Bacterial Causes of Pneumonia

Pneumonia can be caused by viruses, bacteria, or fungus. Bacterial causes of pneumonia are most common. The most common typical bacteria that are found in pneumonia are Streptococcus Pneumoniae, Haemophilus Influenzae, Chlamydia Trachomatis, Mycoplasma Pneumoniae, and Legionella Pneumophila.

Streptococcus Pneumoniae: This is a gram-positive bacterium. It is catalase negative and alpha-hemolytic (which means that it somewhat likes to grow on blood agar). They are in the shape of elongated cocci. Usually, they are seen as pairs called diplococci, but may also occur singly or in short chains. S. Pneumonia are normal inhabitants of the human respiratory tract. Disease happens only when the host is immunocompromised. It is also known to cause otitis media and meningitis. It usually causes lobar pneumonia in the elderly and children or the immunologically compromised. They do not form spores and are non-motile. They ferment lactose. Purulent, "rusty-colored" sputum in patients with S. Pneumonia is usually the hallmark of S. Pneumonia in patients suspected of suffering from Lobar Pneumonia.

Haemophilus Influenzae: This is a small, non-motile, Gram-negative bacterium that resides in the Pasteurellaceae family. In 1890, it was mistakenly thought to be the cause of the disease Influenza, and it was named accordingly. It is presently known that the Rhinovirus is the major cause of the flu, and not a bacterium. The name has stuck with the bacterium, though. It is present in the nasopharynx of approximately 75% of people. It is a blood loving and fastidious bacterium, which means it is picky on where it will grow. This bacterium will grow on chocolate agar. Its shapes range from coccobacillus to filamentous rods on the Gram Stain. Pathogenic species of H. Influenza typically have a capsule. It typically causes infections of the respiratory and genital mucous membranes. Infection can happen from inhalation or fomites. (A fomite is an inanimate object carrying infectious organisms such as a table top or a door knob.) H. Influenzae form "satellite colonies." Initially, H. influenza invades the nasopharyngeal mucosa before spreading to the lower respiratory tract where the organism invades and destroys the mucus membranes producing Pneumonia.

Chlamydia Pneumoniae: This is a less common causative agent of Pneumonia. It is a gram-negative, aerobic, obligate intracellular pathogen. It is the only bacteria known that have a life cycle, the elementary body (infectious form, and the reticulate body (replicative form). It comes in the shape of a small bacillus with no peptidoglycan in its cell wall. The Chlamydiae species is known for its lack of peptidoglycan membrane and only has an outer membrane and an inner membrane. It is therefore, harder to kill by most antibiotics which actually target the peptidoglycan membrane. They cause harm by invading macrophages, but show its virulence when the cause the macrophage to not make the bacterium fuse with lysososme. The lysosome is the main killer of bacteria inside of a macrophage. In this way, the Chlamydiae bacteria escape killing by a macrophage, and actually live inside of a macrophage! When they replicate, they lyse the macrophage cell.

Mycoplasma Pneumoniae: This bacterium is a member of the class called Mollicutes, meaning soft skin (hence, can change shape). They completely lack a bacterial cell wall. Wall-less bacteria are more difficult to kill because our antibiotics target the cell wall. M. Pneumoniae, like Chlamydiae, is an obligate intracellular pathogen. It lives inside of macrophages and other cells. It cannot survive on its own as it is too small. Mycoplasma do not have their own machinery to replicate because they are so small. M. Pneumoniae is currently thought to be responsible for both tracheobronchitis and primary, atypical pneumonia. M. Pneumoniae can be caught through close personal contact via respiratory droplets. It's peak prevalence times are usually in autumn and winter.

Legionella Pneumophila: Legionella Pneumophila was first discovered in 1976 when an outbreak of pneumonia at an American Legion convention killed 29 people. It is motile, rod-shaped, gram-negative, aerobic bacterium. It requires complex nutrition, such as high cysteine levels and low sodium levels to grow optimally. It has always have been found in non-marine aquatic environments such as lakes and ponds. L. Pneumophila can survive in tap water at room temp for over a year! Legionella bacteria are transmitted to the lungs of human beings through a process called aerosilisation. (Air conditioning cooling towers). It is the cause of Legionnaire's Disease and Pontiac Fever (a mild infection which causes flu-like symptoms that goes away without treatment). Legionnaire's Disease incidence has increased over the past decade or so because of the wide use of central air conditioning, especially in office buildings, hotels, and hospitals. The major side effect of Legionnaire's Disease is respiratory failure which requires mechanical ventilation.