Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IV THE EFFECT OF PHYSICAL AND CHEMICAL AGENTS UPON BACTERIA, AND THE EFFECTS OF BACTERIAL GROWTH Like the higher forms of vegetation, bacteria are susceptible to many influences, the physical and chemical conditions which surround them determining whether they shall live and multiply, or lie dormant, or perish. Temperature. — Three points of temperature are considered in the growth of different bacteria: a minimum being the lowest point at which growth occurs, an optimum being the temperature of most luxuriant growth, and the maximum being the highest degree at which growth can take place. The extremes of temperature between which the majority of bacteria are known to grow are 5.5 C. (41.9 F.) and 43 C. (109.4 F.), although species exist which may multiply at 70 C. (158 F.) and others as low as 0 C. (32 F.). The bacteria commonly found in soil and waterand the bacilli of diphtheria and typhoid fever are much less sensitive to low degrees of temperature than to extremes of heat, sometimes surviving weeks of freezing. A certain species of bacteria commonly found in sewage shows no signs of growth in a temperature below 60 C. (140 F.). The favorable degree of temperature for the development of most pathogenic bacteria is that of the human body, 37 C. (98.6 F.). The range between the minimum and maximum degrees of temperature in bacterial resistance is equally great, spores being much more resistant than the vegetative farms; some spores withstanding boiling for sixteen hours, while the vegetative forms are usually killed by ten minutes' exposure to 60 C. (140 F.) with moisture. By thermal death point is meant the degree of temperature necessary to kill the organisms, in a given time; and as this varies with different species, it is used as a means of iden...