Abstract: To investigate the physiological response of S. thermophilus grx02 to ciprofloxacin, the minimal inhibitory concentration (MIC) of ciprofloxacin on S. thermophilus grx02 was determined. The growth curve and pH changes of S. thermophilus grx02 in MRS broth supplemented with 0, 1/2 MIC, 1/10 MIC and 1/50 MIC ciprofloxacin were determined. In addition to this, morphological observations under light microscope and transmittance electron microscope (TEM) were conducted, and key enzyme activities involved in intracellular lactose metabolism were assayed in the presence of ciprofloxacin. The results showed that the MIC of ciprofloxacin on S. thermophilus grx02 was 3.89μg/mL. The growth and acid production rate of S. thermophilus grx02 decreased significantly after addition of 1/2 MIC ciprofloxacin. The bacteria became semi-oval and the capsules became thin and compact after addition of different sub-MIC concentrations of ciprofloxacin. The activity of β-galactosidase and lactate dehydrogenase decreased obviously after addition of 1/2 MIC ciprofloxacin. After addition of 1/50 MIC ciprofloxacian, the β-galactosidase activity decreased but lactate dehydrogenase activity increased. In conclusion, our results showed that, under the stress of ciprofloxacin, S. thermophilus turned on some stress response system to inhibit cell division and lead to complex changes in the physiological mechanism.

Key words: ciprofloxacin, stress, S. thermophilus, physiological characteristics