Mycoplasma pneumoniae: characteristics, morphology, pathogenesis - science - 2023



Mycoplasma pneumoniae it is the main bacterium of the genus Mycoplasma. This species is responsible for producing more than 2 million infections per year in the United States.

While infection by Mycoplasma pneumoniae it is highly contagious, only 3 to 10% of infected individuals develop symptoms consistent with bronchopneumonia.

However, in most cases it presents with mild clinical manifestations such as pharyngitis, tracheobronchitis, bronchiolitis, and croup, while others are asymptomatic.

Infections with this bacteria can occur throughout the year, but the highest incidence is seen in late fall and winter. The infection can appear at any age, however the most susceptible age groups are children older than 5 years, adolescents and young adults.

For reasons that are still unknown, children under 3 years of age tend to develop upper respiratory infections, while older children and adults are more likely to develop pneumonia.


Strains of Mycoplasma pneumoniae they are antigenically homogeneous, this means that only one serotype is known that reproduces by binary fission.

In this species the only known reservoir is man. It is generally isolated from the respiratory tract and its presence is considered pathological.

Nutritional and biochemical characteristics

It is an obligate aerobic microorganism. It grows in culture media containing sterol, purines, and pyrimidines. In crops in vitro they tend to grow very slowly with a recovery time between 4 to 21 days.

From a biochemical point of view Mycoplasma pneumoniae glucose ferments with the formation of acid end products. It does not use arginine and it does not split urea. Its optimum pH ranges from 6.5 to 7.5.


Domain: Bacteria.

Phylum: Firmicutes.

Class: Mollicutes.

Order: Mycoplasmatales.

Family: Mycoplasmataceae.

Genus: Mycoplasma.

Species: pneumoniae.


Mycoplasma pneumoniae it is among the smallest microorganisms capable of living and reproducing extracellularly. Its size ranges from (150 to 200 nm.

This bacterium is characterized by not having a cell wall, being limited by a trilaminar membrane that provides flexibility and polymorphic capacity, that is, it can take a variety of forms.

The absence of a wall means that these microorganisms cannot be stained with the Gram stain.

They have a very small DNA genome (0.58 to 2.20Mb) compared to other bacteria that have 4.64Mb genomes.

The colonies of Mycoplasma pneumoniae they have a grainy surface with a dense center typically buried in agar (inverted fried egg appearance).

Virulence factor

Mycoplasma pneumoniae it has a membrane-associated 169 kDa protein called P1, which has an adhesin function. These adhesins bind to complex oligosaccharides that contain sialic acid and are found in the apical part of the cells of the bronchial epithelium.

Adhesin affects ciliary action and initiates a process that leads to the desquamation of the mucosa and later to an inflammatory reaction and secretion of exudates.

Inflammation is characterized by the presence of lymphocytes, plasma cells, and macrophages that can infiltrate and cause thickening of the walls of bronchioles and alveoli.

On the other hand, M. pneumoniae produces hydrogen peroxide locally, causing a cytopathic effect on the epithelium of the respiratory tract and cilia, being responsible for the persistent cough.

No endotoxins or exotoxins have been found in this genus.

Pathogenesis and clinical manifestations of pneumonia

Mycoplasma pneumoniae it is transmitted from one person to another through aerosols of infected respiratory secretions. As transmission is associated with shed cells, the expelled droplets of saliva must be large for dissemination to occur.

The incubation period is long; varies between two to three weeks.


The infection begins by the adherence of the microorganism to a receptor on the surface of the epithelial cells or to the cilia and microvilli of the cells of the bronchial epithelium and stay there on the surface, stimulating cell desquamation and inflammation.

Because the disease has been seen to be more severe in adults, clinical manifestations and complications are believed to be due to an exaggerated immune response to the organism.

A modulated cytokine production and lymphocyte activation can minimize the disease, but if it is exaggerated the disease is exacerbated through the development of immune lesions.

That is, the more vigorous the cell-mediated immune response and cytokine stimulation, the more severe the clinical disease and lung injury.

On the other hand, immunopathogenic factors are probably involved in many of the additional pulmonary complications given the cross-reactivity between human antigens and microorganism antigens.

Clinical manifestations

Pneumonia can affect the upper or lower respiratory tract, or both. Symptoms usually appear gradually, over a few days, and can persist for weeks or months.

The infection is characterized by an insidious onset, fever, headache, inflammation of the pharynx, hoarseness and persistent cough (tracheobronchitis) during the day and at night, it can even present with earache.

The cough is dry at first and choppy, with minimal production of sputum, which later may be mucopurulent and very rarely contain blood.

