A simple statistical approach that represents the frequency distribution of plasmids in clinical isolates of the enterobacteria

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Abstract
The frequency distribution of plasmids in a representative collection of Escherichia coli and other enterobacteria was compared with the frequencies predicted by the Poisson distribution. The distribution of E. coli plasmids did not differ significantly (p > 0.2) whereas the difference between observed and predicted distributions of plasmids in combined populations of other enterobacteria was significant (p < 0.001). Previous studies had suggested that plasmid-free strains contributed disproportionately to the overall frequency distribution. Therefore, plasmid-free strains were excluded and the frequency distribution of plasmids in plasmid-containing strains compared with frequencies predicted by a modified Poisson distribution conditional upon n .noteq. 0. The results obtained showed that the frequency distribution of plasmids in E. coli and in other enterobacteria did not differ significantly form the predicted distribution (p > 0.2 and p > 0.6 respectively). The frequency distribution of plasmids in previously published studies was compared with predicted frequencies by means of (a) the Poisson distribution and (b) the modified Poisson distribution conditional upon n .noteq. 0. The results indicate that the modified Poisson distribution described provides an adequate description of plasmid frequency distributions and represent the observed frequencies better than the distributions predicted directly from the Poisson formula.

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