The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Surface Proteins of Gram-Positive Bacteria and Mechanisms of Their Targeting to the Cell Wall Envelope

Recent studies have indicated that the attachment of bacteria to mucosal surfaces is the initial event in the pathogenesis of most infectious diseases due to bacteria in animals and humans. An understanding of the mechanisms of attachment and a definition of the adhesive molecules on the surfaces of bacteria (adhesins) as well as those on host cell membranes (receptors) have suggested new approaches to the prevention of serious bacterial infections: (1) application of purified adhesion or receptor materials or their analogues as competitive inhibitors of bacterial adherence; (2) administration of sublethal concentrations of antibiotics that suppress the formation and expression of bacterial adhesins; and (3) development of vaccines against bacterial surface components involved in adhesion to mucosal surfaces. Progress has already been made in the development of antiadhesive vaccines directed against the fimbrial adhesins of several human bacterial pathogens.

Bacterial Adherence: Adhesin-Receptor Interactions Mediating the Attachment of Bacteria to Mucosal Surfaces

Publisher Summary Several hundred strains of staphylococci and related organisms have been tested for the production of cell surface and/or extracellular SPA. Techniques for detecting the cell-bound protein include agglutination of erythrocytes sensitized with SPA-reactive antibody. Results have demonstrated the presence of extracellular as well as cell-bound SPA and implied that SPA on the membrane had easy access to Ig molecules in the fluid phase or bound to cell-surface antigens in such a way that the Fc region could interact with SPA receptors. Indeed, studies have shown that SPA is located primarily on the surface of Cowan strain I and is linked covalently to the peptidoglycan portion of the membrane. Studies on biosynthesis have been carried out in strains that produce cell-bound, as well as extracellular SPA (e.g., Cowan I) or only the extracellular product. The physicochemical properties of SPA prepared by lysostaphin treatment of Cowan strain I are summarized. The high frictional ratio (2:1) and intrinsic viscosity (29 ml/gm) compared to values normally obtained for globular proteins (1.1- 1.25 and 3-4 mg/gm, respectively) suggest that SPA is a relatively elongated molecule. The molecular weight of 42,000 is the average of several determinations and is the generally accepted value. This compares to the molecular weight of 29,500 found in the most active fraction obtained after chromatography of heat-extracted material. This chapter discusses the occurrence, properties, applications, biological activity, and potential significance of Ig receptors produced mainly by staphylococci and streptococci. Although they act primarily as Fc receptors, for this discussion they have been classified more broadly as Ig receptors. This is because SPA displays secondary binding activity localized in the Fab region of certain species and classes of Ig, and this has been reported to be the sole type of activity associated with polyclonal human IgE.

Protein A of Staphylococcus aureus and related immunoglobulin receptors produced by streptococci and pneumonococci

Molecular and Chemotaxonomic Approaches to the Classification of Streptococci, Enterococci and Lactococci: A Review

Specific binding of the 125I-labeled human S protein (vitronectin) which has been shown to be identical with serum-spreading factor, was observed with group A, C, and G streptococci as well as with Staphylococcus aureus and Escherichia coli. The specific binding of S protein to group A, C, and G streptococci was high, whereas the binding to S. aureus and E. coli cultures was moderate. In contrast, group B streptococci and a number of other bacterial species tested did not interact with S protein. The binding of S protein to bacteria was saturable and could be inhibited only by unlabeled S protein but not by albumin. Trypsinization and heat treatment of bacteria destroyed the S-protein binding capacity for group G streptococci, S. aureus, and E. coli but not for group A and C streptococci. Likewise, unlabeled human fibronectin and heparin inhibited the binding of labeled S protein to group G streptococci, S. aureus, and E. coli, but did not influence the binding to group A and C streptococci. Double-reciprocal plots of S-protein binding to group G streptococci indicated that fibronectin inhibited the binding in a competitive manner, while heparin acts in a noncompetitive manner. Moreover, the binding of S protein to G streptococci could be partially by the synthetic peptide Gly-Arg-Gly-Asp-Ser, which contains the cell attachment site of S protein. Trypsin-treated S protein had similar binding activity as untreated S protein for group G streptococci, S. aureus, and E. coli, but showed reduced binding to group A and C streptococci. The present data are indicative of two different types of bacterial binding sites in S protein. The binding to group G streptococci, S. aureus, and E. coli is mediated in part through a domain in the S protein containing the sequence Arg-Gly-Asp, whereas a different site is responsible for the binding to group A and C streptococci.

https://iai.asm.org/content/iai/55/8/1878.full.pdf

Specific binding of the human S protein (vitronectin) to streptococci, Staphylococcus aureus, and Escherichia coli.

A total of 297 bacterial strains belonging to 27 species was tested for quantitative uptake of radiolabeled human serum albumin. Specific binding sites with high affinity for human serum albumin were found exclusively in group C and G streptococci. The albumin binding was found to be a time-dependent, saturable, and displaceable process which obeyed simple kinetic equations. Scatchard analysis revealed that human serum albumin bound to a homogeneous population of receptors with an affinity in the order ot 10(7) liters/mol and that the average bacterial cell carried more than 80,000 binding sites. The albumin receptor is a heat-stable component susceptible to proteolytic digestion. It has a surface localization separate from the receptors for immunolgobulin G, fibrinogen, aggregated beta 2-microglobulin, and haptoglobin. In individual strains, albumin reactivity was also detected independently of these other types of interactions with human proteins.

https://iai.asm.org/content/iai/27/1/6.full.pdf

G Kronvall

Papers

Demonstration of specific binding sites for human serum albumin in group C and G streptococci.