Effect of Growth at Sub-lethal Concentrations of Kanamycin on the Cell Membrane Integrity and Amount of Capsular Glucuronic Acid in Wild-type Escherichia coli and Strain with a cpsB Mutation

It has previously been reported that the aminoglycoside antibiotic kanamycin induces the production of extracellular capsular polysaccharide and loss of cell membrane integrity in Escherichia coli due to the introduction of misfolded membrane proteins. However, it is not known if the increased production of the polysaccharide capsule protected the bacterium by allowing the recovery of membrane integrity. We attempted to examine this possibility in this study by growing E. coli B23, CLN08w1 (a cpsB mutant not expected to produce capsule), and BW2511-3 (parent strain of CLN08w1) with sub-lethal concentrations of kanamycin, determined by minimal inhibitory concentration test, in M9 minimal medium with 0.4% w/v glycerol. The growth of each strain was followed by turbidity readings, and no difference in growth rates was observed between the treatment and controls. We isolated the capsular material at an initial and late stage of growth to perform a glucuronic acid assay, and observed an increase in capsular production of all three strains in the presence of kanamycin. No consistent pattern of change in membrane integrity was observed as measured by propidium iodide. From the results, we concluded that either production of capsules is not correlated to change in membrane integrity, or that the propidium iodide assay is not sensitive enough to monitor the change in membrane integrity.