Time‐kill studies with a ceftazidime‐treated mixed culture consisting of P seudomonas aeruginosa , B urkholderia cepacia and S taphylococcus aureus

So far, time‐kill analyses were mostly done with isolates of bacteria. Here, we used a mixed culture consisting of P seudomonas aeruginosa , B urkholderia cepacia , and S taphylococcus aureus to investigate the impact of ceftazidime treatment. Following an earlier study with the same strains, the influence of different ceftazidime concentrations as well as repeated ceftazidime treatment was tested. In order to assess the influence of substrate competition, which might be relevant to interpret mixed‐culture time‐kill studies, the major metabolites of the chemically defined cultivation medium were measured additionally. Time‐kill experiments were conducted in shake flasks with the chemically defined and modified medium M 199. The cell concentration in the mixed culture was analyzed on the single‐species level using a quantitative terminal restriction fragment (qT‐RFLP) method. The amount of gluconate produced in mixed culture positively correlated with increased ceftazidime concentrations (5, 15, 30, 60 mg/L). B urkholderia cepacia developed resistance after repeated ceftazidime addition and reached the highest cell concentration of the three cultivated strains. P seudomonas aeruginosa showed a pronounced regrowth phase after removal of ceftazidime, while growth of S . aureus was not influenced by medium exchange. In conclusion, growth of B . cepacia was dominant in the ceftazidime‐treated mixed culture over the observed time range, due to low susceptibility against ceftazidime as well as advantages in substrate usage.