Role of autolysins in the killing of bacteria by some bactericidal antibiotics

Methylarsenite is a broad-spectrum antibiotic disrupting cell wall biosynthesis and cell membrane potential

Methylarsenite (MAs(III)), a product of arsenic biomethylation or bioreduction of methylarsenate (MAs(V)), has been proposed as a primitive antibiotic. However, the antibacterial property and the bactericidal mechanism of MAs(III) remain largely unclear. In this study, we found that MAs(III) is highly toxic to 14 strains of bacteria, especially against 9 strains of Gram-positive bacteria with half maximal inhibitory concentration (IC50) in the sub micromolar range for Staphyloccocus aureus, Microbacterium sp., Pseudarthrobacter siccitolerans and several Bacillus species. In a co-culture of B. subtilis 168 and MAs(III)-producer Enterobacter sp. CZ-1, the later reduced non-toxic MAs(V) to highly toxic MAs(III) to kill the former and gain a competitive advantage. MAs(III) induced autolysis of B. subtilis 168. Deletion of the autolysins LytC, LytD, LytE, and LytF suppressed MAs(III)-induced autolysis in B. subtilis 168. Transcriptomic analysis showed that MAs(III) downregulated the expression of the major genes involved in the biosynthesis of the cell wall peptidoglycan. Overexpression of an UDP-N-acetylglucosamine enolpyruvyl transferase gene murAA alleviated MAs(III)-induced autolysis in B. subtilis 168. MAs(III) disrupted the membrane potential of B. subtilis 168 and caused severe membrane damage. The results suggest that MAs(III) is a broad-spectrum antibiotic preferentially against Gram-positive bacteria by disrupting the cell wall biosynthesis pathway and cell membrane potential.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

Streptococcus pneumoniae's Virulence and Host Immunity: Aging, Diagnostics, and Prevention

Streptococcus pneumoniae is an infectious pathogen responsible for millions of deaths worldwide. Diseases caused by this bacterium are classified as pneumococcal diseases. This pathogen colonizes the nasopharynx of its host asymptomatically, but overtime can migrate to sterile tissues and organs and cause infections. Pneumonia is currently the most common pneumococcal disease. Pneumococcal pneumonia is a global health concern and vastly affects children under the age of five as well as the elderly and individuals with pre-existing health conditions. S. pneumoniae has a large selection of virulence factors that promote adherence, invasion of host tissues, and allows it to escape host immune defenses. A clear understanding of S. pneumoniae's virulence factors, host immune responses, and examining the current techniques available for diagnosis, treatment, and disease prevention will allow for better regulation of the pathogen and its diseases. In terms of disease prevention, other considerations must include the effects of age on responses to vaccines and vaccine efficacy. Ongoing work aims to improve on current vaccination paradigms by including the use of serotype-independent vaccines, such as protein and whole cell vaccines. Extending our knowledge of the biology of, and associated host immune response to S. pneumoniae is paramount for our improvement of pneumococcal disease diagnosis, treatment, and improvement of patient outlook.

New Found Hope for Antibiotic Discovery: Lipid II Inhibitors

Research into antibacterial agents has recently gathered pace in light of the disturbing crisis of antimicrobial resistance. The development of modern tools offers the opportunity of reviving the fallen era of antibacterial discovery through uncovering novel lead compounds that target vital bacterial cell components, such as lipid II. This paper provides a summary of the role of lipid II as well as an overview and insight into the structural features of macrocyclic peptides that inhibit this bacterial cell wall component. The recent discovery of teixobactin, a new class of lipid II inhibitor has generated substantial research interests. As such, the significant progress that has been achieved towards its development as a promising antibacterial agent is discussed.

Screen for agents that induce autolysis in Bacillus subtilis

The growing prevalence of antibiotic-resistant infections underscores the need to discover new antibiotics and to use them with maximum effectiveness. In response to these needs, we describe a screening protocol for the discovery of autolysis-inducing agents that uses two Bacillus subtilis reporter strains, SH-536 and BAU-102. To screen chemical libraries, autolysis-inducing agents were first identified with a BAU-102-based screen and then subdivided with SH-536 into two major groups: those that induce autolysis by their direct action on the cell membrane and those that induce autolysis secondary to inhibition of cell wall synthesis. SH-536 distinguishes between the two groups of autolysis-inducing agents by synthesizing and then releasing β-galactosidase (β-Gal) in late stationary phase at a time that cells have nearly stopped growing and are therefore tolerant of cell wall synthesis inhibitors. Four hits, named compound 2, compound 3, compound 5, and compound 24, obtained previously as inducers of autolysis by screening a 10,080-compound discovery library with BAU-102, were probed with SH-536 and found to release β-Gal, indicating that their mode of action was to permeabilize the B. subtilis cell membrane. The four primary hits inhibited growth in Staphylococcus aureus, Enterococcus faecium, Bacillus subtilis, and Bacillus anthracis, with MICs in the 12.5- to 25-μg/ml (20 to 60 μM) range. The four primary hits were further used to probe B. subtilis, and their action was partially characterized with respect to the dependence of induced autolysis on specific autolysins.

[Tolerance and heteroresistance in Gram-positive microorganisms]

During the last few years, insufficient efficacy of currently recommended antimicrobial agents has been observed, mainly in the case of glycopeptides, during the treatment of infections due to methicillin-resistant Staphylococcus aureus, even if these isolates show MIC values within the susceptible range. The phenomena associated with this observation are tolerance (a genetic event in which a bactericidal antibiotic fails to kill a bacterial population), persistence (a non-inherited and transient phenotypic phenomenon in which a bacterial subpopulation -0.1%-10%- survive lethal antimicrobial concentrations irrespective of the mechanisms of action) and heteroresistance (an epigenetic event in which less susceptible isogenic subpopulations are recovered when the entire population is challenged with concentrations exceeding MIC values). New antimicrobials, including daptomycin, are less affected by these phenomena and should be considered as the treatment of choice when these events are demonstrated or suspected.

