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

Desiccation Survival of Salmonella enterica,Escherichia coli, and Enterococcus faecium Related to Initial Cell Level and Cellular Components

ABSTRACT

Salmonella enterica is well known for its ability to survive and persist in low-moisture environments. Previous studies have indicated a link between the initial cell level and the population of Salmonella that survives after desiccation and subsequent storage; however, how the initial cell concentration affects survival is unknown. This study was conducted to examine this phenomenon and to determine whether it occurs in other microorganisms, specifically Shiga toxigenic Escherichia coli (STEC) and Enterococcus faecium. Salmonella, STEC, and E. faecium were grown as sessile cells on Trypticase soy agar with yeast extract (TSAYE) and harvested in buffered peptone water (BPW). To determine recovery at different initial cell levels, cultures were diluted to 9, 7, and 5 log CFU/mL and applied to filters. Filters were dried for 24 h and then stored for 28 days at 25°C and 33% relative humidity. During storage, cells were recovered from filters with BPW and cultivated on TSAYE. Recovery of both Salmonella and E. coli, but not E. faecium, was nonproportional. Lower initial populations were less viable after 24 h of desiccation; ≥10 log CFU/mL was recovered when 11 log CFU/mL was desiccated, but <3 log CFU/mL was recovered when 5 log CFU/mL was desiccated. Once dried, persistence did not appear affected by initial cell concentration. When inactivated (heat-treated) cells were added to the diluent, recovery of Salmonella was proportional with respect to the initial cell level. To further examine the response to desiccation, Salmonella was diluted in BPW containing 1 of 11 test cell components related to quorum sensing or known to affect desiccation resistance to assess recovery and persistence. Of the 11 additions, only cell debris fractions, cell-free extract, and peptidoglycan improved recovery of Salmonella. Desiccation survival appears related to cell wall components; however, the exact mechanism affecting survival remains unknown.

HIGHLIGHTS

Structural Changes in Cell-Wall and Cell-Membrane Organic Materials Following Exposure to Free Nitrous Acid

Previous studies demonstrate that free nitrous acid (FNA, i.e., HNO₂) is biocidal for a range of microorganisms. The biocidal mechanisms of FNA are largely unknown. In this work, it is hypothesized that FNA will break bonds in molecules found in the cell envelope, thus causing cell lysis. Selected molecules representing components found in the cell envelope were treated with FNA at 6.09 mg N/L (NO₂^- = 250 mg N/L, pH 5.0) for 24 h (conditions typically used in applications) to evaluate the hypothesized chemical interactions. Molecular changes were observed using analytical techniques including proton (¹H) nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization mass spectrometry (ESI-MS). It was found that FNA broke down a range of cell envelope molecules. The spectral data demonstrated that the FNA reactions proceeded via two general pathways. One consisted of electrophilic substitution, whereby the nitrosonium ion (NO⁺) was the reactive electrophile. The other was via oxidative reactions involving nitrogen radicals (e.g., •NO₂ and •NO) formed from the decomposition of FNA. We further revealed that it was HNO₂ that caused the breakdown, rather than the exclusive action of the acid (H⁺) or nitrite (NO₂^-) counterparts. The fragmentation of these representative cell envelope molecules provides insight into the biocidal effects of FNA on microorganisms.

Bacteriocin protein BacL1 of Enterococcus faecalis targets cell division loci and specifically recognizes L-Ala2-cross-bridged peptidoglycan

Bacteriocin 41 (Bac41) is produced from clinical isolates of Enterococcus faecalis and consists of two extracellular proteins, BacL1 and BacA. We previously reported that BacL1 protein (595 amino acids, 64.5 kDa) is a bacteriolytic peptidoglycan D-isoglutamyl-L-lysine endopeptidase that induces cell lysis of E. faecalis when an accessory factor, BacA, is copresent. However, the target of BacL1 remains unknown. In this study, we investigated the targeting specificity of BacL1. Fluorescence microscopy analysis using fluorescent dye-conjugated recombinant protein demonstrated that BacL1 specifically localized at the cell division-associated site, including the equatorial ring, division septum, and nascent cell wall, on the cell surface of target E. faecalis cells. This specific targeting was dependent on the triple repeat of the SH3 domain located in the region from amino acid 329 to 590 of BacL1. Repression of cell growth due to the stationary state of the growth phase or to treatment with bacteriostatic antibiotics rescued bacteria from the bacteriolytic activity of BacL1 and BacA. The static growth state also abolished the binding and targeting of BacL1 to the cell division-associated site. Furthermore, the targeting of BacL1 was detectable among Gram-positive bacteria with an L-Ala-L-Ala-cross-bridging peptidoglycan, including E. faecalis, Streptococcus pyogenes, or Streptococcus pneumoniae, but not among bacteria with alternate peptidoglycan structures, such as Enterococcus faecium, Enterococcus hirae, Staphylococcus aureus, or Listeria monocytogenes. These data suggest that BacL1 specifically targets the L-Ala-L-Ala-cross-bridged peptidoglycan and potentially lyses the E. faecalis cells during cell division.

