Autolytic defective mutant of Streptococcus faecalis

Antimicrobial tolerance and its role in the development of resistance: Lessons from enterococci

Bacteria have developed resistance against every antimicrobial in clinical use at an alarming rate. There is a critical need for more effective use of antimicrobials to both extend their shelf life and prevent resistance from arising. Significantly, antimicrobial tolerance, i.e., the ability to survive but not proliferate during antimicrobial exposure, has been shown to precede the development of bona fide antimicrobial resistance (AMR), sparking a renewed and rapidly increasing interest in this field. As a consequence, problematic infections for the first time are now being investigated for antimicrobial tolerance, with increasing reports demonstrating in-host evolution of antimicrobial tolerance. Tolerance has been identified in a wide array of bacterial species to all bactericidal antimicrobials. Of particular interest are enterococci, which contain the opportunistic bacterial pathogens Enterococcus faecalis and Enterococcus faecium. Enterococci are one of the leading causes of hospital-acquired infection and possess intrinsic tolerance to a number of antimicrobial classes. Persistence of these infections in the clinic is of growing concern, particularly for the immunocompromised. Here, we review current known mechanisms of antimicrobial tolerance, and include an in-depth analysis of those identified in enterococci with implications for both the development and prevention of AMR.

Contribution of autolysin and Sortase a during Enterococcus faecalis DNA-dependent biofilm development

Biofilm production is a major attribute of Enterococcus faecalis clinical isolates. Although some factors, such as sortases, autolysin, and extracellular DNA (eDNA), have been associated with E. faecalis biofilm production, the mechanisms underlying the contributions of these factors to this process have not been completely elucidated yet. In this study we define important roles for the major E. faecalis autolysin (Atn), eDNA, and sortase A (SrtA) during the developmental stages of biofilm formation under static and hydrodynamic conditions. Deletion of srtA affects the attachment stage and results in a deficiency in biofilm production. Atn-deficient mutants are delayed in biofilm development due to defects in primary adherence and DNA release, which we show to be particularly important during the accumulative phase for maturation and architectural stability of biofilms. Confocal laser scanning and freeze-dry electron microscopy of biofilms grown under hydrodynamic conditions revealed that E. faecalis produces a DNase I-sensitive fibrous network, which is important for biofilm stability and is absent in atn-deficient mutant biofilms. This study establishes the stage-specific requirements for SrtA and Atn and demonstrates a role for Atn in the pathway leading to DNA release during biofilm development in E. faecalis.

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).

Uberolysin: a novel cyclic bacteriocin produced by Streptococcus uberis

Streptococcus uberis is commonly found in the environment and in association with various bovine body sites and is a major cause of bovine mastitis. Moreover, S. uberis is known to produce a variety of bacteriocin-like inhibitory substances, antimicrobial agents that generally inhibit closely related bacterial species. In this respect, S. uberis strain 42 has previously been shown to produce a novel nisin variant named nisin U. This paper reports that, in addition to nisin U, S. uberis strain 42 produces a second bacteriocin that induces the lysis of metabolically active, susceptible target bacteria and which has therefore been named uberolysin. Isolation of the native active antimicrobial agent revealed that uberolysin is a 7048 Da peptide that is refractory to sequence analysis by Edman degradation. Transposon mutagenesis was used to generate a uberolysin-negative mutant of S. uberis 42 and sequencing of DNA flanking the insertion site revealed, in addition to the structural gene (ublA), several open reading frames likely to be involved in post-translational modification, transport and producer self-protection (immunity), and possibly in regulation of the biosynthetic gene cluster. In addition, a pair of direct repeats that may be involved in bacteriocin acquisition were identified; indeed, ublA could be identified in 18 % of tested S. uberis strains. Enzymic hydrolysis of uberolysin was used to confirm that ublA does indeed encode the precursor of uberolysin, that an unusually short leader sequence of only six amino acids is cleaved during processing of the mature peptide and that uberolysin is post-translationally covalently modified to form a head-to-tail monocycle. Thus, uberolysin is a unique cyclic bacteriocin, belonging to the same family of bacteriocins as enterocin AS-48 and circularin A.

