450 THE PASTEURELLA GROUP 



are examined a small unstained refractile area is seen in the centre. In rapidly 

 growing young cultures of the highly virulent strains, the rounded forms 

 predominate and the appearance is suggestive of diplococci ; in older or in 

 less virulent cultures the elongated, bacillary forms are chiefly found. 



The fowl pasteurella does not form spores. 



Staining methods. The bacillus is easily stained with the ordinary dyes, 

 and is gram-negative. 



(a) Cultures. Stain with carbol-thionin, Kiihne's carbol-blue or dilute 

 carbol-fuchsin. 



(b) Scrapings of organs and blood films. 1 . Simple staining. Stain for a few 

 minutes with carbol-blue or carbol-thionin, wash, dry and mount. The leuco- 

 cytes, the nuclei of the red cells and the bacilli are stained deep blue or violet. 



2. Double staining. Drop a little 1 per cent, aqueous solution of eosin on 

 a blood film, leave for 2 or 3 minutes, wash, then pass through carbol-blue, 

 wash again, dry and mount. This method gives exceedingly pretty prepara- 

 tions ; the cytoplasm of the red cells is stained red by the eosin while their 

 nuclei and the micro-organisms are blue (fig. 225). Unfortunately the 

 technique is a little difficult : the action of the blue must be carefully 

 watched under the microscope and the dye washed off as soon as the 

 differentiation is complete. 



(c) Sections. Nicolle's tannin method should be used. 



2. Cultural characteristics. 



Conditions of growth. The fowl pasteurella is primarily an aerobic organism. 

 It will also grow in media deprived of oxygen, but anaerobic cultures are 

 always very poor and can only be obtained under certain 

 conditions, e.g. by sowing freely in serum broth. In aerobic 

 cultures the organism grows feebly between 20 and 25 C. ; 

 the optimum temperature is from 35-39 C. 



The most useful media are chicken- or veal-broth made 

 neutral or slightly alkaline, and especially broths containing 

 serum. 



Cultures in peptone broth produce indol but only after 

 incubating for about a fortnight (Porcher and Panisset). 



Characters of growth. Broth. At 37 C. growth takes 

 place rapidly in chicken-broth. A slight cloudiness is 

 visible in about 10 hours which increases for the next 

 10-12 hours, then the growth deposits in the form of a 

 scanty precipitate leaving the medium clear. Growth 

 ceases in about a week. 



The virulence of the culture reaches its maximum in about 24 

 hours ; at this stage the rounded forms predominate, while in 

 older cultures the long forms are the more numerous. When 

 growth has ceased, the precipitate consists of granular debris 

 in which no definite structure can be made out ; if sown on a fresh 

 medium, however, it will for some time give rise to virulent 

 cultures. 



FIG. 226. Pasteur- Agar. Growth is rapid at 37 C. ; a thin white glistening 



eiia gaiiincK. stab streak is formed, thicker in the centre than at the edges. 



^ e j > If a drop of blood be smeared on the surface of the agar, 



single colonies will usually be obtained which are at first 



transparent and bluish and later semi-opaque. 



Coagulated serum. The growth on this medium has the same appearance 

 as the growth on agar. 



