The Fundamentals of Bacteriology

by Charles Bradfield Morrey

4. It must be recovered from such animals in pure culture and this culture likewise reproduce the disease.

These postulates have not been fully met with reference to any disease, but the principles embodied have been applied as far as possible in all those infections which we recognize as specific, and whose causative agent is accepted. In many diseases recognized as infectious and contagious no organism has been found which is regarded as the specific cause. In some of these the organism appears to be too small to be seen with the highest powers of the microscope, hence they are called “ultramicroscopic” organisms. Because these agents pass through the finest bacterial filters, they are also frequently called “filterable.” The term “virus” or “filterable virus” is likewise applied to these “ultramicroscopic” and “filterable” agents.

The term primary infection is sometimes applied to the first manifestation of a disease, either specific or non-specific, while secondary refers to later developments. For example, a secondary general infection may follow a primary wound infection, or primary lung tuberculosis be followed by secondary generalized tuberculosis, or primary typhoid fever by a secondary typhoid pneumonia. The terms primary and secondary are also used where the body is invaded by one kind of an organism and later on by another kind; thus a primary measles may be followed by secondary infection of the middle ear, or a primary influenza may be followed by a secondary pneumonia, or a primary scarlet fever by a secondary nephritis (inflammation of the kidney). Where several organisms seem to be associated simultaneously in causing the condition then the term mixed infection is used—in severe diphtheria, streptococci are commonly associated with the Corynebacterium diphtheriæ. In many cases of hog-cholera, mixed infections in the lungs and in the intestines are common. Wound infections are usually mixed. Auto-infection refers to those conditions in which an organism commonly present in or on the body in a latent or harmless condition gives rise to an infectious process. If the Bacterium coli normal to the intestine escapes into the peritoneal cavity, or passes into the bladder, a severe peritonitis or cystitis, respectively, is apt to result. “Boils” and “pimples” are frequently autoinfections. Such infections are also spoken of as endogenous to distinguish them from those due to the entrance of organisms from without—exogenous infections. Relapses are usually instances of autoinfection.

Those types of secondary infection where the infecting agent is transferred from one disease focus to another or several other points and sets up the infection there are sometimes called metastases. Such are the transfer of tubercle bacilli from lung to intestine, spleen, etc., the formation of abscesses in internal organs following a primary surface abscess, the appearance of glanders nodules throughout various organs following pulmonary glanders, etc.

The characteristic of a pathogenic microörganism which indicates its ability to cause disease is called its virulence. If slightly virulent, the effect is slight; if highly virulent, the effect is severe, and may be fatal.

On the other hand, the characteristic of the host which indicates its capacity for infection is called susceptibility. If slightly susceptible, infection is slight, if highly susceptible, the infection is severe.

Evidently the degree of infection is dependent in large measure on the relation between the virulence of the invading organism and the susceptibility of the host. High virulence and great susceptibility mean a severe infection; low virulence and little susceptibility a slight infection; while high virulence and little susceptibility or low virulence and great susceptibility might mean a moderate infection varying in either direction. Other factors influencing the degree of infection are the number of organisms introduced, the point where they are introduced and various conditions. These will be discussed in another connection ([Chapter XXV]).

The study of pathogenic bacteriology includes the thorough study of the individual organisms according to the methods already given ([Chapters XVIII–XXI]) as an aid to diagnosis and subsequent treatment, bacteriological or other, in a given disease. Of far greater importance than the treatment, which in most infectious diseases is not specific, is the prevention and ultimate eradication of all infectious diseases. To accomplish these objects involves further a study of the conditions under which pathogenic organisms exist outside the body, the paths of entrance into and elimination from the body and those agencies within the body itself which make it less susceptible to infection or overcome the infective agent after its introduction. That condition of the body itself which prevents any manifestation of a virulent pathogenic organism after it has been once introduced is spoken of as immunity in the modern sense. Immunity is thus the opposite of susceptibility and may exist in varying degrees.

That scientists are and have been for some years in possession of sufficient knowledge to permit of the prevention and eradication of most, if not all, of our infectious diseases can scarcely be questioned. The practical application of this knowledge presents many difficulties, the chief of which is the absence of a public sufficiently enlightened to permit the expenditure of the necessary funds. Time and educative effort alone can surmount this difficulty. It will probably be years yet, but it will certainly be accomplished.