There’s a legion of bacteria out there, but one is stealing the headlines

This week, the UK headlines (the Queen’s Diamond Jubilee aside), have been dominated by an outbreak of Legionnaires’ disease in Edinburgh. It therefore seems fitting that I should join the trend and write my first blog post in a fairly long time on this topic.

Legionnaires’ disease first acquired its name back in 1976 when a Philadelphia based convention of the American Legion was struck by a plague of pneumonia. It took almost 3 years for the causative agent to be discovered, which was traced to a cooling tower containing the bacterium named Legionella pneumophila. The Legionellae bacterial family is large, with roughly 50 different species, yet only a few of these have been found to cause any kind of disease. Legionellae bacteria are classed as being “Gram negative” meaning they do not take up a marker stain for bacteria known as the Gram stain. Gram negative bacteria are characterised by a large protective layer surrounding their cell known as the outer membrane. This outer membrane can help to protect the bacteria from certain antibiotics and thus limit the efficacy of certain treatments, such as penicillin. Legionnaires’ disease outbreaks are sporadic events and are often traced to man-made structures like cooling towers and air conditioning units. The outbreaks are characterised by pneumonia and flu-like symptoms. Those who have read previous blogs of mine may remember the discovery of Mimivirus, which was originally thought to be causing a Legionnaires’ disease outbreak (here’s a link to refresh the memory or for those who haven’t read it before.)

L. pneumophila are predominantly found in freshwater environments. When this was first discovered it seemed odd since growing L. pneumophila in a lab was very difficult due to the need for a very high level of nutrients, which are not naturally found in freshwater. It became apparent that these bacteria do not grow in a completely isolated way; much like a virus, the bacterium needs to parasitize another living organism; in the case of L. pneumophila the main host species are amoeba.

Interestingly, most natural freshwater reservoirs tend to normalise to the ambient temperature, so a collection of rain water inside an old tyre, for instance, would be at the temperature of the surrounding air (I use the tyre example as I will soon be writing a blog on Dengue virus - you’ll have to come back to see why tyres are important). However, L. pneumophila grows best when at 25 ^OC - 42^OC, with the best growth occurring at 35 ^OC. Now I haven’t been to Scotland in a long time, but I’m pretty sure it hasn’t got up to 35^OC any time in the recent past (if ever). While natural freshwater reservoirs are unlikely to reach high enough temperature to support L. pneumophila growth, man-made environments such as cooling towers or air conditioning units are. Legionnaires’ disease is therefore, in essence, a man-made phenomenon due to our alteration of the environment (it’s not alone in this respect).

So the next question to look at is how exactly this little bacterium is causing the outbreak of the disease we are currently seeing in Scotland. There are two main ways L. pneumophila can jump from its usual amoeba hosts into humans; these can be either through small droplets of water containing bacteria that are inhaled into the lungs or through an aspiration route in which water in the mouth containing bacteria gets into the lungs. The aspiration route needs some pre-existing damage as things in the mouth are not usually meant to enter the lungs; this damage can be caused by smoking or chronic lung disease for instance. Our lungs are lined with immune cells known as phagocytes which essentially sit there and wait for things that shouldn’t be there, such as bacteria. Once the phagocytes detect the bacteria, they engulf then and taken them into the cell where they would normally be degraded (a process known as phagocytosis), thus providing protection. L. pneumophila are a bit cheeky. The bacterium allows itself to be phagocytosed but then blocks the destruction step; they simply remain in a compartment of the cell where they are able to grow. Over time the growth of L. pneumophila will lead to death of the infected phagocyte which is often accompanied by bursting of the cell. This bursting, like popping a water-balloon, releases all the contents to the environment. All the bacteria and all the other nasty stuff inside a cell are released out into the lungs causing a large inflammatory response. It is this response, designed to protect, that causes the flu-like symptoms and the pneumonia seen in Legionnaires’ disease. When this becomes severe (which is relatively rare) people need hospitalisation for treatment. The treatment is generally effective, so for most people Legionnaires’ is a nasty disease but one that usually clears up by itself or through medical intervention.

As I finish writing this post, there are currently 24 confirmed cases of Legionnaires’ disease in the current outbreak, with one death. It’s a nasty disease but one that is rare and can very often be treated and I’m sure that within a few days to a week the number of new cases will begin to decrease.