Isolation and partial characterization of the Streptococcus mutans type AII lantibiotic mutacin K8

Streptococcus mutans strain K8 was shown to produce a newly identified type AII lantibiotic, mutacin K8. The mutacin K8-encoding muk locus consists of 13 ORFs, three of which (mukA1, A2 and A3) have close homology to scnA, the structural gene encoding the Streptococcus pyogenes lantibiotic SA-FF22, and another (mukA') resembles scnA', an ORF in the SA-FF22 locus that has no currently assigned function. Inactivation of the muk locus indicated that mutacin K8 is responsible for most of the inhibitory activity produced by strain K8 in deferred antagonism tests on Columbia blood agar base supplemented with 5 % human blood and 0.1 % CaCO(3). By contrast, on tryptic soy agar plus 2 % yeast extract and 0.5 % CaCO(3) most of the inhibitory activity of strain K8 appeared to be attributable either to mutacin IV or to some other inhibitory peptide(s) exported by the mutacin IV transporter nlmT. An inhibitory peptide purified from a derivative of strain K8 in which nlmT had been inactivated had a mass of 2734 Da and an N-terminal sequence identical to the predicted propeptide translation products of mukA1 and mukA3. The muk locus may be widely distributed in S. mutans, since 9 (35 %) of 26 strains tested contained at least part of the locus. In the genome sequence of strain UA159 the muk locus is incomplete, the sole residual components being the ORFs encoding the putative two-component regulatory system mukR (SMU.1815) and mukK (SMU.1814), followed by two transposases (SMU.1813 and SMU.1812) and then the ORFs mukF (SMU.1811), mukE (SMU.1810) and mukG (SMU.1809), thought to encode putative immunity peptides. Strains such as UA159 having incomplete loci did not produce detectable levels of mutacin K8.

The atlA operon of Streptococcus mutans: role in autolysin maturation and cell surface biogenesis

The Smu0630 protein (AtlA) was recently shown to be involved in cell separation, biofilm formation, and autolysis. Here, transcriptional studies revealed that atlA is part of a multigene operon under the control of at least three promoters. The morphology and biofilm-forming capacity of a nonpolar altA mutant could be restored to that of the wild-type strain by adding purified AtlA protein to the medium. A series of truncated derivatives of AtlA revealed that full activity required the C terminus and repeat regions. AtlA was cell associated and readily extractable from with sodium dodecyl sulfate. Of particular interest, the surface protein profile of AtlA-deficient strains was dramatically altered compared to the wild-type strain, as was the nature of the association of the multifunctional adhesin P1 with the cell wall. In addition, AtlA-deficient strains failed to develop competence as effectively as the parental strain. Mutation of thmA, which can be cotranscribed with atlA and encodes a putative pore-forming protein, resulted in a phenotype very similar to that of the AtlA-deficient strain. ThmA was also shown to be required for efficient processing of AtlA to its mature form, and treatment of the thmA mutant strain with full-length AtlA protein did not restore normal cell separation and biofilm formation. The effects of mutating other genes in the operon on cell division, biofilm formation, or AtlA biogenesis were not as profound. This study reveals that AtlA is a surface-associated protein that plays a critical role in the network connecting cell surface biogenesis, biofilm formation, genetic competence, and autolysis.

Peptidoglycan O acetylation and autolysin profile of Enterococcus faecalis in the viable but nonculturable state

The O acetylation of peptidoglycan occurs specifically at the C-6 hydroxyl group of muramoyl residues. Using a combination of high-performance liquid chromatography-based organic acid analysis and carbohydrate analysis by high-pH anion-exchange chromatography, we determined that strains of Entercoccus durans, E. faecalis, E. faecium, and E. hirae produce O-acetylated peptidoglycan. The levels of O acetylation ranged from 19% to 72% relative to the muramic acid content, and they were found to vary with the growth phase of the culture. Increases of 10 to 40% in O acetylation were observed with cultures entering the stationary phase. Cells of E. faecalis in the viable but nonculturable (VBNC) state had the highest levels of peptidoglycan O acetylation. The presence of this modification to peptidoglycan was shown to inhibit the action of hen egg white lysozyme in a concentration-dependent manner. Zymography using sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels containing either O-acetylated or chemically de-O-acetylated peptidoglycan was used to monitor the production of specific autolysins in E. faecalis. Differences in the expression of specific autolysins were observed with the age of the culture, and VBNC E. faecalis produced the highest levels of these enzymes. This technique also permitted classification of the enterococcal autolysins into enzymes that preferentially hydrolyze either O-acetylated or non-O-acetylated peptidoglycan and enzymes that show no apparent preference for either substrate type.

