Mycoplasma Pneumoniae: What is it? Transmission, Symptoms and Diagnosis of G. Bertelli

Generality

Mycoplasma pneumoniae is a bacterium responsible for diseases that mainly affect the respiratory system .

The manifestations deriving from exposure to this pathogen are variable and range from mild upper respiratory tract infections (colds, pharyngitis, etc.) to more severe forms of pneumonia .

Mycoplasma pneumoniae assumes clinical relevance mainly due to its primary responsibility for primary atypical pneumonia . This disease mainly affects children and young adults, in an age range between 5 and 35 years. Primary atypical pneumonia is often asymptomatic, but when immune defenses are reduced, infection can lead to very serious hematological and neurological complications .

What's this

Mycoplasma pneumoniae : what is it?

Mycoplasma pneumoniae is a bacterium belonging to the Mycoplasmataceae family, able to colonize the respiratory tract and produce diseases of varying degrees.

In particular, this organism is known to be the most common etiologic agent of primary atypical pneumonia .

Did you know that…

In humans, Mycoplasma pneumoniae commonly causes respiratory infections, but is often not identified as a causative agent, as it causes mild to moderate, non-specific and self-limiting symptoms. For this reason, the implication of this bacterium is underestimated, in a similar way to the cold of viral origin.
At present, primary atypical pneumonia represents the most severe morbid form produced by Mycoplasma pneumoniae .

Mycoplasma pneumoniae: biological characteristics

Mycoplasma pneumoniae has very particular characteristics.

Absence of the cell wall

First of all, these mycoplasmas are distinguished from other bacteria by the lack of the cell wall (that is, they lack peptidoglycan). The membrane of Mycoplasma pneumoniae is trilaminar, lipoprotein and rich in sterols which give it a rigid consistency. If it did not have this composition, the bacterial cell would be fluid, unsuitable for maintaining a constant volume and sensitive to osmotic shocks.

Being devoid of this structure, Mycoplasma pneumoniae is not sensitive to beta-lactam antibiotics (these drugs precisely inhibit the synthesis of the cell wall), but it is typically vulnerable to macrolides and tetracyclines.

Furthermore, the absence of the wall does not make it possible to identify by Gram stain (a relatively fast test used to detect presence and roughly classify bacteria).

Structural polymorphism

Mycoplasma pneumoniae is an infectious agent with a remarkable structural polymorphism, therefore it is able to take very different forms. Also for this reason, this typology of mycoplasmas has been grouped in the Mollicutes class (from the Latin " mollis cutis " which means "soft skin").

Small size and limited metabolism

Mycoplasma pneumoniae is one of the smallest bacteria (diameter 0.2-0.3 µm) capable of independent living. The genome is extremely small - from 0.58 to 2.20 megabases (Mb) - when compared, for example, to that of E. coli (4.64 Mb).

Mycoplasma pneumoniae has limited metabolic and biosynthetic capabilities. It is, in fact, an obliged aerobic bacterium, therefore it proliferates in the absence of oxygen. Reproduction occurs by binary fission: Mycoplasma pneumoniae multiplies on the surface of the mucous epithelia and shows little tendency to pass them.

It should be noted that Mycoplasma pneumoniae is an extra-cellular pathogen and its survival depends on adherence to the respiratory epithelium . This "fixation" to the ciliary membranes results mainly from the interactive adhesion and from the accessory proteins: the relationship that the Mycoplasma pneumoniae establishes with the host cells is defined pericellular parasitism .

Causes and Risk Factors

Mycoplasma pneumoniae : when is it pathogenic?

Mycoplasma pneumoniae is widely distributed in the environment and is able to colonize various animal and plant species.

In humans, this bacterium is mainly located in the respiratory tract, where, under normal conditions, it is harmless or causes asymptomatic infections. However, in immunosuppressed patients, the Mycoplasma pneumoniae can take on an "aggressive" character and behave as a pathogen, exploiting the situation to its advantage to multiply immensely, to the point of causing serious illnesses.

In detail, the Mycoplasma pneumoniae :

Adheres to the ciliated epithelia of the mucous membranes through a particular protein, called citoadesina P1 ;
It causes ciliostasis, thus blocking the movement of the eyelashes and microvilli of the airway epithelial cells, facilitating bacterial colonization ;
It can lead to desquamation of the epithelium and, consequently, to a persistent cough, irritation and exudate formation.

The adhesin P1 is an immunodominant protein, against which the immune system generates an impressive specific response . Recent scientific studies have shown that the production of cytokines and activation of lymphocytes can minimize infection with Mycoplasma pneumoniae through the enhancement of host defense mechanisms or, on the contrary, exacerbate the disease by inducing the development of immune-mediated lesions at the lung level.

Primary atypical pneumonia from Mycoplasma pneumoniae

Mycoplasma pneumoniae is the most common etiologic agent of primary atypical pneumonia (also known as mycoplasma pneumonia ). This pathology mainly affects children in late childhood and in adults, in a range between the ages of 5 and 35, although infections in the elderly are not uncommon.

