METHODS OF STUDYING BACTERIA 37 



aerobic ; whilst others will grow only in its absence, for ex- 

 ample the anaerobic Bacillus tetani, the cause of lockjaw or 

 tetanus. Many, however, can flourish under either condition. 

 The best or "optimum" temperature, as well as the 

 maximum and minimum at which organisms will grow, has 

 to be studied. Most disease-producing bacteria grow best at 

 the body temperature of their hosts in the case of man, 37 C. 

 Organisms of putrefaction such as are found in water, soil, 

 etc., grow at ordinary temperatures, but flourish most abund- 

 antly at summer heat, and 

 it is, of course, common 

 knowledge that meat, milk, 

 etc., "go bad" much more 

 rapidly in summer than in 

 winter, "keep" best if 

 kept cool, and indefinitely 

 if frozen. Such freezing, 

 however, does not kill the 

 majority of bacteria, but 

 only " inhibits " or pre- 

 vents their activities. Even 

 prolonged exposure to tem- 

 peratures far below C. 

 (32 F.) has little effect 

 on their vitality. Certain 

 thermophilic or heat-loving 

 bacteria can live at com- 

 paratively high tempera- 

 tures, for example those 

 found in hot springs, and 

 in hot beds of decaying 

 organic matter, and to the 

 activities of these, amongst 



FIG. 12. Biological Incubator, for the cul- 

 tivation of micro-organisms in which 

 cultures may be grown at a suitable 

 temperature, usually 37 C. 



other factors, is due the "heating" of haystacks, manure- 

 heaps, etc. 



The thermal death point, or the temperature at which 

 they are destroyed, has already been alluded to under 

 Sterilisation (p. 24). 



The actual size, shape, colour, method of spread, and other 

 physical characters of the colonies or masses of bacterial 

 growth in culture are of considerable importance in identify- 

 ing bacteria. They depend to some extent on physical con- 

 ditions, e.g. temperature, nature of the nutrient medium 

 employed, and the amount of moisture present; but under 

 similar conditions, an organism tends usually to show the 

 same characters more or less constantly. Members of the 



