells in both exponential and stationary growth phases. A reduction of 6 log of exponentially growing cells was achieved with rapid chilling in the presence of 100 IU ml⁻¹ nisin. Cells were more sensitive if nisin was present during stress. Nevertheless, addition of nisin to cell suspension after the rapid chilling produced up to 5 log of cell inactivation for exponentially growing cells and 1 log for stationary growing cells. This suggests that the rapid chilling strongly damaged the cell membrane by disrupting the outer membrane barrier, allowing the sensitization of E. coli to nisin post-rapid chilling. Measurements of membrane permeabilization showed a good correlation between the membrane alteration and nisin sensitivity. Application involving the simultaneous treatment with nisin and rapid cold shock could thus be of value in controlling Gram negatives, enhancing microbiological safety and stability.     

Specific Cell Wall Proteins Confer Resistance to Nisin upon Yeast Cells

The cell wall of a yeast cell forms a barrier for various proteinaceous and nonproteinaceous molecules. Nisin, a small polypeptide and a well-known preservative active against gram-positive bacteria, was tested with wild-type Saccharomyces cerevisiae. This peptide had no effect on intact cells. However, removal of the cell wall facilitated access of nisin to the membrane and led to cell rupture. The roles of individual components of the cell wall in protection against nisin were studied by using synchronized cultures. Variation in nisin sensitivity was observed during the cell cycle. In the S phase, which is the phase in the cell cycle in which the permeability of the yeast wall to fluorescein isothiocyanate dextrans is highest, the cells were most sensitive to nisin. In contrast, the cells were most resistant to nisin after a peak in expression of the mRNA of cell wall protein 2 (Cwp2p), which coincided with the G2 phase of the cell cycle. A mutant lacking Cwp2p has been shown to be more sensitive to cell wall-interfering compounds and Zymolyase (J. M. Van der Vaart, L. H. Caro, J. W. Chapman, F. M. Klis, and C. T. Verrips, J. Bacteriol. 177:3104–3110, 1995). Here we show that of the single cell wall protein knockouts, a Cwp2p-deficient mutant is most sensitive to nisin. A mutant with a double knockout of Cwp1p and Cwp2p is hypersensitive to the peptide. Finally, in yeast mutants with impaired cell wall structure, expression of both CWP1 and CWP2 was modified. We concluded that Cwp2p plays a prominent role in protection of cells against antimicrobial peptides, such as nisin, and that Cwp1p and Cwp2p play a key role in the formation of a normal cell wall.     

Cloning of genes controlling alginate biosynthesis from a mucoid cystic fibrosis isolate of Pseudomonas aeruginosa.

Mucoid strains of Pseudomonas aeruginosa isolated from the sputum of cystic fibrosis patients produce copious quantities of an exopolysaccharide known as alginic acid. Since clinical isolates of the mucoid variants are unstable with respect to alginate synthesis and revert spontaneously to the more typical nonmucoid phenotype, it has been difficult to isolate individual structural gene mutants defective in alginate synthesis. The cloning of the genes controlling alginate synthesis has been facilitated by the isolation of a stable alginate-producing strain, 8830. The stable mucoid strain was mutagenized with ethyl methanesulfonate to obtain various mutants defective in alginate biosynthesis. Several nonmucoid (Alg-) mutants were isolated. A mucoid P. aeruginosa gene library was then constructed, using a cosmid cloning vector. DNA isolated from the stable mucoid strain 8830 was partially digested with the restriction endonuclease HindIII and ligated to the HindIII site of the broad host range cosmid vector, pCP13. After packaging in lambda particles, the recombinant DNA was introduced via transfection into Escherichia coli AC80. The clone bank was mated (en masse) from E. coli into various P. aeruginosa 8830 nonmucoid mutants with the help of pRK2013, which provided donor functions in trans, and tetracycline-resistant exconjugants were screened for the ability to form mucoid colonies. Three recombinant plasmids, pAD1, pAD2, and pAD3, containing DNA inserts of 20, 9.5, and 6.2 kilobases, respectively, were isolated based on their ability to restore alginate synthesis in various strain 8830 nonmucoid (Alg-) mutants. Mutants have been assigned to at least four complementation groups, based on complementation by pAD1, pAD2, or pAD3 or by none of them. Introduction of pAD1 into the spontaneous nonmucoid strain 8822, as well as into other nonmucoid laboratory strains of P. aeruginosa such as PAO and SB1, was found to slowly induce alginate synthesis. This alginate-inducing ability was found to reside on a 7.5-kilobase EcoRI fragment that complemented the alg-22 mutation of strain 8852. The pAD1 chromosomal insert which complements the alg-22 mutation was subsequently mapped at ca. 19 min of the P. aeruginosa PAO chromosome.     