Balance between two transpeptidation mechanisms determines the expression of beta-lactam resistance in Enterococcus faecium

The d,d-transpeptidase activity of high molecular weight penicillin-binding proteins (PBPs) is essential to maintain cell wall integrity as it catalyzes the final cross-linking step of bacterial peptidoglycan synthesis. We investigated a novel beta-lactam resistance mechanism involving by-pass of the essential PBPs by l,d-transpeptidation in Enterococcus faecium. Determination of the peptidoglycan structure by reverse phase high performance liquid chromatography coupled to mass spectrometry revealed that stepwise selection for ampicillin resistance led to the gradual replacement of the usual cross-links generated by the PBPs (d-Ala(4) --> d-Asx-Lys(3)) by cross-links resulting from l,d-transpeptidation (l-Lys(3) --> d-Asx-Lys(3)). This was associated with no modification of the level of production of the PBPs or of their affinity for beta-lactams, indicating that altered PBP activity was not required for ampicillin resistance. A beta-lactam-insensitive l,d-transpeptidase was detected in membrane preparations of the parental susceptible strain. Acquisition of resistance was not because of variation of this activity. Instead, selection led to production of a beta-lactam-insensitive d,d-carboxypeptidase that cleaved the C-terminal d-Ala residue of pentapeptide stems in vitro and caused massive accumulation of cytoplasmic precursors containing a tetrapeptide stem in vivo. The parallel dramatic increase in the proportion of l-Lys(3) --> d-Asx-Lys(3) cross-links showed that the enzyme was activating the resistance pathway by generating the substrate for the l,d-transpeptidase.

Effect of disruption of a gene encoding an autolysin of Enterococcus faecalis OG1RF

A mutant (TX5127) of Enterococcus faecalis OG1RF was generated by disruption mutagenesis of a previously described autolysin gene. TX5127 formed longer chains (2 to 10 cells per chain) than wild-type OG1RF (mainly single cells) during growth in broth even though it had a growth rate similar to that of the parental strain as measured by turbidity and cell count. Autolysin activity, as defined by the ability to lyse heat-killed Micrococcus lysodeikticus cells, was absent in TX5127, while this activity was easily detectable in OG1RF. However, disruption of this autolysin gene did not block the ability of TX5127 to hydrolyze E. faecalis cell walls compared to that of OG1RF. The autolysis rate of cells of TX5127 in 10 mM sodium phosphate buffer (pH 6.8) was slower than that of wild-type OG1RF. TX5127 also showed a decreased rate of lysis in the presence of penicillin, as measured by changes in the turbidity of the culture during 24 h of incubation at 37 degrees C and a slightly decreased effect of penicillin as measured by time-kill curves. The virulence of TX5127 was similar to that of OG1RF in the mouse peritonitis model, indicating that the autolysin of E. faecalis is not important for infection in this model.

Evidence that the PBP 5 synthesis repressor (psr) of Enterococcus hirae is also involved in the regulation of cell wall composition and other cell wall-related properties

psr has been reported by M. Ligozzi, F. Pittaluga, and R. Fontana, (J. Bacteriol. 175:2046-2051, 1993) to be a genetic element located just upstream of the structural gene for the low-affinity penicillin-binding protein 5 (PBP 5) in the chromosome of Enterococcus hirae ATCC 9790 and to be involved in the repression of PBP 5 synthesis. By comparing properties of strains of E. hirae that contain a full-length, functional psr with those of strains that possess a truncated form of the gene, we have obtained data that indicate that psr is involved in the regulation of several additional surface-related properties. We observed that cells of strains that possessed a truncated psr were more sensitive to lysozyme-catalyzed protoplast formation, autolyzed more rapidly in 10 mM sodium phosphate (pH 6.8), and, in contrast to strains that possess a functional psr, retained these characteristics after the cultures entered the stationary growth phase. Cellular lytic properties did not correlate with differences in the cellular contents of muramidase-1 or muramidase-2, with the levels of PBP 5 produced, or with the penicillin susceptibilities of the strains. However, a strong correlation was observed with the amounts of rhamnose present in the cell walls of the various strains. All of the strains examined that possessed a truncated form of psr also possessed approximately one-half of the rhamnose content present in the walls of strains that possessed a functional psr. These data suggest that psr is also involved in the regulation of the synthesis of, or covalent linkage to the cell wall peptidoglycan of, a rhamnose-rich polysaccharide. These differences in cell wall composition could be responsible for the observed phenotypic differences. However, the multiple effects of psr suggest that it is part of a global regulatory system that, perhaps independently, affects several cell surface-related properties.

