PT  - JOURNAL ARTICLE
AU  - Alasdair T. M. Hubbard
AU  - Nicholas Feasey
AU  - Adam P. Roberts
TI  - Evolutionary trajectories to amoxicillin-clavulanic acid resistance in <em>Escherichia coli</em> are affected by growth media
AID  - 10.1101/262691
DP  - 2018 Jan 01
TA  - bioRxiv
PG  - 262691
4099  - http://biorxiv.org/content/early/2018/02/09/262691.short
4100  - http://biorxiv.org/content/early/2018/02/09/262691.full
AB  - Antibiotic resistant bacterial isolates are routinely generated in the laboratory using sub-inhibitory concentrations of the antibiotics in a range of media to study various aspects of evolutionary biology. We wanted to determine whether growth in different media affects the selection of amoxicillin-clavulanic acid resistant Escherichia coli. Using a susceptible clinical isolate of E. coli resistance was selected for after 24-hour exposure to sub-MIC levels of amoxicillin-clavulanic acid in defined (M9), semi-defined (Iso-sensitest broth) and undefined (LB broth) media. Resistant colonies were isolated on LB agar containing 8 (MIC) or 16 μg/ml (2×MIC) amoxicillin-clavulanic acid. Fitness costs of three resistant lineages from each media-type were assessed using a competitive fitness assay and mutations in the ampC promoter region were identified through PCR. No resistant colonies were recovered with pre-incubation in M9, whereas Iso-sensitest and LB broth incubation resulted in growth at 8 and 16 μg/ml of amoxicillin-clavulanic acid, respectively. We observed within media-type variability in the fitness costs associated with resistance and showed that in some cases the fitness is dependent on the media in which the assay is carried out. Our data highlights the importance of media consideration when interpreting the results of evolutionary studies which will ultimately be translated into the clinic.