28 BACTERIOLOGY FOR NURSES 



and 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 develop- 

 ment of most pathogenic bacteria is that of the hu- 

 man 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 forms; some spores withstand- 

 ing 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 identification, the thermal 

 death point of all of the common species being well 

 known. 



Light. That light affects the activities of living 

 cells has long been known; the effects of light upon 

 the higher forms of life have been the subject of much 

 investigation during recent years. Sunlight has a 