An autolysin ring associated with cell separation of Staphylococcus aureus

atl is a newly discovered autolysin gene in Staphylococcus aureus. The gene product, ATL, is a unique, bifunctional protein that has an amidase domain and a glucosaminidase domain. It undergoes proteolytic processing to generate two extracellular peptidoglycan hydrolases, a 59-kDa endo-beta-N-acetylglucosaminidase and a 62-kDa N-acetylmuramyl-L-alanine amidase. It has been suggested that these enzymes are involved in the separation of daughter cells after cell division. We recently demonstrated that atl gene products are cell associated (unpublished data). The cell surface localization of the atl gene products was investigated by immunoelectron microscopy using anti-62-kDa N-acetylmuramyl-L-alanine amidase or anti-51-kDa endo-beta-N-acetylglucosaminidase immunoglobulin G. Protein A-gold particles reacting with the antigen-antibody complex were found to form a ring structure on the cell surface at the septal region for the next cell division site. Electron microscopic examination of an ultrathin section of the preembedded sample revealed preferential distribution of the gold particles at the presumptive sites for cell separation where the new septa had not been completed. The distribution of the gold particles on the surface of protoplast cells and the association of the gold particles with fibrous materials extending from the cells suggested that some atl gene products were associated with a cellular component extending from the cell membrane, such as lipoteichoic acid. The formation of a ring structure of atl gene products may be required for efficient partitioning of daughter cells after cell division.

Identification of endo-beta-N-acetylglucosaminidase and N-acetylmuramyl-L-alanine amidase as cluster-dispersing enzymes in Staphylococcus aureus

Two proteins which are capable of dispersing cell clusters of Staphylococcus aureus have been purified from a S. aureus FDA209P culture supernatant. Both of them were found to have bacteriolytic activity. From the elution profile of column chromatography and Western blot (immunoblot) analysis, one of them was identified as a 51-kDa endo-beta-N-acetylglucosaminidase (GL). The other was a 62-kDa protein on the basis of sodium dodecyl sulfate gel electrophoresis. Analysis of the peptidoglycan fragments following treatment with the 62-kDa protein indicated that this protein is an N-acetylmuramyl-L-alanine amidase (AM). In vitro studies of cluster dispersion activities using S. aureus mutant strains Lyt66 or S. aureus Wood46 grown as clusters demonstrated that these two enzymes act synergistically to disperse clusters into single cells. Antiserum against the 51-kDa GL cross-reacted with the 62-kDa AM, and S. aureus FDA209P grown in the presence of anti-51-kDa-GL immunoglobulin G induced giant clusters. Clusters induced by anti-51-kDa GL and by Cibacron blue F3G-A were dispersed by coincubation with the 51-kDa GL and the 62-kDa AM. Western blot analysis demonstrated that the 51-kDa GL and the 62-kDa AM were missing in culture supernatants of S. aureus Lyt66, Wood46, and RUSAL2 (Tn551 autolysin-defective mutant), which grow in clusters. These results strongly suggest that the 51-kDa GL and 62-kDa AM are involved in cell separation of daughter cells after cell division.

Brochocin-C, a new bacteriocin produced by Brochothrix campestris

Brochotrix campestris ATCC 43754 produces a bacteriocin inhibitory towards Brochothrix thermosphacta, lactobacilli, Listeria spp., and other gram-positive bacteria. This antimicrobial agent is heat stable, sensitive to proteases, catalase insensitive, and free of organic acids. No phage particles were detected by transmission electron microscopy. Muramidase activity was not detected in the preparations. On the basis of established criteria, the antimicrobial agent was classified as a bacteriocin and named brochocin-C.

Production of bacteriocin inhibitory to Listeria species by Enterococcus hirae

A bovine intestinal bacterial isolate, identified as Enterococcus hirae, was found to produce a bacteriocin (designated hiraecin S) inhibitory to Listeria monocytogenes and other Listeria spp. Identification to species level was determined by comprehensive biochemical and morphological tests which were verified by DNA-DNA homology assays. The antimicrobial agent was inactivated by pronase and papain and was insensitive to catalase. The antimicrobial activity was not due to hydrogen peroxide or acid formation, nor was lysozyme or muramidase activity observed in cell-free bacteriocin preparations. Inhibition of selected gram-negative bacteria was not observed. Other enterococci were sensitive to the bacteriocin, and except for Listeria spp., no other gram-positive bacteria tested were inhibited.

Extracellular and cellular distribution of muramidase-2 and muramidase-1 of Enterococcus hirae ATCC 9790

A substantial portion of the second peptidoglycan hydrolase (muramidase-2) activity of Enterococcus hirae ATCC 9790 (formerly Streptococcus faecium) is present in the supernatant culture medium. In contrast, nearly all muramidase-1 activity is associated with cells in the latent, proteinase-activatable form. Muramidase-2 activity is produced and secreted throughout growth, with maximal levels attained at or near the end of exponential growth in a rich organic medium. Muramidase-2 activity in the culture medium remained high even during overnight incubations in the absence of proteinase inhibitors. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of supernatant culture medium concentrated by 60% saturated ammonium sulfate precipitation showed the presence of several Coomassie blue-staining bands. One intensely staining protein band, at about 71 kDa, selectively adsorbed to the insoluble peptidoglycan fraction of cell walls of E. hirae, retained muramidase-2 activity, and reacted in Western immunoblots with monoclonal antibodies to muramidase-2. The mobility of extracellular muramidase-2 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis was indistinguishable from that of muramidase-2 extracted with 6 M guanidine hydrochloride from intact bacteria. Muramidase-2 appears to have only a limited number of binding sites on the peptidoglycan of E. hirae cell walls but binds with high affinity. Although high levels of muramidase-2 activity were present in supernatants of stationary-phase cultures, the bacteria were resistant to autolysis. Thus it appears that the peptidoglycan in walls of intact cells of E. hirae is somehow protected from the hydrolytic action of extracellular muramidase-2.