Autolytic Activity and an Autolysis-Deficient Mutant of Clostridium acetobutylicum

The optimum conditions for autolysis and autoplast formation in Clostridium acetobutylicum P262 have been defined. Autolysis was optimal at pH 6.3 in 0.04 M sodium phosphate buffer, and the bacterium produced latent and active forms of an autolytic enzyme. The ability of cells to autolyze decreased sharply when cultures entered the stationary phase. Autoplasts were induced by 0.25 to 0.5 M sucrose and were stable in media containing sucrose, CaCl(2), and MgCl(2). A pleiotropic autolysis-deficient mutant (lyt-1) was isolated. The mutant produced less autolysin than did the parent P262 strain, and it had an altered cell wall which was more resistant to both its own and P262 autolysins. The mutant formed long chains of cells, and lysozyme was required for the production of autoplasts. Growth of the P262 strain or the lyt-1 mutant was inhibited by the same concentrations of penicillin, ampicillin, and vancomycin. The lyt-1 mutant strain treated with the minimum growth-inhibitory concentration of penicillin autolyzed upon the addition of wild-type autolysin to the autolysis buffer at the same rate as did the untreated P262 strain. Chloramphenicol did not protect the penicillin-treated lyt-1 cells against autolysis enhanced by exogenous wild-type autolysin.

The vertebrate lysozyme inhibitor Ivy functions to inhibit the activity of lytic transglycosylase

The proteinaceous inhibitor of vertebrate lysozymes (Ivy) is produced by a collection of Gram-negative bacteria as a stress response to damage to their essential cell wall component peptidoglycan. A paralog of Ivy, Ivyp2 is produced exclusively by a number of pseudomonads, including Pseudomonas aeruginosa, but this protein does not inhibit the lysozymes, and its function was unknown. In this study, we demonstrate that the production of Ivy (homologs of both Ivyp1 and Ivyp2) correlates with bacteria that do not O-acetylate their peptidoglycan, a modification that controls the activity of the lytic transglycosylases. Furthermore, we show that both Ivy proteins are potent inhibitors of the lytic transglycoslyases, enzymes involved in the biosynthesis and maintenance of peptidoglycan. These data suggest that the true physiological function of the Ivy proteins is to control the autolytic activity of lytic transglycosylases within the periplasm of Gram-negative bacteria that do not produce O-acetylated peptidoglycan and that the inhibition of exogenous lysozyme by Ivy is simply a fortuitous coincidence.

Characteristics of the autolysis of variants ofLactococcus lactissubsp.cremoris

Two slime-forming strains (T5and MLS96) and a non-slime-forming strain (HA)of Lactococcus lactissubsp.cremoriswere investigated for autolysis and optimal conditions for the autolytic process were developed. The rate of autolysis was maximal in exponential phase cells in 0·01 M-Na phosphate buffer, pH 6·5–7·5, at 30–45 °C. Autolysis of the native exponential phase walls was activated by trypsin and inhibited by lipoteichoic acid and cardiolipin. Decreased trypsin activation was found in intact exponential phase cells and no activation in stationary phase cells.N-acetylmuramylhydrolase action in the autolytic system ofLc. lactissubsp.cremorisstrains was indicated by the progressive release of reducing groups. No amidase or endopeptidase action was found. Great variation in the rate of autolysis of whole cells was observed between the two slime-forming strains. Intact cells of strain T5exhibited decreased autolytic activity, but a higher rate of autolysis of isolated exponential phase walls. Autolysins from strain T5walls exhibited lower hydrolytic activity against sodium dodecyl sulphate-treated walls of the other two strains as compared with strains MLS96 and HA. Quantitative analysis revealed higher protein and phosphorus and lower hexosamine and rhamnose in cell wall preparations of strain T5compared with the other two strains. Results suggest that the decreased rate of autolysis of strain T5cells may at least in part be caused by cell surface components other than cell-wall peptidoglycan. It is proposed that the difference in autolytic characteristics should be used as a criterion when selecting starter strains of lactococci (streptococci).

Differential assay for high-throughput screening of antibacterial compounds

The previously described Bacillus subtilis reporter strain BAU-102 is capable of detecting cell wall synthesis inhibitors that act at all stages of the cell wall synthesis pathway. In addition, this strain is capable of detecting compounds with hydrophobic/surfactant activity and alternative mechanisms of cell wall disruption. BAU-102 sequesters preformed beta-gal in the periplasm, suggesting leakage of beta-gal as the means by which this assay detects compound activities. A model is proposed according to which beta-gal release by BAU-102 reflects activation of pathways leading to autolysis. The authors also report a simplified high-throughput assay using BAU-102 combined with the fluorogenic substrate N-methylumbelliferyl-beta-D-galactoside as a single reagent. Cell wall inhibitors release beta-gal consistently only after 60 min of incubation, whereas compounds with surfactant activity show an almost immediate release. A high-throughput screen of a 480-compound library of known bioactives yielded 8 compounds that cause beta-gal release. These results validate the BAU-102 assay as an effective tool in antimicrobial drug discovery.

