Pathogen characteristics
The etiologic agent of the infection is Corynebacterium diphtheriae (Klebs-Löffler bacillus), belonging to the group of coryneform bacteria (diphtheroids).
Morphologically, it is a non-motile Gram-positive bacillus. Its distinctive feature is club-shaped swellings at the rounded ends formed by volutin granules. This makes the bacterium visually resemble a club (from the Greek Coryne). Volutin granules are clearly visible using special staining methods according to Löffler or Neisser.
The microorganism exhibits marked polymorphism: filamentous, coccoid, segmented, and flask-shaped forms may be found in culture. The bacterium does not form true capsules or spores, but is capable of transforming into filterable and L-forms. Fimbriae are present on the cell surface, and toxigenic variants possess a microcapsule containing the cord factor.
The pathogen is a facultative anaerobe with an optimal growth temperature of 37°C. In laboratory practice, elective blood agar media supplemented with tellurite are used for its isolation — accompanying microflora is sensitive to it, whereas the diphtheria bacillus is resistant.
Antigenic structure and classification
The C. diphtheriae population is heterogeneous. Its antigenic structure comprises O- and K-antigens. The species specificity of the bacterium is determined by surface heat-labile K-antigens of protein and nucleoprotein nature.
The detection of K-antigens forms the basis of serological typing, which allows distinguishing about 58 serovars of the pathogen. Based on a complex of antigenic, biochemical, morphological, and cultural characteristics, three main biovars are distinguished:
- gravis
- mitis
- intermedius
Virulence factors and pathogenesis
The epidemic danger and clinical presentation are determined by the bacterium's arsenal of aggressive factors. It is important to note that C. diphtheriae does not possess invasive properties and does not penetrate deep into tissues. Its action is based on surface colonization and toxin release.
- Adhesion factors: surface K-antigens, cord factor, and fimbriae. They ensure fixation and multiplication on the epithelium of mucous membranes (less commonly on wound surfaces).
- Aggressive enzymes: hyaluronidase, neuraminidase, proteases, ribonuclease, and deoxyribonuclease. They destroy parietal mucus and cleave glycoproteins, providing the microorganism with energy for growth.
- Diphtheria exotoxin: a key virulence factor of protein nature. The ability to synthesize it appears only after lysogenization of the bacterium by a temperate bacteriophage that introduces a specific tox gene into the cell. Non-toxigenic strains are incapable of causing the disease.
It is the exotoxin that causes intoxication and toxemia syndrome. Local fibrinous-necrotic inflammation develops at the site of entry, and the circulation of the toxin in the blood leads to severe damage to target organs: the heart, kidneys, peripheral nervous system, and adrenal glands.
Algorithm of anti-epidemic measures for the physician
When identifying a patient with suspected diphtheria in conditions where immediate hospitalization is impossible, the physician must implement the following protocol:
- Isolation: immediately isolate the patient from contact with others.
- Staff protection: physical examination and taking anamnesis must be performed strictly using personal protective equipment (gloves, mask).
- Registration and notification: transmit information to the admission department, register the case in the «Register of Infectious Diseases», and send an «Emergency Notification» to the territorial Center for State Sanitary and Epidemiological Surveillance.
- Diagnostics: perform swabs from the oropharyngeal and nasal mucosa for subsequent bacteriological examination.
- Emergency therapy: initiate antibiotic therapy and administration of diphtheria antitoxin serum (DAS) on-site. Critically important condition: a skin allergy test must be performed prior to administering the therapeutic dose of DAS to rule out sensitization to foreign horse protein.
Treatment and Prevention
The main method of specific population protection is vaccination.
The treatment of patients involves a comprehensive approach including etiotropic, symptomatic, and pathogenetic therapy against the background of a strict diet and regimen. A key role is played by specific detoxification therapy (DAS, fresh frozen immune plasma, immune gamma globulin). In the development of laryngeal diphtheria, surgical treatment may be used according to strict indications. The prognosis of the disease directly depends on the clinical form and severity of the course, which determines the risk of early and late complications.
Briefly: answers to topical questions
What complications can occur in diphtheria?
Diphtheria complications are divided into specific, non-specific bacterial, and exacerbations of non-bacterial concomitant diseases. The following groups of complications are distinguished:
- Specific — infectious-toxic shock, adrenal insufficiency, myocarditis, polyneuropathies, toxic nephrosis, and in croup, laryngeal stenosis and acute respiratory failure.
- Non-specific bacterial — tracheobronchitis, bronchopneumonia, pneumonia, tonsillar or paratonsillar abscesses.
- Non-bacterial nature — exacerbation of chronic diseases of the cardiovascular, endocrine, and urinary systems (coronary artery disease, diabetes mellitus, chronic pyelitis, renal failure).
How is the skin allergy test performed prior to DAS administration?
The skin allergy test to detect sensitization to horse protein prior to serum administration includes two consecutive stages.
- First test — intradermal injection into the forearm of 0.1 ml of diphtheria serum diluted 1:100 with evaluation of the local reaction after 20 minutes.
- Second test — subcutaneous injection into the shoulder of 0.1 ml of undiluted diphtheria serum with evaluation of the reaction after 40 minutes.
The test is considered negative if the diameter of the resulting hyperemia does not exceed 1 cm 40 minutes after subcutaneous administration of the drug.
The genus name Corynebacterium comes from the Greek word "coryne" — club. Remembering the shape of a club makes it easy to memorize the characteristic swellings at the ends of the bacterium formed by volutin granules.
Class and exam questions
Do all strains of the diphtheria bacillus cause disease?
No, the disease is caused exclusively by toxigenic strains. The ability to produce exotoxin appears only after lysogenization of the bacterium by a temperate phage carrying the tox gene.
Why is a skin test necessary before DAS administration?
Diphtheria antitoxin serum (DAS) is produced using horse protein. The skin allergy test is mandatory to identify potential patient sensitization and prevent severe anaphylactic reactions.
Can the diphtheria pathogen penetrate deep tissues?
No, Corynebacterium diphtheriae does not possess invasive properties. The bacterium multiplies on the surface of mucous membranes, and systemic target organ damage is caused by the absorption of exotoxin into the blood (toxemia).
What else to study on this topic
- Clinical classification and specifics of forms of oropharyngeal and laryngeal diphtheria
- Pathomorphological changes in target organs (heart, kidneys, adrenals)
- The phenomenon of bacterial carriage and its epidemiological significance
- Early and late complications of toxic forms of diphtheria
- Mechanisms of the damaging effect of diphtheria exotoxin at the cellular level
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