Frequency of bacteriocin resistance development and associated fitness costs in Listeria monocytogenes

Bacteriocin-producing starter cultures have been suggested as natural food preservatives; however, development of resistance in the target organism is a major concern. We investigated the development of resistance in Listeria monocytogenes to the two major bacteriocins pediocin PA-1 and nisin A, with a focus on the variations between strains and the influence of environmental conditions. While considerable strain-specific variations in the frequency of resistance development and associated fitness costs were observed, the influence of environmental stress seemed to be bacteriocin specific. Pediocin resistance frequencies were determined for 20 strains and were in most cases ca. 10(-6). However, two strains with intermediate pediocin sensitivity had 100-fold-higher pediocin resistance frequencies. Nisin resistance frequencies (14 strains) were in the range of 10(-7) to 10(-2). Strains with intermediate nisin sensitivity were among those with the highest frequencies. Environmental stress in the form of low temperature (10 degrees C), reduced pH (5.5), or the presence of NaCl (6.5%) did not influence the frequency of pediocin resistance development; in contrast, the nisin resistance frequency was considerably reduced (<5 x 10(-8)). Pediocin resistance in all spontaneous mutants was very stable, but the stability of nisin resistance varied. Pediocin-resistant mutants had fitness costs in the form of reduction down to 44% of the maximum specific growth rate of the wild-type strain. Nisin-resistant mutants had fewer and less-pronounced growth rate reductions. The fitness costs were not increased upon applying environmental stress (5 degrees C, 6.5% NaCl, or pH 5.5), indicating that the bacteriocin-resistant mutants were not more stress sensitive than the wild-type strains. In a saveloy-type meat model at 5 degrees C, however, the growth differences seemed to be negligible. The applicational perspectives of the results are discussed.     

Nisin resistance distinguishes Mycoplasma spp. from Acholeplasma spp. and provides a basis for selective growth media

The sensitivity of 11 Mycoplasma and 5 Acholeplasma species to the bacteriocin nisin was determined. When applied on filter paper discs to lawns of acholeplasma cells, nisin (20 nmol per disc) gave 3.5- to 7.0-mm zones of growth inhibition. The inclusion of 0.2 mM nisin in agar medium reduced the number of Acholeplasma laidlawii colonies by a factor of more than 10(6), and in a salts solution, 75 microM nisin killed more than 99.9% of cells within 1 min. Under similar conditions, nisin had no significant effect upon the growth or survival of Mycoplasma species. At low concentrations (1 to 3 microM), nisin stimulated glucose oxidation by A. laidlawii and Acholeplasma oculi. However, in comparison with carbonyl cyanide m-chlorophenylhydrazone (CCCP), a recognized protonophore and uncoupler of respiration, the maximum extent of stimulation was low, < or = 20%, compared with up to 180% for CCCP. Also, in contrast to results obtained with CCCP, at concentrations only slightly above those causing stimulation of acholeplasma oxygen uptake, nisin strongly inhibited respiration. Inhibition of oxygen uptake was greater for A. laidlawii cells grown in the absence of cholesterol, and on agar medium, growth inhibition by nisin decreased with increasing concentrations of cholesterol. Nisin resistance may be a valuable characteristic in the selection and identification of Mycoplasma spp.     

Nisin resistance distinguishes Mycoplasma spp. from Acholeplasma spp. and provides a basis for selective growth media.

