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.