MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
Title: Persistence of somatic and F-specific coliphages, potential indicators of fecal contamination, on spinach foliar tissue
Submitted to: International Association for Food Protection Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: March 2, 2011
Publication Date: July 31, 2011
Citation: Lilly, A., Sapkota, A., Roberts, C.L., Ferguson, S.E., Shelton, D.R., Ingram, D.T., Sharma, M. 2011. Persistence of somatic and F-specific coliphages, potential indicators of fecal contamination, on spinach foliar tissue. International Association for Food Protection Proceedings. [abstract].
Introduction: Recent outbreaks of foodborne illness have been linked to consumption of fresh leafy greens. E. coli is commonly used as the indicator microorganism for fecal contamination on growing leafy greens. However, alternative microorganisms, such as coliphages, may be an alternative indicator of fecal contamination.
Purpose: The persistence and detection of somatic and F+-specific coliphages on spinach plants were evaluated to determine the suitability of these coliphages as indicators of fecal contamination. Additionally, various diluents were assessed for their efficacy in recovering coliphages from spinach.
Methods: Somatic (phiX174) and F+-specific (MS2) coliphages were prepared and spot-inoculated on 4-week old spinach plants, cultivated in growth chambers, at 105 PFU/plant. Foliar tissue was aseptically harvested and homogenized in either sterile water, sterile magnesium (SM) buffer or 3% beef extract (BE). A soft agar (0.60% LB agar) overlay method with E. coli host strains CN13 (for phiX174) and Famp (for MS2) was used to determine phage recovery from spinach plants five times over 20 days. On each day of analysis, three plants inoculated with each coliphage were recovered in each diluent.
Results: On day 0, MS2 recovery was 3.42 log PFU/plant. After 4 days, MS2 phage declined by 1.78 log PFU/plant to below the level of detection (1.65 log PFU/plant), but was recovered by enrichment for two weeks. phiX174 coliphages persisted on the spinach plants for 20 days, declining by 2.22 log PFU/plant from day 0 (5.42 log PFU/plant). Overall, BE was the most effective in recovering phiX174 from spinach foliar tissue, with significantly (P < 0.05) larger titers recovered on day 20 compared to either water or SM.
Significance: The recovery of phiX174 and MS2 with BE after 20 days indicates the potential utility of these coliphages as indicators of fecal contamination on leafy green commodities.