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 greater than 0.2) whereas the difference between observed and predicted distributions of plasmids in combined populations of other enterobacteria was significant (p less than 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 not equal to 0. The results obtained showed that the frequency distribution of plasmids in E. coli and in other enterobacteria did not differ significantly from the predicted distribution (p less than 0.2 and p greater than 0.6 respectively). The frequency distribution of plasmids in previously published studies was compared with predicted frequencies by means of the Poisson distribution and the modified Poisson distribution conditional upon n not equal to 0. The results indicate that the modified Poisson distribution described provides an adequate description of plasmid frequency distributions and represents the observed frequencies better than the distribution predicted directly from the Poisson formula.