Legionella

Legionella is a genus of Gram-negative bacteria that can exist in various environments.

These bacteria are commonly found in soil and water sources. They live in natural bodies of water like lakes and rivers. They also thrive in man-made water systems. These include cooling towers, swimming pools, and domestic water systems. You might even find them in showers, ice-making machines, or whirlpool spas. While they are common, most people who are exposed to them do not get sick.

Legionella causes a group of illnesses known as legionellosis. One form is Legionnaires' disease, which is a type of pneumonia. This illness affects the lungs and can be serious. Another form is Pontiac fever, which is a milder, flu-like illness. In rare instances, the bacteria can cause infections elsewhere. These may include the skin, heart, or even joints. The bacteria are aerobic, meaning they require oxygen to live. They are also motile, which means they can move around.

In nature, Legionella has a unique way of living. Many species are intracellular pathogens, meaning they live inside other cells. Their primary hosts are amoebae, which are tiny organisms found in water and soil.

The bacteria spread to humans through airborne transmission. This happens when tiny droplets of water, called aerosols, are released into the air. These droplets can come from mists or sprays. When a person breathes in these droplets, the bacteria enter the airways. Once inside, the bacteria undergo phagocytosis. This is a process where immune cells take the bacteria into themselves. The bacteria then grow and multiply inside lung cells, specifically alveolar macrophages and monocytes.

Scientists discovered Legionella following a major event in 1976. A mystery disease broke out during a convention of the American Legion in Philadelphia. This association is a group for U.S. military veterans. The outbreak occurred near the 200th anniversary of the Declaration of Independence. Because it was a highly publicized event, it caused great concern in the U.S. On January 18, 1977, researchers identified the cause as a new bacterium. They named the genus Legionella to honor that specific outbreak.

There is a great deal of variety within this genus. Scientists have identified over 50 species and 70 different serogroups, which are specific types of bacteria. The cell wall of the bacteria helps scientists classify them. The structural arrangement and the sugars in the wall are used for this purpose. Researchers have even found 24 conserved signature indels, or CSIs, in various proteins. These are specific markers in the bacteria's DNA that help distinguish one species from another. This genetic diversity is high, with 32% of genes being strain-specific.

Detecting Legionella requires specialized laboratory techniques. Because these bacteria need cysteine and iron to grow, they cannot grow on standard lab media. Scientists use buffered charcoal yeast extract agar to grow them. To test water, they first concentrate the sample. They may use heat or acid treatments to remove other microbes. If colonies grow on agar containing cysteine but not on agar without it, the presence is confirmed.

Some hospitals use a Legionella urinary antigen test. This test is faster than culturing, providing results in hours. However, it only detects one specific type: Legionella pneumophila serogroup 1.

Understanding the risks helps in managing public health. In the United States, Legionella infects between 8,000 and 18,000 people every year. Most cases occur during the summertime. This is often linked to high humidity and rainfall rather than just heat. Heavy rain and flooding can increase the risk of contaminating water sources. Many outbreaks result from poorly maintained cooling towers. Other risks include recreational water like spas or hot springs. Maintaining clean water systems is the best way to prevent the spread of these bacteria.