Legionella pneumophila

Legionella pneumophila is a complex, Gram-negative bacterium. It is the primary cause of Legionnaires’ disease, also known as legionellosis. This bacterium is aerobic, meaning it requires oxygen to grow. It is also pleomorphic, which means it can change its shape. As an intracellular bacterium, it lives inside the cells of other organisms. It prefers to infect freshwater protozoa and soil amoebae to replicate.

The bacterium survives by switching between two distinct life stages. The first is the transmissive phase. This phase occurs when nutrients are low in the environment. In this state, the bacterium acts like a spore to survive and travel. It uses flagella, which are whip-like tails, to move. The second stage is the replicative phase. This phase is used for rapid proliferation, or making many more bacteria. The bacterium switches between these forms based on available metabolic resources.

Legionella pneumophila has a unique cell membrane structure. Being Gram-negative, it possesses an outer membrane made of lipoproteins and phospholipids. A key feature is the presence of lipopolysaccharides, or LPS. These are sugars that extend from the outer membrane. LPS play a vital role in adhesion, which helps the bacteria stick to host cells. These lipopolysaccharides also act as surface antigens. This means they are used to identify the specific type of bacteria. The specific arrangement of these sugars helps scientists classify different strains.

This bacterium thrives in many different environments. It is a mesophilic organism, preferring temperatures between 20 °C and 59 °C. It can tolerate temperatures up to 66 °C, but it dies at or above 70 °C. It can also live in a wide pH range from 5.0 to 8.5. Legionella is often found in man-made water systems like cooling towers and hotels. It frequently lives within biofilms. A biofilm is a community of microbes attached to a surface. These biofilms protect the bacteria from environmental stressors and disinfectants.

When people inhale aerosolized water droplets, the bacteria enter the respiratory tract. They target specific cells called alveolar macrophages in the lungs. Once inside, the bacteria use a system called Dot/Icm to invade the cells. This infection can cause fever, delirium, diarrhea, and decreased kidney or liver function.

Doctors can diagnose the infection using a urine antigen test. Most cases are treated with antibiotics such as azithromycin or fluoroquinolones.

To grow, the bacterium requires specific nutrients. It is auxotrophic for seven essential amino acids. These include cysteine, leucine, methionine, valine, threonine, isoleucine, and arginine. It also needs cysteine and iron to thrive. The bacterium is very efficient at getting food. It can promote the degradation of host proteins to create an excess of amino acids. It also uses various pathways, such as the citric acid cycle, to generate energy. This ability to manipulate host cells makes it a very effective pathogen.

Scientists have studied the genome to understand how this bacterium adapts. The genome is about 3.5 mega base pairs in size. This large genome contains many genes that help it mimic eukaryotic hosts. There are 15 known serogroups, but serogroup 1 is the most common cause of disease. Research on strains like Philadelphia and Paris has helped map its biology. Through horizontal gene transfer, the bacteria can exchange genetic material. This allows it to evolve quickly and potentially develop resistance to drugs.