Induction of In Vitro Resistance to Penicillin in Viridans Group Streptococci and Its Effect on Susceptibility Pattern of Other Antimicrobial Agents

c Abstract: Background: The development of resistance to penicillin in Viridans Group Streptococci (VGS) during therapy has been reported. However, the in vitro development of resistance to penicillin or other antimicrobial agents in VGS is mostly overlooked and rarely reported. Aim & Objectives: To induce in vitro resistance to penicillin in VGS and to study its effect on susceptibility pattern of other structurally related (Beta–lactams) and unrelated antimicrobial agents. Material and Methods: Four isolates of VGS susceptible to all antimicrobial agents were manipulated in vitro to induce resistance to penicillin by sequential exposure to increasing concentrations of penicillin. Results: Increase in MIC values of penicillin from 0.06-0.12 μg/ml to 2-32 μg/ml was observed indicating development of resistance to penicillin. A significant increase in Minimum Inhibitory Concentration (MIC) values of ampicillin and slight increase in MIC values of other antimicrobial agents in some isolates was also noted. Conclusions: Exposure to increasing concentrations of penicillin can promote the development of resistance to penicillin and cross-resistance to other antimicrobial agents suggesting its mutagenic role.