256 PUBLIC HEALTH BACTERIOLOGY 



instead of ox flesh in the preparation of a nutrient 

 medium for the growth of Bacillus pestis. An extract 

 is made from the carcases of rats deprived of skin, 

 head, stomach, and intestines, and the further preparation 

 of the medium is essentially the same as in that of ordinary 

 agar, except that the extract is boiled for half an hour 

 before straining. Rat agar has been used in the (city) 

 laboratory for a number of years as a culture medium 

 for Bacillus pestis, with most satisfactory results, growth 

 taking place on this medium with much greater certainty, 

 rapidity, and profusion than on glycerin agar. It is also 

 noteworthy that on this medium the bacillus closely 

 approximates to the form which it assumes in the body, 

 and is sometimes much elongated. In this elongated 

 form it may be difficult to recognize, so different is it from 

 the familiar cocco-bacillary growth of glycerin agar." 



B. pestis is Gram-negative, non-gelatin-liquefying, non- 

 motile, non-sporing, and non-indol forming. 



Cultures. Grows readily and luxuriantly on all the 

 meat-infusion media. The optimum temperature is 30 C., 

 while below 20 C. and above 38 C. the growth is sparse 

 and delayed. The best reaction is neutrality or slight 

 alkalinity ; but acidity does not prevent growth. 



On agar : growth appears in twenty-four hours as minute 

 colonies, whitish and compact in centre, and showing to 

 hand lens a broad, irregular, indented, granular margin. 

 Kept for a few days at room temperature, some colonies 

 grow faster than others and become more opaque, almost 

 suggesting a mixed growth (Muir and Ritchie). 



On gelatin : a similar growth occurs in two to three days. 

 In stab, a white line of growth takes place along the 

 needle track, and little or no surface growth. 



In broth : growth is slow, and usually forms a slightly 

 granular or powdery deposit at the foot or sides of the tube 

 or flask. If the surface is covered with " ghee " (in India, 

 butter clarified by boiling, and thus converted into a kind 

 of oil), delicate threads of growth extend from the surface 

 downwards, the so-called " stalactite " growth, which 

 however is not specific to the plague bacillus, nor is it 

 shown by all races of the organism. To observe it, the 

 culture must be kept absolutely at rest. 



