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Protein L, a bacterial immunoglobulin-binding protein and possible virulence determinant

Protein L, an immunoglobulin kappa light chain-binding protein, is expressed on the surfaces of certain strains of Peptostreptococcus magnus. Thirty strains of P. magnus were isolated from clinical specimens, and four of them were found to express protein L. Among the 30 strains, 7 were isolated from the vaginas of patients with bacterial vaginosis, and the 4 immunoglobulin-binding strains all belonged to this group, results demonstrating that expression of protein L is correlated to peptostreptococcal virulence (P less than 0.001 in the chi-square test) and indicating that the molecule could be a virulence determinant. Similar amounts of protein L were expressed by the four strains, and when protein L was isolated from three of them and analyzed in Western blots, the same immunoglobulin-binding patterns were obtained. The N-terminal amino acid sequences of tryptic fragments of protein L were determined, and on the basis of these sequence data, oligonucleotides were synthesized and used to screen a genomic library of peptostreptococcal DNA in the lambda ZAP vector. The nucleotide sequence was determined for one of the clones detected in this way. In dot blots and Southern blots of peptostreptococcal DNA, another synthetic oligonucleotide probe based on this sequence showed no hybridization with DNA samples from the nonexpressing strains, whereas similar Southern blot patterns were seen when DNA samples from protein L-expressing strains were analyzed. These results suggest that the protein L gene is missing rather than being down regulated in protein L-negative strains of P. magnus. Finally, the probe did not hybridize with DNA purified from immunoglobulin-binding streptococcal and staphylococcal strains or with Escherichia coli DNA, suggesting that the protein L gene is unique to protein L-expressing strains of P. magnus.