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.

Temperature-dependent effect of sublethal levels of cinnamaldehyde on viability and morphology of Escherichia coli.

AIMS: Effects of sublethal levels of cinnamaldehyde (CIN) on the viability and morphology of Escherichia coli O157:H7 and E. coli 8 WT were investigated at 6 and 37°C. METHODS AND RESULTS: The minimum inhibitory concentration of CIN against E. coli O157:H7 and E. coli 8WT was 400 mg l(-1). At 37°C and ≤300 mg l(-1), CIN delayed the multiplication of both strains, causing a ≤5 and ≤13 h lag, respectively. Delayed multiplication at ≤300 mg l(-1) was partly due to cell elongation and injury as determined by LIVE/DEAD viability, CTC vitality and bis-(1,3-dibutylbarbituric acid) trimethine oxonol staining. The greatest extent of cell elongation (87%) and greatest mean length (6.4 µm) occurred with E. coli O157:H7 at 2-h exposure to 200 mg l(-1) CIN. After initial delays in multiplication, both E. coli O157:H7 and E. coli 8WT returned to exponential growth and normal morphology before reaching the stationary phase. In contrast at 6°C, CIN at ≥100 mg l(-1) prevented cell elongation which occurred in untreated control cells. Treatment with 200 or 300 mg l(-1) CIN at 6°C was lethal to both E. coli strains. At 300 mg l(-1) , CIN caused a ≥5 log CFU ml(-1) reduction at ≤3 days and completely inactivated both of these organisms, causing ≥7 log CFU ml(-1) reduction at 7 days. CONCLUSION: Sublethal levels of CIN at 37°C delayed the multiplication of E. coli cells by causing transient cell elongation, but at 6°C ≥200 mg l(-1) CIN was lethal to E. coli. SIGNIFICANCE AND IMPACT OF THE STUDY: Inhibition of cold-induced cell elongation and the enhanced lethal effect of CIN at 6°C against E. coli O157:H7 suggest that CIN may be useful for control of this pathogen at refrigeration temperatures.