THE FOWL PASTEURELLA 451 



Gelatin. At 22 -23 C. growth is both slow and minimal in amount. 

 Stab culture gives a thin white line spreading out a little on the surface like 

 a nail-head, the growth is very poor in the depth. Stroke culture produces a 

 very fine whitish line which appears blue by transmitted light. The gelatin is 

 not liquefied. 



Potato : Yeast extract. No visible growth occurs on these media- 

 Milk. Growth takes place without coagulation of the medium. 



SECTION III. BIOLOGICAL PROPERTIES. 

 1. Vitality and virulence. 



The fowl pasteurella is a very delicate organism and rapidly dies out in 

 culture. It is easily killed by drying, or by heating to 55 C., as well as by 

 antiseptics or very dilute acids. 



Cultures in liquid media are more resistant than those on solid media, but 

 a broth culture dies out in about 6 weeks or 2 months, and while alive rapidly 

 loses its virulence. The attenuation is due to the action of the oxygen of 

 the air ; infected blood stored in sealed capsules keeps its virulence for a 

 long time, and a virulent culture may be similarly preserved by incubating 

 it in broth for 18 hours and then sealing it up. 



2. Attenuation Vaccination. 



The virulence of broth cultures kept at 37 C. is very materially reduced 

 at the end of a fortnight ; such a culture inoculated into a series of animals 

 does not kill more than two or three out of every ten inoculated. A few 

 days later still the culture has almost entirely lost its virulence : when inocu- 

 lated into fowls it does not set up a fatal disease but merely a temporary 

 indisposition, and if the pectoral muscle be the site of inoculation only a 

 local lesion (gelatinous oedema, muscular sequestrum, or necrosis) results. 

 The virulence of the attenuated virus though slight is however fixed, and by 

 storing it in sealed capsules a culture with this character can be preserved : 

 cultures of different degrees of virulence may also be kept under similar 

 conditions. 



It is always possible to make these cultures fully virulent by passage through 

 sparrows ; a virus which will not kill fowls is still fatal to sparrows and after a few 

 passages will have fully recovered its virulence, so that on inoculation into fowls 

 death results in a few hours. 



It was when working with the bacillus of fowl cholera in 1878 that Pasteur 

 discovered that organisms might lose their virulence, which observation was 

 the basis of the discovery of vaccination with micro-organisms. 



A fowl which has suffered from a mild attack of the disease following the 

 inoculation of an attenuated virus is after recovery no longer susceptible to 

 the disease ; it can be inoculated with the most virulent strains without 

 suffering any harm. Prophylactic vaccination is effected in practice by 

 inoculating into the tip of the wing first a very attenuated virus and a few 

 days later a second stronger vaccine. The animal is then immune for about 

 a year. 



3. Toxin. 



Filtered broth cultures when inoculated into fowls produce a disease 

 characterized by weakness and drowsiness from which the animals always 

 recover and which confers on them a certain degree of immunity (Pasteur). 

 Fowls which have been vaccinated with the micro-organism are still 

 susceptible to the toxin. 



