Production of enterolysin A by a raw milk enterococcal isolate exhibiting multiple virulence factors

Even though enterococci are a common cause of human infection they can readily be isolated from a range of food sources, including various meat and dairy products. An enterococcal strain, DPC5280, which exhibits a broad spectrum of inhibition against many Gram-positive bacteria was recently isolated from an Irish raw milk sample. Characterization of the inhibition revealed that the strain exhibits haemolytic activity characteristic of the two-component lantibiotic cytolysin and also produces a heat-labile antimicrobial protein of 34 kDa. The latter protein displayed cell wall hydrolytic activity, as evidenced by zymogram gels containing autoclaved lactococcal cells. N-terminal sequencing of the purified protein yielded the sequence ASNEWS which is 100 % identical to enterolysin A (accession no. AF249740), a protein which shares 28 and 29 % identity to the Gly-Gly endopeptidases, lysostaphin and zoocin A, respectively. Indeed, amplification of entL from DPC5280 and sequencing revealed that the protein is 100 % identical to enterolysin A. The DPC5280 strain also contained the determinants associated with multiple virulence factors, including gelatinase, aggregation substance and multiple antibiotic resistance. The linkage of this cell-wall-degrading enzyme to other virulence factors in enterococci may contribute to the competitiveness of pathogenic enterococci when found in complex microbial environments such as food and the gastrointestinal tract.