Roles of Phosphate in Influencing Resistance to and Bacterial Uptake of Tetracycline and Streptomycin in Lactic Streptococci .

The effect of buffering growth media with an equimolar concentration of organic phosphate (β-glycerophosphate) or inorganic phosphate such as KHPO or NaHPO on resistance levels of lactic streptococci to tetracycline and streptomycin was tested. Addition of 88 mM KHPO in the medium was most effective in increasing streptomycin resistance as compared to the same molarity of glycerophosphate or NaHPO. With tetracycline, the strain showed maximum sensitivity in a medium buffered with KHPO as compared to a medium with NaHPO or glycerophosphate. The uptake of H-tetracycline by cells increased steadily with time of incubation in the medium with KHPO, while no steady increase was found in the medium without phosphate. The increased uptake of H-tetracycline in the medium where cells showed maximum sensitivity suggests that in lactic streptococci, resistance to the drug develops because of a reduction in the cellular uptake which can be reversed by phosphate. The reverse may be true for streptomycin.