MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
Title: The effect of modified atmosphere packaging on the persistence and expression of virulence factors of Escherichia coli O157:H7 on shredded iceberg lettuce
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2011
Publication Date: May 1, 2011
Citation: Sharma, M., Lakshman, S., Ferguson, S.E., Ingram, D.T., Luo, Y., Patel, J.R. 2011. The effect of modified atmosphere packaging on the persistence and expression of virulence factors of Escherichia coli O157:H7 on shredded iceberg lettuce. Journal of Food Protection. 74(5):718-726.
Interpretive Summary: Outbreaks of Escherichia coli O157:H7 infections have been associated with the consumption of bagged leafy green commodities. Fresh cut lettuce and spinach are placed under modified atmosphere packaging (MAP) to preserve quality attributes. However, it may be possible that these MAP conditions may enhance the virulence of this pathogens when stored at refrigeration or abusive temperatures. Our study investigated shredded lettuce inoculated with E. coli O157:H7 under three different MAP conditions at two temperatures, 4oC and 15oC, for up to 10 days. Our results showed that passive MAP conditions that allowed gaseous exchange promoted the largest reduction of E. coli O157 counts on shredded lettuce when stored at 4oC. At 15oC, these same conditions promoted the slowest growth of E. coli O157:H7. However these conditions at 15oC also promoted the highest expression of genetic virulence factors. Our results indicate that passive modified atmosphere conditions can affected the potential virulence of E. coli O157:H7 at abusive temperatures
Fresh-cut leafy greens contaminated with Escherichia coli O157:H7 have been associated with multiple foodborne outbreaks. Modified atmospheric packaging (MAP) conditions, coupled with abusive storage temperatures of contaminated lettuce which may affect the persistence and expression of E. coli O157:H7 virulence factors on shredded lettuce, were evaluated. Shredded lettuce was inoculated with 5.58 or 3.98 log CFU E. coli O157:H7 / g and stored at either 4oC or 15oC, respectively, for up to 10 days. Lettuce was packaged in treatment A (active MAP, evacuating the package and flushing with N2 in plastic films with 110 Oxygen Transmission Rate (OTR), treatment B (passive MAP 110 OTR with microperforations to promote gas exchange), and treatment C (passive MAP in 0 OTR film). E. coli O157:H7 populations and mRNA were extracted from packages to determine the expression of virulence factors stx2, eae, ehxA, iha, and rfbE under these conditions. E. coli O157:H7 populations on lettuce at 4oC declined in all packaging treatments, but most dramatically under treatment B over 10 days. At 15oC E. coli O157:H7 populations increased by at least 2.76 log CFU/g in all packaging treatment but treatment B supported the smallest increase. At 4oC, on day 1, expression of O157- virulence factors (except stx2) was significantly greater under packaging treatment C than under treatments A or B at 4oC on day 1. At 15oC, expression of eae and iha were significantly greater under treatment B than treatment A and C on day 3. Similarly, treatment B promoted significantly higher expression levels of stx2, eae, ehxA,and rfbE on day 10 compared to treatments A and C at 15oC. These results indicate that passive MAP with gasesous exchange are more inhibitory to the survival and growth of E. coli O157:H7 on shredded iceberg lettuce, but can promote higher expression levels of O157-virulence factors than other MAP treatments. Passive MAP may affect the severity of illness associated with E. coli O157:H7 infections associated with leafy green outbreaks.