Mycobacterium tuberculosis characteristics, morphology, culture

Simon Doyle

The Mycobacterium tuberculosis, also known as Koch's Bacillus, it is a pathogenic bacterium that causes a highly widespread infectious disease throughout the world, known as tuberculosis.

It was first described in 1882 by the German physician and microbiologist Robert Koch. His work earned him the 1905 Nobel Prize in Physiology and Medicine. This discovery was a milestone in medicine, since by knowing the causative agent it was possible to determine its transmission mechanism and what were the favorable conditions for its proliferation..

Mycobacterium tuberculosis cells. Source: By NIAID on Flickr. [Public domain], via Wikimedia Commons

Throughout the years, tuberculosis has meant a disease that has claimed the lives of millions of people. Its origin dates back to prehistoric times, in the Neolithic, when the domestication of animals began. From there and in the different historical stages, epidemics have been unleashed that have greatly reduced the population..

With advances in the area of ​​bacteriology and with the development of antibiotics, it was possible to begin to control the disease. Today its causal agent, the transmission mechanism, the pathogenesis process, as well as the normal course that the disease follows and the factors involved in it are known. This has made it possible to adopt increasingly effective treatment methods.

Article index

  • 1 Taxonomy
  • 2 Morphology
  • 3 General characteristics
    • 3.1 Not mobile
    • 3.2 It is aerobic
    • 3.3 It is neither Gram positive nor Gram negative
    • 3.4 They are alcohol-acid fast bacilli
    • 3.5 It is a parasite
    • 3.6 It is mesophilic
    • 3.7 Its growth is slow
  • 4 Habitat
  • 5 Cultivation
    • 5.1 Synthetic agar medium
    • 5.2 Thickened egg medium
    • 5.3 Required environmental conditions
  • 6 Diseases
    • 6.1 Pathogenesis of tuberculosis
    • 6.2 Virulence factors
  • 7 Symptoms
  • 8 Treatment
  • 9 References


The taxonomic classification of the Mycobacterium tuberculosis is the next:

Domain: Bacterium.

Edge: Actinobacteria.

Order: Actinomycetales.

Family: Mycobacteriaceae.

Gender: Mycobaterium.

Species: Mycobacterium tuberculosis.


The Mycobacterium tuberculosis It is a bacterium that belongs to the group of bacilli. They are rod-shaped, and can be straight or slightly curved cells.

They are extremely small cells, measuring approximately 0.5 microns wide by 3 microns long. When observed under the microscope, individual cells or cells united in pairs are appreciated.

In laboratory cultures, whitish colonies with a multilobular appearance are observed. The bacterium has a single circular chromosome that houses about 4,200,000 nucleotides. The genome contains about 4,000 genes.

The bacterial cell does not produce spores. In addition to this, do not present any protective capsule that surrounds it. It has a thick cell wall that is made up of a polypeptide, peptidoglycan and free lipids.

The cell wall is a complex structure that contains numerous chemical compounds such as mycolic acids, acyl-glycolipids, and sulfolipids..

It also contains integral proteins known as porins, which function as a kind of pores or channels through which certain substances can enter or exit the bacterial cell..

General characteristics

The Mycobacteriurm tuberculosis it is a well-known and widely studied bacterium.

Not mobile

This type of bacteria does not show mobility. This is because in its structure it does not present extensions (cilia or flagella) that stimulate its displacement..

It is aerobic

Likewise, they are strictly aerobic organisms. Due to this, they must be in an environment in which there is ample availability of oxygen. This is the reason why the main infecting organ is the lung.

It is neither Gram positive nor Gram negative

It cannot be classified as gram positive or gram negative bacteria. Despite containing peptidoglycan in its cell wall, when subjected to Gram staining it does not follow the characteristic patterns of either of the two groups..

They are alcohol-acid fast bacilli

When stained, they are able to resist fading with acid or alcohol, without suffering any structural damage. This is due to the integrity of its cell wall and the role of its components, which makes it more resistant than other types of bacteria..

It is a parasite

Another of its characteristics that is decisive in its pathogenesis process is that it is an intracellular parasite. This means that it needs a host to survive. Specifically, the Mycobacterium tuberculosis parasitizes blood cells known as macrophages.

It is mesophilic

Its average optimum growth temperature is located in the ranges 32 to 37 ° C. In addition to this, its optimal pH is between 6.5 and 6.8, which implies that it performs adequately in slightly acidified environments..

Its growth is slow

They have an extremely slow growth rate. Its cell multiplication time is between 15 - 20 hours. Under experimental conditions in the laboratory, this period of time can be shortened a little..