The sensitivity of 11 Mycoplasma and 5 Acholeplasma species to the bacteriocin nisin was determined. When applied on filter paper discs to lawns of acholeplasma cells, nisin (20 nmol per disc) gave 3.5- to 7.0-mm zones of growth inhibition. The inclusion of 0.2 mM nisin in agar medium reduced the number of Acholeplasma laidlawii colonies by a factor of more than 10(6), and in a salts solution, 75 microM nisin killed more than 99.9% of cells within 1 min. Under similar conditions, nisin had no significant effect upon the growth or survival of Mycoplasma species. At low concentrations (1 to 3 microM), nisin stimulated glucose oxidation by A. laidlawii and Acholeplasma oculi. However, in comparison with carbonyl cyanide m-chlorophenylhydrazone (CCCP), a recognized protonophore and uncoupler of respiration, the maximum extent of stimulation was low, < or = 20%, compared with up to 180% for CCCP. Also, in contrast to results obtained with CCCP, at concentrations only slightly above those causing stimulation of acholeplasma oxygen uptake, nisin strongly inhibited respiration. Inhibition of oxygen uptake was greater for A. laidlawii cells grown in the absence of cholesterol, and on agar medium, growth inhibition by nisin decreased with increasing concentrations of cholesterol. Nisin resistance may be a valuable characteristic in the selection and identification of Mycoplasma spp.     

The Lantibiotic Nisin Induces Lipid II Aggregation,Causing Membrane Instability and Vesicle Budding

The antimicrobial peptide nisin exerts its activity by a unique dual mechanism. It permeates the cell membranes of Gram-positive bacteria by binding to the cell wall precursor Lipid II and inhibits cell wall synthesis. Binding of nisin to Lipid II induces the formation of large nisin-Lipid II aggregates in the membrane of bacteria as well as in Lipid II-doped model membranes. Mechanistic details of the aggregation process and its impact on membrane permeation are still unresolved. In our experiments, we found that fluorescently labeled nisin bound very inhomogeneously to bacterial membranes as a consequence of the strong aggregation due to Lipid II binding. A correlation between cell membrane damage and nisin aggregation was observed in vivo. To further investigate the aggregation process of Lipid II and nisin, we assessed its dynamics by single-molecule microscopy of fluorescently labeled Lipid II molecules in giant unilamellar vesicles using light-sheet illumination. We observed a continuous reduction of Lipid II mobility due to a steady growth of nisin-Lipid II aggregates as a function of time and nisin concentration. From the measured diffusion constants of Lipid II, we estimated that the largest aggregates contained tens of thousands of Lipid II molecules. Furthermore, we observed that the formation of large nisin-Lipid II aggregates induced vesicle budding in giant unilamellar vesicles. Thus, we propose a membrane permeation mechanism that is dependent on the continuous growth of nisin-Lipid II aggregation and probably involves curvature effects on the membrane.     

Influence of nisin hinge-region variants on lantibiotic immunity and resistance proteins

The rising existence of antimicrobial resistance, confirms the urgent need for new antimicrobial compounds. Lantibiotics are active in a low nanomolar range and represent good compound candidates. The lantibiotic nisin is well studied, thus it is a perfect origin for exploring novel lantibiotics via mutagenesis studies. However, some human pathogens like Streptococcus agalactiae COH1 already express resistance proteins against lantibiotics like nisin.This study presents three nisin variants with mutations in the hinge-region and determine their influence on both the growth inhibition as well as the pore-forming activity. Furthermore, we analyzed the effect of these mutants on the nisin immunity proteins NisI and NisFEG from Lactococcus lactis, as well as the nisin resistance proteins SaNSR and SaNsrFP from Streptococcus agalactiae COH1.We identified the nisin variant ₂₀NMKIV₂₄ with an extended hinge-region, to be an excellent candidate for further studies to eventually overcome the lantibiotic resistance in human pathogens, since these proteins do not recognize this variant well.     

Isolation and Characterization of Nisin-Resistant Leuconostoc mesenteroides for Use in Cabbage Fermentations

Leuconostoc mesenteroides strains that are resistant to high levels of nisin (up to 25,000 IU/ml in broth) were isolated. These nisin-resistant mutants were evaluated to determine their potential use as starter culture strains for cabbage fermentations. We found that some L. mesenteroides strains could be adapted to high levels of nisin resistance, while others could not. The nisin resistance trait was found to be stable for at least 35 generations, in the absence of nisin selection, for all mutants tested. The effects of nisin and salt, separately and in combination, on growth kinetics of the nisin-resistant strains were determined. Salt was the most influential factor on the specific growth rates of the mutants, and no synergistic effect between nisin and salt on specific growth rates was observed. The nisin-resistant strains were unimpaired in their ability to rapidly produce normal heterolactic fermentation end products. The use of these L. mesenteroides mutants as starter cultures in combination with nisin may extend the heterolactic phase of cabbage fermentations.     