Penicillin tolerance in Streptococcus faecium ATCC 9790

Tolerant strains of Streptococcus faecium had higher levels of muramidase 2 and lower levels of trypsinactivable muramidase 1 than did susceptible strains. Susceptible strains lysed faster than did tolerant strains in buffer and at some antibiotic concentrations. The addition of Triton X-100 produced equal lysis rates for susceptible and tolerant cultures.

Resistance of Streptococcus lactis mutants to beta-lactam antibiotics

Minimal inhibitory concentration and minimal bactericidal concentrations of penicillin and other beta-lactam antibiotics were determined for Streptococcus lactis in milk and trypteine soy broth. The values were always higher in milk than in broth. Minimal inhibitory concentration of penicillin was higher in solid than in liquid media. Two mutants resistant to penicillin and other beta-lactam antibiotics were obtained, and their growth rate, proteolysis, and acidification patterns determined. Tolerance toward these antibiotics was found when the mutants were grown in milk but not when they were grown in broth.

Transition from resistance to hypersusceptibility to beta-lactam antibiotics associated with loss of a low-affinity penicillin-binding protein in a Streptococcus faecium mutant highly resistant to penicillin

Penicillin-binding protein (PBP) 5 of Streptococcus faecium has been shown to have a very low affinity for penicillin, and this PBP was suggested to be responsible for both the natural low susceptibility and high resistance to the antibiotic in this species (R. Fontana, R. Cerini, P. Longoni, A. Grossato, and P. Canepari, J. Bacteriol. 155:1343-1350, 1983). In this study, an S. faecium mutant (Rev 14) hypersusceptible to penicillin was derived from the highly resistant S. faecium R40 treated with novobiocin, and its properties were compared with those of the parent and S. faecium PS, a relatively susceptible strain from which R40 was isolated. The hypersusceptible strain did not synthesize PBP 5, but it did resemble the parent in cell morphology, growth rate, and autolytic activity. In addition, it was highly susceptible to other beta-lactams but remained as susceptible as R40 and PS to antibiotics of a different mechanisms of action. The affinity of individual PBPs for the beta-lactams tested was the same in all the strains. This finding suggested that Rev 14 hypersusceptibility was due to the lack of PBP 5 and strongly supported the role of this protein in the mechanism of both natural low susceptibility and high-level resistance to beta-lactams in S. faecium.

Soluble non-cross-linked peptidoglycan polymers stimulate monocyte-macrophage inflammatory functions

Soluble non-cross-linked peptidoglycan polymers are released by gram-positive bacteria when beta-lactam antibiotics are administered to humans. In this report, we show that this type of peptidoglycan can stimulate monocyte-macrophage functions that cause inflammation. Non-cross-linked peptidoglycan polymers from penicillin-treated Streptococcus faecium were purified and shown to stimulate the production of interleukin 1 by human monocytes and of colony-stimulating factors by a murine macrophage cell line. In addition, the release of plasminogen activator by human monocytes was inhibited by the soluble peptidoglycan. These in vitro results suggest that prolonged treatment with beta-lactam antibiotics, by causing the production of soluble peptidoglycan, may result in interleukin 1-mediated inflammatory reactions, excessive production of monocytes and granulocytes, and increased fibrin deposition.

Bactericidal activity of human lysozyme, muramidase-inactive lysozyme, and cationic polypeptides against Streptococcus sanguis and Streptococcus faecalis: inhibition by chitin oligosaccharides