Peptide antibiotic and actin-binding protein as mixed-type inhibitors of Clostridium difficile CDT toxin activities

CDT from Clostridium difficile is an ADP-ribosyltransferase that causes rapid actin disaggregation and cell death. For efficient catalysis, CDT required specific divalent cations and binding by NAD which can be substituted by ATP but not ADP. Increasing isolation of CDT-producing strains prompted our search for antagonists like the anti-C. difficile agents bacitracin and vancomycin which were effective CDT inhibitors. Other CDT transferase and glycohydrolase inhibitors with consistently low IC50 values were heterocyclic peptide antibiotics containing modified amino acids such as polymyxin B and beta-lactam cephalosporins. The strongest inhibitors were actin-binding proteins which possess extensive interfaces with G-actin, adjoining the CDT-ADP-ribose+ acceptor site and nucleotide cleft. Analysis of the extent and mode of inhibition and actin interaction sites provided fresh evidences on the designation of actin interface domains with actin-binding proteins. Our results uphold ADP-ribosylation as an innate physiologic process in cellular cytoskeletal reorganization regulated by endogenous metabolites.

Antibiotic tolerance in pneumococci

When bacteria such as Staphylococcus aureus and Streptococcus pneumoniae are exposed to lytic antibiotics such as penicillin and vancomycin, a self-induced killing process is initiated in the organism. This killing occurs via both non-lytic and lytic processes. Recent data suggest that the non-lytic killing system, which might affect the cytoplasmic membrane, secondarily activates murein hydrolases that eventually lyse the cell. Disturbances in this suicide pathway can lead to antibiotic tolerance, a process whereby the antibiotic still exerts its bacteriostatic effects but the self-induced killing system is impaired. In mutants obtained in vitro, signaling pathways have been affected that show either increased or decreased antibiotic-induced killing. Among clinical isolates of S. pneumoniae that are tolerant to penicillin and/or vancomycin, we do not yet know whether these signaling pathways are affected. We could, however, demonstrate that the activity of murein hydrolases is negatively controlled by the production of capsular polysaccharides in one vancomycin-tolerant isolate. Hence, type and level of capsular expression might constitute one factor that determines the degree of lysis, once the killing signal has been elicited by the antibiotic.

The bactericidal action of penicillin: new clues to an unsolved mystery

The Staphylococcus aureus lrgAB operon was recently shown to inhibit extracellular murein hydrolase activity and increase tolerance to penicillin. Further characterization of this operon could provide novel insight into the dynamics of S. aureus cell wall metabolism and the mechanism of penicillin-induced lethality.

Lipoteichoic acid as a target for antimicrobial action

Daptomycin, a lipopeptide antibiotic active against gram-positive bacteria, has been found to inhibit lipoteichoic acid (LTA) synthesis as a consequence of membrane binding in the presence of Ca2+. The present study shows that among the bacterial-membrane components, daptomycin binds the protein fraction with a noncovalent bond, as suggested by the instability of the bond in the presence of an ionic detergent such as sodium dodecyl sulfate. Analysis of membrane proteins by isoelectric focusing electrophoresis reveals that 5 bands with isoelectric points ranging from 5.9 to 6.2 bind radioactive daptomycin. These proteins are therefore called daptomycin-binding proteins. In an attempt to correlate these proteins with the main inhibition observed in LTA synthesis, two-dimensional thin-layer chromatography of lipids synthesized during daptomycin treatment was performed. A 3-fold increase in diglucosyl diacylglycerol is demonstrated, while the compounds phosphatidyl-alpha-kojibiosyldiacylglycerol, glycerophosphophosphatidyl-alpha-kojibiosyldiacyl glycerol, and glycerophosphokojibiosyldiacylglycerol, which follow diglucosyl diacylglycerol in LTA synthesis, decrease progressively with time during the course of daptomycin treatment.

Antibacterial action of dipeptides containing an inhibitor of glucosamine-6-phosphate isomerase

Several dipeptides, containing the N3-(4-methoxyfumaroyl)-L-2,3-diaminopropanoic acid (FMDP) moiety linked to protein and non-protein amino acids, exhibited a strong growth-inhibitory and bactericidal effect against Bacillus subtilis. FMDP-dipeptides were efficiently transported into bacterial cells by a di-tripeptide permease and subsequently cleaved by intracellular Mn2+/Co2+-dependent peptidases. Cleavage rates [0.1-5.6 micromol min-1 (mg protein)-1] were about two orders of magnitude lower than transport rates [40-200 micromol min-1 (mg dry wt)-1]. The released FMDP inactivated glucosamine-6-phosphate (GlcN-6-P) isomerase, an enzyme catalysing the first committed step in a biosynthetic pathway leading to amino sugar-nucleotide precursors of bacterial peptidoglycan. Inhibition of GlcN-6-P isomerase precluded peptidoglycan biosynthesis and resulted in a strong bacteriolytic effect. Results of the studies on consequences of GlcN-6-P isomerase inhibition upon the action of FMDP-dipeptides provided evidence demonstrating that the lack of endogenous GlcN-6-P could be a reason for the triggering of bacterial autolysis. Peptides containing the inhibitors of GlcN-6-P isomerase are one of the very few antimicrobial agents known that exhibit both bactericidal and fungicidal effects.

Isolation, purification, and characterization of the major autolysin from Pseudomonas aeruginosa

The major (26 kDa) autolysin from Pseudomonas aeruginosa was purified to apparent homogeneity by a combination of preparative electrophoresis, ion-exchange, and dye-ligand chromatographies. This purification was facilitated by the development of a spot-assay that involved the spotting and subsequent incubation of autolysin samples on polyacrylamide gels containing peptidoglycan. The pl of the 26-kDa autolysin was determined to be between 3.5 and 4 and disulfide bonds within the enzyme were essential for activity. The autolysin catalyzed the release of reducing sugars from the peptidoglycans of Pseudomonas aeruginosa and Escherichia coli indicating it to be a beta-glycosidase. It was ineffective at hydrolysing the peptidoglycan from Gram-positive bacteria and the O-acetylated peptidoglycans from either Proteus mirabilis or Staphylococcus aureus. The N-terminal sequence of the purified autolysin was determined to be His-Glu-Pro-Pro-Gly. The 26-kDa autolysin together with a 29-kDa autolysin was determined to be secreted into the medium by a mechanism that involves the production and release of surface membrane vesicles during normal growth, but the enzymes were not found free and active in culture broth supernatants.

