The influence of subminimal inhibitory concentrations (sub-MIC) of benzylpenicillin on antibiotic susceptibility levels, the cultural-morphological and biochemical properties of C. diphtheriae have been investigated. Two C. diphtheriae strains (biovariants gravis and belfanti) were used. The MICs and the minimal inhibitory concentrations (MBCs) of penicillin have been determined by using the standard broth dilution method (macromethod). The MICs were registered after 24 and 48 years of growth. Initially, both strains were susceptible to the bacteriostatic and bactericidal action of penicillin: MICs were 0,05 and 0,13 mg/L, MBCs - 1,0 and 2,0 mg/L. After passing the broth with sub-MIC penicillin, the increase of MICs and MBCs was observed in both strains. The maximum value of MICs was 1,0 mg/L, MBCs - 128,0 mg/L. In both C. diphtheriae strains the penicillin-tolerance was developed (MBC/MIC = 128). Corynebacteria that had lost penicillin-susceptibility exhibited typical phenotypic properties which are required for the bacteriological identification of this pathogen. The penicillin-resistance culture of corynebacteria began growing slowly, demonstrating the delayed fermentation or did not show the ability to utilize saccharides. At the same time, the ability of toxin production was maintained. The ability of penicillin-susceptibility and penicillin-tolerance of C. diphtheriae to the viability in the presence of high antibiotic concentrations (<$E128,0~symbol О~256,0> mg/L) was tested. After 48-hour presence in high concentrations of penicillin in cultures of susceptible strains, the viability has maintained in no more than 0,001 % of cells. In cultures of corynebacteria with acquired tolerance, the percentage of living cells was more than 10 % of the total number before the end of the exposure period; the ability to colonize in the blood agar was retained in about 0,1 % of cells, forming, however, atypical small colonies. Also the formation of sub-populations of non-culturable persister cells was observed in cultures of penicillin-tolerant C. diphtheriae. Significant changes in the cell wall and the intracellular structure were observed in these cells. The recirculation of the C. diphtheriae from persister-cells occurred after their transfer to a fresh serum broth, holding at a temperature of <$E2~-~6~symbol Р roman C> and the subsequent incubation at <$E37~symbol Р roman C> during 48 - 96 hours. Corynebacteria, being recultivated from non-culturable forms retained the acquired level of penicillin-resistance (including tolerance), atypical culture-morphological properties and slowed down (during 24 - 72 h) the manifestation of the basic biochemical properties. Atypical manifestations of biological properties of C. diphtheriae can lead to the complication of indication and identification of the diphtheria causative agent according to the standard pattern, during the examination of patients after antibiotic therapy, in particular. The persister formation is important for C. diphtheriae population strategy in adverse conditions.

• Капустяк Костянтин Євгенович (біологічні науки)
• Корнійчук Олена Петрівна (біологічні науки)
• Корнійчук Олександр Петрович (економічні науки)

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