RAPID ASSAY FOR GROWTH: DETERMINATION OF MICROBIOLOGICAL SUSCEPTIBILITY AND FUNGISTATIC ACTIVITY

Present methods for evaluating the susceptibility of different materials to microbiological degradation require from two to six weeks. There is a need for more rapid methods. From the analysis of existing procedures it was concluded that modifications such as using more potent organisms or more nearly optimum conditions could lead only to slight improvements. This paper presents a method that is applicable to this type of assay as well as to the testing of germicides. Results can be obtained within one or two days. The method is a manometric one in which growth of the test organism is measured by oxygen absorption. Analysis of some data on the growth of the fungus Myrothecium verrucaria on cotton cloth showed that, while no measurable changes in tensile strength were found during a period of 48 hours, significant growth had occurred. Development of an assay based on a 48-hour period of incubation then hinged on two points: (1) a reliable method for measuring small amounts of growth, (2) verification of the hypothesis that one can predict the susceptibility of a material to fungus attack on the basis of the amount of growth in the first two days. I In the search for a suitable criterion of growth it was concluded that changes in fresh weight or dry weight of the sample plus mycelium are small and too variable to be reliable. The impossibility of separating fungus from material precludes any direct measurement of the amount of mycelium. Measurement of a particular cellular constituent did not offer much promise. For example, the changes in nitrogen were small and the presence of this element in some fungicides or finishing materials would limit the value of such a method. Furthermore the large amount of organic material in the sample renders oxidation difficult. Calculation of the volumes of CO2 evolved and of 02 absorbed indicated a promising lead. Thus 1 mg of cellulose oxidized completely would result in the absorption of about 0.9 ml of 02 or evolution of an equal volume of CO2. The use of a manometric determination of 02 absorption seemed to offer more advantages than chemical or physical measurement of CO2 evolution from the standpoint of simplicity of apparatus, ease of manipulation by untrained technicians, and the like. Conductometric or volunmetric determination of CO2 may be employed advantageously in certain instances. The applicability of standard microrespirometric techniques adapted to a macro scale was next evaluated. Use of Warburg type manometers was not feasible because of the large thermobarometer corrections occasioned by changes in barometric pressure over a 48-hour period and by the more precise tempera-