Why do many ruminal bacteria die and lyse so quickly?

Studies using 15N have indicated that as much as 50% of the microbial mass turns over before N passes to the lower gut, and this N recycling significantly decreases the availability of microbial protein. Protozoa digest bacteria and smaller protozoa, but bacterial protein can turn over even if protozoa are not present. Fibrobacter succinogenes cultures lyse even when they are growing, and the lysis rate is independent of growth rate. When extracellular sugar is depleted, F. succinogenes secretes an extracellular proteinase that inactivates the autolysins. This method of autolytic regulation decreases the turnover of stationary cells. Bacteriophage and anaeroplasma can cause lysogeny, but, as yet, there is little proof that these processes are important determinants of bacterial turnover in vivo. Dietary manipulations (e.g., salt feeding and particle size reduction) that increase liquid and solid dilution rates can increase bacterial flow by decreasing bacterial residence time and turnover. Some dead ruminal bacteria are able to maintain their cellular integrity, and the ratio of dead to live cells in ruminal fluid may be as great as 10:1. Bacterial survival appears to be at least partially explained by the method of sugar transport. When bacteria rely solely on mechanisms of ion-coupled sugar symport, an energized membrane is necessary for the reinitiation of growth. If group translocation (phosphotransferase system) is the mechanisms of transport, uptake can be driven by phosphoenolpyruvate, and an energized membrane and the storage of intracellular reserve materials are not an absolute criteria for survival. In some cases, N deprivation accelerates death. When Prevotella ruminicola was limited for N under conditions of excess energy, methylglyoxal production caused a rapid decrease in viability. The impact of bacterial death in the rumen is not clear-cut. If the rate of fermentation is zero-order with respect to cell concentration (substrate-limited), cell death would have little impact on digestion.

Analysis of the sodium dodecyl sulfate-stable peptidoglycan autolysins of select gram-negative pathogens by using renaturing polyacrylamide gel electrophoresis

For the first time, peptidoglycan autolysins from cellular fractions derived from sonicated cultures of Pseudomonas aeruginosa PAO1, Escherichia coli W7, Klebsiella pneumoniae CWK2, and Proteus mirabilis 19 were detected and partially characterized by zymogram analysis. Purified murein sacculi from P. aeruginosa PAO1 were incorporated into a sodium dodecyl sulfate (SDS)-polyacrylamide gel at a concentration of 0.05% (wt/vol) to serve as a substrate for the separated autolysins. At least 11 autolysin bands of various intensities with M(r)s ranging between 17,000 and 122,000 were detected in each of the homogenated cultures. Some of the autolysins of the four bacteria had similar M(r)s. The zymogram analysis was used to show that a number of the autolysins from E. coli were inhibited by the heavy metals Hg2+ and Cu2+, at 1 and 10 mM, respectively, high ionic strengths, and reagents known to affect the packing of lipopolysaccharides. The activity of an autolysin with an M(r) of 65,000 was also impaired by penicillin G, whereas it was enhanced by gentamicin. A preliminary screen to determine the relationship between penicillin-binding proteins (PBPs) and autolysins was carried out by using a dual assay in which radiolabelled penicillin V bands were visualized on an autolysin zymogram. Radiolabelled bands corresponding to PBPs 3, 4, 5, and 6 from E. coli and P. aeruginosa; PBPs 3, 4, and 6 from Proteus mirabilis; and PBP 6 from K. pneumoniae degraded the murein sacculi in the gels and were presumed to have autolytic activity, although the possibility of two distinct enzymes, each with one of the activities, comigrating in the SDS-polyacrylamide gels could not be excluded. Some radiolabelled bands possessed an Mr of <34,000 and coincided with similar low-Mr autolysin bands.

Initial characterization of two extracellular autolysins from Pseudomonas aeruginosa PAO1

Two extracellular autolysins have been detected in the spent culture supernatants of Pseudomonas aeruginosa PAO1 by using renaturing polyacrylamide gel electrophoresis. The two autolysins were isolated from the culture supernatant by trichloroacetic acid precipitation and were shown to have apparent molecular masses of 26 and 29 kDa. The 26-kDa autolysin first appears during the early exponential phase of growth and then declines sharply, while the 29-kDa autolysin first appears in the late exponential phase of growth and continues well into the stationary phase. Fractionation of whole cells indicated that the 26-kDa enzyme was also localized within the periplasm, with a lesser amount of activity associated with the cytoplasmic membrane. The 29-kDa autolytic activity was distributed within the cell equally between the periplasm and the cytoplasmic membrane. The pH optima of the isolated 26- and 29-kDa autolysins are 6.0 and 5.0, respectively. Further evidence from both protease susceptibility and inhibition studies confirms that these two extracellular autolysins isolated from P. aeruginosa PAO1 are separate and distinct.

Effect of human serum on killing activity of vancomycin and teicoplanin against Staphylococcus aureus

STUDY OBJECTIVE

To investigate the effects of pooled human serum (PHS) on the killing activity of vancomycin and teicoplanin against two isolates of Staphylococcus aureus from patients treated for endocarditis.

DESIGN

An in vitro assessment of antibiotic susceptibility and killing rates.

SETTING

An urban university teaching hospital.

PATIENTS

Pooled human serum from patients treated for endocarditis.

