After freezing and thawing, Pseudomonas aeruginosa harboring a drug resistance plasmid (Hg , Strep ), became acutely sensitive to mercuric ions but not to streptomycin in the plating medium. Its sensitivity to both agents in the plating medium became more pronounced, suggesting a synergistic effect of the mercuric ions with streptomycin. This freeze-thaw-induced sensitivity was transient and ca...