Antimicrobial action of nisin against Salmonella typhimurium lipopolysaccharide mutants.

The antimicrobial activity of nisin against outer membrane lipopolysaccharide mutants of Salmonella typhimurium LT2 was investigated. Nisin sensitivity was associated with the extent of saccharide deletions from the outer membrane core oligosaccharide. The results indicated that the core oligosaccharide in lipopolysaccharide plays a role in nisin sensitivity.     

Antimicrobial action of nisin against Salmonella typhimurium lipopolysaccharide mutants.

The antimicrobial activity of nisin against outer membrane lipopolysaccharide mutants of Salmonella typhimurium LT2 was investigated. Nisin sensitivity was associated with the extent of saccharide deletions from the outer membrane core oligosaccharide. The results indicated that the core oligosaccharide in lipopolysaccharide plays a role in nisin sensitivity.     

Lipid II induces a transmembrane orientation of the pore-forming peptide lantibiotic nisin

Nisin is an antimicrobial peptide produced by Lactococcus lactis and used as a food preservative in dairy products. The peptide kills Gram-positive bacteria via the permeabilization of the membrane, most probably via pore formation using the cell wall precursor Lipid II as its docking molecule. In this study, site-directed tryptophan spectroscopy was used to determine the topology of nisin in the Lipid II containing membrane, as a start to elucidate the mechanism of targeted pore formation. Three single tryptophan mutants were used, which are viable representatives of the wild-type peptide. The emission spectra of tryptophans located at the N-terminus, the center, and the C-terminus as well as quenching by acrylamide and spin-labeled lipids were investigated using model membrane vesicles composed of DOPC containing 1 mol % Lipid II. Nisin was shown to adopt an orientation where the most probable position of the N-terminus was found to be near the Lipid II headgroup at the bilayer surface, the position of the center of nisin was in the middle of the phospholipid bilayer, and the C-terminus was located near the interface between the headgroups and acyl chain region. These results were used to propose a model for the orientation of nisin in Lipid II containing membranes. Our findings demonstrated that Lipid II changes the overall orientation of nisin in membranes from parallel to perpendicular with respect to the membrane surface. The stable transmembrane orientation of nisin in the presence of Lipid II might allow us to determine the structure of the nisin-Lipid II pores in the lipid bilayer.     

Improvement of solubility and stability of the antimicrobial peptide nisin by protein engineering.

Nisin is a 3.4-kDa antimicrobial peptide that, as a result of posttranslational modifications, contains unsaturated amino acids and lanthionine residues. It is applied as a preservative in various food products. The solubility and stability of nisin and nisin mutants have been studied. It is demonstrated that nisin mutants can be produced with improved functional properties. The solubility of nisin A is highest at low pH values and gradually decreases by almost 2 orders of magnitude when the pH of the solution exceeds a value of 7. At low pH, nisin Z exhibits a decreased solubility relative to that of nisin A; at neutral and higher pH values, the solubilities of both variants are comparable. Two mutants of nisin Z, which contain lysyl residues at positions 27 and 31, respectively, instead of Asn-27 and His-31, were produced with the aim of reaching higher solubility at neutral pH. Both mutants were purified to homogeneity, and their structures were confirmed by one- and two-dimensional 1H nuclear magnetic resonance. Their antimicrobial activities were found to be similar to that of nisin Z, whereas their solubilities at pH 7 increased by factors of 4 and 7, respectively. The chemical stability of nisin A was studied in the pH range of 2 to 8 and at a 20, 37, and 75 degrees C. Optimal stability was observed at pH 3.0. Nisin Z showed a behavior similar to that of nisin A. A mutant containing dehydrobutyrine at position 5 instead of dehydroalanine had lower activity but was significantly more resistant to acid-catalyzed chemical degradation than wild-type nisin Z.     

