Plasmid-specified FemABX-like immunity factor in Staphylococcus sciuri DD 4747

Characterization of a novel family of fibronectin-binding proteins with M23 peptidase domains from Treponema denticola

Interactions with fibronectin are important in the virulence strategies of a range of disease-related bacteria. The periodontitis-associated oral spirochaete Treponema denticola expresses at least two fibronectin-binding proteins, designated Msp (major surface protein) and OppA (oligopeptide-binding protein homologue). To identify other T. denticola outer membrane fibronectin-binding proteins, the amino acid sequence of the Treponema pallidum fibronectin-binding protein Tp0155 was used to survey the T. denticola genome. Seven T. denticola genes encoding orthologous proteins were identified. All but two were expressed in Escherichia coli and purified recombinant proteins bound fibronectin. Using antibodies to the N-terminal region of Tp0155, it was demonstrated that T. denticola TDE2318, with highest homology to Tp0155, was cell surface localized. Like Tp0155, the seven T. denticola proteins contained an M23 peptidase domain and four (TDE2318, TDE2753, TDE1738, TDE1297) contained one or two LysM domains. M23 peptidases can degrade peptidoglycan whereas LysM domains recognize carbohydrate polymers. In addition, TDE1738 may act as a bacteriocin based on homology with other bacterial lysins and the presence of an adjacent gene encoding a putative immunity factor. Collectively, these results suggest that T. denticola expresses fibronectin-binding proteins associated with the cell surface that may also have cell wall modifying or lytic functions.

Zif, the zoocin A immunity factor, is a FemABX-like immunity protein with a novel mode of action

Producer cell immunity to the streptococcolytic enzyme zoocin A, which is a D-alanyl-L-alanine endopeptidase, is due to Zif, the zoocin A immunity factor. Zif has high degrees of similarity to MurM and MurN (members of the FemABX family of proteins), which are responsible for the addition of amino acids to cross bridges during peptidoglycan synthesis in streptococci. In this study, purified peptidoglycans from strains with and without zif were compared to determine how Zif modifies the peptidoglycan layer to cause resistance to zoocin A. The peptidoglycan from each strain was hydrolyzed using the streptococcolytic phage lysin B30, and the resulting muropeptides were separated by reverse-phase high-pressure liquid chromatography, labeled with 4-sulfophenyl isothiocyanate, and analyzed by tandem mass spectrometry in the negative-ion mode. It was determined that Zif alters the peptidoglycan by increasing the proportion of cross bridges containing three L-alanines instead of two. This modification decreased binding of the recombinant target recognition domain of zoocin A to peptidoglycan. Zif-modified peptidoglycan also was less susceptible to hydrolysis by the recombinant catalytic domain of zoocin A. Thus, Zif is a novel FemABX-like immunity factor because it provides resistance to a bacteriolytic endopeptidase by lengthening the peptidoglycan cross bridge rather than by causing an amino acid substitution.

Design of a polypeptide FRET substrate that facilitates study of the antimicrobial protease lysostaphin

We have developed a polypeptide lysostaphin FRET (fluorescence resonance energy transfer) substrate (MV11F) for the endopeptidase activity of lysostaphin. Site-directed mutants of lysostaphin that abolished the killing activity against Staphylococcus aureus also completely inhibited the endopeptidase activity against the MV11 FRET substrate. Lysostaphin-producing staphylococci are resistant to killing by lysostaphin through incorporation of serine residues at positions 3 and 5 of the pentaglycine cross-bridge in their cell walls. The MV11 FRET substrate was engineered to introduce a serine residue at each of four positions of the pentaglycine target site and it was found that only a serine residue at position 3 completely inhibited cleavage. The introduction of random, natural amino acid substitutions at position 3 of the pentaglycine target site demonstrated that only a glycine residue at this position was compatible with lysostaphin cleavage of the MV11 FRET substrate. A second series of polypeptide substrates (decoys) was developed with the GFP (green fluorescent protein) domain of MV11 replaced with that of the DNase domain of colicin E9. Using a competition FRET assay, the lysostaphin endopeptidase was shown to bind to a decoy peptide containing a GGSGG cleavage site. The MV11 substrate provides a valuable system to facilitate structure/function studies of the endopeptidase activity of lysostaphin and its orthologues.

Lysostaphin-resistant variants of Staphylococcus aureus demonstrate reduced fitness in vitro and in vivo

Lysostaphin is under development as a therapy for serious staphylococcal infections. During preclinical development, lysostaphin-resistant Staphylococcus aureus variants have occasionally been reported in vitro and in vivo. The acquisition of resistance to this drug, however, leads to a significant increase in beta-lactam antibiotic susceptibility, rendering methicillin-resistant S. aureus (MRSA) strains functionally methicillin susceptible. In this study, we have demonstrated that the development of lysostaphin resistance by two strains of MRSA also led to a loss of fitness in the variants. Consistent with the mutations found in previously reported lysostaphin-resistant S. aureus variants, these two variants had mutations in their femA genes, resulting in nonfunctional FemA proteins and, thus, monoglycine cross bridges in the peptidoglycan. The diminished fitness of the lysostaphin-resistant variants was reflected by (i) a reduced logarithmic growth rate, with the variants being outcompeted in cocultures by their wild-type parental strains; (ii) increased susceptibility to elevated temperatures; and (iii) at least fivefold less virulence of the lysostaphin-resistant variants than their wild-type strains in a mouse kidney infection model, with the lysostaphin-resistant variants being outcompeted in coinfections with their wild-type parental strains. During a 14-day serial passage without selective pressure, the lysostaphin-resistant variants failed to develop compensatory mutations which restored their fitness. These results suggest that should lysostaphin resistance due to an alteration in the FemA function emerge in S. aureus during therapy with lysostaphin, the resistant variants would be less fit and less virulent, and, in addition, infections with these strains would be easily treatable with beta-lactam antibiotics.