INTERVENTIONS

Two clinical isolates of Staphylococcus aureus were obtained from patients treated for endocarditis. Media consisted of cation-supplemented Mueller-Hinton broth alone and in 1:1 dilutions with PHS, 2-hour heat-inactivated PHS (HI-PHS), ultrafiltrate (UF), and 2-hour heat-inactivated ultrafiltrate (HI-UF). Heat inactivation of PHS and UF was accomplished by treatment at 56 degrees C for 2 hours.

MEASUREMENTS AND MAIN RESULTS

Killing curves with vancomycin and teicoplanin were performed using drug concentrations of 45 micrograms/ml and a starting inoculum of approximately 1 x 10(6) colony-forming units (cfu)/ml. Bactericidal rates (-log cfu/ml/hr) were calculated from the slope of the killing curves over 0-12 hours (mean 3-8 replicates).

CONCLUSIONS

The killing activity of vancomycin in PHS and HI-PHS against both isolates was significantly greater than all other media tested (p < 0.0001). Ultrafiltrate tended to reverse this enhancement effect. Addition of PHS or UF did not enhance teicoplanin's killing activity against either isolate. Further investigations in our laboratory will determine if the factor is antibiotic class or organism specific.

Analysis of the autolysins of Bacillus subtilis 168 during vegetative growth and differentiation by using renaturing polyacrylamide gel electrophoresis

The autolysins of Bacillus subtilis 168 were analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis with substrate-containing gels. Four bands of vegetative autolytic activity of 90, 50, 34, and 30 kDa (bands A1 to A4) were detected in SDS and LiCl extracts and in native cell walls by using B. subtilis 168 vegetative cell walls as the substrate incorporated in the gel. The four enzyme activities showed different substrate specificities and sensitivities to various chemical treatments. The autolysin profile was not medium dependent and remained constant during vegetative growth. During sporulation, band A4 greatly increased in activity just prior to mother-cell lysis. No germination-associated changes in the profile were observed, although a soluble 41-kDa endospore-associated cortex-lytic enzyme was found. By using insertionally inactivated mutants, bands A1 and A2 were positively identified as the previously characterized 90-kDa glucosaminidase and 50-kDa amidase, respectively. The common filamentous phenotype of various regulatory mutants could not be correlated to specific changes in the autolysin profile.

Subcellular distribution of the soluble lytic transglycosylase in Escherichia coli

The localization of the major autolytic enzyme, the soluble lytic transglycosylase, in the different cell compartments of Escherichia coli was investigated by immunoelectron microscopy. Ultrathin sections were labeled with a specific antiserum against purified soluble lytic transglycosylase, and the antibody-enzyme complexes were visualized with colloidal protein A-gold. A preferential localization of the lytic transglycosylase in the envelope was observed, with only 20 to 30% of the enzyme left in the cytoplasm. Most of the enzyme associated with the cell wall was tightly bound to the murein sacculus. Sacculi prepared by boiling of cells in 4% sodium dodecyl sulfate could be immunolabeled with the specific antiserum, indicating a surprisingly strong interaction of the lytic transglycosylase with murein. The enzyme-substrate complex could be reconstituted in vitro by incubating pronase-treated, protein-free murein sacculi with purified lytic transglycosylase at 0 degrees C. Titration of sacculi with increasing amounts of enzyme indicated a limiting number of binding sites for about 1,000 molecules of enzyme per sacculus. Ruptured murein sacculi obtained after penicillin treatment revealed that the enzyme is exclusively bound to the outer surface of the sacculus. This finding is discussed in the light of recent evidence suggesting that the murein of E. coli might be a structure of more than one layer expanding by inside-to-outside growth of patches of murein.

Cytolysis of Bacillus subtilis by Fusarium oxysporum

Growth of Fusarium oxysporum on heat-killed Bacillus subtilis cells was accompanied by the loss of bacterial cytoplasmic contents, and this 'cytolysis' could be catalysed in heat-treated bacteria by the fungal culture fluids. In electron micrographs the bacterial walls appeared undamaged, and the absence of wall-lytic enzymes was confirmed by use of isolated bacterial walls as substrate. Appearance of cytolytic activity in cultures was paralleled by the production of proteolytic activity in the cultures. Proteolysis and cytolysis had similar pH optima at 8.8-9.0. Cultures grown on casein, but not glucose, produced high cytolytic activity. Rapid cytolysis occurred when heat-treated B. subtilis cells were incubated with trypsin, subtilisin or pronase E. Viable bacteria, however, were not attacked, either by concentrated culture fluids or by the commercial protease preparations.

Isolation, and morphological and chemical properties of an autolysis-deficient mutant of Clostridium botulinum type A

An autolysis-deficient mutant was isolated from Clostridium botulinum type A 190L by treatment with ethyl methanesulfonate. The cell wall prepared from the mutant autolyzed at much slower rate than that from the parent strain, accompanying with much less liberation of both amino terminals and reducing groups. Electron microscopic observation revealed that the mutant strain was converted to short rod or curved spherical form with thickened cell walls when the growth temperature was shifted from 37 to 45 C. The mutant had a significantly larger amount of non-peptidoglycan-carbohydrate complexes than did the parent strain and became markedly resistant to the autolysin partially purified from the parent, compared with the parent strain. Furthermore, the mutant was fairly tolerant to killing by penicillin. These results suggest that the autolysis deficiency of the mutant was due not only to the deficient production of autolysin but also to the excess accumulation of carbohydrate in the cell wall.

