Impact of persistent and nonpersistent generic Escherichia coli and Salmonella sp. recovered from a beef packing plant on biofilm formation by E. coli O157.

AIMS: To examine the influence of meat plant Escherichia coli and Salmonella sp. isolates on E. coli O157 biofilm formation. METHODS AND RESULTS: Biofilm formation was quantified by crystal violet staining (A570 nm ) and viable cell numbers for up to 6 days at 15°C. All five persistent E. coli genotypes formed strong biofilms when cultured alone or co-cultured with E. coli O157, with A570 nm values reaching ≥4·8 at day 4, while only two of five nonpersistent genotypes formed such biofilms. For E. coli O157:H7 co-culture biofilms with E. coli genotypes 136 and 533, its numbers were ≥1·5 and ≥1 log CFU per peg lower than those observed for its mono-culture biofilm at days 2 and 4, respectively. The number of E. coli O157:NM in similar co-culture biofilms was 1 log CFU per peg lower than in its mono-culture biofilm at day 4 and 6, respectively. Salmonella sp. lowered the number of E. coli O157:NM by 0·5 log unit, once, at day 6. CONCLUSION: Generic E. coli may outcompete E. coli O157 strains while establishing biofilms. SIGNIFICANCE AND IMPACT OF THE STUDY: Findings advance knowledge regarding inter-strain competition for a similar ecological niche and may aid development of biocontrol strategies for E. coli O157 in food processing environments.

Evidence and characterization of a gene cluster required for the production of viscosin, a lipopeptide biosurfactant, by a strain of Pseudomonas fluorescens.

The genetic control of viscosin production was examined in a strain of Pseudomonas fluorescens (PfA7B) that causes broccoli head rot. Viscosin is a potent lipopeptide biosurfactant that enables the bacteria to come into intimate contact with the difficult-to-wet waxy heads of broccoli. Tn5 mutagenesis completely disrupted viscosin production as shown by HPLC analysis of the mutagenized cell lysates. The Vis- mutants retained their pectolytic capability and were able to decay potato slices. On broccoli, however, the Vis- mutants caused decay of wounded florets, but the decay failed to spread to adjacent nonwounded florets as had occurred with the wild-type PfA7B. Triparental matings of the Vis- mutants with their corresponding wild-type clones and the helper Escherichia coli HB101 carrying the mobilization plasmid pPK2013 resulted in three stable viscosin-producing transconjugants that caused typical decay of broccoli tissue. Linkage maps of clones and protein profiles showed that a 25-kb chromosomal DNA region of PfA7B affected the production of three high molecular mass proteins required for viscosin synthesis. These proteins, approximately 218, 215, and 137 kDa in size, likely compose a synthetase complex that assembles the nine amino acid peptide of viscosin and subsequently attaches this to the hydrophobic fatty acid component of the molecule. A probe made from this DNA region hybridized with DNA fragments of other phytopathogenic pseudomonads to varying degrees.