15D-5 |
Methods to recover Escherichia coli O157:H7 and Listeria monocytogenes on strawberries |
Y. HAN and R. H. Linton. Dept. of Food Science, Purdue Univ., 1160 Food Science Bldg., West Lafayette, IN 47907-1160 Better methods to recover healthy or injured pathogens on strawberries are needed. Recovery of pathogens on strawberries can present a challenge due to low pH (3.6) and a susceptible surface. Our objectives were to develop methods to recover inoculated E. coli O157:H7 and L. monocytogenes on strawberries and to study inhibition of L. monocytogenes by acids in strawberries. Strawberries were spot-inoculated with 8 log cfu/strawberry of each pathogen, then stored for 24 hours at 4 oC. The samples were stomached or washed with phosphate buffer saline (PBS, pH 7.2) or with modified PBS (MPBS, pH 8.4). Recovery was determined using a direct surface-plating (SP) with a selective medium, thin-layer plating (TLP), or membrane-transferring plating (MTP) with tryptic soy agar and sorbital MacConkey agar (E. coli O157:H7) or Modified Oxford agar (L. monocytogenes). A spectrophotometer (660nm) was used to study growth inhibition of L. monocytogenes in tryptic soy broth (TSB) at the different pH (3.4-7.3) levels achieved by addition of citric and malic acids or 10% or 50% strawberry juices. There was no significant (P>0.05) difference between recovery of E. coli O157:H7 in the samples stomached or washed with PBS, determined with the SP, TLP, or MTP methods, or between the populations recovered just after inoculation and after one-day storage. Significant differences (P<0.05) in recovery of L. monocytogenes in the samples treated in the same way as E. coli O157:H7 suggested that L. monocytogenes were injured or inactivated. 10g/l citric acid plus 5 g/l malic acids and 50% strawberry juice completely inhibited the growth of 2.7 logcfu/ml L. monocytogenes in TSB. Washing with PBS showed a highest recovery (>80%) for both bacteria and the MTP was the best method to resuscitate stressed bacteria. Development of the methods to recover pathogens will benefit development of food safety prevention programs for produce.
Session 15D, Food Microbiology: Fruits and vegetables
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