In the case of gentamicin a relative difference of approximately three logarithmic orders in CFU was recorded after the first hour of antibiotic treatment, when comparing #Q-VD-Oph ic50 randurls[1|1|,|CHEM1|]# populations of exponential and stationary grown S. suis. Notably, growth to the stationary growth phase did not enhance the tolerance of S. suis to the cyclic lipopeptide daptomycin which completely killed the S. suis population after only one hour of treatment. Taken together, the killing kinetics revealed that under the conditions tested S.

suis develops a growth phase dependent subpopulation showing antibiotic tolerance to a variety of antimicrobial compounds except daptomycin. The persister cell phenotype of S. suis is not inherited and dominated by type I persisters In contrast to genetically encoded antimicrobial

resistance, multidrug tolerance of persister cells is a transient and non-heritable phenotype [10, 26]. To test heritability of antimicrobial tolerance, exponential grown S. suis was treated with 100-fold MIC of gentamicin and the surviving population was used to repeat a new cycle. Four consecutive cycles were tested. Gentamicin was selected for these experiments since this treatment resulted in pronounced biphasic killing curves in the first hours after antibiotic treatment. As depicted in Figure 2A, tolerance to gentamicin of the initial population was not transferred to following S. suis generations. The DMXAA molecular weight characteristic biphasic killing curve upon antibiotic treatment was observed irrespective of the number of passages. These results suggest that the formation why of a S. suis persister cell subpopulation and antimicrobial tolerance is not inherited and of transient nature. Figure 2 Test for the heritability of persistence and elimination of persister cells. (A) Exponential grown S. suis strain 10 was treated with 100-fold MIC

of gentamicin for three hours, and at indicated time points CFU were determined. Subsequently, surviving bacteria were incubated in fresh THB media overnight, then grown to early logarithmic phase and challenged with 100-fold MIC of gentamicin. This procedure was repeated for four consecutive cycles. The values are means of three biological replicates and error bars indicate the standard deviation. (B) S. suis strain 10 was sequentially grown to early exponential growth phase. At each cycle CFU of the initial inoculum and of surviving bacteria after a one-hour 100-fold MIC gentamicin challenge were determined. Data were expressed for each cycle as percentage of surviving bacteria in relation to the initial inoculum before antibiotic treatment. The dotted line represents the limit of detection. Standard deviation is shown for three replicates. In order to dissect whether type I or type II persisters are responsible for gentamicin tolerance, we performed a persister cell elimination assay.