When a culture of this bacterium is carried out, you have to wait approximately 5 or 6 weeks to just begin to appreciate a colony. This is the reason why the signs and symptoms appear after a long time after contact with the bacteria..


This is a bacterium that can be found in a wide variety of environments. It has been found in soil, water, and the gastrointestinal tract of some animals.

The main reservoir is humans, although other primates can be as well. The bacteria have a predilection for lung tissue. However, it can spread through the bloodstream or lymphatic system to other parts of the body..

Likewise, thanks to its morphological characteristics that give it a certain resistance, it can survive for several weeks in dust, clothes and carpets. In sputum it can lie dormant for months.


The Mycobacterium tuberculosis it is a bacterium that to develop in a culture medium needs certain nutritional requirements.

As a carbon source you can use compounds such as glycerol and as a nitrogen source, ammonium ions and asparagine. It also requires albumin, which can be incorporated as an addition to chicken eggs or serum albumin..

Various types of culture medium can be used. Among the most common and functional are: synthetic agar medium and thickened egg medium.

Synthetic agar medium

Contains cofactors, vitamins, oleic acid, glycerol, catalase, albumin, and defined salts. This type of medium is very useful to determine the morphology of the colonies and thus study their susceptibility..

Thickened egg medium

The main ingredient is complex organic substances, such as those contained in fresh eggs and egg yolks. They also have glycerol and defined salts.

Required environmental conditions

Regarding temperature, various studies have shown that the optimum is at 37 ° C. This is because this bacterium has become accustomed to human body temperature. Below 34 ° C it stops growing and above 40 ° C it denatures and dies.

Likewise, it is important to remember that it necessarily requires oxygen to develop, so it must be ensured that this element is available when cultivating..

Depending on the bacterial content of the sample taken for the culture, it may take between 6 and 8 weeks to notice the appearance of colonies.

It is common that antibiotics are added to the culture medium that are innocuous for the Mycobacterium tuberculosis, in order to prevent the proliferation of other types of bacterial cells.


The Mycobacterium tuberculosis it is the main causative agent of an infectious disease known as tuberculosis. The main organ that is affected by this disease is the lung, although cases have been described in which the bacteria have migrated to other parts of the body, causing considerable damage..

Pathogenesis of tuberculosis

The main means of transmission are the secretions expelled by people with the disease, mainly when they cough..

When coughing, they release small, imperceptible particles of liquid, in which a large number of bacterial cells are contained. When evaporating, bacteria remain that can be inhaled by healthy subjects.

As the gateway to the body is inhalation, they pass directly to the respiratory tract, which they travel until they reach their accommodation site: the pulmonary alveoli..

As with all pathogens that enter the body, they stimulate the production of chemical messengers known as lymphokines and cytosines. The function of these molecules is to attract macrophages, cells of the immune system that fight infections.

The bacterium infects macrophages and begins to proliferate in them, causing the characteristic lesions of this pathology in lung tissue..

Virulence factors

Virulence factors are a determining element in the development of an infection. They are defined as the various mechanisms that a pathogen has to infect the host.

In the case of Mycobacterium tuberculosis, virulence factors are as follows:

Chord factor: Its function is to make the bacterial cells group together, thus forming cords.

LAM (Lipo-arabic-mannan): its function is to prevent macrophages from being activated, in addition to promoting the entry of bacteria into them through biochemical mechanisms.

Sulfatides: they prevent the phagosomes in which the bacteria is contained from fusing with the lysosomes for their disintegration.


As in many other pathologies, in the case of tuberculosis it can happen that the person is a carrier of the bacteria, without showing symptoms. This is known as latent tuberculosis..

X-ray with lungs affected by Mycobacterium tuberculosis. Source: By UnknownUnknown author [Public domain], via Wikimedia Commons

On the other hand, a significant number of people who acquire the bacteria manifest a set of symptoms. This is what is called active tuberculosis. In this case, the symptoms that manifest are the following:

  • General malaise (fever, fatigue)
  • Weightloss
  • Constant cough
  • Night sweats
  • Bloody expectoration
  • Pain in the chest, when breathing and coughing.


Treatment for tuberculosis lasts for a long time. When a person suffers from the disease, they must take medications for a period of time ranging from 6 to 9 months.

Among the most common medications to treat this disease are:

  • Pyrazinamide
  • Rifampcin
  • Isoniazid
  • Ethambutol

Of course, the dose and the selection of the drug is made by the doctor taking into account certain parameters such as the age of the patient, their general state of health and the possible resistance to drugs of the infectious strain..

It is vitally important to comply with the treatment in its entirety. If it is suspended prematurely, it could run the risk of generating resistance in bacteria that are still alive, which could lead to an increase in virulence and severity of the disease.


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