Koch's bacillus: what kind of bacterium is it and how does it cause tuberculosis

A cough that has lasted for the third week is easy to attribute to fatigue or a long-standing smoking habit. Sometimes behind it hides a bacterium known for more than a century — Koch's bacillus. Let's calmly examine what this microorganism is, how it gets into the lungs, and why encountering it does not always mean disease.

Koch's bacillus: what it is

Koch's bacillus is the common name for Mycobacterium tuberculosis. Robert Koch isolated it on March 24, 1882, and this date is observed as World Tuberculosis Day. It is a prokaryote: the cell lacks a Golgi apparatus and lysosomes, but has a single circular DNA. The shape is a straight or slightly curved rod 1–10 µm long and 0.2–0.6 µm thick, non-motile, without spores or capsules. A dense wall of waxes protects it on the outside, so it is stained by the Ziehl–Neelsen method: on a blue background, thin crimson-red rods are visible, sometimes in pairs or in the shape of the Roman numeral 'V'.

Mycobacteria: types and dangers of tuberculosis mycobacteria

The causative agent of tuberculosis is not a single species but a group of closely related mycobacteria, MTBC, which are 99.9% identical: M. tuberculosis, M. bovis, M. africanum, and several other species. All of them cause tuberculosis in humans and animals; M. tuberculosis is the best studied.

The mycobacterium does not produce toxins, so there are no striking symptoms in the first days. It divides slowly, with a cycle of 14–18 hours, and ivory-colored colonies on solid media become visible only after 34–55 days. Outside the body it survives for a long time: in water — up to 5 months. Direct sunlight kills it in an hour and a half, ultraviolet — in 2–3 minutes, boiling water — in 5 minutes in moist sputum and 25 minutes in dried sputum, chlorine-containing agents — in 5 hours.

How the causative agent of tuberculosis is transmitted

Tuberculosis spreads through the air. A person with active disease, when the bacterium grows in the lungs or throat, releases small droplets with mycobacteria when coughing, sneezing, talking, and singing. Infection occurs through regular and prolonged inhalation of such droplets — as if day after day you breathe someone else's exhaled air. Therefore, the risk is higher with those you live or work with than with a brief encounter. The disease is not transmitted through touch, kissing, shared food, or utensils.

Why not every infection leads to disease

Most people with mycobacteria in their body do not become ill. This is latent infection: there are no symptoms, the person cannot infect others, but when immunity weakens, the bacterium activates after weeks or years. Active disease means that mycobacteria are multiplying and the person is ill. This is more common with HIV, with infection in the last two years, with diabetes, alcohol dependence, in children under 5 years, in the elderly, in those taking drugs that weaken immunity, in people without housing, and in places of detention.

How the tuberculosis bacillus is detected

First, a Mantoux skin test or a blood test is offered — they are done both for symptoms and for risk due to contact. If the test shows mycobacteria, it is determined whether they are growing: for lung involvement, sputum is examined and a chest X-ray is done. For extrapulmonary forms, ultrasound, echocardiogram, CT, tissue biopsy, and urine tests are possible. Microscopy with Ziehl–Neelsen staining shows the rods themselves, but one smear is not enough — the result is confirmed by culture.

When to see a doctor

The guideline is simple — three weeks. If the cough lasts longer, see a doctor. The list of situations that should not be delayed:

• cough for more than 3 weeks, with or without sputum;
• blood in sputum;
• constant fatigue without cause;
• fever or night sweats that do not go away;
• prolonged contact with a person who has symptoms of tuberculosis.

In children, a warning sign is difficulty gaining weight and height. If the disease has spread beyond the lungs, enlarged lymph nodes, body and joint pain, abdominal pain, constipation, dark urine, headache, vomiting, confusion, and rash appear.

A cough alone is not a reason to call an ambulance. Call 103 or 112 if neck stiffness appears together with severe headache, it is painful to look at bright light, a seizure has occurred, behavior has suddenly changed or confusion has set in, weakness or loss of movement in part of the body has occurred: this can happen when tuberculosis has spread to the brain.

The material is for reference and does not replace a doctor's consultation — if in doubt, discuss your symptoms with a specialist.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer