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Keywords:

  • antimicrobials;
  • cell elongation;
  • cell injury;
  • cinnamaldehyde;
  • Escherichia coli ;
  • inactivation

Abstract

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.