In young people, this is the most common form of pneumonia after that caused by Streptococcus pneumoniae .

Other diseases caused by Mycoplasma pneumoniae

The Mycoplasma pneumoniae is responsible for other, less serious diseases of the respiratory system, such as:

Cold;
Pharyngitis;
Laryngitis;
tracheitis;
Bronchitis.

Mycoplasma pneumoniae is implicated in a wide range of manifestations outside the lower and upper respiratory tract.

These include:

Erythema multiforme;
polyarthritis;
Hemolytic anemia;
Liver disease;
Heart disease;
Neurological disorders.

Transmission mode

Infections with Mycoplasma pneumoniae spread by inter-human infection, usually by aerosol . The pathogen tends to associate with desquamated cells of the respiratory epithelium, so transmission can occur through the inhalation of relatively large droplets of suspended saliva, when an infected person coughs or sneezes. My coplasma pneumoniae is believed to be responsible for 40% of community-acquired pneumonia .

Furthermore, this mycoplasma can cause epidemics, as it can spread rapidly in crowded and promiscuous environments, such as schools, hospital wards and military barracks.

The incubation period of Mycoplasma pneumoniae infections can vary from one to three weeks.

Who is most at risk?

In healthy people, the immune system is generally able to counteract the proliferation of Mycoplasma pneumoniae, before it becomes an infection.

On the other hand, diseases tend to develop in people who are immunosuppressed or have a previous pneumopathy. Other more vulnerable categories are children under the age of 5 and the elderly. Once the infection is overcome, immunity against Mycoplasma pneumoniae is transient, so the infection can be contracted again.

Symptoms and Complications

The symptoms of an infection with Mycoplasma pneumoniae depend on the site affected (respiratory system or other district of the body), the age and general health conditions of the patient. In most cases, the symptoms are mild or moderate and often self-limiting. Other times, infection with Mycoplasma pneumoniae can cause fever, pharynx-tonsillitis, headache and muscle pain ; when the infection involves the lower airways it can cause pneumonia and, rarely, it can spread to other parts of the body.

How does atypical pneumonia occur with Mycoplasma pneumoniae ?

Infection with Mycoplasma pneumoniae develops without obvious symptoms for the first few weeks (1 to 3) after infection and subsequently causes non-specific manifestations such as bronchitis or rhinitis.

The most common sign of respiratory infection with Mycoplasma pneumoniae is the dry cough, then fatty, with yellowish-white sputum emission.

Other symptoms start abruptly and include:

Mild fever;
Chills;
General malaise;
Shortness of breath, especially after an effort;
Headache;
Loss of appetite.

Mycoplasma pneumoniae pneumonia generally has a favorable course: symptoms tend to resolve within a week (except for coughs that can persist for a long time), while radiological signs may disappear one month after the onset of the disease.

Possible complications

The main complication of atypical pneumonia with Mycoplasma pneumoniae is represented by autoimmune hemolytic anemia .

Other less common consequences of infection include:

Rare ear problems, such as bullous myringitis;
Arthritis;
pericarditis;
Rare and severe skin conditions, such as Stevens-Johnson syndrome and toxic epidermal necrolysis;
Guillain-Barré syndrome;
Encephalitis;
Meningitis;
Kidney failure.

Diagnosis

The diagnosis of a Mycoplasma pneumoniae infection is established by:

Direct search for mycoplasma ;
Demonstration of the increase in serum of the rate of specific antibodies against the infectious agent at least 4 times compared to the value present at the beginning of the disease.

Isolation of the infectious agent can be performed on multiple biological samples (such as sputum, bronchial lavage or pharyngeal swab) collected from three to seven days after the first symptoms appear. The detection of Mycoplasma pneumoniae is performed by culture examination, then the growth of the microorganism in an appropriate culture medium or the search for genetic material .

In order to highlight the antibody response, on the other hand, serological tests like ELISA and complement fixation tests are useful .

During Mycoplasma pneumoniae pneumonias, the blood cell count and the leukocyte formula examination are normal (note: on the contrary, an increase in neutrophil granulocytes is typically found in bacterial pneumonia).

During the physical examination, the doctor may find the presence of respiratory gasps . The chest X-ray often reveals the presence of a dense pulmonary infiltrate, frequently limited to a lung lobe.

Treatment and Prevention

Treatment of Mycoplasma pneumoniae infections is based on specific antibiotic therapy . To manage inflammation, the use of corticosteroids (prednisolone or methylprednisolone) may also be indicated, sometimes in combination with intravenous immunoglobulin administration.

Mycoplasma pneumoniae : antibiotics

The treatment of infections with Mycoplasma pneumoniae involves the use of some antibiotics, such as macrolides and tetracyclines . On the other hand, penicillin is ineffective, which acts by inhibiting the synthesis of some fundamental constituents of the bacterial rigid wall of which, as mentioned, Mycoplasma pneumoniae is lacking.