Pneumonia: viral or bacterial

Pneumonia is an inflammation of the pulmonary alveoli, it can be the result of a bacterial or viral infection, but there are also other causes.


Bacterial pneumonia

Bacteria are mainly classified according to their colorability into:

Gram-positive – these gram-positive bacteria have a thick, compact cell wall made of a peptidoglycan layer:

  • Staphylococcus
  • Micrococcus
  • Lactococcus
  • Clostridium
  • Bacillus
  • Streptococcus pneumoniae (Pneumococcus): most common
  • Enterococcus

Gram negative – these have a much thinner and less compact cell wall than gram positives. Most gram-negative bacteria are surrounded by a capsule that makes them resistant to the effects of antibiotics:

  • Enterobacteriaceae (Klebsiella)
  • Vibrionaceae
  • Pseudomonas (is the most problematic form).

Most common bacteria

  • Pneumococcus
  • Staphylococcus aureus
  • Haemophilus influenzae
  • Mycoplasma

Less common causes

  • Anaerobic bacteria
  • Tuberculosis.

15% of community-acquired pneumonia is polymicrobial type.

Pneumonia caused by pneumococcus

Pneumococcus (Streptococcus pneumoniae) is an encapsulated gram-positive microorganism found in the nasal cavity of 20-40% of all healthy individuals.
However, one finds here mainly the non-harmful serotypes of this microorganism.
The more serotypically aggressive serotypes can be isolated in the paranasal sinuses and throat of healthy people:

  • in winter,
  • in an epidemic of pneumonia.

However, at this level, the mere presence of the germs is not enough to cause pneumonia.

As a rule, there must be a viral infection that precedes pneumonia caused by pneumococci.

This is particularly important because it induces the appearance on the alveolar cells of receptors of an inflammatory mediator: PAF (platelet activating factor), which contains phosphocholine.
Phosphocholine is located in the cell wall of the pneumococcus, so these microorganisms can benefit from the PAF receptors to attach themselves to the wall of the alveoli (this adhesion is not prevented by the bacterial capsule).

The polysaccharide capsule of pneumococci is very important because it protects these microorganisms from neutrophils.
Phagocytosis is possible only after the formation of specific antibodies to the antigens of the polysaccharides of the capsule.
Therefore, pneumococci without antibiotics can multiply freely over several days until a sufficient level of antibodies in the blood is reached.
However, capsule polysaccharides are not dangerous.

Some components of the bacterial cell wall are important because:

  • They have an important pro-inflammatory effect,
  • They exert a clearly positive chemotactic effect (attraction) on neutrophils,
  • They promote the accumulation of inflammatory fluid and the migration of blood cells (including red blood cells) from the capillaries.

The result is the separation of endothelial (mucosa) and alveolar cells.

The soluble components resulting from the breakdown of the pneumococcal cell wall are particularly active and this explains why the symptoms and signs of pneumonia may worsen during:

  • phagocytosis (digestion) of microorganisms (after formation of anticapsular antibodies),
  • of destruction by antibiotics.

Therefore, mortality in pneumococcal pneumonia does not depend on hypoxemia (lack of oxygen in the blood), which is determined by the spread of inflammation in the lungs, but on the concentration of cell wall components.
If this is increased, the destruction of microorganisms causes the death of the patient by septic shock.
After all, an intracellular protein of pneumococci (pneumolysin) is toxic in virtually all cells of the lungs.

There are different serotypes of Streptococcus pneumoniae. For this reason, the development of an effective vaccine is very lengthy and complicated.
The most commonly used vaccine today is:

  • the 23-valent polysaccharide for adults, directed against the 23 most common serotypes in Western countries,
  • the heptavalent conjugated vaccine for children.

Streptococcus pneumoniae is common in patients:

  • with cirrhosis,
  • with chronic cardiopulmonary diseases, in which the transition from the area of the nose (where there is usually a bacterial flora) to the deeper areas is obviously facilitated.

Mortality is 30 out of 100,000 people, and even higher in the elderly.

Pneumonia caused by Haemophilus influenzae

Haemophilus influenzae type B: is the second most common species.
It often occurs in patients with chronic bronchial disease.
A vaccine is available, but it is only used for certain people because its effectiveness is limited.
It is a common cause of meningitis, especially in children.
Starting from the lungs, this bacterium can reach the central nervous system via the bloodstream and lead to serious complications there.

Pneumonia caused by gram-negative bacteria

Gram-negative bacteria mainly affect:

  • very sick and emaciated persons,
  • Elderly
  • Patients with chronic diseases.

Pseudomonas: mainly affects elderly and very sick patients.
Often develops in patients who have been given antibiotic treatment for a long time.
The result is the loss of normal bacterial flora in the throat and nasal cavities, which limits the penetration of harmful bacterial structures.

