Impact of selected environmental factors on E.coli growth in river water and an investigation of carry-over to fresh produce

The consumption of fresh produce has increased over the past few decades, but it has also resulted in an increase in foodborne outbreaks. Once fresh produce gets contaminated, microbes can survive or even multiply on the produce. There is, however, very little information available on what impact environmental conditions will have on the survival and growth of potential pathogens. Thus the purpose of the first phase of the study was to determine the impact of different environmental factors: carbon levels; temperature; incubation time; and initial microbial load on the growth of E.coli and other "indigenous" microbes present in the Plankenburg river water. Water from the Plankenburg River was incubated at 10° to 35°C and the E.coli and aerobic (ACC) loads determined at 0, 6, 12 and 24 h. The impact of different COD:N:P ratios on growth were also evaluated by altering the water values. It was found that in non-sterile river water the E.coli levels increased with increase in incubation temperature. Minimal growth took place between 10° and 20°C and the largest growth increases at 35°C. The data showed that E.coli dieoff was rapid when the nutrient levels were low. It was concluded that the carbon (COD) level is a major growth limiting factor in river water. The impact of adjusted carbon levels on the growth of E.coli present was then evaluated. The E.coli was found to die-off faster at the higher temperatures when the carbon levels were low (COD = <10 mg.L), possibly due to nutrient limitations. At higher carbon levels (COD = >100 mg.L), the growth profiles of E.coli showed major growth increases with no die-off during the 24 h period, probably because of the availability of sufficient nutrient levels. Even though standard methods can be used to identify if irrigation water is faecally contaminated it is important to confirm that organisms on fresh produce are from the faecally polluted irrigation water. The purpose of the second study was to determine the effect of daily irrigation on carry-over, the effect of “once-off” irrigation on the survival of E.coli on the produce, identifying types of E.coli in the irrigation water and those on irrigated fresh produce, and then linking the E.coli types. In the first trial, green beans planted in a tunnel and irrigated on a daily basis (14 days) with Plankenburg water and a “pure wild-type” E.coli at a concentration of 10 cfu.mL were evaluated. The results showed that E.coli is carried over from irrigation water to the irrigated green beans, especially when the E.coli levels in the river water were high. After the enumeration steps, colonies from both the irrigation water and from the irrigated beans were selected (67 isolates from the green beans and 72 from the irrigation water). Unique phenotypic (API) profiles were generated for each isolate. The first indication of linking was confirmed based on the degree of phenotypic similarity using numerical clustering systems. The results showed that these isolates were related and originated from the same pollution source. The use of PCR further confirmed (uidA positive) that all the phenotypically identified strains were E.coli. With multiplex PCR, further linking confirmation was shown when strains Stellenbosch University http://scholar.sun.ac.za