The influence of biotic and abiotic factors on plasmid transfer between Escherichia coli strains in terms of the variation in the number of transconjugants formed and the variation in transfer frequency was investigated. The density of parent cells affected the number of transconjugants, reaching a maximum when the cell density was on the order of 108 CFU ml-'. As the donor-to-recipient ratios varied from 104 to 104, the number of transconjugants varied significantly (P < 0.001), reaching a maximum with donor-to-recipient ratios between 1 and 10. The concentration of total organic carbon in the mating medium affects both the number of transconjugants and the transfer frequency, being significantly higher (P < 0.001) when the total organic carbon concentration was higher than 1,139 mg of C liter-'. However, the transconjugants were detected even with less than 1 mg of C liter-. Linear regression of loglo transconjugants versus mating temperature showed a highly significant regression line (P < 0.001). Neither the transfer frequency nor the transconjugant number varied significantly in the range of pHs assayed. We can conclude that plasmid transfer by conjugation can take place within a wide range of conditions, even in such adverse conditions as the absence of nutrients and low temperatures.

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Survival of Campylobacter jejuni at 4 and 20°C was investigated by using cellular integrity, respiratory activity, two-dimensional (2D) protein profile, and intact DNA content as indicators of potential viability of nonculturable cells. Intact DNA content after 116 days, along with cellular integrity and respiring cells, was detected for up to 7 months at 4°C by pulsed-field gel electrophoresis. Most changes in 2D protein profiles involved up- or down-regulation.