Refrigeration has been found to cause cell elongation in mesophilic enteric organisms like commensal Escherichia coli and E. coli O157:H7. As elongated cells may divide into multiple daughter cells, they may result in underestimation of pathogen numbers in foods when plate counts are used. When E. coli cultures were incubated at 6°C for ≤10 days, cells grew by elongation, did not divide, and lost viability (LIVE/DEAD vitality stain) at similar rates. Substantial fractions of cells in cultures elongating at 6°C were inactivated by an abrupt shift to 37°C. Direct microscopic observation of cells transferred to 37ºC after 5 days at 6°C showed that few or no cells of normal size (≤4µm) divided, while elongated cells (>4 µm) formed multiple daughter cells. Thus the threat from mesophilic pathogens with a low infective dose may be underestimated in refrigerated foods. It was also found that E. coli O157:H7 cultures containing elongated cells prepared at 6 or 15 °C have greater potential to attach to food contact surfaces than those grown at higher temperatures. Interestingly, at 6°C cell elongation was inhibited by ≥ 100 mg/l cinnamaldehyde and ≥ 200 mg/l cinnamaldehyde was lethal. In contrast, at 37°C 200 mg/l cinnamaldehyde initially delayed multiplication of E. coli cells by causing cell elongation, but from 2 to 4 h, growth resumed and cells reverted to normal length. To understand this transient behaviour, genome-wide transcriptional analysis of E. coli O157:H7 was performed at 2 and 4 h exposure to cinnamaldehyde in conjunction with reverse phase-high performance liquid chromatography analysis for cinnamaldehyde and other cinnamic compounds. At 2 h exposure, cinnamaldehyde induced expression of many oxidative stress-related genes, reduced expression of genes involved in DNA replication, synthesis of protein, O-antigen and fimbriae. At 4 h many repressive effects of cinnamaldehyde on E. coli O157:H7 gene expression were reversed. Data indicated that by 4 h, E. coli O157:H7 was able to convert cinnamaldehyde into the less toxic cinnamic alcohol using alcohol dehydrogenase or aldehyde reductase enzymes (YqhD and DkgA). The results also showed that the antimicrobial activity of cinnamaldehyde was likely attributable to its carbonyl aldehyde group.

Refrigeration has been found to cause cell elongation in mesophilic enteric organisms like commensal Escherichia coli and E. coli O157:H7. As elongated cells may divide into multiple daughter cells, they may result in underestimation of pathogen numbers in foods when plate counts are used. When E. coli cultures were incubated at 6°C for ≤10 days, cells grew by elongation, did not divide, and lost viability (LIVE/DEAD vitality stain) at similar rates. Substantial fractions of cells in cultures elongating at 6°C were inactivated by an abrupt shift to 37°C. Direct microscopic observation of cells transferred to 37ºC after 5 days at 6°C showed that few or no cells of normal size (≤4µm) divided, while elongated cells (>4 µm) formed multiple daughter cells. Thus the threat from mesophilic pathogens with a low infective dose may be underestimated in refrigerated foods. It was also found that E. coli O157:H7 cultures containing elongated cells prepared at 6 or 15 °C have greater potential to attach to food contact surfaces than those grown at higher temperatures. Interestingly, at 6°C cell elongation was inhibited by ≥ 100 mg/l cinnamaldehyde and ≥ 200 mg/l cinnamaldehyde was lethal. In contrast, at 37°C 200 mg/l cinnamaldehyde initially delayed multiplication of E. coli cells by causing cell elongation, but from 2 to 4 h, growth resumed and cells reverted to normal length. To understand this transient behaviour, genome-wide transcriptional analysis of E. coli O157:H7 was performed at 2 and 4 h exposure to cinnamaldehyde in conjunction with reverse phase-high performance liquid chromatography analysis for cinnamaldehyde and other cinnamic compounds. At 2 h exposure, cinnamaldehyde induced expression of many oxidative stress-related genes, reduced expression of genes involved in DNA replication, synthesis of protein, O-antigen and fimbriae. At 4 h many repressive effects of cinnamaldehyde on E. coli O157:H7 gene expression were reversed. Data indicated that by 4 h, E. coli O157:H7 was able to convert cinnamaldehyde into the less toxic cinnamic alcohol using alcohol dehydrogenase or aldehyde reductase enzymes (YqhD and DkgA). The results also showed that the antimicrobial activity of cinnamaldehyde was likely attributable to its carbonyl aldehyde group.