Turbidimetric assay procedures for antibiotic substances have been proposed by Foster (1942) ; McMahan (1944) ; Lee, Foley, and Epstein (1944) ; and Joslyn (1944). Standard methods for the assay of penicillin and streptomycin are used by the Food and Drug Administration (Federal Register, 1947, and recent revisions) and by manufacturers of these antibiotics. Not one of these procedures is exactly applicable to the assay of antibiotic materials for which no standards have been established.

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... MENTAL DEVELOPMENT In order to develop a method that would be suitable for determining the antibiotic strength of experimental preparations, such as broth filtrates and chemical fractions as well as the finally purified substance, growth curves of many pathogenic bacteria were studied under varying conditions. Variables that were investigated include the amount of inoculum, the type of nutriment, and the presence or absence of antibacterial substances. The effect of varying the size of the inoculum is shown in figure 1. Figure 2 illustrates the differences between three different nutrients. The effect of the presence of small amounts of an antibacterial agent may be seen in figure 3. One characteristic of these data is that, after a period of approximately 4 hours, turbidity, which is indicative of active growth, begins to level off. This is interpreted as signifying that the period of most active growth occurs before the cultures are 4 hours old. In order to demonstrate this more conclusively and to point out any significant secondary increase in growth during a 24-hour period, a series of cultures were studied by ascertaining their growth according to turbidimetric readings over a 26-hour period. Figure 4 presents typical results of this portion of the study. These curves demonstrate that the period of most active growth in each case occurred during the first 4-hour period. Examination of growth curves altered by the presence of antibacterial agents shows that there are apparently two different types of modification. In one, the growth curves of cultures containing increasing amounts of antibiotic fan out in a fairly regular manner. Each curve that contains a greater portion of antibiotic agent departs further from the "normal" in a regular and definite manner. In the other, the fanning out is less orderly, and not until the amount of antibacterial substance is relatively large does the growth curve depart markedly from the "normal" and growth of the culture suddenly become nearly negligible. In the latter instance there is a great difference in the behavior of growth curves of cultures of bacteria containing only slightly different concentrations of antibacterial substance. 711 on May 9, 2020 by guest