Comparison of spontaneous antibiotic resistance frequency of Salmonella Typhimurium growth in glucose amended continuous culture at slow and fast dilution rates

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 microg oxytetracycline, 4 and 16 microg tetracycline, 2 and 64 microg kanamycin, and 0.25 and 2 microg enrofloxacin. Regardless of growth rate, CFU of resistant ST from the TSA containing antibiotics was less than 2 x 10(1) except for 2 microg kanamycin and 0.25 microg enrofloxacin treatments (higher than 1 x 10(9) and 4 x 10(7) 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 10(9) for all antibiotics, but was higher on the media containing 2 microg kanamycin and 0.25 microg enrofloxacin at both slow and fast growth rates. In general, minimal susceptibility differences were detected for isolates from slow and fast dilution rates.     

Effects of temperature,nutritional factors and salinity on the uptake of L-methionine by the Sydney rock oyster Saccostrea commercialis

The effects of physiological and nutritional factors and of temperature on the uptake of L-methionine by Saccostrea commercialis were investigated on cultured rock oysters from Port Stephens, New South Wales, Australia, in 1983. Optimum conditions for L-methionine accumulation were 30°C, 18 h exposure and concentrations of other amino acids less than that of L-methionine. The uptake of L-glycine was inhibited by L-methionine in a reciprocal manner. There was no effect of salinity on the accumulation of L-methionine by acclimated oysters. During the latter investigations the range of osmoconformity was found to be 15 to 45 S. Oysters take 2 d to conform to new media osmolarities after salinity changes of 15. Amino acid supplements in oyster diets should be kept within the same order of magnitude to reduce inhibition of uptake.