Introduction
Legionella are gram negative bacilli which do not grow on routine media used in Bacteriology. It requires complex nutritional requirements such as high cysteine levels and low sodium levels to grow. As a result it is often difficult to isolate and identify outside of the host.4 This bacterium is found in rod-like formations usually, 0.3-0.9 micrometersX2-20micrometers long. Many different species of Legionella (at least 34) have been discovered, and more than fifty serogroups of Legionella have been identified. The strain of this bacterium that causes Legionnaire's Disease is called Legionella pneumophila serogroup 1, or the Pontiac Strain.
Occurrence in Natural Environments
Legionella pneumophila have always been found in non-marine aquatic environments such as lakes and ponds. The optimum growth temperature range for this bacteria is 20-45 degrees Celsius. It thrives in areas where there are high concentrations of rust, algae, and organic particles; for these supply the bacteria with a source of iron as well a nitrogen. These are necessary for providing nutrients for growth, as well as protection from harsh chemicals. The organism has been found to possess the ability to survive in tap water at room temperature for over a year. Legionella, in their natural environments, grow within other organisms. For example, the bacteria have frequently been found to be in very close association, possibly an endosymbiotic relationship, with certain protozoan. Hartmanella vermiformis, Tetrahymena thermophila, and Acanthamoeba castellani are just a few of the protozoa that have been identified to have Legionella growing within them. The reason why Legionella grow inside these amoebae is because the bacteria multiply intracellularly, and therefore require the amoebae to aid in reproduction. When Legionella pneumophila adapt to living inside an amoebae many changes have been found to occur. For example, the membrane lipid content changes, the protein profile changes, and a 15-kDa protein thought to be of amebic origin becomes tightly associated with the bacteria. It has been determined that one amoebae can house enough Legionella bacteria to infect a human being, and this single amoebae is small enough to fit in one droplet of aerosolized water.
Occurrence in Man-Made Environments
Legionella bacteria are transmitted to the lungs of human beings through a process called aerosilization. The most common devices that have been found to transmit the bacteria are air conditioning cooling towers; however, there are many other devices that can transmit the bacteria such as: shower heads, pipes, heat exchange bumpers, whirlpools, humidifiers, respiratory therapy devices, and grocery store misters. The bacteria especially favor the average relative humidity level of sixty-five percent. This is because the humidity enables the bacteria to live for longer periods of time in an airborne environment. In this type of humid environment larger droplets can also evaporate forming smaller ones which are more favorable to the bacteria since smaller droplets can penetrate deep into the lungs without being easily expelled.
