190 PASTEURISED OR HEATED MILK 



BACILLUS ABORTUS 



Since 1897 when Bang and Stribald 2 isolated B. abortus as 

 the causative agent of the infectious abortion in cattle, con- 

 siderable study has been given to this organism in various parts 

 of the world. McFadyean and Stockman 3 corroborated 

 Bang's findings, but later work has resulted in the discovery 

 of several allied forms with the consequence that B. abortus is 

 now regarded as a species and not as a distinct biotype. 



During the last decade several workers have found B. abortus 

 in milk by the inoculation method and in some instances as 

 many as 60 per cent of the samples gave positive results. The 

 lesions produced by these samples were not usually sufficient 

 to cause death. 



Although the descriptions of B. abortus as given by various 

 workers showed considerable variations, it remained for Evans 4 

 to classify the various forms and to indicate the relative fre- 

 quency of certain varieties in normal udders. By plating milk 

 on agar containing 10 per cent of bovine serum, Evans isolated 

 B. abortus from 45 (23.4 per cent) of the 192 samples exam- 

 ined. These samples were obtained from 5 dairies. Thirty- 

 three cultures exhibited a marked lipolytic action on milk fat 

 and were, consequently, designated as B. abortus variety 

 lipolyticus. Twelve cultures (variety 6) differed from the 

 pathogenic varieties in their 'ability to ferment the usual test 

 substances, and morphology. The reactions of the varieties 

 isolated by Evans are given in Table LXV, together with those 

 of the typical pathogenic varieties for comparison. 



B. abortus in young cultures shows the typical slender rod 

 form but involution forms are often found in older ones and 

 foetal exudates often contain coccoid varieties. Ordinary 

 aniline dyes may be used for staining purposes, carbol fuchsin 

 followed by 1 per cent acetic acid, dilute carbol fuchsin, and 

 Loeffler's methylene blue giving very satisfactory results. 

 The organism is decolourised during Gram's method of staining. 

 For cultural preparations agar containing 10 per cent of serum 