The basis of the bactericidal activity of human lysozyme against Streptococcus sanguis was studied. Experiments were designed to evaluate the role of lysozyme muramidase activity in its bactericidal potency. Inactivation of the muramidase activity of lysozyme was achieved by reduction of essential disulfides with dithiothreitol (DTT) or by incubation with the chitin oligosaccharides chitotriose and chitobiose. Muramidase-inactive lysozyme, prepared by reduction with DTT, was equal in bactericidal potency to native lysozyme. Solutions of native chicken egg white lysozyme and human lysozyme exhibited equal bactericidal potency yet differed ca. fourfold with respect to lytic (muramidase) activity. The above results suggested that the bactericidal activity of lysozyme is not dependent upon muramidase activity. Chitotriose and chitobiose were found to inhibit both lytic and bactericidal activities of lysozyme. The bactericidal activity of muramidase-inactive lysozyme (reduction with DTT) was also inhibited by chitotriose and chitobiose. Further investigations demonstrated that chitotriose and chitobiose were also potent inhibitors of the bactericidal activity of the cationic homopolypeptides poly-L-arginine and poly-D-lysine. These latter results suggested that the essential bactericidal property of lysozyme was its extreme cationic nature and that some bacterial endogenous activities, inhibitable by chitotriose and chitobiose, were essential for expression of the bactericidal activity of either native or muramidase-inactive lysozyme or of the cationic homopolypeptides. Experiments with Streptococcus faecalis whole cells, cell walls, and crude autolysin preparations implicated endogenous autolytic muramidases as the bacterial targets of chitotriose and chitobiose. The essentially identical responses of S. sanguis and S. faecalis to chitotriose in bactericidal assays with muramidase-inactive lysozyme and polylysine suggested that muramidase-like enzymes exist in S. sanguis and, furthermore, play an essential role in cationic protein-induced loss of viability of the oral microbe.

Isolation and characterization of soluble peptidoglycan from several strains of Streptococcus faecium

Two phenotypically autolysis-deficient strains of Streptococcus faecium ATCC 9790 were shown to produce high-molecular-weight, soluble, linear, uncross-linked peptidoglycan when incubated with benzylpenicillin in a wall medium which permits cell wall synthesis (wall thickening) but not balanced growth. This high-molecular-weight s-peptidoglycan was shown to have a molecular weight of 46,000 to 54,000, lack peptide cross-links, and be virtually devoid of accessory wall polymers. It was hydrolyzed by hen egg white lysozyme and the endogenous, autolytic N-acetylmuramidase of S. faecium, but was not attacked by proteinases. Chemical analyses of the polymer are consistent with the following structure, where n is the number of repeating disaccharide units: (formula; see text).

Hydrolysis of soluble, linear, un-cross-linked peptidoglycans by endogenous bacterial N-acetylmuramoylhydrolases

Soluble, linear, uncross-linked peptidoglycans, prepared from two autolysis-defective mutants of Streptococcus faecium ATCC 9790 and from Micrococcus leuteus, were used as substrates for studies of hydrolysis by an N-acetylmuramoylhydrolase (muramidase). The kinetics of hydrolysis of these substrates and the ability of the muramidases isolated from S. faecium ATCC 9790 and from two autolysis-defective mutants, Lyt-14 and Aut-3, to carry out transglycosylation reactions were compared with the action of hen egg white lysozyme (EC 3.2.1.17). Hydrolysis of these substrates by the endogenous streptococcal muramidases resulted in the production of disaccharide-peptide monomers with the structure (formula; see text) as nearly the sole product. As estimated from increases in reducing groups, hydrolysis proceeded at a linear rate for extended intervals, with consumption of up to 75% of the substrate, even at substrate concentrations well below the Km value. Apparent Km and relative Vmax values for the three streptococcal enzymes were indistinguishable from each other or from those for hen egg white lysozyme. These results indicate that the autolysis-defective phenotype of these mutants cannot be attributed to differences in their muramidases. In contrast to the action of hen egg white lysozyme, the streptococcal muramidase failed to catalyze transglycosylations. The extended periods of hydrolysis at constant rates are consistent with the occurrence of multiple catalytic events after the formation of the enzyme-substrate complex.

Monoclonal antibodies to immunodeterminants of lipoteichoic acids

Murine hybrid cell lines producing monoclonal antibodies directed against determinants present on lipoteichoic acids were generated. Hapten inhibition studies showed that one group of monoclonal antibodies was inhibited by deacylated cardiolipin, and the second group was inhibited by kojibiose. Thus, antibodies directed against the polyglycerophosphate chain, which is common to the lipoteichoic acids of many gram-positive species, and against the streptococcal group D antigen were obtained.

Division blocks in temperature-sensitive mutants of Streptococcus faecium (S. faecalis ATCC 9790)

Two hundred nine temperature-sensitive growth or division (or both) mutants of Streptococcus faecium ATCC 9790 were isolated. These strains were examined for timing of the division block in the cell division cycle. About 42% of the isolates were blocked at terminal stages of cell division. A second large group appeared to be blocked at various stages of septation. Only five of the temperature-sensitive isolates were blocked at a stage before the completion of chromosome replication. Thirty temperature-sensitive isolates lysed after one or more doublings at the nonpermissive temperature.

