I have a background in microbiology through both education and my work, and as a result of this I am often looking at microbes and learning some of the fascinating tricks they are able to play to either (a). avoid detection, or (b) survive and thrive. With that in mind I wanted to talk about one such bacteria that has adapted to have some incredible defence mechanisms.

Before I get started, it is worth covering what is Legionella. This is a type of bacteria that is often found is water systems and soil and was discovered in around 1976 as a severe case of pneumonia. Later on the causative agent was isolated and discovered to be Legionella pneumophilia, the type of pneumonia these people developed is now referred to as Legionnaires disease.

Legionella proliferates in water systems most commonly, and when people breath in contaminated water droplets (usually in the form of aerosols) it can cause significant illness and in those who are sensitive (i.e., immunosuppressed, or have co-morbidity) it can also lead to death.

Putting aside the impact of this bacteria on people, it has developed an incredible defence mechanism against one of its common predators in the wild, the amoebae. For those who are not aware, the amoebae is a type of cell or unicellular organism that has the ability to alter its shape by extending or retracting parts of its body. These microbes ingest their food by wrapping around the item (a process called phagocytosis) and digesting them in the phagosome (through the addition of enzymes and other items into the phagosome), see Image 1 (Below).

This process is very common, and is a highly complex pathway. Once the Phagosome is formed a complex chemical cascade occurs that signals to the Amoeba's internal systems to begin to break down the items within the Phagosome. In some cases the process is aiming to digest dead materials, whilst in other cases it is aiming to break down and consume bacteria and other protists.

This is where Legionella bacteria have developed an incredible trick which in my mind has no doubt aided their survival. As would normally happen in nature, Legionella will be detected by the amoebae who will aim to consume these through phagocytosis. The bacteria will be encapsulated by the pseudopods and captured inside the Phagosome however, Legionella has a few tricks up its sleeves (metaphorically). Once the Phagosome is formed, Legionella acts to block some of those complex chemical cascades I mentioned earlier, this means that the Legionella is captured within the Phagosome but it is not actually digested.

That in and of itself is a bit of a dirty trick, but Legionella has another trick up its sleeve. Within the phagosome Legionella begins to secrete hundreds of proteins in the infected cell which aim to force the cell to redirect its resources for the benefit of the bacteria. This means that the host cell (the amoebae) is in effect suppling all of the materials the Legionella bacteria needs which can grow within the cell and will literally eventually burst and allow the bacteria to then infect other cells.

As an added benefit, in many cases amoebae can also go into a state known as a cyst which is a highly protective stage. When nutrient stress or other stress conditions occur (i.e., chlorination) Legionella can be protected within this state until conditions allow for its return.

Whilst in this article I have been talking about amoebae, in reality one of the major lines of defence in the human body is what is known as a macrophage. These are very similar in how they function to amoebae (there are differences, but for the purpose of this I have simplified it), and when a macrophage tries to consume Legionella the bacteria can replicate in these and destroy them, the impact is that the body in effect loses its first line of defence and in some cases the immune system can't cope with the infection and a life-threatening pneumonia may develop.

This is an incredible ability of Legionella which sadly has devastating impacts on some people, what I have always pondered with this pathway is that in humans it is an evolutionary dead-end as Legionella cannot transmit from Human to Human, but I guess given Amoebae are so common in the environment, and so similar to macrophages in humans that it still serves an evolutionary purposes and is retained. Nonetheless it is quite an incredible ability to block the pathways and use the cell for protection and nutrient supply.