The Fundamentals of Bacteriology

by Charles Bradfield Morrey

CHAPTER XXVIII.
RECEPTORS OF THE SECOND ORDER.

AGGLUTININS.

Charrin and Rogers appear to have been the first (1889) to observe the clumping together of bacteria (Pseudomonas pyocyanea) when mixed with the blood serum of an animal immunized against them. Gruber and Durham (1896) first used the term “agglutination” in this connection and called the substance in the blood-serum “agglutinin.” Widal (1896) showed the importance of the reaction for diagnosis by testing the blood serum of an infected person against a known culture (typhoid fever).

It is now a well-known phenomenon that the proper injection of cells of any kind foreign to a given animal will lead to the accumulation in the animal’s blood of substances which will cause a clumping together of the cells used when suspended in a suitable liquid. The cells settle out of such suspension much more rapidly than they would otherwise do. This clumping is spoken of as “agglutination” and the substances produced in the animal are called “agglutinins.” If blood cells are injected then “hemagglutinins” result: if bacterial cells “bacterial agglutinins” for the particular organism used as “glanders agglutinin” for Pfeifferella mallei, “abortion agglutinin” for Bacterium abortus, “typhoid agglutinin” for Bacterium typhosum, etc.

The phenomenon may be observed either under the microscope or in small test-tubes, that is, either microscopically or macroscopically.

In this case the cells introduced, or more properly, some substances within the cells, act as stimuli to the body cells of the animal injected to cause them to produce more of the specific cell receptors which respond to the stimulus. The substance within the introduced cell which acts as a stimulus (antigen) to the body cells is called an “agglutinogen.” That “agglutinogen” is present in the cell has been shown by injecting animals experimentally with extracts of cells (bacterial and other cells) and the blood serum of the animal injected showed the presence of agglutinin for the given cell. It will be noticed that the receptors which become the free agglutinins have at least two functions, hence at least two chemical groups. They must combine with the foreign cells and also bring about their clumping together, their agglutination. Hence it can be stated technically that an agglutinin possesses a haptophore group and an agglutinating group.

It is probable that the agglutination, the clumping, is a secondary phenomenon depending on the presence of certain salts and that the agglutinin acts on its antigen as an enzyme, possibly a “splitting” enzyme. This is analogous

to what occurs in the curdling of milk by rennet and in the coagulation of blood. This probability is substantiated by the fact that suspensions of bacteria may be “agglutinated” by appropriate strengths of various acids.

The formation of agglutinin in the body for different bacteria does not as yet appear to be of any special significance in protecting the animal from the organism, since the bacteria are not killed, even though they are rendered non-motile, if of the class provided with flagella, and are clumped together. The fact that such bodies are formed, however, is of decided value in the diagnosis of disease, and also in the identification of unknown bacteria.

In many bacterial diseases, agglutinins for the particular organism are present in the blood serum of the affected animal. Consequently if the blood serum of the animal be mixed with a suspension of the organism supposed to be the cause of the disease and the latter be agglutinated, one is justified in considering it the causative agent, provided certain necessary conditions are fulfilled. In the first place it must be remembered that the blood of normal animals frequently contains agglutinins (“normal agglutinins”) for many different bacteria when mixed with them in full strength. Hence the serum must always be diluted with physiological salt solution (0.85 per cent.). Further, closely related bacteria may be agglutinated to some extent by the same serum. It is evident that if they are closely related, their protoplasm must contain some substances of the same kind to account for this relationship. Since some of these substances may be agglutinogens, their introduction into the animal body will give rise to agglutinins for the related cells, as well as for the cell introduced. The agglutinins for the cell introduced “chief agglutinins,” will be formed in larger quantity, since a given bacterial cell must contain more of its own agglutinogen than that of any other cell. By diluting the blood serum from the animal to be tested the agglutinins for the related organisms (so-called “coagglutinins” or “partial agglutinins”) will become so much diminished as to show no action, while the agglutinin for the specific organism is still present in an amount sufficient to cause its clumping. Agglutinins are specific for their particular agglutinogens, but since a given blood serum may contain many agglutinins, the serum’s specificity for a given bacterium can be determined only by diluting it until this bacterium alone is agglutinated. Hence the necessity of diluting the unknown serum in varying amounts when testing against several known bacteria to determine for which it is specific, i.e., which is the cause of the disease in the animal.