The newly-released antibiotic, carbenicillin (Pyopen, Geopen), opens a new dimension in our ability to cope with Pseudomonas and indole-positive Proteus infections, two notoriously difficult organisms to wrestle with in the past from the antibiotic standpoint. It was first synthesized a number of years ago, but its use was not pursued at that time. Later, it was independently rediscovered in England and therapeutic trials in humans were initiated.
Chemically, carbenicillin is disodium alpha-carboxybenzyl penicillin and thus closely related to ampicillin, an aminobenzyl penicillin. This kinship to ampicillin is a convenient way of remembering the antibacterial spectrum of carbenicillin because, in general, it is active against the same array of gram-positive and gramnegative bacteria. The notable exceptions are Pseudomonas and Proteus. Ampicillin is generally active against indole-negative Proteus (P mirabilis) but ineffectual against the indole-positive species (P vulgaris, P rettgeri and P morgani). These organisms are not frequent causes of serious infections
NELSON JD. Carbenicillin— A Major New Antibiotic. Am J Dis Child. 1970;120(4):382-383. doi:10.1001/archpedi.1970.02100090156024