Development of gradient diffusion strips to identify the potentiating effect of antimicrobial compounds against multidrug-resistant Klebsiella pneumoniae strains

Objective. The aim of the study was to develop gradient diffusion strips with substances capable of potentiating the action of antibiotics for the treatment of infections caused by multidrug-resistant strains of K. pneumoniae. Materials and Methods. The substances azidothymidine and baicalin were used to evaluate the joint action of combinations of antibiotics with substances that potentiate their action. Determination of susceptibility to combinations of antibiotics (gentamicin, cefotaxime, ciprofloxacin and chloramphenicol) with the listed substances was carried out by a modified gradient-diffusion method (cross-test). We used gradient diffusion strips, which was made in the Saint-Petersburg Pasteur Institute. We investigated clinical multidrug-resistant K. pneumoniae strains (n = 20). The result of the joint action of a combination of two compounds was assessed by calculating the fractional inhibitory concentrations ΣFIC. Results. The values of MIC decreased eightfold when azidothymidine was combined with gentamicin and two times when combined with ciprofloxacin. MIC values were not reduced by the combination of azidothymidine with cefotaxime or chloramphenicol. It has been established that azidothymidine has antibacterial activity against strains of K. pneumoniae: MIC50 – 1 μg/ml and MIC90 – 2 μg/ml. The MIC50 and MIC90 values for baicalin were > 256 μg/ml. A synergistic antibacterial effect was detected when azidothymidine was combined with gentamicin (ΣFIC 0.33–0.50). An additive effect (ΣFIC 0.65–0.84) was detected when azidothymidine was combined with ciprofloxacin. Baikalin reduced the MIC value of cefotaxime by half, and the MIC50 values for gentamicin also decreased by half. Baicalin did not affect the susceptibility of multidrug-resistant K. pneumoniae strains to ciprofloxacin and chloramphenicol. Synergism was detected in the combination of baicalin and cefotaxime (ΣFIC 0.52 – 0.75 – additive effect). Conclusions. The development of gradient diffusion strips with azidothymidine and baicalin makes it possible to simplify methods for assessing the joint action of combinations of these substances with antibiotics and to avoid time-consuming preparatory steps. Our study demonstrated that azidothymidine exhibits synergistic activity in combination with gentamicin and ciprofloxacin and baicalin – in combination with cefotaxime. Further studies are needed to evaluate the potential use of these combinations in practical healthcare.