The Relationship Between pH Tolerance, Virulence, and Proteolytic Enzymes in Bacteria I. Bacillus Anthracis *

The nature of the virulence of micro-organisms has been investigated by many workers in an effort to determine the influences and mechanisms involved. Such investigations have, in the main, dealt with the study of various factors associated with virulence, but there is no evidence that such factors contribute to the make-up of virulence. The pigment of staphylococcus is an example of a factor indirectly related to virulence. Other factors, such as the capsular polysaccharides of Diplococcus pneumoniae, are related more directly, for these substances must be present if virulence is shown. However, like every factor so far found to be associated with virulence, these compounds may be present in a bacterium without virulence being significant. They are necessary but not sufficient to induce virulence. Virulence, while commonly used in referring to a property of an organism, actually refers to the results of the interaction between the invaded host and the invading organism as has been stated by Dubos.9 When the organism is invasive, that is, can break through the first lines of body defense and establish itself in the body, and when it is toxic, the organism is said to be virulent. Thus we have two main conditions which the organism must fulfill to be virulent, the loss of either one destroying virulence and the presence of either one alone being insufficient for virulence. The mouse is relatively resistant to the Shigella group of bacteria, although such bacteria produce a toxin to which the mouse is susceptible. S. dysenteriae cannot break through the defense barriers and initiate growth and thus cannot produce the toxin to which the mouse is susceptible. This bacterium is toxic but not invasive and thus cannot kill mice. However, when mucin is injected with the bacteria the defense reactions of the mouse are broken down, as has been shown by Ercoli, Lewis, and Harker,10 and the bacteria can now grow in the body. Invasiveness has been acquired and together with the toxicity al-