Disease
C. diphtheriae causes diphtheria (Figure 1). Other Corynebacterium species (diphtheroids) are implicated in opportunistic infections.
Fig1. Diphtheria. Note whitish-gray pseudomembrane covering posterior pharynx and marked inflammation of palate and pharynx. Caused by diphtheria toxin, an exotoxin that inhibits protein synthesis by inhibiting elongation factor-2. (Used with permission from Dr. Peter Strebel.)
Important Properties
Corynebacteria are gram-positive rods that appear club-shaped (wider at one end) and are arranged in palisades or in V- or L-shaped formations (Figure 2). The rods have a beaded appearance. The beads consist of granules of highly polymerized polyphosphate—a storage mechanism for high-energy phosphate bonds. The granules stain metachromatically (i.e., a dye that stains the rest of the cell blue will stain the granules red).
Fig2. Corynebacterium diphtheriae—Gram stain. Arrow points to a “club-shaped” gram-positive rod. Arrowhead points to typical V- or L-shaped corynebacteria. (Reproduced with permission from Public Health Image Library, Centers for Disease Control and Prevention.)
Transmission
Humans are the only natural host of C. diphtheriae. Both toxigenic and nontoxigenic organisms reside in the upper respiratory tract and are transmitted by airborne droplets. The organism can also infect the skin at the site of a preexisting skin lesion. This occurs primarily in the tropics but can occur worldwide in indigent persons with poor skin hygiene.
Pathogenesis
Although exotoxin production is essential for pathogenesis, invasiveness is also necessary because the organism must first establish and maintain itself in the throat. Diphtheria toxin inhibits protein synthesis by ADP-ribosylation of elongation factor-2 (EF-2). The toxin affects all eukaryotic cells regardless of tissue type but has no effect on the EF-2 in prokaryotic cells.
The toxin is a single polypeptide with two functional domains. The binding (B) domain mediates binding of the toxin to glycoprotein receptors on the cell membrane. The active (A) domain possesses enzymatic activity that cleaves nicotinamide from nicotinamide adenine dinucleotide (NAD) and transfers the remaining ADP-ribose to EF-2, thereby inactivating it.
The DNA that codes for diphtheria toxin is part of the DNA of a temperate bacteriophage called beta phage. During the lysogenic phase of viral growth, the DNA of this virus integrates into the bacterial chromosome and the toxin is synthesized. C. diphtheriae cells that are not lysogenized by this phage do not produce exotoxin and are nonpathogenic.
The host response to C. diphtheriae consists of the following:
(1) A local inflammation in the throat, with a fibrinous exudate that forms the tough, adherent, gray pseudomembrane characteristic of the disease.
(2) Antibody that can neutralize exotoxin activity by blocking the interaction of the binding domain with the receptors, thereby preventing entry into the cell. The immune status of a person can be assessed by Schick’s test. The test is performed by intradermal injection of 0.1 mL of purified standardized toxin. If the patient has no antitoxin, the toxin will cause inflammation at the site 4 to 7 days later. If no inflammation occurs, antitoxin is present and the patient is immune. The test is rarely performed in the United States except under special epidemiologic circumstances.
Clinical Findings
Although diphtheria is rare in the United States, physicians should be aware of its most prominent sign, the thick, gray, adherent pseudomembrane over the tonsils and throat (see Figure 1) .The other aspects are nonspecific: fever, sore throat, and cervical adenopathy. There are three prominent complications:
(1) Extension of the membrane into the larynx and trachea, causing airway obstruction.
(2) Myocarditis accompanied by arrhythmias and circulatory collapse.
(3) Nerve weakness or paralysis, especially of the cranial nerves. Paralysis of the muscles of the soft palate and pharynx can lead to regurgitation of fluids through the nose. Peripheral neuritis affecting the muscles of the extremities also occurs.
Cutaneous diphtheria causes ulcerating skin lesions covered by a gray membrane. These lesions are often indolent and often do not invade surrounding tissue. Systemic symptoms rarely occur.
Laboratory Diagnosis
Laboratory diagnosis involves both isolating the organism and demonstrating toxin production. It should be emphasized that the decision to treat with antitoxin is a clinical one and cannot wait for the laboratory results. A throat swab should be cultured on Loeffler’s medium, a tellurite plate, and a blood agar plate. The tellurite plate contains a tellurium salt that is reduced to elemental tellurium within the organism. The typical gray-black color of tellurium in the colony is a telltale diagnostic criterion. If C. diphtheriae is recovered from the cultures, either animal inoculation or an antibody-based gel diffusion precipitin test is performed to document toxin production. A PCR assay for the presence of the toxin gene in the organism isolated from the patient can also be used.
Smears of the throat swab should be stained with both Gram stain and methylene blue. Although the diagnosis of diphtheria cannot be made by examination of the smear, the finding of many tapered, pleomorphic gram-positive rods can be suggestive. The methylene blue stain is excellent for revealing the typical metachromatic granules.
Treatment
The treatment of choice is antitoxin, which should be given immediately based on clinical impression because there is a delay in laboratory diagnostic procedures. The toxin binds rapidly and irreversibly to cells and, once bound, cannot be neutralized by antitoxin. The function of antitoxin is therefore to neutralize unbound toxin in the blood. Because the antiserum is made in horses, the patient must be tested for hypersensitivity, and medications for the treatment of anaphylaxis must be available. Serum sickness may occur after administration of antiserum made in horses.
Treatment with penicillin G or erythromycin is also recommended, but neither is a substitute for antitoxin. Antibiotics inhibit growth of the organism, reduce toxin production, and decrease the incidence of chronic carriers.
Prevention
Diphtheria is very rare in the United States because children are immunized with diphtheria toxoid (usually given as a combi nation of diphtheria toxoid, tetanus toxoid, and acellular pertussis vaccine, often abbreviated as DTaP).
Diphtheria toxoid is prepared by treating the exotoxin with formaldehyde. This treatment inactivates the toxic effect but leaves the antigenicity intact. Immunization consists of three doses given at 2, 4, and 6 months of age, with boosters at 1 and 6 years of age. Because immunity wanes, a booster every 10 years is recommended. Immunization does not prevent nasopharyngeal carriage of the organism.
