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Objectives: Streptococcus tigurinus is a new member of the Streptococcus mitis group. It causes various invasive infections. Its isolation from the oral cavity has been reported recently for the first time in our previous study. An attempt has been made to develop a novel in vitro model for the study of development of resistance to penicillin and to find out its effect on susceptibility pattern of other antimicrobial agents in an isolate of S. tigurinus . Materials and Methods: An in vitro model mimicking in vivo conditions was developed to induce resistance in S. tigurinus with MIC value of 0.12 µg/ml to penicillin by 10 serial passages by sequential exposure to increasing concentration of penicillin. Results: The in vitro manipulation of original strain of S. tigurinus resulted in an increase in MIC value to 8 µg/ml indicating development of resistance to penicillin and a major increase in MIC value of ampicillin from 0.25 to 16 µg/ml, and increase in MIC values of cefepime (0.12 to 0.25 µg/ml), erythromycin (0.12 to 0.25 µg/ml), azithromycin (0.5 to 1 µg/ml) and clarithromycin (0.25 to 0.5 µg/ml) indicating development of cross-resistance to these antimicrobial agents. Conclusion: Results indicate that exposure to increasing concentrations of penicillin can induce the development of resistance to penicillin and cross-resistance to other antimicrobial agents. As in vitro conditions are quite similar to in vivo situation, the results obtained are more appreciable than routine methods using artificial media.

A Newer in Vitro Model for the Study of Development of in vitro resistance to penicillin in a novel isolate of Streptococcus tigurinus