Report on the Bacteriology of Water. 



203 



ferments which they produce : this causes local fioodings, and the 

 running together of neighbouring colonies, or the submergence of 

 some of them, and seriously interferes with the counting and estima- 

 tion of the numbers. 



The only mode of combating this difficulty consists in using such a 

 volume of the infecting water as will yield a manageable number of 

 such centres of liquefaction. 



But these are by no means the only sources of fallacy incidental to 

 the methods of gelatine-plate cultures. It has been implied by some 

 of the earlier workers, rather than definitely assumed, that all the 

 living germs in the sample of water mixed with the nutrient gelatine* 

 will give rise to colonies, provided the plate culture is thin enough 

 to ensure the access of oxygen to all parts, the sample small enough 

 to ensure isolation of the individual bacteria or spores, and the 

 temperature a suitably high one to promote rapid growth, without 

 preventing the proper solidification of the medium. 



As matter of fact, there are serious fallacies traceable to all these 

 implications. Many bacteria are now known which are incapable of 

 growing in presence of the oxygen of the air, while others will only 

 withstand partial pressures of that gas ; it may be safely concluded 

 that the gelatine-plate cultures give no account whatever of these 

 forms, although they may and often do occur in tap waters,f &c. 



Moreover, even the thinnest layer of gelatine may so far hinder the 

 access of oxygen to completely submerged aerobic forms as to retard 

 their growth, and so they become dominated by the more rapid de- 

 velopment of other colonies. This domination is not necessarily due 

 to the mere flooding of the suppressed forms with liquefied gelatine : 

 GarreJ showed a short time ago that some bacteria, growing on 

 gelatine side by side with other species, can inhibit the life- actions of 

 the latter by the poisonous influence of their metabolic products, and 

 Miquel§ claims to have proved similar actions in water, and even to 

 have isolated the toxic principles, and rendered other water immune 

 by their aid. 



* The quality of gelatine and peptone varies abo. For hints in this connexion 

 see Beinsch, " Zur bakteriolog. Unters. des Trinkwassers " (' Centr. f . Bakt.,' vol. 10, 

 1891, p. 415). 



f A good instance has recently been investigated by Perdrix (" Sur les fermenta- 

 tions produites par un Microbe anaerobie de l'Eau," ' Ann. Inst. Pasteur,' vol. 5, 

 1891, pp. 286— 311). 



X " Ueber Antagonisten unter den Bakterit-n " (' Correspondenzbl. f. Schweizer. 

 Aerzte,' Jahrg. 17, 1887). Also Blagovestchensky ("Sur l'Antagonisme entre les 

 Bacilles du Charbon et ceux du Pus Bleu," 'Ann. de l'lnst. Pasteur,' 1890, vol, 4, 

 pp. 689—715). 



§ "Dixieme Memoire sur les Poussieres organisees de l'Air et des Eaux " 

 (' Annuaire de l'Observatoire de Montsouris,' 1888), and ' Manuel Pratique d' Analyse 

 Baetenologiqiie des Eaux,' 1891, pp. 153 — 155. 



