Pneumonia is an inflammation or infection of the lungs most commonly caused by a bacteria or virus, but can also be caused by inhaling foreign substances.

What Is Pneumonia?

Pneumonia occurs when the lungs' air sacs and small airways fill with liquid and cannot function properly. When gas exchange is impeded, the body cannot take in oxygen as it normally would. Oxygen is vital to the body’s cells, where it is needed for aerobic respiration; the breakdown of glucose to obtain energy for cellular work.

Causes of Pneumonia

Pneumonia can have many different causes: viral, bacterial, fungal or, in the case of aspiration pneumonia, may result from the inhalation of substances such as caustic chemicals, food or vomit.

Bacterial Pneumonia

All bacteria are prokaryotes (Pro = “before”, karyon = “nucleus”). Prokaryotes were the first type of cell to evolve, and still lack a membrane bound nucleus; their genetic material is naked within the cytoplasm.

Of different types of pneumonias, bacterial tend to be the most serious and, in adults, the most common. Bacterial pneumonia may follow influenza or even a common cold, particularly the elderly or immune compromised.

Gram-Positive and Gram Negative Bacteria

Bacterial cell walls are the target of many antibiotics, and a primary component of these cell walls is a molecule called peptidoglycan, a huge polymer that is found only in bacteria.

From the peptidoglycan inwards all bacterial cells are very similar. Going further out, the bacterial world divides into two major classes: Gram positive and Gram negative.

Gram-positive Cells

In Gram-positive cells, peptidoglycan makes up as much as 90% of the thick, compact cell wall, which is the outermost structure of Gram + cells.

Gram-negative Cells

The cell walls of gram-negative bacteria are more chemically complex, thinner and less compact. Peptidoglycan makes up only 5 – 20% of the cell wall, and is not the outermost layer, but lies between the plasma membrane and an outer LPS membrane.

Antibiotics and Peptidoglycan

Penicillins and cephalosporins interfere with the structure of peptidoglycan, but because of the LPS membrane, these antimicrobials can’t access the peptidoglycan of gram-negative bacteria. Gram-positive bacteria, with no membrane outside the peptidoglycan cell wall, are more susceptible to these antibiotics.

Some Types of Gram Positive Bacterial Pneumonia

Pneumococcal Pneumonia

Organism: Streptococcus pneumoniae

The causative agent of pneumococcal pneumonia is coccus (round shaped) bacteria that occurs in pairs or short chains.

S. pneumoniae is the most common pneumonia-causing bacterium in adults, responsible for approximately 85% of pneumonia cases.

S. pneumoniae is part of the body’s normal flora. It grows in the mouth and pharynges of approximately 75% of the human population, but can cause illness when it travels to the lungs particularly when immune function is compromised. Pneumococcal pneumonia most often occurs in the fall and winter months.

Symptoms are similar to those seen in other types of bacterial pneumonia, including cough, chest pain, fever, and difficulty breathing; but additionally blood may be seen phlegm.

Jim Henson (the Muppets creator) died in 1990 of a virulent strain of group A beta-hemolytic streptococcus.

Staphylococcal Pneumonia

Organism: Staphylococcus aureus

S. aureus is another round shaped bacterium, but it occurs in grape-like clusters.

S. aureus, normally occurs in and on the human body, but can be an opportunistic pathogen when a person’s immune system is compromised. Pneumonia can result when bacteria from the mouth and is inhaled into the lungs. Staphylococcal pneumonia does not typically occur in the absence of a predisposing viral infection, except in children under the age of 2

Staphylococcus causes typical pneumonia symptoms, but the chills and fever are more persistent in staphylococcal pneumonia than in pneumococcal pneumonia. Sometimes symptoms can worsen rapidly, with severe and potentially fatal deterioration in lung function.

Primary Atypical Pneumonia

Organism: Mycoplasma pneumoniae

Mycoplasmas are very simple unicellular organisms most likely descended from Gram-positive bacteria. They cannot perform many metabolic functions, and are considered the perfect model of the minimalist cell. Mycoplasmas are strictly parasitic.

The early symptoms of pneumonia caused by M. pneumoniae--fever, malaise, headache and sore throat--are not typical signs of pneumonia, hence the name Primary Atypical Pneumonia.

Symptoms can last for several weeks, but are rarely severe enough to result in hospitalization or death. Because the symptoms are mild, this disease is also known as ‘Walking Pneumonia’.

Unlike pneumococcal pneumonia, primary atypical pneumonia is not seasonal, and cases occur throughout the year.

This article contains a brief summary of some of the causes of pneumonia. It s not intended for use in self-diagnosis. If you are ill, see your health care provider.

Pneumonia Resources

For more information on pneumonia, see the Mayo Clinic: Infectious Diseases or Science Prof Online. For additional material on other types of pneumonia see the Suite101.com articles Gram Negative Bacterial Pneumonia, Primary Atypical Pneumonia and Endemic Fungal Pneumonia.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.