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

Abstract

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. INTRODUCTIONDespite the fact that foods containing enterococci have a long history of safe use, these organisms are not considered as GRAS (generally recognized as safe) organisms. They can readily be isolated from a range of food sources, including raw milk, and are often a constituent of some mixed starter strains used commercially. However, many strains can act as opportunistic pathogens, causing a variety of infections such as urinary tract infections, bacteremia and infective endocarditis (Murray, 1990;Jett et al., 1994), and are of major importance in community-acquired and in hospital-acquired (nosocomial) infections (Jett et al., 1994;Low et al., 1994;Jones et al., 1997;Simjee & Gill, 1997). A contributing factor to their pathogenesis is their evolving resistance to antibiotics. For example, resistance to the antibiotic vancomycin is now widespread among members of the genus, which leaves few options for disease management (Aguirre & Collins, 1993;Knudtson & Hartman, 1993;Facklam & Sahm, 1995;Klein et al., 1998). Considerable progress has recently been made in determining the traits responsible for pathogenesis of enterococci.For example, studies have shown that phenotypes such as production of cytolysin, which has both haemolytic activity (by lysing a broad spectrum of cells, including human, horse and rabbit erythrocytes) and bactericidal activity (against Gram-positive bacteria) play a role in the progression of enterococcal infection (Ike et al., 1984;Jett et al., 1992). Cytolysin enhances the virulence of Enterococcus faecalis in animal models, such as muri...

Controlled Autolysis and Enzyme Release in a Recombinant Lactococcal Strain Expressing the Metalloendopeptidase Enterolysin A

“…In another example, de Ruyter et al (4) used the nisin-inducible system (3) to clone the holin and lysin genes of phage US3 into Lactococcus lactis under the control of the nisin-inducible promoter. The addition of nisin resulted in cell lysis in this case, as evidenced by a fourfold increase in L-lactate dehydrogenase (LDH) release into the curd relative to the results seen with control strains.The enterococcal metalloendopeptidase enterolysin A (EntL) exhibits cell wall-degrading activity and was characterized in two separate studies (6,12). One of the producing strains, E. faecalis DPC5280, was also found to produce the lantibiotic cytolysin and displayed a broad spectrum of inhibition which can be attributed to production of both antimicrobials (enterolysin and cytolysin).…”

“…The enterococcal metalloendopeptidase enterolysin A (EntL) exhibits cell wall-degrading activity and was characterized in two separate studies (6,12). One of the producing strains, E. faecalis DPC5280, was also found to produce the lantibiotic cytolysin and displayed a broad spectrum of inhibition which can be attributed to production of both antimicrobials (enterolysin and cytolysin).…”

Partial Diversity Generates Effector Immunity Specificity of the Bac41-Like Bacteriocins of Enterococcus faecalis Clinical Strains

“…In contrast, class III bacteriocins, which are also referred to as bacteriolysins, are heat-labile antimicrobial proteins showing enzymatic bactericidal activity (7,19,20). To date, the only enterococcal bacteriolysins to be identified are enterolysin A and bacteriocin 41 (Bac41) (21)(22)(23).…”

Bacteriocin Protein BacL1of Enterococcus faecalis Targets Cell Division Loci and Specifically Recognizes l-Ala2-Cross-Bridged Peptidoglycan

“…Unlike the low-molecular-weight peptide-type class I and II bacteriocins, heat-labile antimicrobial proteins are referred to as bacteriolysins, previously named class III bacteriocins, and show enzymatic bactericidal activity (22,23). In enterococci, the bacteriolysins enterolysin A and bacteriocin 41 (Bac41) have been identified (24)(25)(26).…”