AcmA of Lactococcus lactis is an N-acetylglucosaminidase with an optimal number of LysM domains for proper functioning

AcmA, the major autolysin of Lactococcus lactis MG1363 is a modular protein consisting of an N-terminal active site domain and a C-terminal peptidoglycan-binding domain. The active site domain is homologous to that of muramidase-2 of Enterococcus hirae, however, RP-HPLC analysis of muropeptides released from Bacillus subtilis peptidoglycan, after digestion with AcmA, shows that AcmA is an N-acetylglucosaminidase. In the C-terminus of AcmA three highly similar repeated regions of 45 amino acid residues are present, which are separated by short nonhomologous sequences. The repeats of AcmA, which belong to the lysine motif (LysM) domain family, were consecutively deleted, removed, or, alternatively, one additional repeat was added, without destroying the cell wall-hydrolyzing activity of the enzyme in vitro, although AcmA activity was reduced in all cases. In vivo, proteins containing no or only one repeat did not give rise to autolysis of lactococcal cells, whereas separation of the producer cells from the chains was incomplete. Exogenously added AcmA deletion derivatives carrying two repeats or four repeats bound to lactococcal cells, whereas the derivative with no or one repeat did not. In conclusion, these results show that AcmA needs three LysM domains for optimal peptidoglycan binding and biological functioning.

Antibiotic-induced cell wall fragments of Staphylococcus aureus increase endothelial chemokine secretion and adhesiveness for granulocytes

Antibiotics release inflammatory fragments, such as lipoteichoic acid (LTA) and peptidoglycan (PG), from the cell wall of Staphylococcus aureus. In this study, we exposed S. aureus cultures to a number of beta-lactam antibiotics (imipenem, flucloxacillin, and cefamandole) and protein synthesis-inhibiting antibiotics (erythromycin, clindamycin, and gentamicin) and investigated whether supernatants of these cultures differ in their capacity to stimulate endothelial cells (EC). After 24 h of incubation, endothelial adhesiveness for leukocytes, surface expression of various adhesion molecules, and secretion of the chemokines interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1) were measured. Supernatants of beta-lactam-exposed cultures (designated beta-lactam supernatants) enhanced the adhesiveness of EC for granulocytes, whereas those of protein synthesis-inhibiting antibiotic-exposed cultures (designated protein synthesis-inhibitor supernatants) did not. This hyperadhesiveness coincided with a higher intercellular adhesion molecule-1 expression on the surface of the stimulated EC. In addition, EC stimulated with beta-lactam supernatants secreted significantly higher concentrations of the chemokines IL-8 and MCP-1 than those stimulated with protein synthesis-inhibitor supernatants. The finding that the concentrations of LTA and PG in beta-lactam supernatants were much higher than those in protein synthesis-inhibitor supernatants suggests that the observed differences in stimulatory effect between these supernatants are a result of differences in the release of cell wall fragments, although the presence of other stimulatory factors in the supernatants cannot be excluded. In conclusion, our results argue for a release of LTA and PG from S. aureus after exposure to beta-lactam antibiotics that enhances the development of a systemic inflammatory response by stimulating EC such that adhesiveness for granulocytes is increased and large amounts of IL-8 and MCP-1 are secreted.

Antibiotic-induced release of lipoteichoic acid and peptidoglycan from Staphylococcus aureus: quantitative measurements and biological reactivities

Antibiotics with different mechanisms of action may vary with respect to their effects on the release and immunostimulatory activities of cell wall fragments from gram-positive bacteria. Therefore, after Staphylococcus aureus was cultured for 4 h in the absence of antibiotics (control) and in the presence of beta-lactam antibiotics (imipenem, flucloxacillin, or cefamandole) and protein synthesis-inhibiting antibiotics (erythromycin, clindamycin, or gentamicin), the lipoteichoic acid (LTA) and peptidoglycan (PG) levels in the bacterial supernatants were measured. beta-Lactam antibiotics greatly enhanced the release of LTA and PG (4- to 9-fold and 60- to 85-fold, respectively), whereas protein synthesis inhibitors did not affect PG release and even inhibited the release of LTA compared to the amount of LTA released in control cultures. The capacity of beta-lactam supernatants to stimulate the production of tumor necrosis factor alpha and interleukin-10 in human whole blood was significantly higher than that of protein synthesis inhibitor or control supernatants; the amounts of these cytokines released were directly proportional to the concentrations of PG and LTA in the supernatants. Enzymatic degradation of PG in the supernatants indicated that PG was mainly responsible for the observed biological reactivity.

