4M REPORT— 1893. 



To what ti'ibunal must the bewildered bacteriologist have recourse ? In 

 Dr. Koch's last paper, ' Der augeDblickliche Stand der Choleradiagnose ' 

 (Zeitsch. f. Hygiene, xiv. [1893], p. 335), the final referees in this diagnosis 

 are (1) the so-called indol reaction and (2) the pathogenic effects of 

 inoculation into animals. Thus the morphological have had to give way 

 to chemical and physiological tests. 



An almost precise parallel is presented by the history of the diagnosis 

 of the typhoid bacillus. In the first instance morphological tests for its 

 identification were in vogue, more especially its great motility in broth 

 and its almost invisible growth on potatoes ; both of these criteria have 

 had to be abandoned, inasmuch as they are possessed also by closely allied 

 organisms, and the tests which at present serve at any rate for its ready 

 distinction from the bacillus coli communis are (1) the absence of indol 

 reaction, (2) the non-coagulation of milk, and (3) the non-fermentation 

 of dextrose and meat extract. 



Thus, whilst morphological methods may serve to distinguish the 

 typhoid bacilli from a number of other forms, it is to chemical tests that 

 we must have recourse in order to differentiate it from its closest allies. 



I do not, however, for a moment wish to convey the impression that 

 such chemical tests are altogether unassailable — far from it, for I have 

 had abundant opportunities of observing their inconstancy and treacherous 

 variation. It is, however, highly significant that in the diagnosis of the 

 • two micro-organisms, upon which almost more attention has been recently 

 showered than upon any others, the tests universally acknowledged to be 

 the most reliable are in both cases chemical ones. It is, moreover, 

 -obvious that these chemical differences will in the future have to be far 

 more closely and systematically studied than in the past, as they are 

 doubtless capable of very great extension for purposes of diagnosis. 



Thus, the only other chemical tests which have hitherto been in any 

 way extensively introduced are — 



(1) The reduction of nitrates to nitrites. 



(2) The ammoniacal fermentation of urea. 



Of these the first is particularly available, as a comparatively large 

 number of bacteria have the power of effecting this change, whilst 

 hitherto, curiously, only one organism has been found possessing the 

 ■power of bringing about the oxidation of nitrites to nitrates. 



Fermentations. — These chemical tests to which, as I have pointed out, 

 we are now so often obliged to resort in bacteriological diagnosis naturally 

 lead us to a consideration of some of the more striking chemical changes 

 induced by micro-organisms, and which we generally group together 

 Tinder the name of Fermentations. 



Of these fermentations the most important, from a practical point of 

 -view, is still, of course, the alcoholic fermentation induced by yeast, and, 

 as is so well known, the practical application of this fermentation has 

 been put on a sound scientific basis through the researches of Chr. Hansen, 

 whose pure yeasts have, however, hitherto found less favour in this 

 country than elsewhere, although they have been employed on a large 

 experimental scale by Mr. Horace Brown and Dr. Mon-is, whilst more 

 recently, in a few English breweries, the pure yeasts have been adopted 

 to some extent in actual practice. The principal diflBculty in the way of 

 these pure yeasts being employed for English beers appears to be that, 

 until recently, none of them was capable of bringing about that ' after- 



