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 in the stationary phase proteome. E. coli 933D
(stxII-) was cultured in Soil Extracted Soluble Organic Matter (SESOM) and the
culturable count determined for 24 d. 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 24 d, indicating an altered
physiological state with superior longevity. This was not due to decreased cell density on
entry to stationary phase as 24h 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.
