Antimicrobial substances produced by Staphylococcus aureus strains isolated from cattle in Brazil.

Among 46 isolates of Staphylococcus aureus obtained from cattle in the State of Paraíba, Brazil, four were shown to produce antimicrobial substances (AMS). The two best AMS producers carried single plasmids of about 8.0 kbp and 50 kbp, respectively, which were designated pRJ34 and pRJ35. Curing experiments and molecular analysis associated the AMS production with the presence of these plasmids in the cells. The biochemical properties exhibited by the AMS suggested that they might be bacteriocins (Bac). The bacteriocin encoded by pRJ34 showed properties identical to those of the bacteriocins encoded by other small staphylococcal Bac plasmids. However, the bacteriocin encoded by the large plasmid pRJ35 has shown some properties which distinguish it from the other bacteriocins of Staph. aureus described so far, suggesting it may be a new member of the staphylococcal bacteriocin family.

Genetic analysis of the bacteriocin-encoding plasmids pRJ6 and pRJ9 of Staphylococcus aureus by transposon mutagenesis and cloning of genes involved in bacteriocin production.

pRJ6 and pRJ9, small Staphylococcus aureus plasmids which code for bacteriocins, exhibited a bactericidal activity against several lactic acid bacteria and strains of Listeria monocytogenes, an important food-borne pathogen. Filter-mating experiments using plasmid derivatives tagged with either Tn551 or Tn917-lac showed that pRJ6, but not pRJ9, could be mobilized by staphylococcal conjugative plasmids. Transposon mutagenesis of both plasmids was also performed. The bacteriocin and immunity structural genes of pRJ6 are part of the same operon, which is located around co-ordinate 4.0, being transcribed from right to left. However, gene cloning experiments using a staphylococcal vector showed some evidence for the involvement of additional functions of pRJ6 in bacteriocin expression. One function involved in pRJ6 mobilization mapped around co-ordinate 5.2, and it appears to be transcribed from left to right. The bactericidal action exerted by strains harbouring pRJ9 appears to reflect the activity of at least two bacteriocins, whose combined action results in a broader spectrum of activity and in a higher antagonistic activity. Gene cloning experiments also supported these assumptions.