Koch’s Bacillus: Morphology, Characteristics, Resistance, Laboratory Staining and Discovery

Robert Koch, a German physician and scientist, discovered Mycobacterium tuberculosis in 1882.

The Koch bacillus or Mycobacterium tuberculosis , is an infectious microorganism that causes tuberculosis in humans.

It is a type of microbacterium that includes two species that cause diseases in humans: Mycobacterium tuberculosis and Mycobacterium leprae , which is the bacteria that causes leprosy.

A 1/3 of the world population is infected with Mycobacterium Tuberculosis and millions of people with active tuberculosis.

There are approximately 9 million new cases each year and 3 million people die annually from the disease .


The structure of Mycobacterium Tuberculosis when viewed under the microscope shows thin rods or bacilli.

Staining of this bacterium is difficult, but once stained, it becomes resistant to discoloration by diluted mineral acids, this is the reason why Mycobacterium tuberculosis is called Acid-resistant bacilli (BAR).

Characteristics of Koch’s Bacillus

Mycobacterium tuberculosis belongs to the genus Mycobacterium (this is category M. Tuberculosis in taxonomy).

The following characteristics are found in any organism located taxonomically in this genus.

  • They are aerobic microorganisms: this means that they require oxygen to live and cause infection; This is the reason why Mycobacterium tuberculosis lodges mainly in the lungs, since there it obtains all the oxygen necessary for its survival.
  • They have no movement: because they do not have structures to move, like cilia.
  • They do not form spores.
  • They are not encapsulated: they do not present capsules that cover the cell wall.
  • Its growth is slow: this is the reason why it takes time to show the signs and symptoms of Mycobacterium tuberculosis and it also takes time to treat them. When growing it , the tuber colonies appear after 2 weeks or 6-8 weeks.
  • They are forced to parasite as they depend on others to survive.
  • They are opportunistic pathogens that can cause serious infections when the immune system is suppressed by the presence of diseases or drugs (immunosuppression).


It is relatively heat resistant but is destroyed when heated to 60 degrees Celsius for 20 minutes.

It can survive in sputum for approximately 20 to 30 hours.

Microbacteria are relatively resistant to disinfectants and can survive after exposure to any of these chemicals: 5% phenol, 15% sulfuric acid, 3% nitric acid, 5% oxalic acid and 4% sodium hydroxide. % (NaOH).

And its constant mutation makes it resistant to the medications used, which is why its treatment is based on the application of two or more antibiotics.

Staining in the laboratory

The procedure for dyeing “Mycobacterium tuberculosis” consists of taking a sample as a sputum stain and covering it with a red mixture known as a fuchsin-phenolic stain (Ziehl-Neelsen technique).

It is then heated to help the penetration of the added dye.

Subsequently, a mixture of acid and alcohol in the proportion: 95% ethanol and 3% hydrochloric acid (HCl) is poured onto the heated sputum smear.

Then a counter methylene blue stain is applied.

When this is done to other bacteria that are not acid resistant, their cell walls usually dissolve and the red color of the stain is washed and they also take on the blue color of the added stain.

Culture media

1.- Solid media

  • Egg yolk: this includes: Lowenstein-Jensen medium, Petragnini medium and half Dorset.
  • Blood example is the means of Tarshi.
  • Serum: example is the slope of Loeffler’s serum.
  • Popes: the example is the Pawlowsky medium.

2.- Liquid media

  • Dubos media.
  • Medios de Middlebrooks.
  • Means of Proskauer and Beck.
  • Sula means.
  • Middle of Sauton.

Therefore, the cultivation of Mycobacterium tuberculosis can be carried out using any of these means: either the liquid medium or the solid medium.

Transmission of Mycobacterium tuberculosis – Koch bacillus

Droplet infection

This occurs by transmission from person to person by inhalation of aerosols when an infected person coughs, speaks, points or laughs.

Abrasion contamination of the skin

This occurs mainly in laboratory workers.

Symptoms and signs of Mycobacterium tuberculosis infection

Generally, people infected with tuberculosis harbor the bacteria without developing any symptoms.

This condition is known as “Latent Tuberculosis Infection”.

In this period, the bacteria are inactive and kept under control by the body’s immune system.

The person, even when he harbors the bacteria, does not feel sick.

It is estimated that one third of all living beings are latently infected with the bacteria.

The typical clinical picture of the infection includes intense fevers, with night sweats and chills.

Symptoms that involve other areas of the body vary, depending on the affected organ.

