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Growth kinetics of Streptococcus thermophilus at subbacteriostatic penicillin G concentrations
F Yöndem1, M Ozilgen, T F Bozoglu
1Food Engineering Department, Middle East Technical University Ankara, Turkey.
Abstract:
Streptococcus thermophilus may be subjected to the effects of penicillin G in contaminated milk used for yogurt production. Sensitivity of this microorganism to penicillin G has been conventionally determined by the help of penicillin G-impregnated disks placed on solid media. It was observed that the bacteriostatic penicillin G concentration was much greater in liquid media than in solid media. The conventional disk method may not be appropriate for antibiotic sensitivity determinations if the microorganisms will be used in liquid culture. A simple mathematical model simulated the growth of S. thermophilus in liquid culture. Numerical values of this model's parameters were regarded as the measure of the antibiotic effect on the culture. In penicillin G containing fresh medium, small concentrations of antibiotic decreased the specific growth rate considerably. Increasing the antibiotic concentration caused only slight additional decline. Antibiotic shock, i.e., rapidly introducing penicillin G into an actively growing antibiotic-free culture, stopped growth of the penicillin G-resistant microorganisms, and no death was observed, but a fraction of the microorganisms were killed in the wild culture. Both the wild and the resistant cultures recovered from the shock in a few hours. Addition of penicillin G-resistant microorganisms together with the antibiotic dosage into the wild culture prevented death.
Insights
Penicillin G sensitivity testing for Streptococcus thermophilus in liquid cultures requires different methods than standard disk diffusion. Mathematical modeling revealed penicillin G significantly impacts bacterial growth rate and survival in liquid media.
Area of Science:
- Microbiology
- Food Science
- Pharmacology
Background:
- Streptococcus thermophilus is crucial for yogurt production.
- Penicillin G contamination in milk can affect S. thermophilus.
- Conventional antibiotic sensitivity testing uses solid media (disk diffusion).
Purpose of the Study:
- To evaluate the effectiveness of penicillin G on S. thermophilus in liquid culture.
- To determine if conventional disk diffusion methods are suitable for liquid culture applications.
- To model the impact of penicillin G on S. thermophilus growth dynamics.
Main Methods:
- Comparison of penicillin G bacteriostatic concentrations in liquid versus solid media.
- Development and application of a mathematical model to simulate S. thermophilus growth in liquid culture.
- Observation of bacterial response to both gradual and sudden (shock) penicillin G exposure.
Main Results:
- Bacteriostatic penicillin G concentrations are higher in liquid media than solid media.
- Low penicillin G concentrations significantly reduce S. thermophilus growth rate.
- Antibiotic shock initially halts growth but cultures recover; some cell death occurs in wild cultures.
- Addition of resistant strains mitigates cell death during shock exposure.
Conclusions:
- Conventional disk diffusion may underestimate penicillin G's effect in liquid cultures.
- Mathematical modeling provides insights into S. thermophilus response to penicillin G.
- Understanding penicillin G's impact is vital for ensuring yogurt quality and safety.