In previous studies we have shown that CcpA is a pleiotropic regulator of S. suis carbon metabolism, virulence gene expression and the expression of
the arginine deiminase (AD) system [37–39]. The latter is crucial for bacterial survival in acidic environments and is most likely required for alternative ATP generation. Hence, we tested respective S. suis mutant strains 10ΔccpA and 10ΔAD for gentamicin tolerant persister cells. CFU of bacterial strains grown to the exponential growth phase were determined over time after treatment with 100-fold MIC gentamicin. The gentamicin MIC values of the mutant strains did not differ from those of the wild type strain. No change in persister levels was observed for exponential grown strain 10ΔccpA, whereas the AD mutant strain 10ΔAD showed an approximately two log-fold higher persister cell level over time compared to the wild type (Figure 4A). This difference was abrogated
when stationary Selleckchem MK-4827 growth phase cultures were challenged by gentamicin this website (Figure 4B). Interestingly, GDC-0068 clinical trial during the later growth phase the persister level of strain 10ΔccpA decreased as compared to the wild type and strain 10ΔAD. Figure 4 Effect of specific gene inactivation on S. suis persister formation. Exponential (A) or stationary (B) grown S. suis strains were treated with 100-fold MIC of gentamicin over time. Persister cell levels were determined for the wild type strain 10, and its knock-out mutant strains 10∆ccpA and 10∆AD, which lack the genes coding for the global transcriptional regulator CcpA and the catabolic arginine deiminase system, respectively. The values are means of three biological replicates and error bars indicate the standard deviation. Significant differences to wildtype persister levels were calculated by a
one-tailed t-test (*, P < 0.05; **, P < 0.01). Persister cell formation occurs in different S. suis strains and streptococcal species Next, we tested antibiotic tolerance and persister cell formation in other S. suis strains and Nintedanib (BIBF 1120) streptococcal species. For this, we analyzed a human serotype 2 isolate (strain 05ZYH33) originating from a S. suis outbreak in China and a serotype 9 strain (strain A3286/94) isolated from a pig with meningitis [40, 41]. The MIC values of gentamicin for strain 05ZYH33 and strain A3286/94 are given in Additional file 1: Table S1. In all strains, treatment with 100-fold MIC of gentamicin induced the characteristic biphasic killing curve and resulted in a complete killing of bacteria after 24 hours. No substantial differences could be observed between strains in the exponential growth phase (Figure 5). On the other hand, using stationary cultures strain 10 showed the highest degree of drug tolerance. Strains A3286/94 and 05ZYH33 were killed more efficiently, especially during the first hour of antibiotic treatment, with persister cell differences of up to two log-fold CFU.