Klebsiella pneumoniae is a gram-negative bacillus that can lead to severe pneumonia, with mortality reaching as high as 20-50%.
Cause of Friedländer’s pneumonia.
As a rule, this germ develops a special aggressiveness in:

  • Alcoholic
  • elderly people admitted to hospital,
  • Patients with chronic bronchitis or diabetes.

In these individuals, Klebsiella pneumoniae can permanently affect the entire bronchial tree.

Staphylococcus aureus: rare in communities (2%), more often in hospital (10-15% of nosocomial pneumonia) because, unfortunately, it is a resistant germ that develops multiple drug resistance.
Staphylococcus aureus is also a saprophyte (bacterium that feeds on dead or decomposing organisms) and is enormously common, especially on the skin.
Like pneumococcus, staphylococcus can cause pneumonia in previously healthy individuals, although it is usually a complication of flu.


Legionella pneumophila: is a bacterium that lives in water bodies, usually found in protozoa, which are very abundant in humid environments. It also survives very low temperatures and is transmitted by contaminated aerosols (from water systems, showers, wells, air conditioners, etc.).

Transmission from person to person has not yet been detected.

It can also lead to the inevitably fatal outcome if it does not:

  • is detected,
  • is treated quickly.

There are different species of Legionella, but the most dangerous species is certainly Legionella pneumophila serotype 1:

Legionella can manifest itself in 2 forms:

  1. Form of simple flu (Pontiac fever)
  2. Legionnaires’ disease (systemic form, a serious form of pneumonia)

Bacteria that cause pneumonia in context

  • Previously healthy individuals: Streptococcus pneumoniae
  • Complication of viral disease: Streptococcus pneumoniae and Staphylococcus aureus
  • Pre-existing chronic lung diseasediabetes or alcoholism: Streptococcus pneumoniae, Haemophilus influenzae and Klebsiella pneumoniae
  • Immunocompromised patients (neoplasia, corticosteroid or immunosuppressant therapy): Streptococcus pneumoniae, Pseudomonas aeruginosa, Enterobacteriaceae and anaerobic bacteria
  • Aspiration pneumonia: Enterobacteriaceae and anaerobic bacteria
  • Inpatients: pneumococcus, mycoplasma or Haemophilus influenzae
  • Patients of one area: Streptococcus, Mycoplasma and Haemophilus
  • Drug-dependent patients: Staphylococcus
  • Patients during a hotel stay: Legionella

In any case, there are no predictive clinical features of a pathogen.

In addition, in 50% of cases, the responsible microorganism is not found.
Today, the doctor also prescribes therapy without knowing exactly the bacterium that caused pneumonia, drawing on empirical experience, which usually leads to success.

Viral pneumonia

Viruses can:

  • the lungs are severely affected,
  • cause a very serious clinical picture.

Reasons are:

  • There are few effective antiviral drugs.
  • They usually cause interstitial pneumonia and thus affect the entire lungs.

There are two possibilities:

  • Bacterial pneumonia is the result of a viral infection.
  • The virus directly causes pneumonia.

These viruses attack the alveoli by causing an initial inflammation in the area of the interstitium. In addition, they can lead to the formation of hyaline membranes (rich in fibers), which:

  • block the exchange of breath,
  • cause respiratory distress syndrome or ARDS (a severe form of respiratory failure).

It usually affects young children, rarely adults.

The course is:

  • onset with a viral form of the upper respiratory tract (pharyngitis, cold),
  • Then the patient begins to deteriorate more and more, with a dry cough and a number of ailments, especially when hyaline membranes form.

For diagnosis:

  • Auscultation of the chest is negative.
  • The results of the laboratory test are non-specific.
  • The chest X-ray is crucial for the diagnosis.

Since it is a systemic disease, it can cause:

  • lymph node enlargement,
  • Splenomegaly
  • other symptoms, such as inflammation of the meninges.

It can be complicated by:

  • a bacterial infection,
  • ARDS (is not specific to these viruses and can also occur due to inhalation of gas, heroin overdose and narcotics).

It is an influenza pneumonia:

  • The lungs are inflamed and full of fluid.
  • There is an increased blood circulation.
  • Hyaline membranes are formed, caused by extensive alveolar damage.

Pneumonia caused by SARS

SARS is a serious, acute viral respiratory syndrome that emerged worldwide a few years ago.
The first epidemic caused by animals occurred in a Chinese province. Everything went unnoticed until 2002, when a doctor from this area stayed in a hotel in Hong Kong.
All those who stayed on the same floor fell ill and through their onward journey the disease spread.
There were over 3000 cases and 165 deaths.

It is a coronavirus (discovered much later) that usually does not cause symptoms in humans. However, it can cause very special problems in animals and only through the cattle has this infection occurred.
Certainly, humans do not have carrier status, one thinks mainly of animals, but still one has no certainty about it.


  • Incubation period: 2 weeks.
  • Dry cough (as caused by smoke).
  • SARS has been successfully treated with ribavirin (antiviral, which is also used for hepatitis).
  • There are no reports of SARS endemics.

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