Escherichia coli strains have extended-spectrum beta-lactamases (ESBL) genes that are frequently encoded on conjugative plasmids. In our study, we analyzed the conjugative plasmid pES6, isolated from ESBL-positive E. coli strains from selected poultry farms in Slovenia, that confer resistance to low concentrations of fluoroquinolones (qnrS1) and cefotaxime (blaSHV-12). We analyzed the frequency of conjugative transfer of resistance to cefotaxime from source donors VF2017-A5 (A5) and VF2017-A6 (A6). The frequency of A5 conjugation experiments varied from 10-7 to 10-5, and the frequency of A6 conjugation experiments from 10-5 to 10-3. By sequencing the plasmid DNA (pDNA) of selected transconjugants (TK), we found that in addition to plasmid pES6, which is identical to plasmid pEC-243 described in the literature, additional plasmids pV5, pV6, and pK6 are present. Supposed plasmid pV5, present in TK derived from the A5 donor, encodes the colicins B and M. Supposed plasmid pV6, encoding the colicin Ia and microcin V and cryptic plasmid pK6 are present in TK derived from the A6 donor. Their presence in TK was confirmed by PCR with plasmid-specific primers. For plasmids pV5 and pV6, their presence was also confirmed phenotipically by colicin activity. Comparison of the results of bioinformatic analysis with data obtained from analysis of isolated, uncut and cut pDNA after gel electrophoresis, led to the conclusion that pDNA electrophoresis methods are not sufficient to visually detect all plasmids in the isolated pDNA of a single strain.
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