360 METHODS OF OBTAINING PURE CULTURES 



(1) The bacteria that are susceptible to low degrees of heat, and low concen- 

 trations of chemical disinfectants ; this class includes the non-sporing bacteria 

 and the vegetative forms of the spore-bearing bacteria. They are destroyed by 

 moist heat at 60° C. in half an hour and by 1 per cent, phenol within an hour. 



(2) The bacteria that are susceptible to low degrees of heat, but are resistant 

 to low concentrations of disinfectants ; this class includes the acid-fast bacteria, 

 which are killed at 60° C. in half an hour, but resist destruction by chemical agents 

 in the cold often for several hours. 



(3) The bacteria that are resistant both to low degrees of heat and low 

 concentrations of disinfectants ; this class includes the sporing forms of the 

 spore-bearing bacteria. To kill them with certainty, steam under pressure at a 

 temperature of 120° C. for half an hour should be employed, or high concentra- 

 tions of disinfectants, for example, 5 per cent, phenol, maintained for several 

 hours. 



A study of the resistance of a given bacterium will, as a rule, merely serve to 

 confirm the conclusions already reached by the three previous methods of exam- 

 ination, but occasionally it is in itself of some diagnostic importance. Thus certain 

 of the non-sporing vegetative bacteria, for example the enterococcus, are not 

 destroyed at 60° C. in half an hour ; they require a temperature of 65° C. ; 

 this abnormal heat resistance is of value in differentiating this species of strepto- 

 coccus from other species, which are readily killed at the lower temperature. 

 Whenever an organism is suspected of forming spores, the heat resistance must 

 be tested, and not till the suspected spores have definitely been found to be 

 resistant to heat should the conclusion be reached that they really are spores. 

 Many forms have been interpreted in the past as being true spores, which on 

 subsequent examination have been found to be devoid of the characteristic property 

 of heat resistance. 



E. Metabolism.- — Under this heading is included a study of the oxygen pressure 

 required for growth, the optimum temperature for growth, pigment formation, 

 haemolysin production, and the elTect on growth of adding different substances to 

 the medium. It is usual to divide bacteria into 3 classes according to their 

 oxygen requirements • (1) Strict aerobes : these organisms will grow only in the 

 presence of free oxygen. (2) Strict anaerobes : these will grow only in the absence 

 of free oxygen. It must be noted, however, that growth will occur in the 

 presence of molecular oxygen, provided the medium contains a reducing system 

 capable of bringing about a sufficiently low 0-E. potential. (See Chapters 3 

 and 36.) (3) Facultative anaerobes : these grow best under aerobic conditions, 

 but are able to grow under anaerobic conditions. To these may be added a fourth 

 class, the microaerophiles, comprising those organisms that grow best under a 

 pressure of oxygen lower than that of the atmosphere. According to their tem- 

 perature requirements bacteria may be divided into (1) the mesophilic, which 

 have an optimum temperature between 20° C. and 40° C, and (2) the thermophilic, 

 which have an optimum temperature between 60° C. and 70° C. In medical 

 bacteriology, the important distinction lies between those organisms that will 

 grow at room temperature as well as at 37° C, and those that will grow at 37° C. 

 but not at room temperature. The latter class includes many of the highly 

 parasitic organisms. The power to hsemolyse may be studied by growing the 

 organism on blood agar plates, or by mixing varying dilutions of a broth culture 

 with a suspension of washed red cells. This property is of considerable import- 



