Genetic studies of Staphylococcus aureus virulence factors

Crystal structure of a Staphylococcus aureus protein A domain complexed with the Fab fragment of a human IgM antibody: structural basis for recognition of B-cell receptors and superantigen activity

Staphylococcus aureus produces a virulence factor, protein A (SpA), that contains five homologous Ig-binding domains. The interactions of SpA with the Fab region of membrane-anchored Igs can stimulate a large fraction of B cells, contributing to lymphocyte clonal selection. To understand the molecular basis for this activity, we have solved the crystal structure of the complex between domain D of SpA and the Fab fragment of a human IgM antibody to 2.7-A resolution. In the complex, helices II and III of domain D interact with the variable region of the Fab heavy chain (V(H)) through framework residues, without the involvement of the hypervariable regions implicated in antigen recognition. The contact residues are highly conserved in human V(H)3 antibodies but not in other families. The contact residues from domain D also are conserved among all SpA Ig-binding domains, suggesting that each could bind in a similar manner. Features of this interaction parallel those reported for staphylococcal enterotoxins that are superantigens for many T cells. The structural homology between Ig V(H) regions and the T-cell receptor V(beta) regions facilitates their comparison, and both types of interactions involve lymphocyte receptor surface remote from the antigen binding site. However, T-cell superantigens reportedly interact through hydrogen bonds with T-cell receptor V(beta) backbone atoms in a primary sequence-independent manner, whereas SpA relies on a sequence-restricted conformational binding with residue side chains, suggesting that this common bacterial pathogen has adopted distinct molecular recognition strategies for affecting large sets of B and T lymphocytes.

Cell cycle regulation of mammary epithelial cell detachment by Staphylococcus aureus

The effect of Staphylococcus aureus on detachment of bovine mammary epithelial cells in culture was examined. Mammary epithelial cells became detached from fresh monolayers following a 3 h incubation in the presence of Staph. aureus M60. Two different procedures indicated that cell detachment coincided with the S-phase of the cell cycle. The roles of proteinases, toxins and Ca availability in inducing cell detachment were examined. Addition of the proteinase inhibitor phenylmethylsulphonyl fluoride (1 mM) to the culture medium prevented cell detachment. Addition of a combination of purified staphylococcal proteinases XVI and XVII-B to the culture medium of mammary epithelial cells induced cell detachment in the absence of Staph. aureus. Cell detachment may be caused by a staphylococcal proteinase. However, addition of Ca (10 mM) to the culture medium abolished Staph. aureus-induced cell detachment, despite the fact that proteinase activity was still apparently present. Isogenic mutants of Staph. aureus M60, expressing either alpha or beta toxins but not both, induced cell detachment, but to a lesser extent than the wild type. Thus, Ca and toxins play some role during cell detachment. Clones established from detached cells that were washed and replated showed the same susceptibility to Staph. aureus-induced cell detachment as the parental cells. This indicated that there is no subclone of mammary epithelial cells more sensitive to this effect.

Effect of staphylococcal beta toxin on the cytotoxicity, proliferation and adherence of Staphylococcus aureus to bovine mammary epithelial cells

The effect of staphylococcal beta toxin on the cytotoxicity, proliferation and adherence of S. aureus. to bovine mammary epithelial cells was studied. Bovine erythrocytes and mammary epithelial cells were incubated with purified staphylococcal alpha and beta toxins and with culture supernatants from S. aureus M60 and two mutant strain that are negative for either the production of alpha (DU5789 alpha-) or beta (DU5846 beta-) toxin. Lysis of bovine erythrocytes was due primarily to beta toxin. Alpha toxin increased the lysis of bovine erythrocytes by purified beta toxin, but the presence of alpha toxin in culture supernatants from S. aureus did not increase the lysis of bovine erythrocytes. Purified beta toxin was cytotoxic to mammary secretory epithelial cells, but to a lesser extent than alpha toxin. Together they exhibited an additive effect on mammary epithelial cells. Inactivation of the alpha toxin-gene of S. aureus M60 decreased the cytotoxic effect on mammary epithelial cells to a greater extent than the inactivation of the beta toxin-gene. Also, the relative percentages of DU5789 alpha- and DU5846 beta- adhering to mammary cell monolayers, the number and size of colonies and the number of infected epithelial cells decreased. This in vitro study showed that beta toxin damages bovine mammary secretory epithelial cells, increased the damaging effects of alpha toxin, increases the adherence of S. aureus to mammary epithelial cells and increases the proliferation of S. aureus.

Effects of Staphylococcus aureus toxins on the growth of bovine mammary epithelial cells (MAC-T) in culture

The effects of Staphylococcus aureus M60 culture supernatant on growth of mammary epithelial cells were tested in vitro. Exposure of MAC-T cells to S. aureus culture supernatant reduced cell proliferation and colony-forming ability because of reduced ability to adhere to plastic. Growth-inhibiting effects of S. aureus culture supernatant were abolished by pretreatment with trypsin. Lysis of S. aureus cells with lysostaphin demonstrated that the inhibition was also present in cell lysates. Treatment of MAC-T cells with culture supernatant from isogeneic mutants of S. aureus M60 that produced either alpha or beta toxins implicated alpha toxin as the main factor inhibiting cell proliferation.

Proliferation of a bovine mammary epithelial cell line in the presence of bacterial virulence factors

Effects of bacterial virulence factors on bovine mammary cell structure and function are not well defined. In this study, we evaluated the influence of specific bacterial virulence factors on proliferation of a bovine mammary epithelial cell line. The MAC-T cells were cultured in the presence of medium only, Staphylococcus aureus alpha-toxin, Staph. aureus beta-toxin, Escherichia coli endotoxin, Streptococcus uberis capsule, or hyaluronidase. Cells were cultured in the presence of virulence factors for 48 h at 37 degrees C. The MAC-T cell proliferation was inhibited by all concentrations of endotoxin and alpha-toxin and by most concentrations of hyaluronic acid capsule and hyaluronidase > 7.8 micrograms/ml. Staphylococcus aureus beta-toxin had no effect on MAC-T cell proliferation. Virulence factors produced by mastitis pathogens may influence mammary epithelial cell proliferation in vivo, which could be important during the periparturient period, when mammary tissue undergoes rapid differentiation and growth.

The continuing importance of staphylococci as major hospital pathogens

Rates of hospital-acquired staphylococcal infection have risen substantially in the United States over the last decade. Moreover, it has been shown that bloodstream infections caused by these organisms account for significant morbidity and mortality. It is likely that the changes in medical practice and in types of patient account for these changes, and current antibiotic therapy has helped select more resistant organisms. The increasing use of implantable and temporary medical devices and the increased use of immunosuppressive therapies correlate strongly with the rising incidence of these organisms. Likewise, more patients are predisposed to these infections because of poor immune status and factors favouring colonization. Intrinsic microbiological factors such as glycocalyx production and the presence of bacterial surface proteins allow these organisms to adhere to protein coated foreign bodies and basement membranes thus enabling them to initiate infection and cause disease. Moreover, virulence factors such as encapsulation, slime production and elaboration of extracellular enzymes aid their resistance to host defences. As a group, staphylococci are a leading cause of hospital-acquired infection, and species identification is required for both treatment and control.