Zymographic characterization of Staphylococcus aureus cell wall

Lysostaphin: Engineering and Potentiation toward Better Applications

Lysostaphin is a potent bacteriolytic enzyme with endopeptidase activity against the common pathogen Staphylococcus aureus. By digesting the pentaglycine crossbridge in the cell wall peptidoglycan of S. aureus including the methicillin-resistant strains, lysostaphin initiates rapid lysis of planktonic and sessile cells (biofilms) and has great potential for use in agriculture, food industries, and pharmaceutical industries. In the past few decades, there have been tremendous efforts in potentiating lysostaphin for better applications in these fields, including engineering of the enzyme for higher potency and lower immunogenicity with longer-lasting effects, formulation and immobilization of the enzyme for higher stability and better durability, and recombinant expression for low-cost industrial production and in situ biocontrol. These achievements are extensively reviewed in this article focusing on applications in disease control, food preservation, surface decontamination, and pathogen detection. In addition, some basic properties of lysostaphin that have been controversial and only elucidated recently are summarized, including the substrate-binding properties, the number of zinc-binding sites, the substrate range, and the cleavage site in the pentaglycine crossbridge. Resistance to lysostaphin is also highlighted with a focus on various mechanisms. This article is concluded with a discussion on the limitations and future perspectives for the actual applications of lysostaphin.

Peptidoglycan derived from Staphylococcus epidermidis induces Connexin43 hemichannel activity with consequences on the innate immune response in endothelial cells

Gram-positive bacterial cell wall components including PGN (peptidoglycan) elicit a potent pro-inflammatory response in diverse cell types, including endothelial cells, by activating TLR2 (Toll-like receptor 2) signalling. The functional integrity of the endothelium is under the influence of a network of gap junction intercellular communication channels composed of Cxs (connexins) that also form hemichannels, signalling conduits that are implicated in ATP release and purinergic signalling. PGN modulates Cx expression in a variety of cell types, yet effects in endothelial cells remain unresolved. Using the endothelial cell line b.End5, a 6 h challenge with PGN induced IL-6 (interleukin 6), TLR2 and Cx43 mRNA expression that was associated with enhanced Cx43 protein expression and gap junction coupling. Cx43 hemichannel activity, measured by ATP release from the cells, was induced following 15 min of exposure to PGN. Inhibition of hemichannel activity with carbenoxolone or apyrase prevented induction of IL-6 and TLR2 mRNA expression by PGN, but had no effect on Cx43 mRNA expression levels. In contrast, knockdown of TLR2 expression had no effect on PGN-induced hemichannel activity, but reduced the level of TLR2 and Cx43 mRNA expression following 6 h of PGN challenge. PGN also acutely induced hemichannel activity in HeLa cells transfected to express Cx43, but had no effect in Cx43-deficient HeLa OHIO cells. All ATP responses were blocked with Cx-specific channel blockers. We conclude that acute Cx43 hemichannel signalling plays a role in the initiation of early innate immune responses in the endothelium.

Identification and molecular characterization of anN-acetylmuramyl-l-alanine amidase Sle1 involved in cell separation ofStaphylococcus aureus

We purified a peptidoglycan hydrolase involved in cell separation from a Staphylococcus aureus atl null mutant and identified its gene. Characterization of the gene product shows a 32 kDa N-acetylmuramyl-L-alanine amidase that we designated Sle1. Analysis of peptidoglycan digests showed Sle1 preferentially cleaved N-acetylmuramyl-L-Ala bonds in dimeric cross-bridges that interlink the two murein strands in the peptidoglycan. An insertion mutation of sle1 impaired cell separation and induced S. aureus to form clusters suggesting Sle1 is involved in cell separation of S. aureus. The Sle1 mutant revealed a significant decrease in pathogenesis using an acute infection mouse model. Atl is the major autolysin of S. aureus, which has been implicated in cell separation of S. aureus. Generation of an atl/sle1 double mutant revealed that the mutant cell separation was heavily impaired suggesting that S. aureus uses two peptidoglycan hydrolases, Atl and Sle1, for cell separation. Unlike Atl, Sle1 is not directly involved in autolysis of S. aureus.

