Activity of Combinations of Antibiotics

The bactericidal activity of combinations of antibiotics have been studied by two methods; in onie, li(quid media are used (Kirby, 1944; Hobby and Dawson, 1946; Price et al., 1949; Bigger, 1950; Bliss et al., 1952; Jones and Finland, 1956) and in the second, solid media are preferred. A simplified method using li(quid media, suggested by Jawetz and co-workers (1955), can be used in the clinical laboratory. It has been further modified by us (Chabbert, 1953b). In this modification (the disc tube method) which we use routinely, the percentage of survivors is estimated by streaking on a solid medium. These simplified techniques permit the study of only a small number of arbitrarily selected concentrations of antibiotics, but the methods utilizing diffusion on solid media permits one to obtain gradients of concentrations (Bonifas, 1952; Chabbert, 1953a; Knapp and Jacherts, 1955; Dye, 1956; Weil and Kotsevalov, 1956). Dye (1956) suggested the use of two strips of filter paper, deposited at right angles on the surface of the agar to permit the study of the bacteriostatic effect of varying concentrations of two different antibiotics. To study the bactericidal effect of combinations of antibiotics, the method of Dye had to be modified to permit the transfer of survivors to an antibiotic-free medium. This transfer can be done by the velveteen replica plating method of Lederberg and Lederberg (1952) (Elek and Hilson, 1954; Pilkington et al., 1956; Manten, 1956) or by growing the organisms to be studied on cellophane. This method proposed by Gratia (1944) and Heatley (1947) has been modified further in our work (Chabbert, 1953a). Recently we described a method permitting the growth of bacteria on a diaphragm of cellophane (Chabbert, 1957). The combination of this method with that of Dye (1956) permitted us to study the bactericidal effect of combinations of antibiotics in the clinical laboratory.