Corynebacteria are Gram-positive, aerobic, nonmotile, rod-shaped bacteria related to the Actinomycetes. They do not form spores or branch as do the actinomycetes, but they have the characteristic of forming irregular shaped, club-shaped or V-shaped arrangements in normal growth. They undergo snapping movements just after cell division which brings them into characteristic arrangements resembling Chinese letters. The genus Corynebacterium consists of a diverse group of bacteria including animal and plant pathogens, as well as saprophytes.
Some corynebacteria are part f the normal flora of humans, finding a suitable niche in virtually every anatomic site. The best known and most widely studied species is Corynebacterium diphtheriae, the causal agent of the disease diphtheria. History and Background No bacterial disease of humans has been as successfully studied as diphtheria. The etiology, mode of transmission, pathogenic mechanism and molecular basis of exotoxin structure, function, and action have been clearly established. Consequently, highly effective methods of treatment and prevention of diphtheria have been developed.
The study of Corynebacterium diphtheriae traces closely the development of medical , immunology and molecular biology. Many contributions to these fields, as well as to our understanding of host-bacterial interactions, have been made studying diphtheria and the diphtheria toxin. Hippocrates provided the first clinical description of diphtheria in the 4th century B. C. There are also references to the disease in ancient Syria and Egypt. In the 17th century, murderous epidemics of diphtheria swept Europe; in Spain “El garatillo” (the strangler”), in Italy and Sicily, “the gullet disease”.
In the 18th century, the disease reached the American colonies and reached epidemic proportions in 1735. Often, whole families died of the disease in a few weeks. The bacterium that caused diphtheria was first described by Klebs in 1883, and was cultivated by Loeffler in 1884, who applied Koch’s postulates and properly identified Corynebacterium diphtheriae as the agent of the disease.
In 1884, Loeffler concluded that C. diphtheriae produced a soluble toxin, and thereby provided the first description of a bacterial exotoxin. In 1888, Roux and Yersin demonstrated the presence of the toxin in the cell-free ulture fluid of C. iphtheriae which, when injected into suitable lab animals, caused the systemic manifestation of diphtheria. Two years later, von Behring and Kitasato succeeded in immunizing guinea pigs with a heat-attenuated form of the toxin and demonstrated that the sera of immunized animals contained an antitoxin capable of protecting other susceptible animals against the disease.
This modified toxin was suitable for immunizing animals to obtain antitoxin but was found to cause severe local reactions in humans and could not be used as a vaccine. In 1909, Theobald Smith, in the U. S. emonstrated that diphtheria toxin neutralized by antitoxin (forming a Toxin-Anti-Toxin complex, TAT) remained immunogenic and eliminated local reactions seen in the modified toxin.
For some years, beginning about 1910, TAT was used for active immunization against diphtheria. TAT had two undesirable characteristics as a vaccine. First, the toxin used was highly toxic, and the quantity injected could result in a fatal toxemia unless the toxin was fully neutralized by antitoxin. Second, the antitoxin mixture was horse serum, the components of which tended to be allergenic and to sensitize individuals to the serum.
In 1913, Schick designed a skin test as a means of determining susceptibility or immunity to diphtheria in humans. Diphtheria toxin will cause an inflammatory reaction when very small amounts are injected intracutaneously. The Schick Test involves injecting a very small dose of the toxin under the skin of the forearm and evaluating the injection site after 48 hours. A positive test (inflammatory reaction) indicates susceptibility (nonimmunity). A negative test (no reaction) indicates immunity (antibody neutralizes toxin).
In 1929, Ramon demonstrated the conversion of diphtheria toxin to its nontoxic, ut antigenic, equivalent (toxoid) by using formaldehyde. He provided humanity with one of the safest and surest vaccines of all time-the diphtheria toxoid. In 1951, Freeman made the remarkable discovery that pathogenic (toxigenic) strains of C. diphtheriae are lysogenic, (i. e. , are infected by a temperate B phage), while non lysogenized strains are avirulent. Subsequently, it was shown that the gene for toxin production is located on the DNA of the B phage.
In the early 1960s, Pappenheimer and his group at Harvard conducted experiments on the mechanism of a action of the diphtheria toxin. They studied the effects of the toxin in HeLa cell cultures and in cell-free systems, and concluded that the toxin inhibited protein synthesis by blocking the transfer of amino acids from tRNA to the growing polypeptide chain on the ribosome. They found that this action of the toxin could be neutralized by prior treatment with diphtheria antitoxin. Subsequently, the exact mechanism of action of the toxin was shown, and the toxin has become a classic model of a bacterial exotoxin.
Human Disease Diphtheria is a rapidly developing, acute, febrile infection which involves both local and systemic pathology. A local lesion develops in the upper respiratory tract and involves necrotic injury to epithelial cells. As a result of this injury, blood plasma leaks into the area and a fibrin network forms which is interlaced with with rapidly-growing C. diphtheriae cells. This membranous network covers over the site of the local lesion and is referred to as the pseudomembrane. The diphtheria bacilli do not tend to invade tissues below or away from the surface epithelial cells at the site of the local lesion.
At this site they produce the toxin that is absorbed and disseminated through lymph channels and lood to the susceptible tissues of the body. Degenerative changes in these tissues, which include heart, muscle, peripheral nerves, adrenals, kidneys, liver and spleen, result in the systemic pathology of the disease. In parts of the world where diphtheria still occurs, it is primarily a disease of children, and most individuals who survive infancy and childhood have acquired immunity to diphtheria. In earlier times, when nonimmune populations (i. e. , Native Americans) were exposed to the disease, people of all ages were infected and killed.