Pesticides as a source of microbial contamination of salad vegetables.

Ten commercially available pesticides (insecticides, herbicides and fungicides), used during the production of vegetable produce, were examined as potential sources of microbial contaminants. As purchased, none of the pesticides showed the presence of viable microorganisms (< 5 CFU/ml). Using an agar plate diffusion assay, they did not inhibit a range of bacteria of spoilage and public health significance on vegetable produce. After reconstitution in sterile water to their recommended concentration, two of the pesticides supported the survival and growth of inoculated species of Pseudomonas, Salmonella and Escherichia coli. Listeria monocytogenes did not survive after inoculation into any of the pesticides. Pesticides were reconstituted in different sources of agricultural water (bore, dam and river) and examined for survival and growth of microorganisms naturally present in these waters. On storage at 30 degrees C for 48 h, nine of the pesticides supported the growth of bacterial species present in these waters. Predominant species in the pesticide solutions, before and after storage, varied according to the source, but species of Pseudomonas, Acinetobacter and Aeromonas and various coliforms exhibited significant growth. Unless managed properly (reconstituted in potable water, and used without lengthy storage), pesticides could contribute to the microbial load of vegetable produce, thereby affecting their shelf-life and public health safety.

Application and evaluation of denaturing gradient gel electrophoresis to analyse the yeast ecology of wine grapes.

The performance of denaturing gradient gel electrophoresis (DGGE) for analysing yeasts associated with wine grapes was compared with cultural isolation on malt extract agar (MEA). After optimisation of PCR and electrophoretic conditions, the lower limit of yeast detection by PCR-DGGE was 10(2) cfuml(-1), although this value was affected by culture age and the relative populations of the species in mixed culture. In mixed yeast populations, PCR-DGGE detected species present at 10-100-fold less than other species but not when the ratio exceeded 100-fold. Aureobasidium pullulans was the main species isolated from immature, mature, and both damaged and undamaged grapes. It was not detected by PCR-DGGE when present at populations less than 10(3) cfug(-1). When approaching maturity, damaged grapes gave a predominance of Metschnikowia and Hanseniaspora species (10(5)-10(7) cfug(-1)), all detectable using PCR-DGGE. However, various species of Rhodotorula, Rhodosporidium and Cryptococcus were not detected by this method, even when populations were as high as 10(4) cfug(-1). PCR -DGGE was less sensitive than culture on MEA for determining the yeast ecology of grapes and could not reliably detect species present at populations less than 10(4) cfug(-1). However, this method detected a greater diversity of species than agar plating.

Occurrence and significance of Bacillus thuringiensis on wine grapes.

Wine grapes harvested at different stages during cultivation from several vineyards in New South Wales, Australia, harboured Bacillus thuringiensis at viable populations of 10(2)-10(6) cfu/g. Commercial preparations of B. thuringiensis had been sprayed onto the grapes as a biological insecticide. B. thuringiensis (10(1)-10(3) cfu/ml) was isolated from grape juice and fermenting grape juice in a commercial winery. Although B. thuringiensis remained viable when inoculated at 10(3)-10(4) cfu/ml into grape juice and wine (pH 3.0-6.0), it did not grow. Using in vitro agar culture assays, B. thuringiensis inhibited several grape-associated yeasts and bacteria as well as various species of fungi associated with grape spoilage and ochratoxin A production. B. thuringiensis did not inhibit Saccharomyces cerevisiae in agar culture or during alcoholic fermentation of grape juice. B. thuringiensis inhibited the malolactic bacterium, Oenococcus oeni, in agar culture but not during mixed cultures in a liquid medium.