E strips

Overnight cultures of P. aeruginosa were prepared in MH broth and diluted to 0.5 MacFarland units. Uniform swabbing of cultures onto MH agar plates with or without 0.3% or 0.03% bile was followed by manual placing of antibiotic discs (Thermo Scientific) or E-strips (Thermo Scientific) on the surface of the inoculated agar. Plates were incubated overnight at 37 °C and scored by measurement of the diameter (disk assays) or visualisation (E-strip) of the bacterial growth. Growth kinetic experiments were performed on a BioScreen C plate reader (Oy Growth Curves Ab). Cells were inoculated from overnight cultures to OD_600 nm 0.05 and treated with increasing concentrations of polymyxin B or erythromycin, either in the presence or absence of bile. Growth kinetics were measured over 24 hr at 37 °C.

Overnight cultures of P. aeruginosa were adjusted to 0.5 MacFarland units which is the standard recommendation of the Clinical Laboratory Standard Institute in MH broth. MH agar plates supplemented with or without 0.3% (w/v) bile were uniformly swabbed with culture. E-strips (Thermo Fisher scientific) were placed on to the surface of the agar manually. Plates were left to incubate at 37°C overnight after which the zone of inhibition was recorded.