Streptococcus faecium ATCC 9790 penicillin-binding proteins and penicillin sensitivity are heavily influenced by growth conditions: proposal for an indirect mechanism of growth inhibition by beta-lactams

The effects of variations in growth conditions on the penicillin response of Streptococcus faecium ATCC 9790 were studied. Changes in the growth temperature and medium composition were found to cause striking changes in the bacterial generation time, cellular penicillin sensitivity (minimum inhibitory concentration), sensitivity of peptidoglycan synthesis to inhibition by penicillin, rate of autolysis, and labeling pattern of penicillin-binding proteins. However, no constant relationship between these parameters and the minimum inhibitory concentration could be observed. Similar electrophoretic patterns for penicillin-binding proteins were observed in cells grown in different media at the optimal growth temperature. Inhibition of cell division by penicillin in cells grown at this temperature (but not at higher or lower temperatures) caused filamentation of the bacteria. In cells grown in a chemically defined medium at the optimal temperature (but not at temperatures above or below), complete inhibition of cell division was associated with only partial inhibition (34% after 150 min) of peptidoglycan synthesis. It is suggested that the status and physiological importance of individual penicillin-binding proteins in S. faecium are heavily influenced by growth conditions. Depending on the growth conditions, different penicillin-binding proteins may perform the cellular function, indispensible for bacterial growth.

Effects of penicillin on synthesis and excretion of lipid and lipoteichoic acid from Streptococcus mutans BHT

Cultures of Streptococcus mutans BHT grown for at least eight generations in a chemically defined medium containing [1(3)-14C]glycerol, when treated with growth-inhibitory concentrations (0.2 micrograms/ml) of benzylpenicillin (Pen G), produced and excreted increased amounts of lipid and lipoteichoic acid per unit of cells. Cellular lysis was not observed. Compared with untreated controls, lipid excretion increased 15-fold, and lipoteichoic acid excretion increased 6-fold, 4 h after the addition of Pen G. All lipid species showed increased synthesis and excretion after exposure to Pen G. Although the same lipid types were found in both the Pen G-treated and the untreated cultures, the percent composition was altered after treatment with Pen G. The most dramatic example of this was the percentage of intracellular diphosphatidylglycerol found in the Pen G-treated cultures, 22.6%, in contrast to 5.3% found in the untreated cultures.

Lipids and lipoteichoic acid of autolysis-defective Streptococcus faecium strains

Two of four previously isolated autolysis-defective mutants of Streptococcus faecium (Streptococcus faecalis ATCC 9790) incorporated substantially more [14C]glycerol into lipids and lipoteichoic acid than did the parent strain. Consistent with increased accumulation of lipids and lipoteichoic acid, significantly higher levels of phosphorus were found in the corresponding fractions of the two mutant strains than in the wild type. Although the autolysis-defective mutant strains contained the same assortment of lipids as the wild type, the relative amount of [14C]glycerol incorporated into diphosphatidylglycerol increased, accompanied by a decreased fraction of phosphatidylglycerol. These results suggested that increased cellular content of two types of substances, acylated lipoteichoic acid and lipids (notably diphosphatidylglycerol), which previously had been shown to be potent inhibitors of the N-acetylmuramoylhydrolase of this species, contributed to the autolysis-defective phenotype of these mutants. Consistent with this interpretation are observations that (i) cerulenin inhibition of fatty acid synthesis increased the rates of benzylpenicillin-induced cellular lysis and that (ii) Triton X-100 or Zwittergent 3-14 treatment could reveal the presence of otherwise cryptic but substantial levels of the active form of the autolysin in cells of three of four mutants and of the proteinase-activable latent form in all four mutants.

Inhibition of peptidoglycan, ribonucleic acid, and protein synthesis in tolerant strains of Streptococcus mutans

Exposure of exponentially growing cultures of Streptococcus mutans strains FA-1 and GS-5 to various concentrations of benzylpenicillin (Pen G) resulted in inhibition of turbidity increases at low concentrations (0.02 to 0.04 mug/ml). However, in contrast to some other streptococcal species, growth inhibition was not accompanied by cellular lysis or by a rapid loss of viability. In both strains, synthesis of insoluble cell wall peptidoglycan was very sensitive to Pen G inhibition and responded in a dose-dependent manner to concentrations of about 0.2 and 0.5 mug/ml for strains GS-5 and FA-1, respectively. Higher Pen G concentrations failed to inhibit further either growth or insoluble peptidoglycan assembly. Somewhat surprisingly, Pen G also inhibited both ribonucleic acid (RNA) and protein syntheses, each in a dose-dependent manner. Compared with inhibition of peptidoglycan synthesis, inhibition of RNA and protein syntheses by Pen G was less rapid and less extensive. Maximum amounts of radiolabeled Pen G were specifically bound to intact cells upon exposure to about 0.2 and 0.5 mug/ml of Pen G for strains GS-5 and FA-1, respectively, concentrations consistent with those that resulted in maximum or near-maximum inhibitions of the synthesis of cellular peptidoglycan, RNA, and protein. Five polypeptide bands that had a very high affinity for [(14)C]Pen G were detected in a crude cell envelope preparation of strain FA-1. After exposure of cultures of strain FA-1 to the effects of saturating concentrations of the drug for up to 3 h, addition of penicillinase was followed by recovery of growth after a lag. The length of the lag before regrowth depended on both Pen G concentration and time of exposure. On the basis of these and other observations, it is proposed that the secondary inhibitions of cellular RNA or protein synthesis, or both, are involved in the tolerance of these organisms to lysis and killing by Pen G and other inhibitors of insoluble peptidoglycan assembly.

