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Appl Environ Microbiol. 1968 June; 16(6): 877-880
Copyright © 1968 American Society for Microbiology. All Rights Reserved.

Measurement of Three Antibiotics (Penicillin, Cephalothin, and Chloramphenicol) When Present Together in Mixtures

Thorndike Memorial Laboratory, Second and Fourth (Harvard) Medical Services, Boston City Hospital, Boston, Massachusetts 02118
Department of Medicine, Harvard Medical School, Boston, Massachusetts 02118

ABSTRACT

Benzylpenicillin, cephalothin, and chloramphenicol were measured individually and quantitatively when present together in mixtures at concentrations found in patients. The assay depended on the selective inactivation of two of the antibiotics, permitting the third to be assayed as if present alone. Agar seeded with a chloramphenicol-resistant, penicillinase-negative Staphylococcus aureus was used for a cylinder-plate assay of appropriately diluted fluid to determine the activity of either benzylpenicillin or cephalothin after either ß-lactamase inactivation of the former or ultraviolet light inactivation of the latter. Chloramphenicol was assayed with Sarcina lutea after appropriate ß-lactamase inactivation of both cephalothin and benzylpenicillin. Fluids containing various amounts of the three antibiotics were assayed by this method with less than 8% error. Procedures that fail to measure each antibiotic individually may give larger errors.