BACTERIAL CULTURES 



123 



POTENTIALS OF DIFFERENT BACTERIA 



Hcemolytic streptococci and pneuniococci. —^These organisms, as described in an 

 earlier section, are deficient in catalase activity and hence peroxide is formed in 

 aerated cultures. For this reason the electrode potential does not remain at a low 

 level after the cessation of active proliferation, but begins to rise to the high positive 

 potential characteristic of peroxide. These phenomena are not apparent when 

 catalase preparations are added to the cultures (Hewitt, 1930, 1931). The bacteria 

 are deficient in various oxidative mechanisms as well as catalase, and although they 

 are fairly active in effective fermentations of sugars, they display httle oxidising 

 activity towards the fermentation products (Hewitt, 1932). Oxidation-reduction 

 conditions affect the behaviour of the hsemolysin of those hsemolytic streptococci 

 which are of importance in human infections and the maintenance of the virulence 

 of these organisms also is apparently related to the electrode potentials of the 

 cultures as described in a later section. 



— 0-4 



24 48 72 



Time (hours) 



Fig. 35 Aerated broth cultures 



96 



120 144 



-0-2 



120 



24 48 72 



Time (hours) 

 Fig. 36 Aerated broth cultures 



144 



C. diphthericB. — Diphtheria bacilli display very marked catalase activity, as 

 may be demonstrated by letting fall a drop of hydrogen peroxide upon a cultm'e 

 on solid medium and observing the effervescence which is frequently vigorous enough 

 to lift the colonies off the surface. It is evident, therefore, that peroxide cannot 

 accumulate in such cultures, and it is found indeed that the electrode potential 

 of diphtheria cultures remains at a low level long after the logarithmic phase of 

 growth, and does not ever rise to the high level characteristic of peroxide formation 

 (Hewitt, 1930). Diphtheria bacilli are able readily to oxidise substances such as 

 fatty acids, etc., converting them to carbonates, hence aerobic cultures may become 

 very alkaUne (well over pH 8*0 or even 8-5). For toxin production it is neces'^ary 

 to ensure a generous oxygen supply in the cultures as, for example, by using a shallow 

 layer only of medium. No such conditions have been found necessary for the soluble 

 toxins of haemolytic streptococci. 



