Influence of Salts on the Uptake of Streptomycin by Escherichia coli

Abstract

A PREVIOUS report from this laboratory¹ described the kinetics of uptake of streptomycin labelled with carbon-14 by Escherichia coli W growing in mineral-citrate-glucose medium A². In the procedure used, the cells were separated from the medium on a membrane filter and washed with distilled water before determination of radioactivity. We now find that the uptake had been markedly enhanced by a step not reported previously: dilution of each sample with several volumes of distilled water immediately before filtration. (This predilution in water was considered trivial because the subsequent washing with water removed little of the radioactivity on the filter, although almost all of it could be removed by washing with medium A.) When this predilution step was omitted the radioactivity retained on the filter was lower by 50 to 80 per cent (Fig. 1), but there was still, as previously reported, an immediate ‘primary’ uptake and, after a lag, a gradually increasing ‘secondary’ uptake. (Szybalski and Mashima³ have reported kinetics of streptomycin uptake which are difficult to compare with ours since their first sample was not taken until 1 hr. had elapsed and they measured the radioactivity retained after washing with non-radioactive streptomycin.)