Although the hanging-drop method for the detection of bacterial motility can be employed successfully, the procedure has several distinct disadvantages. It is so tedious that the determination of this bacterial characteristic is frequently neglected in the routine laboratory. Furthermore, the results are often uncertain because it is difficult to observe motility when only a few of the cells in a culture are motile. Finally, it is necessary to provide relatively young cultures for the examination. The use of semi-solid media for the determination of bacterial motility, on the other hand, eliminates the shortcomings of a hanging-drop technique. The results are macroscopic. They are cumulative, thereby particularly qualifying the method for use in the routine laboratory, where examinations cannot always be carried out at a specific time. Moreover, this method practically eliminates the possibility of overlooking motility when only a small proportion of motile cells are present, because the localized out-growths, which occur wherever motile cells are deposited along the stab, can hardly escape notice. Semi-solid media have been employed for many years in the study of bacterial motility. Rosenthal (1895) reported marked differences in the size and shape of bacterial colonies which had developed in semi-solid nutrient gelatin. Klie (1896) called attention to the spreading and thread-like appearance of colonies 1 This work was supported in part by contributions from the General Education
Although it has been repeatedly observed that the thermal death time of bacteria is dependent upon the temperature employed (the influence of the initial concentration of bacteria, have been ignored by a number of workers. When this factor is considered, the thermal death time follows a regular order which is adequately described by applying the equation for the "mono-molecular reaction rate" (Baker and Mc-Clung, 1939;Bigelow, 1922;Chick, 1910;Madsen and Nyman, 1907). This has been generally ignored by workers interested in thermal death, and as a result the literature contains a number of conflicting statements in this regard. This has made it difficult to evaluate some of the bactericidal aspects of the pasteurization process. As an aid in quantitative determinations of bacterial death by heat, the authors have proposed the concept of "decimal reduction time" (Katzin and Sandholzer, 1942) which is based upon the application of the monomolecular reaction rate constant to bacterial death under uniform conditions. The purpose of this study is to illustrate the application of this principle to the problem of the use of coliform bacteria as indicators of the sanitary status of pasteurized milk.
MATERIALS AND METHODS
CulturesIn all, 66 strains of Escherichia were employed. These were originally isolated from a variety of sources including human, monkey, bovine and sheep feces, human blood, soil, milk, water, urine and oysters. The initial source of some of the older laboratory strains is uncertain.
Outlines of the changes in shape and size of bacteria under the influence of bacteriophage have been drawn from numerous visual and photographic records, and this composite material has given some insight into the mechanism of the lytic process. These observations, amply reviewed by Bronfenbrenner (1), do not require recapitulation here. Most observers have confirmed d'Herelle's (2) statement that Gram-negative bacteria in a moist medium enlarge by swelling and eventually burst. Whether Gram-positive bacteria swell or burst is still an open question, as shown by the contradictory evidence in the recent papers of Bronfenbrenner (3) and Krueger and Northrop (4). The time intervals of the morphological changes preceding lysis, the relation of these changes to stages of growth and the time occupied by the final bursting or disintegration have not been precisely determined. The amount, character, distribution and persistence of the residue remaining after lysis have not been adequately described. Very few measurements of dimensions of cells or of time intervals have been presented in reports on the bacteriophagic process. In their lack of quantitative data on serial changes in cells and their residues, the available records are deficient in the material needed for the construction of generalizations.Recognizing this deficiency, Bronfenbrenner, Muckenfuss and Hetler (5) were the first to attempt to supply the required data by making 279 on May 7, 2018 jem.rupress.org Downloaded from
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