Internalization and Fate of Escherichia coli O157:H7 in Leafy Green Phyllosphere Tissue Using Various Spray Conditions

Abstract

In the past decade, leafy greens have been implicated in several outbreaks of foodborne illness, and research has focused on contamination during preharvest operations. Concerns have been raised that internalization of pathogens into the edible tissue occurs where postharvest chemical interventions would be ineffective. This study was initiated to measure the degree and fate of Escherichia coli O157:H7 internalized in the phyllosphere tissue of leafy greens when spray conditions, inoculum level, and type of leafy green were varied. Two spraying treatments were applied: (i) spraying individual spinach or lettuce leaves on plants once with a high dose (7 to 8 log CFU/ml) of E. coli O157:H7 and (ii) spraying spinach, lettuce, or parsley plants repeatedly (once per minute) with a low dose (2.7 to 4.2 log CFU/ml) of E. coli O157:H7 over a 10- to 20-min period. With the high-dose spray protocol, no significant differences in the prevalence of internalization occurred between Shiga toxin-negative E. coli O157:H7 isolates and virulent isolates (P > 0.05), implying that the Shiga toxin virulence factors did not influence internalization or the subsequent fate of those populations under these test conditions. Significantly greater internalization of E. coli O157:H7 occurred in spinach leaves compared with lettuce leaves when leaves were sprayed once with the high-dose inoculum (P < 0.05), whereas internalization was not observed in lettuce leaves but continued to be observed in spinach and parsley leaves following repeated spraying of the low-dose inoculum. Based on these results, it is surmised that a moisture film was generated when spraying was repeated and this film assisted in the mobilization of pathogen cells to plant apertures, such as stomata. E. coli O157:H7 cells that were internalized into spinach tissue using a low-dose repeat-spray protocol were temporary residents because they were not detected 2 days later, suggesting that plant-microbe interactions may be responsible.

Survival and internalization of Salmonella and Escherichia coli O157:H7 sprayed onto different cabbage cultivars during cultivation in growth chambers

“…With the exception of the 'Capture' cultivar, the percentage of plants with internalized cells in their phyllosphere tissue at 24 h decreased from that which had been observed at 1 h and those decreases ranged from 7% to 43% (Table 1). In contrast to a previous study, the prevalence of internalized E. coli O157:H7 in spinach had dropped more dramatically, initially being 100% and dropping to 7.5% after 24 h. 28 In fact, due to the limited number of samples analyzed at 24 h in this experimental trial, the only cultivar that exhibited a significant drop in prevalence of internalized Salmonella within 24 h after exposure was 'Fast Vantage'. In that case, the probability that a sample positive for internalized Salmonella would be identified at 24 h was 10.5 times less likely than at 1 h [95% confidence interval (CI) from 0.7 to 153, P = 0.0468].…”

Plant species affects establishment ofEscherichia coliO157:H7gfp+on leafy vegetables

“…; Erickson et al . ). Salmonella and Shiga toxigenic E. coli have been the main focus (Brandl ; Brandl et al .…”

“…The prevalence of artificially inoculated human pathogens has been investigated on many different types of leafy vegetables, such as lettuce, spinach and rocket (Cooley et al 2006;Alam et al 2014;Erickson et al 2014). Salmonella and Shiga toxigenic E. coli have been the main focus (Brandl 2004;Brandl et al 2013;Moyne et al 2013; and various routes of transmission have been studied (Brandl 2006;Mandrell 2009;Farrar and Guzewich 2014).…”

Effect of the food production chain from farm practices to vegetable processing on outbreak incidence

“…Van der Linden and colleagues (2013) demonstrated that overhead sprinkle irrigation was associated with increased enteric pathogen contamination of butterhead lettuce. Repeated spray irrigation with a low dose (approximately 3.5 log CFU ml −1 ) of E. coli O157:H7 resulted in internalization of the pathogen in spinach and parsley leaves, but not in lettuce leaves (Erickson et al, 2014). The pathogen persisted for 2 days post-exposure, suggesting that irrigation immediately prior to harvest presents an increased microbiological risk.…”