90 and 0 95 of the maximum density K R The solid gray line descri

90 and 0.95 of the maximum density K R The solid gray line describes the prediction for maximum density K T being a fraction of 0.80 of K R . Also, the experimental results of the long term experiment 3 did not show a decrease in the proportion of T in comparison to T + R (Figure 3). This means that the population of T did not decline more than 10 fold compared to T + R, which would have been visible. Because the experiment did not allow distinction between T alone and R + T together, we

cannot determine if R was replaced or if R and T coexisted with R at low numbers. Discussion Fitness costs resulting in a lower bacterial growth rate or a lower maximum density due to the presence of the plasmid IncI1 carrying the bla CTX-M-1 gene were not observed here. No differences were found between donor D, recipient R and transconjugant T in growth rate ψ, maximum density Geneticin cell line K or lag-phase λ in single population experiments 1a-j. Fitness costs might have arisen in a

competition setting with mixed populations of D and R[19] due to competition for resources or inhibition by the competitor. However, also in the mixed populations of the conjugation experiments 2a-b, we could not find a difference in growth parameters Quisinostat between the recipient R and donor D. San Millan et al.[20] neither found a difference in percentage of plasmid free and plasmid carrying bacteria for their pB1000 plasmid in the first 12 hours. However, starting at day 2 they observed a clear decrease in Buspirone HCl the fraction of plasmid carrying bacteria. Also in our experiments, the fitness costs of the plasmid carrying bacteria were not evident in the early phase. Small fitness costs may not be observable at all in experiments with a short duration, but when the experiments are maintained longer, fitness costs other than costs related to the growth rate can play a role. In

12 or 24 hours experiments, these differences might be too small to measure. This is why we conducted the long term experiment 3 both with intervals of 24 and 48 hours, as the duration of our experiments 1 and 2 (up to 24 hours) may have been too short to observe fitness costs. We showed by simulation (illustrated in Figure 3) that only for large fitness costs resulting in a 20% smaller maximum density K by carrying the IncI1 plasmid, a distinct decrease in population size would have been observed within the time-frame of experiment 3. This was, however, not observed in experiment 3, underlining the conclusion that this plasmid does not infer sufficient fitness costs to its host bacterium to let it go extinct in the absence of antimicrobials. Thus, our results suggest that reduction of the use of antimicrobials might not result in a decrease, let alone extinction, of such a plasmid. This is in accordance with the conclusions of Poole et al.[21].

Comments are closed.