Survival of Escherichia coli O157:H7 in Soils from Vegetable Fields with Different Cultivation Patterns

Abstract

Escherichia coli O157:H7 survived longer in soils from plastic-greenhouse cultivation than soils from the open field. Soil pH, organic carbon levels, and the ratio of bacterial phospholipid fatty acids (PLFAs) to fungal PLFAs played the significant roles in survival of O157:H7. Greater attention should be paid to the control of pathogen contamination under conditions of plasticgreenhouse cultivation. E scherichia coli O157:H7 is dangerous because of its resistance to low pH (2.5), its low infective dose (as few as 10 cells), and its high pathogenicity (1). Raw fruits and vegetables, especially leafy greens that are consumed raw, are well-known vehicles for transmitting O157:H7 and may be polluted via field application of raw manure or contaminated irrigation water (2, 3). O157:H7 survival in soils from vegetable field should be considered an important factor for the likelihood of vegetable contamination. In China, plastic-greenhouse cultivation of vegetables is becoming more popular because of the higher growth rate and yield. However, little is known about the effects of various soil cultivation practices on the survival of O157:H7. The aim of this study was therefore to assess its survival in soils from a vegetable field under conditions of different cultivation patterns.Five soils (S0, S1, S4, S7, and S15) were collected from a depth of 0 to 20 cm in fields (sites) that were under conditions of various lengths of time (0, 1, 4, 7, and 15 years, respectively) of plasticgreenhouse cultivation in the same area. Three replicates were sampled for each site. The selected soil properties are listed in Table 1. O157:H7 cells were inoculated into the soil samples and gently mixed to establish a cell density of about 10 6 CFU per gram of dry weight (CFU · g Ϫ1 ). Forty grams (measured on the basis of analysis of oven-dried samples) of inoculated soil was placed in a 50-ml sterile plastic tube. All tubes (triplicates for each site, plus uninoculated controls) were incubated in the dark at 25 Ϯ 1°C with the field capacity (soil water content) at Ϫ33 kPa. Sterile deionized water was added to the tubes every 2 days to maintain constant soil moisture status during the incubation. A soil subsample was taken from each tube at 0, 0.04, 1, 3, 5, 7, 10, 15, and 25 days after inoculation and subjected to 10-fold serial dilutions with 0.1% peptone buffer. The last three of the serial dilutions per sample were plated onto sorbitol SMAC-BCIG (sorbitol MacConkey-5-bromo-4-chloro-3-indoxyl-␤-D-glucuronide) agar containing 100 g · ml Ϫ1 of rifampin and 25 g · ml Ϫ1 of nalidixic acid for enumeration (37°C, 16 h). The sampling was stopped after O157:H7 cells below the detection limit (approximately 100 CFU · g Ϫ1 ) appeared twice in succession during the incubation. Survival of O157:H7 was modeled by fitting the experimental data to the Weibull survival function proposed by Mafart et al. (4). The time needed to reach the detection limit (t d ) was calculated from the Weibull survival function.Enumeration analysis indicated that there wa...

Interaction between the Microbial Community and Invading Escherichia coli O157:H7 in Soils from Vegetable Fields

“…For each composite soil sample, a portion of sieved subsample was freeze-dried and was then subjected to PLFA analysis according to the method described by He et al (3). The individual PLFAs of the test soils are shown in Table 1, and other soil properties (pH, water-soluble organic carbon, available phosphorus, available potassium, organic carbon, total nitrogen, field capacity [soil water content at Ϫ33 kPa], clay, total PLFAs, bacterial PLFAs, and fungal PLFAs) have been reported previously by Yao et al (9).…”

“…The soil samples used in this study (S0, S1, S4, S7, and S15) were from fields under plastic-greenhouse cultivation for various lengths of time (0, 1, 4, 7, and 15 years, respectively) in Shandong Province, China, and were collected from the surface horizon (0 to 20 cm) (9). The samples were of the same soil type, and the sampling sites shared similar weather conditions, including mean annual temperature (13.1°C) and precipitation (578.4 mm), due to the closeness of their locations.…”

“…Invading E. coli O157:H7 has been observed to survive in soil or soil-related (manure) environments for days to more than 1 year (9,(15)(16)(17)(18)(19). The fate of E. coli O157:H7 in soil can be affected by both biotic and abiotic factors (9,12,15).…”

“…Invading E. coli O157:H7 has been observed to survive in soil or soil-related (manure) environments for days to more than 1 year (9,(15)(16)(17)(18)(19). The fate of E. coli O157:H7 in soil can be affected by both biotic and abiotic factors (9,12,15). Our previous study revealed that the soil microbial community, especially bacteria and fungi, played an important role in the survival of E. coli O157:H7 (9), a finding consistent with those of other studies (15,19,20).…”

“…Many researchers have reported that soil microbial communities respond to the presence of heavy metals and organic pollutants (1)(2)(3)(4)(5)(6). Increasing numbers of studies in recent years have also been conducted on soil biopollution by pathogenic microorganisms, including viruses, protozoa, and bacteria such as Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes (7)(8)(9). E. coli O157: H7, a significant pathogen carried naturally by animals, has been shown to pose a significant threat to environmental safety and public health because of its low infective dose (as few as 10 cells) and its high pathogenicity in watery diarrhea, hemorrhagic colitis, hemorrhagic-uremic syndrome, thrombotic thrombocytopenic purpura, etc.…”