Structure of vancomycin and a vancomycin/D-Ala-D-Ala complex in solution

Restrained molecular dynamics simulations were used to study the interactions between the glycopeptide antibiotic vancomycin and the dipeptide Ac-D-Ala-D-Ala. Restraints were obtained from a combination of homonuclear and heteronuclear two-dimensional NMR experiments (NOESY, ROESY, 1H-15N inverse correlation). The comparison between the structures obtained for vancomycin alone and for the complex suggests a new hypothesis on the binding mode of this system. The numerical simulations were not straightforward because vancomycin is made of building blocks for which standard force-fields are not available. The representation of unusual chemical environments is also mandatory. We believe that our extension of the force-field parameters to our system could be of more general interest. Furthermore, we consider vancomycin and its complex a good example for exploring the more general problem of molecular recognition, a challenge that has been widely approached in the past few years but for which no unique and general methodology has, so far, been recognized.

Paradoxical response of Enterococcus faecalis to the bactericidal activity of penicillin is associated with reduced activity of one autolysin

Ten clinical isolates of Enterococcus faecalis were examined for susceptibility to the bactericidal activity of penicillin. Four of these had MBCs of penicillin equal to 2 to 4 x the MIC, and six exhibited a paradoxical response to penicillin, i.e., the bactericidal activity of the antibiotic had a concentration optimum at 2 to 4 x the MIC and decreased significantly at concentrations above this. We found that the paradoxical response to penicillin was an intrinsic and stable property of a strain, but that its phenotypic expression was not homogeneous; only a fraction of the cell population that died at low concentrations was able to survive at high penicillin concentrations. The size of this fraction increased with increasing antibiotic concentration and reached a maximum in the late-log phase of growth. All 10 strains produced a lytic enzyme that was active on Micrococcus luteus heat-killed cells, whereas only some strains lysed E. faecalis heat-killed cells. Strains producing large amounts of the latter enzyme did not show the paradoxical response to penicillin, whereas mutants of these strains that lacked this enzymatic activity paradoxically responded to the antibiotic activity. In addition, from strains that showed paradoxical response to penicillin and produced only the enzyme that was active on M. luteus, it was possible to isolate mutants that were also capable of lysing E. faecalis cells and that were killed with similar efficiency by all concentrations above the MBC. On the basis of these findings, the paradoxical response to penicillin is explained as a property of certain strains of E. faecalis; this property is genetically characterized by alterations in synthesis or activity of one autolysin but phenotypically expressed only by a few cells that are in a particular physiological condition when exposed to high concentrations of antibiotics.

Reverse inoculum effect in bactericidal activity and other variables affecting killing of group B streptococci by penicillin

Variables of the effect of penicillin G on the numbers of viable group B streptococci in broth cultures were studied. One-fourth of the MIC was the lowest concentration that reduced the viable count compared with antibiotic-free controls. The rate of killing increased with the concentration of penicillin up to 4 X MIC, but no further. During the first 2 or 3 h, the bactericidal activity was more rapid than later on. The MIC and supraoptimal concentrations of penicillin killed an inoculum of 10(6) organisms more rapidly than an inoculum of 10(4) organisms. The MIC was not inoculum dependent. The reverse inoculum effect was revealed by the killing curves but not by the MBC. There were reproducible differences among strains as to the rate of killing by penicillin; these did not correlate with the rate of multiplication, which also varied among strains. Among the 11 strains tested, there were no tolerant ones.

Involvement of autolysin in cellular lysis of Bacillus subtilis induced by short- and medium-chain fatty acids

The addition of saturated C6, C8, C10, and C12 fatty acids appeared to lyse actively growing cells of Bacillus subtilis 168, as judged by a decrease in the optical density of the culture. Of these fatty acids, dodecanoic acid was the most effective, with 50% lysis occurring in about 30 min at a concentration of 0.5 mM. These conditions also decreased the amount of peptidoglycan estimated by the incorporated radioactivity of N-acetyl-D-[1-14C]glucosamine. At concentrations above 1 mM, however, bacterial lysis was not extensive. Dodecanoic acid did not affect autolysis of the cell wall. The lytic action of dodecanoic acid was greatly diminished in cells in which protein synthesis was inhibited and in an autolytic enzyme-deficient mutant. The results suggest that fatty acid-induced lysis of B. subtilis 168 is due to the induction of autolysis by an autolytic enzyme rather than massive solubilization of the cell membrane by the detergent-like action of the fatty acids.

Solid-state 15N NMR studies of the effects of penicillin on cell-wall metabolism of Aerococcus viridans (Gaffkya homari)

Lyophilized whole cells and isolated cell walls from Aerococcus viridans (Gaffkya homari) grown on a synthetic medium containing benzylpenicillin, and either L-[epsilon-15N]lysine or 15N-ammonium ion as the only source of label, have been studied using cross-polarization magic-angle spinning 15N nuclear magnetic resonance. The lysine is incorporated directly into protein and cell-wall peptidoglycan and was used to measure cell-wall cross-links. The ammonium ion acts as a non-specific label monitoring general metabolism. Inhibition of cell-wall cross linking by penicillin occurs, but may not be the exclusive cause of cell death and lysis in this microorganism. Instead, the disruption of the mechanism for control of peptidoglycan synthesis probably is a contributing factor.

