Effects of Sitagliptin on Serratia marcescens

The effect of sub-inhibitory concentration of sitagliptin on the growth of the tested strain of S. marcescens was detected according to Nalca et al. [25 (link)]. Overnight culture from S. marcescens was prepared in LB broth and adjusted to 0.5 McFarland Standard. The prepared suspension was used to inoculate LB broth containing 1 mg/ml of sitagliptin and control LB broth without sitagliptin so that the final inoculum is approximately 1×10⁸ CFU/ml. After overnight incubation at 37°C, the optical densities of both cultures were measured at 600 nm by using Biotek Spectrofluorimeter (Biotek, USA). The experiment was performed in triplicate and data are presented as the mean ± standard error. A P value < 0.05 was considered statistically significant using Student's t-test with (Graphpad Prism 5 software).
Sub-inhibitory concentration of sitagliptin was used to investigate its anti-virulence and anti-quorum sensing activities on S. marcescens. The reason for the use of this concentration is to avoid any effect on the growth of the tested bacterial strain. The OD₆₀₀ of sitagliptin sub-MIC-treated (1 mg/ml) and untreated cultures of S. marcescens were compared to show that the growth was not affected by sitagliptin treatment. For normalizing the results in all the next experiments, the sitagliptin treated or untreated bacterial cultures were adjusted to the growth density OD₆₀₀ of 0.4 (1×10⁸ CFU/ ml).