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SUMMARY : The organisms whose antibacterial powers are to be investigated are grown in parallel streaks on cellophan superposed on an agar plate. The cellophan is stripped off and the test organisms are streaked on the sterile agar surface thereby revealed, at right angles to the first streaks. After incubation, growth of the test organisms will be interrupted in those areas in which an inhibitor has diffused through the cellophan from the first organisms. There are some limitations in the application of the method. During some recent work (Heatley & Florey, 1946) a strain of Bact. coEi (Escherichia coli) was found to produce an antibiotic which inhibited some coliform organisms at a very high dilution, other coliform organisms being unaffected. Examination of a number of other strains of Bact. coli (M. A. Jennings, unpublished) showed that a few of them also formed antibiotics which were highly specific in the same way, inhibiting some strains of coliforms but not others, but differing from each other and from the first inhibitor examined in regard to the particular strains against which they were active. The fact that the usual methods of detecting antibiotic action, employing only a few different strains, revealed a number of different highly specific inhibitors, suggested that there might be inhibitors formed by other strains of Bact. coli which had escaped detection because the test organisms used were not susceptible. A simple test was required by which several strains of Bact. coli could be tested for antibacterial activity against several different test organisms. The method described below, which embodies more or less well-known principles (e.g. Gratia, 1944), was found to work satisfactorily, and may have wider applications, not only for the detection of antibiotic activity or the comparison of range of antibacterial action, but perhaps also for the selection of strains of bacteria which have special nutritional requirements, or of those which form an excess of certain growth factors, etc. Principle of the method The surface of a rather deep nutrient agar plate is covered with a circular sheet of cellophan and is then dried. The organisms to be tested are sown in parallel streaks on the surface of the cellophan (primary streaks), and the plate is incubated for 16-20 hr. (Pl. 1 A). The cellophan, together with the organisms growing on it, is stripped off, and the test organisms (secondary streaks) are then sown across the surface of the agar at right angles to the position formerly

A Simple Plate Method for Multiple Tests of the Antibacterial Activity of many Bacteria against other Bacterial Strains