Transient exposure to a physiologically-relevant concentration of calcium confers tobramycin resistance upon sessile cells of Pseudomonas aeruginosa.

Sessile populations of a mucoid Pseudomonas aeruginosa isolate generated in M-56 medium of 'low' (0.02 mM) or 'high' (2.5 mM) Ca2+ were treated with tobramycin (250 micrograms.ml-1), as were biofilms exposed transiently to 'high' Ca2+ medium. Viability decreased by over 99.9% within 8 h in the 'low' and 'high' samples, while transient Ca2+ exposure was protective. Dispersed sessile bacteria were as tobramycin sensitive as planktonic bacteria.