THE DETECTION OF BACTERIAL PATHOGENICITY 225 



cause death or readily recognizable symptoms. The determination of 

 pathogenicity of such undoubted pathogens as anthrax usually presents 

 no special problem. However, a number of the bacteria which may be 

 pathogenic for man, such as the hemolytic Group A Streptococci, fre- 

 quently lose virulence when cultivated on artificial media or even on 

 initial isolation. Such organisms may require repeated animal passage, 

 etc., before their lethal properties are demonstrable (see below). 



GENERAL FACTORS UNDERLYING VIRULENCE 



The pathogenicity of microorganisms represents the resultant of a 

 number of single components shared by both the bacterium and its host. 

 Thus, specific toxicity, speed of multiplication, tissue tropisms, strain, 

 and age of the cell all contribute to the virulence of a microorganism and 

 the outcome of infection. In addition, an array of host factors, as, 

 for example, portal of entry, species or age resistance, avitaminosis, 

 partially determines whether the test animal will live or die. In certain 

 instances those factors which determine virulence for animals are the 

 same as for man; in others they are not. 



Some of the most prominent components of pathogenicity are the 

 following : 



1. Septicemia is an important factor in fatal infections caused by most 

 of the gram-positive cocci and certain bacilli, such as Pasteurella pestis. 

 Massive hemal multiplication of the invading bacterium results in an 

 overwhelming infection, and in general, experimental animals developing 

 septicemia succumb after a short illness. In contrast, a bacteremia per 

 se is not indicative of pathogenicity, and bacteria can be readily isolated 

 from the circulation of many healthy animals or those inoculated with an 

 attenuated or nonpathogenic strain of certain microorganisms. In local- 

 ized infection showers of organisms may appear in the blood stream at 

 intervals. This is usually, but not always, associated with a rise in body 

 temperature. 



2. Toxin production is an important feature of bacterial virulence and 

 sometimes one of the most difficult to establish. A toxic death may be 

 considered when animals succumb shortly after infection and often 

 without evidence of extensive bacterial multiplication. The inoculation 

 of culture filtrates will then be required to characterize the agent further. 

 In this connection one should remember that there need not be a correla- 

 tion between pathogenicity and high toxin production and, as in the case 

 of diphtheria or plague, strains isolated from mild cases may be very 

 toxigenic when grown on artificial media. 



The exotoxins of gram-positive bacteria are ready demonstrable in the 

 filtrates of autolysis-free cultures. Among the major toxins are those of 



