Robert Premier

Efficacy of chlorine for inactivation of Escherichia coli on vegetables

Abstract The effectiveness of calcium hypochlorite on inactivation of Escherichia coli inoculated on fresh produce was investigated. Different exposure times and concentrations of chlorine were studied. Dipping was not effective at eliminating E. coli populations although it significantly reduced the E. coli counts compared with inoculated, undipped lettuce. Dipping inoculated cos lettuce leaves into hypochlorite solutions containing 50 mg/l or greater free chlorine for times of 30 s or greater reduced E. coli cells by approximately 1.9–2.8 log10 CFU/g from an initial population of approximately 6.8 log10 CFU/g. Dipping inoculated broccoli florets into hypochlorite solution reduced E. coli cells by approximately 1.7–2.5 log10 CFU/g, depending on the time and concentration of the free chlorine. Dipping lettuce or broccoli in water alone reduced cell numbers by 1.5–1.8 log10 CFU/g. Dipping broccoli florets for 2 min in a 100 mg/l free chlorine solution at temperatures between 4 and 25°C reduced E. coli cells by approximately 2.4 log10 CFU/g. No significant effect of temperature on the rate of cell reduction was observed.

Effect of heating and aging of poultry litter on the persistence of enteric bacteria

Food-borne illnesses have rarely been associated with the reuse of poultry litter as an organic fertilizer and soil amendment in agriculture. Yet farming practices in many countries have come under increased scrutiny because of heightened consumer awareness of food safety and environmental issues. This study was conducted to determine whether simple on-farm management practices could improve the microbiological safety of poultry litter. First, the effects of heat and moisture on the survival of Escherichia coli and Salmonella enterica serovar Typhimurium in poultry litter were investigated under laboratory conditions. Second, the persistence and regrowth of enteric bacteria were examined in poultry litter that had been aged for up to 12 wk in either a turned or static (unturned) windrow. Escherichia coli and Salmonella counts in poultry litter were reduced by >99% in 1 h at 55 or 65°C under laboratory conditions. At 35°C, both persisted longer under moist (65% wt/wt, wet basis) than dry (30% wt/wt) conditions. Poultry litter aged for 3 wk in a turned windrow, and up to 6 wk in a static windrow, supported increased E. coli densities when incubated in the laboratory at 37°C for 21 d. Peak temperatures >65°C were observed in both windrows within the first 3 wk of aging; after this point, the turned windrow was more consistently exposed to temperatures >45°C than the static windrow. By 12 wk, however, E. coli counts were very similar (3 to 3.6 log(10)) in the outside edge of both windrows. This study highlights the need for a better understanding of the interrelationship between spontaneous heating in organic waste streams, organic matter stabilization, and pathogen reduction.

Persistence of Escherichia coli on injured vegetable plants.

Minor shoot injury to glasshouse celery, Cos lettuce and chive plants significantly increased the persistence of applied Escherichia coli (P<0.05). After 1 week, mean counts of about 5 log(10) CFU/g decreased to fewer than 0.5 log(10) CFU/g on the uninjured plants, compared to 4 log(10) CFU/g or more on injured plants. By the end of the 3-week long experiments, counts from the uninjured plants were 0.21 log(10) CFU/g or fewer, but 2.8, 2.3 and 5.1 log(10) CFU/g on injured Cos lettuce, celery and chive plants, respectively. A field experiment using Cos lettuce also showed that shoot injury increased E. coli persistence. Counts from the injured plants on days 1, 3, and 7 were, 4.2, 4.1 and 3.3 log(10) CFU/g, respectively, whereas the uninjured plants returned significantly (P<0.05) lower counts on those days, and were 2.8, 2.0 and 1.2 log(10) CFU/g, respectively. These findings reveal that increased E. coli persistence on injured tissue is common to different vegetables and can occur in the glasshouse and the field. The implications of this study on vegetable production practices are presented.

Distribution of antigen specific memory T cells in lymph nodes after immunization at peripheral or mucosal sites

The distribution of antigen‐specific memory T cells in different lymph nodes of sheep was determined using an antigen‐specific in vitro proliferation assay. Lymph nodes were collected from sheep immunized simultaneously with avidin or ovalbumin in a peripheral tissue site (hind leg muscle) and keyhole limpet haemocyanin (KLH) in an intestinal tissue site (gut wall or colonic mucosa). The results showed a consistently high proliferative response in typical peripheral lymph nodes (popliteal and prescapular) and a low or negative response in gastrointestinal lymph nodes (abomasal and jejunal) while the response in other nodes was variable. The low proliferative response in the gastrointestinal lymph nodes was not due to the presence of supressor CD8+ lymphocytes and the proliferative response could not be raised to peripheral lymph nodes levels with the addition to cultures of IL‐2 or mitomycin‐C treated peripheral lymph node cells. The high proliferative response in the peripheral lymph nodes was not suppressed by the addition of mitomycin‐C‐treated gastric lymph node cells but was dramatically reduced by the addition of mAb against the IL‐2‐receptor or by depletion of CD4+ T cells. The results suggest that antigen‐specific proliferative memory T cells, which may be Th1‐like memory cells, preferentially migrate to peripheral lymph nodes independent of their site of induction.

Shoot Injury Increases the Level of Persistence of Salmonella enterica Serovar Sofia and Listeria innocua on Cos Lettuce and of Salmonella enterica Serovar Sofia on Chive.

Minor shoot injury significantly (P < 0.05) increased the level at which Salmonella enterica serovar Sofia persisted on cos lettuce in the greenhouse. Initial mean counts of the Salmonella on the injured and uninjured cos lettuce were on the order of 6 log CFU/g. After 3 days, the mean count decreased to 4.8 log CFU/g on the injured plants compared with the significantly (P < 0.05) smaller count of 3.4 log CFU/g on the uninjured plants. By the end of the 3-week experiment, the count from the injured plants was 2.9 log CFU/g compared with a count of below the level of detection from the uninjured plants. A similar pattern of bacterial persistence was observed on injured versus uninjured plants by using Listeria innocua on cos lettuce and S. enterica serovar Sofia on chive. The findings reaffirm earlier results with Escherichia coli and increase the impetus to avoid shoot injury during the production of cos lettuce and chive, if bacteria of food safety concern are present.