CLASSIFICATION OF BACTERIA 



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(ii) Peroxide producers. Devoid of catalase. Relatively insensitive to 

 peroxide. Pneumococci and certain streptococci. 



(iii) Non-peroxide producers. Devoid of catalase. Shiga's dysentery 

 bacillus, certain streptococci. 



(iv) Contain catalase. Majority of aerobes and facultative anaerobes. 

 It is obviously difficult, if not impossible, to establish the essential nature, or 

 fortuitous arrangement of such a classification. Organisms of class IV might be 

 sensitive or insensitive to peroxide if they did not contain catalase, and they might 

 or might not form peroxide if they were without this enzyme. This group, therefore, 

 may contain organisms of very different respiratory functions masked by their 

 general property of producing catalase, although it must be conceded that the presence 

 of catalase in many cells of aerobic habit must have the effect of protecting them from 

 the destructive effects of peroxide. With regard to class (iii) it is possible that the 

 absence of peroxide accumulation in cultures of organisms such as B. dysenterice 



o 



20 



120 140 



40 60 80 100 



Time (hours) 



Fig. 23 



Aerated broth cultures 



(Shiga), which do not contain catalase, may be due to the presence of peroxidase 

 (Hewitt, 1931). It seems reasonable to suppose that when peroxidase is present any 

 peroxide formed will rapidly disappear and not accumulate. 



Two main problems emerge from this survey : — 



(i) Does peroxide formation denote a different mechanism of respiration 



from that of non-peroxide forming organisms ? 



(ii) Are obligate anaerobes such because, if grown aerobically, they would 



form peroxide which would immediately stop growth ? 



The alternatives to these are that non-peroxide formation is an accidental 

 phenomenon due to catalase production, etc... and that anaerobic organisms cannot 

 grow in air for reasons other than peroxide production and its harmful effects. 



To return to the experimental data given above, streptococci and pneumococci 

 may be sharply differentiated in behaviour from staphylococci and C. diphthericB. 

 With the two former organisms the electrode potential commences to rise imme- 

 diately after the cessation of active proliferation, whilst, with the latter organisms, 

 the potentials remain at a low (i.e., reducing) level long after the logarithmic phase 

 of growth. In aerated cultures (fig. 23) also the behaviour of the two different types 

 of organisms may also be readily distinguished. With streptococci and pneumo- 



