Effects of recovery, plating, and inoculation methods on quantification of Escherichia coli O157:H7 and Listeria monocytogenes from strawberries

Abstract

Effects of different recovery and inoculation methods on quantification of Escherichia coli O157:H7 and Listeria monocytogenes from strawberries were studied. Strawberries were spot or dip inoculated with 7 to 8 log CFU per strawberry of each pathogen, air dried for 2 h, and stored for 1, 3, and 7 days at 4°C. The inoculated samples were stomached or washed with phosphate-buffered saline (PBS; pH 7.2) or with modified PBS (pH 8.4). Bacterial levels were determined using a direct selective plating, thin agar layer plating, or membrane-transferring plating (MTP) with tryptic soy agar and sorbital MacConkey agar (E. coli O157:H7) or modified Oxford agar (L. monocytogenes). Under most test conditions, washing with PBS followed by MTP had significantly higher (P < 0.05) recovery for both bacteria compared with other tested methods. Within a 7-day storage period for spot-inoculated strawberries, a stomaching step resulted in an injury of 0.9 to 1.4 log CFU for E. coli O157: H7 and 1.4 to 1.7 log CFU for L. monocytogenes. When a washing step was used instead, this resulted in an injury of only 0.2 to 0.6 log CFU for E. coli O157:H7 and 0.2 to 0.7 log CFU for L. monocytogenes. Both bacteria could survive on strawberry surfaces, but their recovered levels decreased with the increase of storage time at 4°C for both spot and dip inoculation methods. Dip inoculation generally had a lower recovery than spot inoculation. An ideal protocol to recover and enumerate E. coli O157:H7 and L. monocytogenes from strawberries involved shaking and washing samples with 100 ml of PBS for 15 min at 22°C coupled with a MTP enumeration method.