Pseudomonas resistance due to inactivated susceptibility disks.

The deterioration of carbenicillin is similar to that of other penicillins. Susceptibility test results, by using disks which have been exposed to humidity, may indicate false resistance. Proper storage and handling methods are suggested.

Drug resistance as influenced by inactivated sensitivity discs.

Reports of staphylococci resistant to the semisynthetic penicillins stimulated a study of the factors influencing the stability of the drugs in discs. The behavior of penicillin G, methicillin, oxacillin, cloxacillin, and cephalothin discs under different humidity and temperature conditions is described. Humidity was found to be the most significant factor in drug inactivation. Storage of discs in a vacuum desiccator at -20 C provides maximal antibiotic stability.