The symptoms of Tuberculosis in the lungs include persistent cough, with sputum sometimes accompanied by blood chest pain.

Although the lungs are the main organ where the damage caused by tuberculosis occurs, other organs and tissues may be affected.

The progress of the disease tends to spread the infection to other organs and in some cases the first sign of the disease can occur affecting the brain, kidneys or bones, so the symptoms that involve these areas will vary, depending on the organ that is affected.


The treatment regimen for Mycobacterium tuberculosis infection is the same as for pulmonary tuberculosis.

After the identification of the bacteria causing tuberculosis, medical therapy was based on rest, and a good climate with a lot of fresh cold air, good nutrition and isolation.

Currently the treatment is focused in the first place on the reduction of the number of bacilli quickly, minimizing the risk of disease transmission.

Another of the treatment approaches is to prevent the development of resistance to medications, for this reason multiple medications administered at the same time and in the long term are applied to prevent relapse.

The most commonly used medications for the treatment of tuberculosis: are: Izoniazine, rifampin, pyrazinamide, streptomycin, and ethambutol.

Monitoring involves the control of side effects and monthly sputum tests.

Prevention of Mycobacterium tuberculosis infections

Vaccination is an important preventive measure against tuberculosis. A vaccine called BCG (Bacillus Calmette-Guérin) is currently administered and is made from a weakened micro-bacterium that infects livestock.

This vaccine protects against tuberculosis and even when it is infected, it will be milder and avoid having forms of TB skeletal, meningeal and miliary tuberculosis.

It is administered at an early age and serves to strengthen the immune system to withstand the most severe forms of the disease.

Being composed of live bacteria, it is not possible to administer it to immunosuppressed babies.

Discovery of Koch’s Bacillus

The German bacteriologist Robert Koch is generally considered the founder of modern bacteriology.

It is responsible for devising or adapting many of the basic principles and techniques, in particular bacteriology staining methods.

He isolated and obtained in 1876 a pure culture of the bacterium Bacillus anthracisthat causes anthrax, he discovered the cholera vibrio Vibrio cholerae in 1883, and he identified and isolated the tubercle bacillus Mycobacterium tuberculosis in 1882.

In 1905, when Koch received the Nobel Prize in Medicine, he dedicated his acceptance speech to promote greater understanding of tuberculosis and its causal agent.

Koch died in 1910, leaving the scientific community and the world in general with a valuable heritage of knowledge and understanding as a result of his work on anthrax, cholera, trypanosomiasis and especially tuberculosis.

Although it was known that tuberculosis was due to an infectious agent, the organism had not yet been isolated and identified.

Koch began his research, with a tuberculous tissue from the body of an individual of thirty-six years.

This man had been in perfect health until three weeks before his death, when he developed cough, chest pains and severe weight loss.

Koch ground the tuberculous material and injected it into the eyes and under the skin of the guinea pigs.

While waiting for the infection to appear, he smeared the infected tissue on glass slides and looked at it through the microscope.

For days, he did not notice anything since the tuberculosis bacillus is extremely small, about one third the size of the anthrax bacillus, for this reason, other microscopists had not been able to find it.

But after hours of immersion in various tints, they finally took on enough color to stand out from their surroundings like tiny blue rods.

The infected animals began to die one by one, and in the samples observed the presence of yellowish tubers identical to those that had previously observed the sample of tissue of the individual.

Now, sure of having identified the tuberculosis bacillus, Koch took a portion of infected tissue from the bodies of patients who had died of tuberculosis.

He injected this tissue into several species of animals and only when the animal or man developed the disease, Koch could find the rods dyed blue.

So on March 24, 1882, Koch announced to the Physiological Society of Berlin, which had isolated and cultivated the tubercle bacillus.

Three weeks later, on April 10, he published an article entitled “The etiology of tuberculosis.”

In 1884, in a second article with the same title, he exhibited for the first time the “postulates of Koch”, which since then have become basic for the studies of all infectious diseases.

He had observed the bacillus in association with all the cases of the disease, had developed the organism outside the body of the host, and had reproduced the disease in a susceptible host inoculated with a pure culture of the isolated organism.

Koch continued his studies on tuberculosis, hoping to find a cure.

In 1890, he announced the discovery of tuberculin, a substance derived from tubercle bacilli, that he thought was capable of stopping bacterial development in vitro and in animals.

Tuberculin turned out to be an ineffective therapeutic agent, but a valuable diagnostic tool.