BACTERIAL RESPIRATION 199 



water and oxygen, even in the absence of any oxidisable 

 compound to take up the liberated oxygen. On the 

 basis of the presence of catalase McLeod has divided 

 the bacteria into four groups : — 



(a) Strict Anaerobes. — These organisms have no cata- 

 Jase and are very sensitive to the presence of hydrogen 

 peroxide. Since they produce the latter in presence of 

 oxygen they are incapable of growth aerobically. They 

 are not sensitive to cyanide. 



{b) Micro-aerophilic Organisms. — Members of this 

 group have no catalase but produce hydrogen peroxide ; 

 however, they are only moderately sensitive to it, and. 

 can therefore survive if the oxygen tension is not too 

 great. As examples may be quoted the pneumococcus, 

 most streptococci and the lactic acid bacteria. 



(c) Non-peroxide Producers. — These produce neither 

 catalase nor hydrogen peroxide and can tberefore grow 

 aerobically in spite of the absence of catalase. Examples 

 are Shigella dysenterice and Str. fcecalis. 



(d) Catalase Producers. — Bacteria of this group pro- 

 duce hydrogen peroxide, which is immediately broken 

 down by catalase. The aerobes and most facultative 

 anaerobes belong to this class. They are sensitive to 

 cyanide, which inhibits catalase action. 



Hydrogen peroxide and organic peroxides can also be 

 decomposed by the enzyme peroxidase, which differs 

 from catalase in that an oxidisable substance must be 

 present to take up the liberated oxygen or to donate 

 hydrogen, as the case may be . If such a second substance is 

 absent no decomposition of the peroxide takes place. The 

 organic peroxides usually arise from the oxidation by atmo- 

 spheric oxygen of di- or trihydric phenols, such as catechol, 



