Comparison of Spontaneous Antibiotic Resistance Frequency of Salmonella Typhimurium Growth in Glucose Amended Continuous Culture at Slow and Fast Dilution Rates

Abstract

The objective of the study was to determine the frequency of spontaneous acquisition of resistance to select antibiotics by Salmonella Typhimurium (ST) when grown in glucose amended continuous flow culture at slow (D = 0.025 h^{− 1}) or fast (D = 0.27 h^{− 1}) dilution rates. The bacterium was grown in LB minimal medium (pH 6.25) containing no antibiotics. Upon achieving steady state, samples were plated to tryptic soy agar (TSA) alone or supplemented (per ml) with 2 and 16 μg oxytetracycline, 4 and 16 μg tetracycline, 2 and 64 μg kanamycin, and 0.25 and 2 μg enrofloxacin. Regardless of growth rate, CFU of resistant ST from the TSA containing antibiotics was less than 2 × 10¹ except for 2 μg kanamycin and 0.25 μg enrofloxacin treatments (higher than 1 × 10⁹ and 4 × 10⁷ CFU of resistant ST for trials 1 and 2, respectively). Frequency of recovering resistant ST from the TSA containing the higher antibiotic concentrations was less than 1 in 10⁹ for all antibiotics, but was higher on the media containing 2 μg kanamycin and 0.25 μg enrofloxacin at both slow and fast growth rates. In general, minimal susceptibility differences were detected for isolates from slow and fast dilution rates.

Keywords:

• Antibiotic resistance
• Dilution rate
• Continuous culture
• Salmonella Typhimurium.

ACKNOWLEDGMENT

This research was supported by the United States Department of Agriculture (USDA-NRI grant number 2001-02675), USDA FAS #58-3148-3-038, Hatch grant H8311, and U.S. Poultry and Egg Association grant #485.

Notes

¹Dilution rate.

¹Dilution rate.

*Resistance breakpoints.

*Resistance breakpoints.

*Resistance breakpoints.

¹Minimum inhibition concentration.

²Dilution rate.

³Not available.