by Josh Koh & Seyed Alipour
Corynebacterium diphtheriae was identified as the causative agent of diphtheria by Edwin Klebs in 1883. By 1884, Friedrich Löffler isolated C. diphtheriae. Due to Klebs’ and Löffler’s collective work, this bacterium was then known as Klebs-Löffler bacillus. C. diphtheriae is a Gram-positive bacteria surrounded by a thick outermost layer of peptidoglycan (polymer of sugars and amino acids). This bacterium is found throughout the world but is more prevalent in subtropical and developing countries that lack active vaccination programs against its toxin. Most commonly, C. diphtheriae infects humans, but there have been records of rare animal infections. In humans, C. diphtheriae infects and grows outside cells on the skin or the nasopharyngeal cavity, the upper part of the respiratory tract near the throat that lies behind the nose and above the mouth.
C. diphtheriae is transmitted between humans by contaminated aerosols or by contact with contaminated objects. Even if the infected person is asymptomatic, they can be infectious for up to four weeks. Once on the skin or in the upper respiratory tract, C. diphtheriae attaches to the cells and kills them by releasing its toxin, the Diphtheria Toxin (DT). DT will kill the cells by stopping protein production, allowing C. diphtheriae to use the released resources for its growth and multiplication. Through this mechanism, C. diphtheriae damages the skin and the lining of the upper respiratory tract and throat.
Symptoms of C. diphtheriae infections usually arise 2-5 days post-infection. DT released by C. diphtheriae causes open sores or shallow ulcers on the skin. The respiratory damage from DT results in sore throat, swollen regions in the neck, and the formation of a thick and grey coating of dead tissue in the affected area – called a “pseudo-membrane” (Figure 1). This pseudo-membrane may cause difficulty breathing or choking. A respiratory infection may also result in mild fever and weakness. In such cases, the disease is referred to as diphtheria. In some rare cases, DT can enter the circulatory system and cause heart, nerve, and kidney damage, as well as organ death.
Diphtheria diagnosis can be done through the identification of the common symptoms mentioned above and doing a swab test. The swab test includes taking a sample from the back of the throat or the skin lesion and trying to identify if C. diphtheriae and DT are present.
While skin diphtheria rarely causes severe disease, respiratory diphtheria remained the number one cause of death in children under 14 years of age across Canada until the mid-1920s. Before the introduction of the diphtheria vaccine in 1926, Canada recorded 9057 cases of diphtheria in 1924 – its highest number of annual cases ever. For more information regarding the vaccine, refer to the ‘Treatment’ section. Routine immunization in infants and children began in 1930, leading to a steep decline in diphtheria morbidity and mortality rates in Canada. Consequently, from 1993 to 2018, only 19 cases were reported, and the last recorded death by diphtheria was in 2010. Other industrialized and developed countries began infant and childhood immunization shortly after World War II. Like Canada, immunizations led to a dramatic reduction of diphtheria cases and transmission in those countries.
Recent respiratory outbreaks in developed regions such as Europe and North America are associated with disease-carrying travelers returning from regions where diphtheria is endemic (regularly found among populations of defined locations). Diphtheria is endemic in developing countries with insufficient immunization of children under 15, typically with high population densities and unsanitary living conditions. For instance, countries of the former Soviet Union with low immunization coverage endured over 150,000 cases and 5,000 deaths in the 1990s via respiratory diphtheria. Furthermore, regions where diphtheria is endemic usually have subtropical climates, such as many Asian and African countries (e.g., Haiti, Bangladesh, and Yemen).
Recent studies have suggested that sporadic (irregular and infrequent) cases of respiratory diphtheria occurring in countries with higher immunization coverages mostly affect the population older than 15 years of age, indicating waning vaccine immunity. Even so, the most susceptible populations to diphtheria are children below the age of 15 and the elderly. New outbreaks occur mainly among alcohol and drug abusers. The case mortality rate of respiratory diphtheria without treatments (refer to ‘Treatment’ section) is 30–50%. The introduction of respiratory diphtheria treatments significantly reduced the mortality rate to 5-10%.
The first step necessary for C. diphtheriae infection is the colonization of the host (Figure 2). C. diphtheriae uses pili, a sort of hair-like protrusion on its surface, to attach to the host cells. The bacterial DNA encodes for nine different types of pili. The bacteria can switch between these nine different pili to confuse the human immune system so that it can live and grow longer at the infection site. This is called antigenic variation.
The most important virulence factor for C. diphtheriae is the DT. C. diphtheriae will secrete DT once it attaches to the cells lining the throat, the upper respiratory tract, or the skin. As DT is secreted from C. diphtheriae, it is referred to as an exotoxin. DT is composed of two subunits, A and B. The A subunit is the enzymatic subunit that speeds up reactions, and the B subunit is responsible for binding to target cells and releasing the A subunit into the cell. The B subunit attaches to a receptor, a molecule that recognizes specific targets, on the surface of the host cells. This receptor is called the heparin-binding epidermal growth factor precursor (HB-EGF). The binding of DT via the B subunit to the receptor causes the cell to take in the DT-receptor complex. This process is called endocytosis. At this point, DT is in an endosome, a sort of bag, inside the host cell. Proteases, which are enzymes that degrade proteins, then cut DT as the environment in the endosome becomes progressively more acidic. The acidic endosome causes DT to change shape and the B subunit pushes the A subunit into the host cell cytoplasm, which is the solution that fills cells.
Within the cytoplasm, the A subunit will modify a component of the host cell machinery that is involved in protein synthesis. This component is called EF-2. By modifying this component, DT will make the host cell unable to make proteins needed for its survival. As a result, the DT affected cell will die, providing resources for the growth of C. diphtheriae.
Antibiotic treatments in conjunction with the diphtheria antitoxin are very effective in treating skin and respiratory C. diphtheriae infections. C. diphtheriae is susceptible to antibiotics such as penicillin and erythromycin. Penicillin inhibits the enzyme that adds rigidity to the peptidoglycan layers of the bacterium’s cell wall. By inhibiting this enzyme, penicillin will destabilize C. diphtheriae’s cell wall and kill it. Contrarily, erythromycin will stop the synthesis of proteins, namely DT, to neutralize C. diphtheriae’s ability to stop the host cell’s protein production. Furthermore, the diphtheria antitoxin deactivates the unbound DT to prevent damage to the host cells. As respiratory diphtheria may proceed to cause damage to organs – as mentioned in the ‘disease’ section – or death if left untreated, early diagnosis and subsequent treatment mean a better chance of survival.
While treatments for diphtheria have proven effective, the best method for controlling diphtheria morbidity and mortality is through preventive measures such as immunization. Immunization is established against C. diphtheriae’s toxin via the Diphtheria, Tetanus, and Pertussis (DTaP) vaccine, which contains an inactivated version of the DT called a toxoid. The inactivated DT will not cause an illness but will trigger the human immune system to form antibodies against it for protection against future exposures to DT-producing C. diphtheriae. The vaccine will only confer immunity to DT (the disease-causing agent) and not C. diphtheriae specifically. In 2014, it was estimated that 86% are vaccinated against DT globally. The DTaP vaccine also confers immunity to tetanus and pertussis, two other deadly human diseases.
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