CHAPTEE XIII 



THE DETECTION OF PATHOGENIC BACTERIA IN MILK 



I. The Detection and Isolation of Tubercle Bacilli 

 FROM Milk 



Two methods are available — microscopic, and by animal in- 

 oculation. Isolation from the milk by direct cultivation is 

 impracticable. 



Microscopic Method. — Tubercle bacilli are usually present 

 in only small numbers in market milk, therefore positive find- 

 ings are only likely when the bacilli are concentrated. To do 

 this centrifugalise the milk and microscopically examine the 

 sediment. The larger the bulk of milk centrifugalised and the 

 more thoroughly it is done {i.e. the faster the rotation and the 

 longer the time) the greater the chances of obtaining any tubercle 

 bacilli present, in the sediment. Since many of the bacilli 

 are retained in the cream this should be thoroughly broken 

 up after a first centrifugalisation and the mixture again 

 rotated. If this is not done the cream should be separately 

 examined. For practical routine purposes 50 to 100 c.c. is a 

 convenient amount to centrifugalise in a machine rotating 

 about 3000 times per minute, the centrifugalisation to last 

 at least 30 minutes and preferably longer. 



Minute portions of the sediment are spread upon cover-slips, 

 dried, fixed, and stained by the Ziehl Neelsen method. 

 Del^pine recommends placing the films when dry and before 

 staining in a covered capsule containing equal parts of absolute 

 alcohol and ether, in which they are kept for at least two 

 hours, after which the capsule is placed in a dish of water 

 previously brought to a temperature of 80° to 90° C. The 

 mixture of absolute alcohol and ether boils at once, and after 



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