A new approach to determination of bactericidal activity of benzylpenycylin on Corynebacterium diphtheriae

Abstract

Information on the bactericidal activity of benzylpenycylin is important when planning antibiotic therapy for a number of diseases caused by Corynebacterium diphtheriae (endocarditis, bacteremia, septicemia, etc.). However, methods for determining the minimum bactericidal concentration (MBC) of antibiotics are complicated. The aim of the study was to develop a method for testing of diphtheria causative agent susceptibility to the bactericidal action of penicillin without MBC determination. The minimum inhibitory concentrations (MICs) and MBCs in 80 strains of C. diphtheriae were determined using the standard broth dilution method (macromethod). The MICs were registered after 24 and 48 years of growth. After the first day of growth the MIC of penicillin was in the range of 0.017 to 0.5 mg / L, after second day — in the range of 0.035 to 0.5 mg / liter. The increase of the MICs for the second day of growth was observed in 47.5±5.6% of strains. MIC50 and MIC90 both after 24 hours and after 48 hours of growth were 0.13 and 0.25 mg/L, respectively. MBCs of penicillin was in the range of 0.5 to 32.0 mg/L, MBC50 was 4.0 mg/l, MBC90 – 8.0 mg/l. In 35,0±5,3% of the studied strains, high MBCs (8.0 mg / l or higher) was detected. It has been established that in C.diphtheriae the relationship between the MIC and the MBC of penicillin is clearly expressed, taking into account the two values of the MIC – for the first and second day of growth. Investigated strains are divided into three conditional groups: 1) with MIC 0.13 mg/L and basically with low MBCs; 2) with MIC 0.25 mg/L, in most cases are not susceptible to bactericidal action, and 3) strains with MIC 0.5 mg/L and high MBCs. The most pronounced were differences in the prevalence of corynebacteria with high MBCs in two groups of strains: MICs which did not exceed 0.13 mg/L after 24 and 48 hours, and those in whom the MIC was 0.25 mg/L or higher already on the first day of incubation (Student's coefficient t=4.13, p<0.001). The obtained results can be used to improve the methods for determining of corynebacteria susceptibility to antimicrobials.