Fate of Escherichia coli O157:H7 and Salmonella spp. on Bruised and Intact Strawberries

Introduction:  Strawberries are harvested at or near full ripe maturity for superior eating quality. These fruit tend to be less firm and are more susceptible to bruising during harvest and transport, which may increase risks of foodborne pathogen proliferation.

Purpose:  The objective of this research was to quantify the fate of Escherichia coli O157:H7 and Salmonella on bruised and intact strawberries at shipping (2°C) and retail display (15.5°C) temperatures.

Methods:  Strawberries were bruised by dropping a 32.6 g steel ball into a 23 cm PVC pipe directly onto a whole strawberry.  Twenty microliters of a five-strain cocktail of rifampicin-resistant E. coli O157:H7 or Salmonella, at a concentration of ca. 10⁶ CFU/ml, was spot inoculated onto either bruised or nonbruised portions of the strawberry and dried for 1 h.  Strawberries, stored at 2°C and 15.5°C, were sampled at 0, 2, 5, and 24 h and days 0, 1, 3, and 7, respectively.  Pathogen populations were enumerated on selective and non-selective media supplemented with rifampicin.  

Results:  Populations of E. coli O157:H7 and Salmonella decreased under all experimental conditions.  At 2 °C, E. coli O157:H7 populations decreased by 1 and 0.7 log CFU/berry over 24 h on bruised and intact strawberries, respectively. Salmonella populations decreased by 1.3 and 1.5 log CFU/berry over 24 h on bruised and intact strawberries, respectively at 2 °C. At 15.5 °C, E. coli O157:H7 and Salmonella showed similar trends over 7 days, where both populations decreased by >2.3 and >2 log CFU/berry on bruised strawberries and >1.9 and >1.6 log CFU/berry on intact strawberries, respectively.

Significance:  Bruising did not significantly affect the fate of E. coli O157:H7 or Salmonella populations on mature strawberries. The current practice of harvesting strawberries at full ripe maturity, whether bruising occurs or not, does not impose any additional food safety risks.