146 PRINCIPLES OF BACTERIOLOGY 



some part of the reaction of the tissues which in other portions gives rise 

 to the antitoxins and antibactericidal substances. It is also a sign that 

 the body cells as a whole are not yet overwhelmed by the infection. 



With few exceptions the bacterial poisons produce an increase in the 

 number of leukocytes and a lessening in the amount of haemoglobin 

 in the blood. In uncomplicated infection with typhoid bacilli there is 

 a hypoleukocytosis. The different varieties of leukocytes are increased 

 in varying proportions in different infections. The red-blood cells are 

 directly injured by a number of bacterial substances. The deleterious 

 effects on the nutrition are partly due to the direct effect of the poison 

 and partly to the diseased conditions of the organs of the body, such as 

 the spleen, kidney, and liver. Degeneration of the nerve cells is fre- 

 quently noticed after infectious diseases; especially is this true of diph- 

 theria. Several bacterial poisons have been found to produce convul- 

 sions; the best example of this is the tetanus toxin. 



Influence of Quantity in Infection. With pathogenic bacteria the 

 number introduced has an immense influence upon the probability of 

 infection taking place. 



If we introduce into a culture medium containing some fresh human 

 blood or serum a few bacteria it is probable that they will all die; whereas 

 if a greater number are introduced, while there will at first be a great 

 diminution of these, those that die, having combined with the bacteri- 

 cidal substances in the serum, neutralize them ; then those bacteria which 

 survive begin to increase, and soon they multiply enormously. The 

 same is true for parasitic bacteria in the body. A few only gaining 

 entrance, they may die; a larger number being introduced, some may 

 or may not survive; but if a still greater quantity is injected it is almost 

 certain that there will be some surviving members, which, because of 

 the antagonistic substances, having been neutralized or of their having 

 some peculiar properties, will begin to grow and excite disease. 



With those bacteria whose virulence is great a very few organisms 

 will produce disease almost as quickly as a million, allowance only 

 being made for the short time required for the few to become equal in 

 number to the million. At the other extreme of virulence, however, 

 many millions may have to be introduced to permit of the development 

 of any of the organisms in the body. With these bacteria we are thus 

 able to produce either no effect whatever, a local effect, or in some 

 cases a general septicaemia, by regulating the amount of infection intro- 

 duced. In the majority of cases in man the number of bacteria re- 

 ceived is comparatively small ; but by the rupture of an abscess into 

 a body cavity or into the circulation, or by the opening of the intes- 

 tinal contents into the. peritoneum, the quantity introduced may be 

 enormous. 



Variation in Degree of Virulence Possessed by Bacteria. Bacteria 

 differ, as has already been stated, as to the ease and rapidity with 

 which they grow in any nutritive substance and the amount of poison 

 they produce. Both of these properties not only vary greatly in 

 different members of the same species, but each variety of bacteria 



