220 MILK AND THE PUBLIC HEALTH CHAP, xm 



are, however, in the main distinctive, and afford a convenient, 

 practical means of classification. The essential differences are, 

 that the human type possesses a lower virulence for the 

 bovine animal and a greater degree of cultural luxuriance 

 when grown on artificial media. As Eastwood states : 1 



These two differences, taken in conjunction, afford strong prima 

 fade evidence as to whether or not a particular human bacillus is 

 likely to have been derived from an infected bovine (or other animal 

 capable of harbouring similar bacilli). This evidence is particularly 

 valuable owing to the high degree of stability which the tubercle 

 bacillus usually possesses, as shown by the fact that its individual 

 characteristics are not readily changed by transplantation into the 

 tissues of animals of different species. 



Eastwood finds that the parallelism between high virulence 

 and scanty cultural growth is only roughly true. He finds 

 that : " Whereas there is a gradual and unbroken transition 

 between the cultural characters of the strains which grow least 

 well on artificial media, and those which grow most luxuriantly, 

 the transition in the scale of virulence is, on the whole, 

 abrupt." Bearing in mind these reservations, the following 

 tabular summary of the differences may be accepted. 



Bovine. Human. 



Morphology. Cannot be differentiated. In cultures the bovine bacilli 



are, in general, shorter, thicker, and more regular in their staining. 

 Cultural Characteristics. 



Serum. Grows slowly, and at Grows more rapidly. 

 the end of two to three weeks 

 shows on the surface of the 

 medium a thin greyish uniform 

 growth, not wrinkled and not 

 pigmented. 



Glycerine Agarand Broth. Grows Grows readily often from the start, 

 comparatively feebly and with Growth tends to become wrinkled, 

 difficulty. 



The Royal Tuberculosis 

 Commission divided their 

 cultures into 3 grades ac- 

 cording to the rate and 

 luxuriance of their growth. 

 Pathogenic Properties. 



Calves. Highly pathogenic. Not pathogenic. 



"In calves the subcutaneous ''In calves, a subcutaneous in- 



injection under the skin of the oculation of 50 milligrammes of 



1 Royal Commission on Tuberculosis, 1907, vol. iv., partii., Appendix. 