Inhibition of peptidoglycan synthesis of Streptococcus faecium ATCC 9790 and Streptococcus mutans BHT by the antibacterial agent dodecyl glycerol

Dodecyl glycerol inhibits the synthesis of the peptidoglycans of Streptococcus faecium ATCC 9790 and Streptococcus mutans BHT. This metabolic regulation represents the second known mode by which dodecyl glycerol expresses antibacterial activity. The first mode of action of dodecyl glycerol was shown to stimulate autolysin activity which degrades cell-wall peptidoglycan (Ved HS, Gustow E, Mahadevan V and Pieringer RA, 1984, J. Biol. Chem. 259, 8115-8121).

pH-dependent oxacillin tolerance of Staphylococcus aureus

A proportion of clinical isolates of Staphylococcus aureus exhibit resistance to bactericidal activity of certain antibiotics, despite normal susceptibility to inhibition. This phenomenon is termed "tolerance." The methodology used to determine tolerance varies greatly. To clarify the relationship between laboratory methodology and tolerance, we determined the minimal bactericidal concentrations and minimal inhibitory concentrations for 20 clinical isolates by two methods. Inocula were prepared by either 3-h growth of the organism in Mueller-Hinton broth or overnight (22 to 24 h) cultures in Trypticase soy broth (BBL Microbiology Systems, Cockeysville, Md.). All inocula were plated for colony counts and tested for pH. An American Type Culture Collection (Rockville, Md.) reference strain was included in all tests for standardization. Tolerance was defined by the strictest criterion, i.e., a minimal bactericidal concentration/minimal inhibitory concentration ratio of greater than or equal to 100. With the first method, none of the 20 isolates displayed tolerance (mean inoculum pH, 7.15). When inocula were grown in Trypticase soy broth with overnight incubation, 35% (7 of 20) showed tolerance (mean inoculum pH, 6.22). There was a significant association between the decreased bactericidal capacity at high oxacillin concentrations and overnight incubation in Trypticase soy broth (P less than 0.01). We suggest that tolerance in staphylococci is in some way related to the pH value of the inoculating culture. Such pH-induced tolerance may have a clinical corollary in sequestered infections where the pH is acidic.

Synthesis of peptidoglycan in vivo in methicillin-resistant Staphylococcus aureus

The cell-wall composition and degree of cross-linking of peptidoglycan in a strain of Staphylococcus aureus (strain MR-1) which is highly resistant to methicillin were similar to those of other strains of S. aureus. When the organism was grown in the presence of very low concentrations of methicillin (equivalent to 3 x 10(-4) x minimum growth-inhibitory concentration [MGIC] there was a large decrease in the degree of cross-linking of the peptidoglycan. Increasing concentrations of methicillin (up to 1.25 x 10(-2) x MGIC) caused a further decrease in cross-linkage but thereafter a minimum value was reached. This remained unchanged even after growth of the organisms in much higher concentrations of the antibiotic up to 0.3 x minimum growth-inhibitory concentration. S. aureus MR-1 was able to grow normally for many generations under these conditions and reduction in cross-linkage of peptidoglycan was the only change detected in wall chemistry. Growth in the presence of methicillin (up to 0.3 x MGIC) (or other beta-lactam antibiotics) did not lead to an imbalance in the biosynthesis of peptidoglycan since no soluble polymers were secreted into the growth medium and nucleotide-linked precursors did not accumulate intracellularly. High concentrations of beta-lactam antibiotics (5 x MGIC) were bacteriostatic not bactericidal and this may be related to an apparent deficiency in the endogenous autolytic enzymes of strain MR-1. Studies of the penicillin-binding proteins after growth in the presence of methicillin suggest that one of these proteins remains resistant to very high concentrations of the antibiotic. We propose that this protein acts as the primary transpeptidase responsible for the incorporation of newly synthesised peptidoglycan into the growing wall.

Cell wall turnover in growing and nongrowing cultures of Bacillus subtilis

Cell wall turnover was studied in cultures of Bacillus subtilis in which growth was inhibited by nutrient starvation or by the addition of antibiotics. Concomitantly, the synthesis of wall, as measured by the incorporation of radioactively labeled N-acetylglucosamine, was followed in some of these cultures. In potassium- or phosphate-starved cultures, growth stopped, but wall turnover continued at a rate slightly lower than that in the control cultures. Lysis of cells did not occur. In glucose-starved cultures, continued wall turnover caused lysis of cells, since wall synthesis apparently was inhibited. The same phenomenon was observed after growth arrest by the addition of wall synthesis inhibitors such as fosfomycin, cycloserine, penicillin G, and vancomycin. Growth arrest by the addition of chloramphenicol allowed the continuation of wall synthesis; therefore, the observed turnover generally did not cause cell lysis.

Antibiotic-induced lysis of enterococci

Enterococci are resistant to penicillin killing in vivo and in vitro. Because some bacteria resistant to penicillin killing have reduced autolytic activity, we examined the lysis of clinical enterococcal isolates suspended in buffer (spontaneous lysis), and compared it with their susceptibility to antibiotic-induced lysis and killing. We found significant correlations between spontaneous and antibiotic-induced lysis, using five antibiotics that inhibit cell wall synthesis (penicillin, cephalothin, bacitracin, cycloserine, and vancomycin). Among isolates, strains more rapidly lysed by one antibiotic were more rapidly lysed by the other antibiotics, and more susceptible to spontaneous lysis. In studies involving a single strain grown in different media, spontaneous lysis also correlated closely with antibiotic-induced lysis. These results are consistent with a common mechanism for spontaneous and antibiotic-induced lysis, such as the autolytic enzyme system. Human serum was one of the least permissive media tested for enterococcal growth and antibiotic-induced lysis and killing. We suggest that the inhibitory effect of human serum on growth and the activation of the enterococcal autolytic enzyme system may be a critical factor in the resistance of enterococcal endocarditis to treatment with penicillin alone.

pH-dependent penicillin tolerance of group B streptococci

Group B streptococci lose viability without apparent lysis during treatment with beta-lactam antibiotics and vancomycin. Rapid loss of viability was observed in early-exponential-phase cultures. Cultures in the mid-exponential growth phase exhibited various degrees of resistance to the bactericidal effect of the antibiotics, whereas their susceptibilities to the growth-inhibitory effect remained unchanged. This growth-phase-dependent tolerance was caused by the gradual increase in acidity of the cultures as the cell concentration increased. Retitration of the pH to neutrality made the formerly tolerant bacteria again fully susceptible to the killing effect of penicillin. Conversely, lowering the pH value of the medium resulted in antibiotic tolerance throughout culture growth. The penicillin-binding proteins of whole bacteria and their labeling pattern were found to be independent of culture pH. It is suggested that the mechanism of Ph-dependent tolerance is indirect and may be mediated by an autolysin. The tolerance of group B streptococci for penicillin could be clinically relevant in view of the relatively low pH values known to prevail in the natural host environments colonized by these bacteria.