The autolytic ('suicidase') system of Enterococcus hirae: from lysine depletion autolysis to biochemical and molecular studies of the two muramidases of Enterococcus hirae ATCC 9790

Autolysis of Enterococcus hirae ATCC 9790 is the result of the action of endogenous enzymes that hydrolyze bonds in the protective and shape-maintaining cell wall peptidoglycan. It is thought that these potentially suicidal enzymes play a positive role(s) in wall growth and division and are expressed as autolysins when cell wall assembly and/or repair are inhibited. E. hirae possesses two potentially autolytic enzymes, both of which are muramidases. Although they hydrolyze the same bond as hen egg-white lysozyme, both are high-molecular-mass, complex enzymes. Muramidase-1 is synthesized as a zymogen, requiring protease activation. It is a glucoenzyme that is also multiply nucleotidylated with an unusual nucleotide, 5-mercaptouridine monophosphate. Muramidase-2 is almost certainly a product of a separate gene. The deduced amino acid sequence of a cloned gene for extracellular muramidase-2 showed several unusual features. It appears to be a two-, or perhaps three-domain protein with a putative glycosidase-active site near the N-terminal end and six 45-amino-acid-long repeats at the C-terminal end which are presumed to be involved with high-affinity binding to the insoluble peptidoglycan substrate. Muramidase-2 binds penicillin with low affinity. The presence of several amino acid groupings characteristic of serine-active site beta-lactam-interactive proteins is consistent with the possible presence of a penicillin-binding, third domain. Indirect evidence consistent with a role(s) for these enzymes in cell wall growth and division has been obtained. However, proof of such role(s) awaits modern genetic, molecular, and biochemical analyses.

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.

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.

Peptidoglycan cross-linking and teichoic acid attachment in Streptococcus pneumoniae

Autolysin-defective pneumococci continue to synthesize both peptidoglycan and teichoic acid polymers (Fischer and Tomasz, J. Bacteriol. 157:507-513, 1984). Most of these peptidoglycan polymers are released into the surrounding medium, and a smaller portion becomes attached to the preexisting cell wall. We report here studies on the degree of cross-linking, teichoic acid substitution, and chemical composition of these peptidoglycan polymers and compare them with normal cell walls. peptidoglycan chains released from the penicillin-treated pneumococci contained no attached teichoic acids. The released peptidoglycan was hydrolyzed by M1 muramidase; over 90% of this material adsorbed to vancomycin-Sepharose and behaved like disaccharide-peptide monomers during chromatography, indicating that the released peptidoglycan contained un-cross-linked stem peptides, most of which carried the carboxy-terminal D-alanyl-D-alanine. The N-terminal residue of the released peptidoglycan was alanine, with only a minor contribution from lysine. In addition to the usual stem peptide components of pneumococcal cell walls (alanine, lysine, and glutamic acid), chemical analysis revealed the presence of significant amounts of serine, aspartate, and glycine and a high amount of alanine and glutamate as well. We suggest that these latter amino acids and the excess alanine and glutamate are present as interpeptide bridges. Heterogeneity of these was suggested by the observation that digestion of the released peptidoglycan with the pneumococcal murein hydrolase (amidase) produced peptides that were resolved by ion-exchange chromatography into two distinct peaks; the more highly mobile of these was enriched with glycine and aspartate. The peptidoglycan chains that became attached to the preexisting cell wall in the presence of penicillin contained fewer peptide cross-links and proportionally fewer attached teichoic acids than did their normal counterparts. The normal cell wall was heavily cross-linked, and the cross-linked peptides were distributed equally between the teichoic acid-linked and teichoic acid-free fragments.

The autolytic peptidoglycan hydrolases of Streptococcus faecium

Streptococcus faecium ATCC 9790 possesses two peptidoglycan hydrolase activities. The first enzyme, an N-acetylmuramoylhydrolase, has been purified and has been shown to be a glucoenzyme. Studies of hydrolysis of soluble, linear uncross-linked peptidoglycan chains showed that the enzyme bound strongly to the non-reducing ends of the chains and then sequentially (processively) hydrolysed susceptible bonds in that chain. The second peptidoglycan hydrolase does not appear to be a glycoprotein and differs from the first enzyme in substrate specificity and mechanism of hydrolysis. The presence of two partially redundant activities which may play different roles in surface growth and division could, at least in part, explain previous difficulties in obtaining mutants which completely lack autolytic activity.

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.