Release of autolytic enzyme from Streptococcus, faecium cell walls by treatment with dilute alkali

The autolytic enzyme (endo-beta-1,4-N-acetylmuramoylhydrolase) of Streptococcus faecium (S. faecalis ATCC 9790) was released in a soluble form from insoluble cell wall-autolytic enzyme complexes by treatment with dilute NaOH at 0 degree C. Treatment of cell wall-enzyme complexes, obtained from either exponential- or stationary-phase cells, with 0.008 to 0.01 N NaOH gave maximum yields of autolytic enzyme activity. At a fixed concentration of NaOH, the yield of autolysin increased with increasing wall densities and was accompanied by the release of methylpentose and phosphorus in amounts proportional to the autolysin. Since extraction of wall-enzyme complexes with 4.5 M LiCl at 0 degree C also removed methylpentose and phosphorus, release of enzyme with NaOH did not appear to result from hydrolysis of covalent linkages. The autolytic enzyme activity released from intact cells, or cell walls, was predominantly in the later (proteinase activable) form which could be activated by trypsin or a proteinase present in commerical bovine plasma albumin.

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.

Effects of temperature on the autolytic enzyme system of Streptococcus faecalis

The cellular autolytic reaction system in Streptococcus faecalis ATCC 9790 was analyzed for relative increases in reaction rates with increasing temperature by determination of Arrhenius activation energies (E). The systems examined were: (i) an isolated wall-enzyme complex in 0.01 M sodium phosphate, pH 6.9; (ii) exponential-phase cells suspended in 0.01 or o.3 M sodium phosphate pH 6.8, or in 0.04 M ammonium acetate, pH 6.8, (iii) growing cultures deprived of glucose or lysine; and (iv) cultures treated in growth media with the nonionic detergent, Triton X-100. For detergent-treated cells, E values were between 23.9 and 27.4 kcal/mol (ca. 100.1 to 174.7 kJ/mol) at concentrations of Triton X-100 between about 0.03 and 0.072 mg/ml. E values dropped sharply to 11.5 to 13.0 kcal/m-l (ca. 48.2 to 54.4 kJ/mol) at Triton X-100 concentrations of 0.12 mg/ml or higher. For the remaining systems, E values ranged from 16 to 20 kcal/mol (ca. 67.0 to 83.7 kJ/mol) (wall lysis, cellular autolysis in 0.01 M sodium phosphate or in 0.04 M ammonium acetate, and autolysis of glucose-starved cells) to 31 to 38 kcal/mol (ca 129.8 to 159.1 kJ/mol) (cellular autolysis in 0.3 M sodium phosphate or autolysis of lysine-starved cells). High concentrations of Triton X-100 appear to lower the E values below the 16 to 20 kcal/mol observed for the autolysis of isolated walls. This effect may be related to disruption by the detergent of a hydrophobic complex regulating cellular autolysis in vivo.

Cellular lysis of Streptococcus faecalis induced with triton X-100

Lysis of exponential-phase cultures of Streptococcus faecalis ATCC 9790 was induced by exposure to both anionic (sodium dodecyl sulfate) and nonionic (Triton X-100) surfactants. Lysis in response to sodium dodecyl sulfate was effective only over a limited range of concentrations, whereas Triton X-100-induced lysis occurred over a broad range of surfactant concentrations. The data presented indicate that the bacteriolytic response of growing cells to Triton X-100: (i) was related to the ratio of surfactant to cells and not the surfactant concentration per se; (ii) required the expression of the cellular autolytic enzyme system; and (iii) was most likely due to an effect of the surfactant on components of the autolytic system that are associated with the cytoplasmic membrane. The possibility that Triton X-100 may induce cellular lysis by releasing a lipid inhibitor of the cellular autolytic enzyme is discussed.