148 BACTERIOLOGY FOR NURSES 



centrifuging the suspension in a special tube. A sediment of or- 

 ganisms up to a certain mark gives an approximate number when 

 diluted with a given quantity of salt solution. 



Sensitized Vaccines. The bacteria living or dead are left in 

 contact for some time with the serum of an animal immunized 

 against that particular species in order that a combining of 

 antigen and antibody may take place, after which the serum is 

 removed by centrifuging. It is claimed that immunity produced 

 by sensitized vaccines is more quickly developed and of longer 

 duration ; also the local and general reactions are lessened. 



Polyvalent Vaccines. Cultures of several different species of 

 bacteria may be mixed in definite proportions and administered 

 at the same time. A vaccine containing typhoid, paratyphoid, 

 A and B bacilli, and cholera spirilla is reported to have been used 

 with success. 



Bacterial vaccines are always given in subcutaneous injections. 

 Three or four doses are usually given at intervals of about five 

 to ten days. 



In addition to being used as a prophylactic measure, vaccines 

 are often employed therapeutically in local infections such as acne, 

 pustule, or a boil. It is assumed that while the local resistance 

 has been lowered it is probable that the general antibody produc- 

 ing tissues have not commenced to react ; the vaccine may thus 

 stimulate the latter and cause the infected area to be flooded with 

 antibodies. 



Frequently an autogenous vaccine is prepared for such cases ; 

 that is, the infecting organism is isolated from the discharge, 

 grown in pure culture, and prepared as a vaccine. In most in- 

 stances immunity produced by the introduction of a bacterial 

 vaccine lasts from two to five years and may, of course, be 

 renewed. 



It is not definitely known how long a vaccine may be effectively 

 used after the date of its preparation ; usually after a period of 

 from four to six months it is supposed to lose its potency. 



Immunization with Toxin. Soluble toxin such as produced 

 by the diphtheria bacillus may be obtained free from bacteria 