The role of lipid II in membrane binding of and pore formation by nisin analyzed by two combined biosensor techniques

Nisin, a peptide antibiotic, efficiently kills bacteria through a unique mechanism which includes inhibition of cell wall biosynthesis and pore formation in cytoplasmic membranes. Both mechanisms are based on interaction with the cell wall precursor lipid II which is simultaneously used as target and pore constituent. We combined two biosensor techniques to investigate the nisin activity with respect to membrane binding and pore formation in real time. Quartz crystal microbalance (QCM) allows the detection of nisin binding kinetics. The presence of 0.1 mol% lipid II strongly increased nisin binding affinity to DOPC (k(D) 2.68 x 10(-7) M vs. 1.03 x 10(-6) M) by a higher association rate. Differences were less pronounced while using negatively charged DOPG membranes. However, lipid II does not influence the absolute amount of bound nisin. Cyclic voltammetry (CV) data confirmed that in presence of 0.1 mol% lipid II, nanomolar nisin concentrations were sufficient to form pores, while micromolar concentrations were necessary in absence of lipid II. Both techniques suggested unspecific destruction of pure DOPG membranes by micromolar nisin concentrations which were prevented by lipid II. This model membrane stabilization by lipid II was confirmed by atomic force microscopy. Combined CV and QCM are valuable to interpret the role of lipid II in nisin activity.     

The increase of O-acetylation and N-deacetylation in cell wall promotes acid resistance and nisin production through improving cell wall integrity in <Emphasis Type="Italic">Lactococcus lactis</Emphasis>

Cell wall is closely related to bacterial robustness and adsorption capacity, playing crucial roles in nisin production in Lactococcus lactis. Peptidoglycan (PG), the essential component of cell wall, is usually modified with MurNAc O-acetylation and GlcNAc N-deacetylation, catalyzed by YvhB and XynD, respectively. In this study, increasing the two modifications in L. lactis F44 improved autolysis resistance by decreasing the susceptibility to PG hydrolases. Furthermore, both modifications were positively associated with overall cross-linkage, contributing to cell wall integrity. The robust cell wall rendered the yvhB/xynD-overexpression strains more acid resistant, leading to the increase of nisin production in fed-batch fermentations by 63.7 and 62.9%, respectively. Importantly, the structural alterations also reduced nisin adsorption capacity, resulting in reduction of nisin loss. More strikingly, the co-overexpression strain displayed the highest nisin production (76.3% higher than F44). Our work provides a novel approach for achieving nisin overproduction via extensive cell wall remodeling.     

Translational regulation by an intramolecular stem-loop is required for intermolecular RNA regulation of the par addiction module

The par stability determinant of Enterococcus faecalis plasmid pAD1 is the only antisense RNA-regulated addiction module identified to date in gram-positive bacteria. par encodes two small, convergently transcribed RNAs, designated RNA I and RNA II, that function as the toxin (Fst)-encoding and antitoxin components, respectively. Previous work showed that structures at the 5′ end of RNA I are important in regulating its translation. The work presented here reveals that a stem-loop sequestering the Fst ribosome binding site is required for translational repression but a helix sequestering the 5′ end of RNA I is not. Furthermore, disruption of the stem-loop prevented RNA II-mediated repression of Fst translation in vivo. Finally, although Fst-encoding wild-type RNA I is not toxic in Escherichia coli, mutations affecting stem-loop stability resulted in toxicity in this host, presumably due to increased translation.     

Nisin A depletes intracellular ATP and acts in bactericidal manner against Mycobacterium smegmatis

Nisin is a bacteriocin produced by many strains of Lactococcus lactis. This study examined the effect of nisin on Mycobacterium smegmatis, a non-pathogenic species of Mycobacterium. Nisin had a minimum inhibitory concentration of 8.0 micrograms ml-1 and a minimum inhibitory dose of 7.5 micrograms ml-1 against Myco. smegmatis. Treatment with 25.0 micrograms ml-1 nisin caused partial inhibition of Myco smegmatis; the survivors were nisin-sensitive when tested in a separate experiment. Mycobacterium smegmatis cells exposed to 50.0 micrograms ml-1 of nisin, lost their viability. the effect of nisin on the growth of Myco. smegmatis was both time- and concentration-dependent. Nisin (10.0 micrograms ml-1) caused 97.7 +/- 2.0% reduction in internal ATP and leakage of intracellular ATP out of Myco. smegmatis cells after several hours of treatment. These data suggest that nisin inhibits Myco. smegmatis by the same mechanism by which it inhibits other bacteria and warrants further investigation as a possible antitubercular agent.