STUDIES RELATING TO THE SERUM RESISTANCE OF CERTAIN GRAM-NEGATIVE BACTERIA.

Paracolobactrum ballerup, an organism considered completely insusceptible to the bactericidal action of the antibody-complement system, became extremely sensitive to immune serum and even to normal serum, in conjunction with complement, when cultivated at temperatures above 37 degrees C. This conversion to serum sensitivity was associated with the loss of the organism's Vi antigen and its tendency to assume a rough state. It did not result in a genetic change in the organism, however, since reincubation of serum-sensitive P. ballerup at 37 degrees C resulted in a restoration of its resistance. The loss of serum resistance as a result of cultivation temperatures above 37 degrees C may be of significance as an example of the enhancement of host defense mechanisms induced by fever. Special conditions of testing indicated that P. ballerup cultured at 37 degrees C and S. paratyphosa C, organisms considered insusceptible to serum bactericidal action, were not entirely refractory to serum. These conditions included simply a relatively low ratio of the number of test organisms to serum volume and an extended incubation period of the organisms with serum bactericidal substances, or even without these substances, prior to the initiation of bactericidal action. It is likely, therefore, that an absolute distinction between serum-sensitive and serum-resistant Gram-negative organisms does not exist, but rather that there is a very broad distribution of serum sensitivity among these organisms.