This balance can tip either way. For some plasmids, it is impossible to be maintained solely by conjugation [7] and so they require different mechanisms of maintenance [8]. For other plasmids and systems the disadvantages of plasmid carriage, however, does not outweigh the spread by conjugation [9], which enables maintenance of

the plasmid by conjugation. Addiction systems, of which IncI1 plasmids have several present [10, 11], can prevent the loss of the plasmid, but cannot prevent selective disadvantages of the carriage of a plasmid. We aim to determine the fitness costs of this plasmid for the bacterium. Here, we used in vitro experiments, analysed by use of a mathematical model, to assess whether a combination of plasmid IncI1 and ESBL-gene bla CTX-M-1 can persist in vitro in a population of a broiler field isolate of E. coli. The mathematical model described combines a growth model with conjugation and plasmid INCB018424 purchase loss processes. The growth was modelled with three growth parameters: a lag-phase, an intrinsic growth rate, and a maximum density. The intrinsic growth rate is

the maximum growth rate of the population, which is inhibited during the lag-phase and at high bacterial densities. The maximum density is the maximum bacterial density in the medium. First, we estimated the bacterial growth parameters, conjugation coefficients and plasmid loss rate from experiments with a short duration (i.e. 24 or 48 hours). Then, we buy PD-0332991 compared single and mixed cultures to determine selective buy CAL-101 disadvantage and a difference in conjugation

coefficients between the donor and the newly acquired transconjugant strain [9, 12]. Finally we compared long-term predictions of our model to a 3-months experiment in which a mixed culture was regularly transplanted to fresh medium. Methods Bacterial isolates and plasmids All isolates used in the in vitro experiments were derived from the Dutch national monitoring program for antimicrobial resistance and antimicrobial usage in food-producing animals in 2006 [13] and 2010 [14]. The isolates used in this study were isolated from broiler faeces collected at slaughterhouses in the Netherlands. The bacterial isolates and plasmids used in the study are listed in Additional file 1. E38.27 was used as plasmid donor (D) in the experiments. Fossariinae E38.27 carries bla CTX-M-1 on an IncI1 plasmid of sequence type 7, and is therefore resistant to cefotaxime. Isolate E75.01 was used as recipient (R). This isolate is resistant to ciprofloxacin, due to mutations in the bacterial chromosome. Both isolates were analysed for plasmid content as described earlier [5, 15]. E. coli sequence types were determined by Multi Locus Sequence Typing [16]. The transconjugant (T), called T38.27, consisted of E75.01 that acquired the IncI1 plasmid with bla CTX-M-1 from E38.27, and is resistant to ciprofloxacin due to the presence of mutations in the chromosome (present in strain E75.