The infection affects the trachea, bronchi, bronchioles, and peribronchial tissue and can spread into the alveoli and alveolar walls.

In uncomplicated cases, the acute febrile period lasts for about a week, while the coughing and laziness can last two weeks or even longer.

Children under five years of age are more likely to develop coryza and wheezing.

Chest x-ray

The chest radiograph shows a mononuclear cell infiltrate around the bronchi and bronchioles. However, radiographic patterns can vary widely. They may show peribronchial pneumonia, ateletacsia, nodular infiltrates, and hilar lymphadenopathy.

In 25% of cases there may be small pleural effusions.

Generally, the infection is usually severe in immunosuppressed, sickle cell or Down syndrome patients, the cause being unknown in the latter case.

Pulmonary complications

Complications are rare, among them are:

  • Pleuritis,
  • Pneumothorax,
  • Respiratory distress syndrome,
  • Lung abscess.

On the other hand, Mycoplasma pneumoniae it can exacerbate other lung diseases such as asthma and chronic lung disease.

Extrapulmonary complications

As extrapulmonary complications, the following have been described:

  • Skin condition: severe erythema multiforme, erythema nodosum, maculopapular or urticarial eruptions, Stevens-Johnson syndrome, toxic epidermal necrolysis, and pityriasis rosea.
  • Peripheral vasospasm: Raynaud's phenomenon.
  • Hemolytic anemia and jaundice: due to hemolytic antibodies, paroxysmal cold hemoglobinuria.
  • Cardiovascular disorders: pericarditis, myocarditis.
  • Central nervous system involvement: encephalitis, myelitis, meningoencephalitis, neuropathies, motor deficits, Guillain-Barre syndrome.
  • Joint involvement: myalgia, arthralgia, arthritis.
  • Ocular disorders: swelling of the papilla, atrophy of the optic nerve, retinal exudation and hemorrhages.
  • Renal disorders (these are rare): membranoproliferative glomerulonephritis, nephrotic syndrome, transient massive proteinuria, acute interstitial nephritis, acute renal failure, hemolytic uremic syndrome, isolated hematuria, cystitis or urethritis.

Infection by Mycoplasma pneumoniae in immunosuppressed patients

In the case of individuals with humoral and / or cellular immunodeficiency, they are more predisposed to suffer a more severe disease due to this microorganism.

Patients with hypogammaglobulinemia usually present with severe upper and lower airway symptoms, with little or no infiltrate on chest radiographs that present with complications such as rashes, joint pain, and arthritis.

Mycoplasma pneumoniae it can cause severe illness in HIV positive patients, who have depressed cellular immunity.

It should be noted that infection by M. pneumoniae Fulminant disseminated is rare but can occur in these patients.


The microorganisms are capable of recovering in cultures in the incubation phase, during the disease and after it, even in the presence of specific antibodies.

Mycoplasma pneumoniae they grow in special media such as PPLO (Pleuropneumonia Like Organism) at 37 ° C for 48 to 96 hours or more.

However, because the culture is very slow and the sputum Gram stain also does not help, the diagnosis is mainly made through serological methods or by conventional or real-time molecular biology tests (PCR).

At the serological level, the determination of specific IgG and IgM antibodies is available.

Further M. penumoniae induces the formation of cold agglutinins, nonspecific antibodies that agglutinate human erythrocytes when cold. These antibodies aid in the diagnosis as they rise in convalescence.


Initial symptoms usually resolve within 3 to 10 days without antimicrobial treatment, while recovery from radiological abnormalities is usually slow (3 to 4 weeks or more).

However, fatal cases are rare, that is, their evolution is generally benign and self-limited. However, its improvement can be accelerated with proper treatment.

However, although treatment improves the signs and symptoms of infection, the microorganism is not eradicated from the respiratory tract, as it has been possible to isolate Mycoplasma pneumoniae after 4 months of recovery from infection. This may explain recurrences and relapses despite appropriate treatment.

All Mycoplasmas are naturally resistant to beta-lactams and glycopeptides, because they do not have a cell wall; target site of these antibiotics.

The sulfonamides, trimethoprim, polymyxins, nalidixic acid, and rifampicin are also inactive.

Mycoplasma pneumoniae it is susceptible to antibiotics that interfere with protein or DNA synthesis, such as tetracyclines, macrolides, and some quinolones.

Among the macrolides, azithromycin is the most useful because it has fewer side effects.

Prevention and control

Immunity to Mycoplasma is transient, for this reason it has not been possible to develop a vaccine and consequently recurrences are frequent.

As a preventive measure, the patient is isolated and biosecurity measures taken when handling objects and waxes of the sick patient.


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