Abstract
Enterohaemorrhagic Escherichia coli, such as serotype O157:H7, are a leading cause of food-associated outbreaks. While the primary reservoir is associated with cattle, plant foods have been associated as sources of human infection. E. coli is able to grow in the tissue of food plants such as spinach. While fecal contamination is the primary suspect, soil has been underestimated as a potential reservoir. Persistence of bacterial populations in open systems is the product of growth, death, predation, and competition. Here we report that E. coli O157:H7 can grow using the soluble compounds in soil, and characterize the effect of soil growth on the stationary phase proteome. E. coli 933D (stxII−) was cultured in Soil Extracted Soluble Organic Matter (SESOM) and the culturable count determined for 24d. The proteomes of exponential and stationary phase populations were characterized by 2D gel electrophoresis and protein spots were identified by MALDI-TOF mass spectrometry. While LB controls displayed a death phase, SESOM grown population remained culturable for 24d, indicating an altered physiological state with superior longevity. This was not due to decreased cell density on entry to stationary phase as 24 h SESOM populations concentrated 10-fold retained their longevity. Principal component analysis showed that stationary phase proteomes from SESOM and LB were different. Differences included proteins involved in stress response, motility, membrane and wall composition, nutrient uptake, translation and protein turnover, and anabolic and catabolic pathways, indicating an altered physiological state of soil-grown cells entering stationary phase. The results suggest that E. coli may be a soil commensal that, in absence of predation and competition, maintains stable populations in soil.
Highlights
Escherichia coli O157:H7 and related enterohaemorrhagic strains have been associated with many serious food-associated outbreaks (Hilborn et al, 1999; Currie et al, 2007; Grant et al, 2008; King et al, 2009)
Nutrient carryover from the LB-pre-culture was avoided by extensive washing and inoculating to a low initial density of A546 = 0.005. These results indicate that, in the absence of competition and predation, populations of E. coli O157:H7 933D should be able to grow and divide in soils, as supported by population increases in the various Soil Extracted Soluble Organic Matter (SESOM) evaluated
1/40th LB–grown cells were more likely to survive than LB-grown ones, irrespective of cell density post-stationary phase. These results indicated that conditions upon entry into stationary phase affect the condition of the cells, thereby determining their potential for survival over long term incubation
Summary
Escherichia coli O157:H7 and related enterohaemorrhagic strains have been associated with many serious food-associated outbreaks (Hilborn et al, 1999; Currie et al, 2007; Grant et al, 2008; King et al, 2009). More recently various plant foods such as spinach, tomato, lettuce, and fresh fruits have been identified as sources (Grant et al, 2008; Herman et al, 2015; Denis et al, 2016). These foods were thought to be fecally contaminated, but recent reports suggest growth of E. coli O157:H7 (Brandl, 2008; Wright et al, 2013, 2017) in tissues of salad leaves and tomatoes. These crop plants would need to be infected during growth
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