Legionella pneumophila (Legionnaires disease and Pontiac fever)


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Legionella pneumophila (Legionnaires disease and Pontiac fever)

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Legionella pneumophila (Legionnaires disease and Pontiac fever)

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Legionella pneumophila (Legionnaires disease and Pontiac fever)


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Legionella pneumophila (Legionnaires disease and Pontiac fever)

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USMLE® Step 1 style questions USMLE

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A 20-year-old man presents to the emergency department due to fever, dyspnea, productive cough and headache for the past four days. Today the patient also developed abdominal pain and diarrhea. The patient returned from a tournament in Indiana with his college wrestling team one week ago. During this trip, the team used an indoor hot tub. Three other team members have developed a flu-like illness. Temperature is 40 ºC (104 ºF), pulse is 120/min, respirations 22/min, and blood pressure is 100/50 mmHg. The chest radiograph shows a dense left lower lobe consolidation. Initial laboratory workup is shown below. Sputum testing shows abundant neutrophils but no organisms are visualized. Which of the following is the most likely cause of this patient’s underlying lung condition?  

 Laboratory value  Result 
 Serum chemistries 
 Hemoglobin  12.6 g/dL 
 Hematocrit  39 % 
 Leukocyte count  19,000/mm3 
 Platelet count  50,000/mm3 
 Sodium   130 mEq/L 
 Potassium   4.2 mEq/L 
 Chloride  97 mEq/L 

External References

First Aid








Charcoal yeast extract culture

Legionella pneumophila p. , 124, 141

Legionella pneumophila p. , 141

Macrolides p. 190

Legionella pneumophila p. , 141

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Content Reviewers

Rishi Desai, MD, MPH


Alexandru Duhaniuc, MD

Legionella pneumophila is a Gram-negative bacillus, which means is shaped like a rod, and can be found in many water systems, such as hot water tanks, cooling towers, large air conditioning systems or hot tubs.

It is typically transmitted by inhaling infected aerosols, like contaminated water sprays, jets, or mists and causes a disease called legionellosis with two distinct entities.

The first one, Legionnaires’ disease got its name from a deadly outbreak of pneumonia in 1976 among people attending a convention of the American Legion, and the second one, Pontiac fever, got its name from Pontiac, Michigan, where the first case was recognized.

Now, Legionella pneumophila has a very thin peptidoglycan layer, so it stains like a Gram-negative bacteria.

But it stains pretty weakly as a Gram-negative bacteria, so it’s best visualized with silver stain.

Legionella pneumophila is non-spore forming, aerobic, which means it needs oxygen to survive, facultative intracellular, which means it can survive both inside and outside the cell, and oxidase and catalase positive, which means it produces both of these enzymes.

Finally, it needs special nutrients to grow, such as cysteine and iron, so it grows on a special medium called buffered charcoal yeast extract, or BCYE, which contains cysteine and iron that are essentially for growth of Legionella.

So, on this medium, it forms grey-white colonies with a cut-glass appearance.

Now, Legionella pneumophila can enter the body through inhalation of contaminated water droplets.

Once it reaches the lungs, it gets ingested by alveolar macrophages and inside macrophages, it gets wrapped up in a vesicle called phagosome, which normally merges with lysosomes to kill invading bacteria.

However, Legionella has a type IVB secretion system, which uses effector proteins to prevent phagolysosomal fusion.

As a result, Legionella is able to survive and replicate inside macrophages.


Legionella pneumophila is a Gram-negative rod-shaped bacteria that can be found in many water systems, such as hot water tanks, cooling towers, large air conditioning systems, or hot tubs. It is typically transmitted by inhaling infected aerosols, like contaminated water sprays, jets, or mists, and causes a disease called legionellosis with two distinct forms.

First, there is Legionnaires' disease which causes severe pneumonia with high-grade fever. It can also cause some gastrointestinal symptoms such as vomiting and diarrhea and neurological symptoms such as headache and confusion. The second form is Pontiac fever, which is a much milder disease without pneumonia, but with some flu-like symptoms.

Legionella pneumophila is diagnosed by identifying Legionella in a culture from respiratory tract secretions and using a urinary antigen test. Legionnaires' disease is treated with macrolides and fluoroquinolone antibiotics, whereas Pontiac fever may resolve spontaneously.


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