34 2 VIRULENCE OF BACTERIA 



Thus in pneumonia the number of leucocytes per cubic millimetre 

 has a most important prognostic value, and in almost all cases it 

 will be found that a patient with a high leucocytosis will recover, 

 even if the attack is a severe one and the clinical signs unfavour- 

 able. This leucopaenia may be attributed either to negative 

 chemotaxis, or to the production of paralysis or death of the 

 leucocytes, or to inhibition of the functions of the bone-marrow, 

 or to a combination of these causes. In any case, it argues a high 

 grade of virulence on the part of the bacteria, which, by lessening 

 the action of the main natural protective forces, increases the 

 chance of a lethal issue to the disease. Let us, therefore, discuss 

 some of the main facts (many of which have been referred to 

 before) concerning the nature of the mechanism of an increase in 

 virulence. 



For an organism to be virulent it must produce a toxin which 

 has a profound action on the cells (and, it may be added, on the 

 important cells) of the animal in question. This, of course, is 

 obvious, and without it the organism could only produce pathogenic 

 effects by such means as deprivation of oxygen or food-stuffs from 

 the host or the production of bacterial emboli results which are 

 of no practical importance. Without an active toxin the bacteria 

 would either lie latent, as occurs in typhoid fever, gonorrhoea and 

 tubercle, or circulate in the blood as a harmless parasite, as is the 

 case with many protozoa in the lower animals. 



As regards the special actions of toxins which render the bacteria 

 especially virulent, we may distinguish two (i) a leucocytic or 

 leucolytic action, and (2) negative chemotaxis. The former is a 

 very common function of highly virulent bacteria ; its action is 

 best seen in the production of pus and necrosis in local lesions, 

 but its presence may be inferred from the degenerative changes 

 frequently present in leucocytes in general infections. And these 

 leucocytic substances need not be true toxins, for there is 

 reason to believe that the non-specific enzymes and other 

 substances which the bacteria produce have this action, and 

 that the death and destruction of leucocytes which is so marked 

 a feature of abscess production may be due in part to their 

 action. 



We have already referred to the question of negative chemotaxis. 

 It probably actually occurs, but the proof is hardly convincing, 

 and the subject needs investigation by modern methods in vitro. 

 There is no doubt, however, that in virulent infections the regions 