Effect of cerulenin on cellular autolytic activity and lipid metabolism during inhibition of protein synthesis in Streptococcus faecalis

Cellular autolytic activity as well as lipid and lipoteichoic acid metabolism have been studied in cultures of Streptococcus faecalis receiving various combinations of the following treatments: chloramphenicol addition, starvation for an essential amino acid (valine), and cerulenin treatment. Lipoteichoic acid initially accumulated in chloramphenicol-treated and amino acid-starved cells and decreased relative to the cellular mass in cerulenin-treated cells. The relative phosphatidylglycerol content of amino acid-starved cultures or of cultures treated with either antibiotic rapidly decreased upon initiation of each treatment. In all cases, cerulenin initially stimulated diphosphatidylglycerol synthesis. Pretreatment of cultures with cerulenin prevented the inhibition of cellular synthesis autolysis normally observed during chloramphenicol treatment, but did not affect amino acid starvation-induced inhibition of autolytic activity. Variations in the levels of the nonionic lipid fraction, predominantly diglycerides, correlated best with the patterns of autolytic activity observed during chloramphenicol treatment, whereas variations in the levels of diphosphatidylglycerol and lipoteichoic acid correlated best with the patterns of autolytic activity observed during amino acid starvation. Components of the nonionic lipid fraction were demonstrated to inhibit autolytic activity 50% in whole cell and in cell wall assays at 60 and 120 nmol/mg (dry weight), respectively.

Antibiotic-tolerant mutants of Streptococcus pneumoniae that are not deficient in autolytic activity

Several mutants of Streptococcus pneumoniae were isolated that appeared tolerant, to varying extents, to the lytic and bactericidal effects of some antibiotics that inhibit peptidoglycan synthesis, but were not deficient in autolytic activity. The method used to select the mutants was based on the survival of tolerant mutants during treatment with either bacitracin, benzylpenicillin, D-cycloserine plus beta-chloro-D-alanine, or vancomycin. Most (60 to 80%) of the surviving isolates were found to be deficient in autolytic activity, and these were rejected. The smaller proportion that had wild-type sensitivity to deoxycholate-induced lysis was studied further with respect to tolerance to the other antibiotics used in the selection procedures. Two of these mutants (selected by treatment with benzylpenicillin) were tolerant to either benzylpenicillin or D-cycloserine plus beta-chloro-D-alanine, but were supersusceptible, in terms of initiation of lysis, to either bacitracin or vancomycin. The minimal inhibitory concentration values of several antibiotics for these two mutants were identical to those for the wild-type strain. Moreover, the interaction of radioactive benzylpenicillin with the penicillin-binding proteins, examined in whole organisms, also appeared the same as previously found for either wild-type or autolytic-deficient strains of S. pneumoniae.

Lethal effect of a heterologous murein hydrolase on penicillin-treated Streptococcus sanguis

Nine strains of Streptococcus sanguis exhibited tolerance to benzylpenicillin: the growth of each strain was susceptible to penicillin with minimal inhibitory concentrations of 0.1 mug/ml or lower, but the bacteriolytic and bactericidal effects were limited in each case. The tolerance of these bacteria was also reflected in the large discrepancies between the minimal inhibitory and minimal bactericidal concentrations for benzylpenicillin. The hypothesis that a natural deficiency of endogenous murein hydrolase (autolysin) in this species accounts for the penicillin tolerance was tested by using a heterologous murein hydrolase, the C-phage-associated lysin. In seven of the strains, addition of the lysin to the culture together with penicillin or other cell wall inhibitors resulted in lysis and rapid loss of viability. The enzyme alone did not appreciably affect normally growing cultures. The irreversible effects of penicillin plus lysin were drastically reduced in the presence of the bacteriostatic agents chloramphenicol and cerulenin. Speculations based on experiments are presented for the mechanisms by which penicillin treatment sensitizes these bacteria to an exogenous lytic enzyme. Similar phenomena requiring cooperation of host factors and penicillin may occur during infection, since somewhat similar although less pronounced results were obtained by addition of human lysozyme to penicillin-treated S. sanguis.

Effects of cerulenin on antibiotic-induced lysis of streptococcus faecalis (S. faecium)

Addition of the antibiotic cerulenin to cultures lowered the minimal effective concentration of penicillin G or methicillin required to produce bacterial lysis and killing. This effect was most pronounced at subinhibitory antibiotic concentrations. Cerulenin had no significant effects on lysis or killing induced in the presence of D-cycloserine, fosfomycin, bacitracin, or vancomycin.

Morphological and physiological study of autolytic-defective Streptococcus faecium strains

Three autolytic-defective mutants of Streptococcus faecium (S. faecalis ATCC 9790) were isolated. All three autolytic-defective mutants exhibited the following properties relative to the parental strain: (i) slower growth rates, especially in chemically defined medium; (ii) decreased rates of cellular autolysis and increased survival after exposure to antibiotics which block cell wall biosynthesis; (iii) decreased rates of cellular autolysis when treated with detergents, suspended in autolysis buffers, or grown in medium lacking essential cell wall precursors; (iv) a reduction in the total level of cellular autolytic enzyme (active plus latent forms of the enzyme); (v) an increased ratio of latent to active forms of autolysin; and (vi) increased levels of both cellular lipoteichoic acid and lipids.