Temperature‐dependent effect of sublethal levels of cinnamaldehyde on viability and morphology of E scherichia coli

Aims Effects of sublethal levels of cinnamaldehyde ( CIN ) on the viability and morphology of E scherichia coli O 157: H 7 and E. coli 8 WT were investigated at 6 and 37°C. Methods and Results The minimum inhibitory concentration of CIN against E. coli O 157:H7 and E . coli 8 WT 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 O 157: H 7 at 2‐h exposure to 200 mg l −1 CIN . After initial delays in multiplication, both E . coli O 157: H 7 and E . coli 8 WT 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 O 157: H 7 suggest that CIN may be useful for control of this pathogen at refrigeration temperatures.