Induction of inflammatory cytokines and nitric oxide in J774.2 cells and murine macrophages by lipoteichoic acid and related cell wall antigens from Staphylococcus epidermidis

Staphylococcus epidermidis causes infections associated with medical devices including central venous catheters, orthopaedic prosthetic joints and artificial heart valves. This coagulase-negative staphylococcus produces a conventional cellular lipoteichoic acid (LTA) and also releases a short-glycerophosphate-chain-length form of LTA (previously termed lipid S) into the medium during growth. The relative pro-inflammatory activities of cellular and short-chain-length exocellular LTA were investigated in comparison with peptidoglycan and wall teichoic acid from S. epidermidis and LPS from Escherichia coli O111. The ability of these components to stimulate the production of pro-inflammatory cytokines [interleukin (IL)-1β, IL-6 and tumour necrosis factor (TNF)-α] and nitric oxide was investigated in a murine macrophage-like cell line (J774.2), and in peritoneal and splenic macrophages. On a weight-for-weight basis the short-chain-length exocellular LTA was the most active of the S. epidermidis products, stimulating significant amounts of each of the inflammatory cytokines and nitric oxide, although it was approximately 100-fold less active than LPS from E. coli. By comparison the full-chain-length cellular LTA and peptidoglycan were less active and the wall teichoic acid had no activity. As an exocellular product potentially released from S. epidermidis biofilms, the short-chain-length exocellular LTA may act as the prime mediator of the host inflammatory response to device-related infection by this organism and act as the Gram-positive equivalent of LPS in Gram-negative sepsis. The understanding of the role of short-chain-length exocellular LTA in Gram-positive sepsis may lead to improved treatment strategies.

A scavenger function for a Drosophila peptidoglycan recognition protein

Recent studies of peptidoglycan recognition protein (PGRP) have shown that 2 of the 13 Drosophila PGRP genes encode proteins that function as receptors mediating immune responses to bacteria. We show here that another member, PGRP-SC1B, has a totally different function because it has enzymatic activity and thereby can degrade peptidoglycan. A mass spectrometric analysis of the cleavage products demonstrates that the enzyme hydrolyzes the lactylamide bond between the glycan strand and the cross-linking peptides. This result assigns the protein as an N-acetylmuramoyl-l-alanine amidase (EC ), and the corresponding gene is thus the first of this class to be described from a eukaryotic organism. Mutant forms of PGRP-SC1B lacking a potential zinc ligand are enzymatically inactive but retain their peptidoglycan affinity. The immunostimulatory properties of PGRP-SC1B-degraded peptidoglycan are much reduced. This is in striking contrast to lysozyme-digested peptidoglycan, which retains most of its elicitor activity. This points toward a scavenger function for PGRP-SC1B. Furthermore, a sequence homology comparison with phage T7 lysozyme, also an N-acetylmuramoyl-l-alanine amidase, shows that as many as six of the Drosophila PGRPs could belong to this class of proteins.

Increased glycan chain length distribution and decreased susceptibility to moenomycin in a vancomycin-resistant Staphylococcus aureus mutant

A vancomycin-resistant Staphylococcus aureus mutant, COL-VR1 (MIC, 16 microg/ml), was isolated from methicillin-resistant S. aureus COL by exposure to vancomycin. COL-VR1 also showed decreased susceptibility to teicoplanin (8-fold), methicillin (2-fold), macarbomycin (8-fold), and moenomycin (16-fold). Macarbomycin and moenomycin are thought to directly inhibit transglycosylase activity. Characterization of the mutant revealed a thickened cell wall and suppression of penicillin-induced lysis, although the amounts of the five penicillin-binding proteins (PBPs 1, 2, 3, 4, and 2') and the profiles of peptidoglycan hydrolases were not altered. Analysis of muropeptide profile and glycan chain length distribution by reversed-phase high-pressure liquid chromatography revealed slightly decreased peptide cross-linking and an increased average glycan chain length compared to those of the parent. These results together suggest that a transglycosylase activity was enhanced in the mutant. This may represent a novel mechanism of glycopeptide resistance in S. aureus.

Staphylococcus aureus produces autolysin-susceptible cell walls during growth in a high-NaCl and low-Ca2+ concentration medium

The growth of Staphylococcus aureus 209P becomes unusually sensitive to a high-NaCl concentration by decreasing the Ca2+ concentration in growth media, and cells either autolyze or transform into protoplast-like forms when grown standing in high-NaCl and low-Ca2+ concentration media below 37 C (Ochiai, T., Microbiol. Immunol. 43 (7): 705-709, 1999). To assess the role of Ca2+ in the salt tolerance of this organism, cells grown in the presence of different concentrations of Ca2+ were treated with boiling SDS, and their susceptibilities to crude autolysin (3 M LiCl extract of S. aureus 209P cells) were evaluated by turbidimetric assay and zymographic analysis. Susceptibilities of SDS-treated cells (SDS-cells) to crude autolysin were significantly influenced by Ca2+ concentration in the culture, and SDS-cells prepared from cultures grown in high-NaCl and high-Ca2+ concentration media exhibited marked resistance to crude autolysin when the assay system contained a high concentration of NaCl. On the contrary, SDS-cells prepared from cultures grown in high-NaCl and low-Ca2+ concentration media were rather susceptible to crude autolysin under the same assay conditions. A zymographic analysis revealed that the constitution of cell-associated autolysins was not influenced by the concentration of exogenous Ca2+. These results suggested that at least part of the mechanism of salt-induced autolysis in S. aureus 209P might be related to the synthesis of an autolysin susceptible cell wall.