Life Cycle – Deadly Bacteria Visits American Convention- Legionella pneumophila.

The life cycle of any bacteria consists of 4 main phases.

In the Lag phase, bacteria do not grow but are metabolically active. In this phase, bacteria try to understand their environment and prepare for replication. They produce the proteins and vitamins needed for division in this phase and even make copies of DNA, but do not actually grow. The duration of this phase depends on the environmental factors such as nutrient availability and temperature. If there are enough nutrients available, the duration of this phase will be short. The optimal environment for Legionella is between 20°and 55°Celsius. Legionella pneumophila bacteria usually spend around 3 hours in this phase adapting to the environment. Initial culturing of Legionella was done on laboratory blood agar, but scientists were wondering why the bacteria wasn’t growing properly. Turns out that Legionella require the presence of iron and a lot of amino acids including cysteine to grow.

In the Log phase, also known as the exponential phase, the bacteria multiply very rapidly by a process called Binary Fission. It’s called the Log phase because they grow logarithmically, which is insanely fast. In favorable conditions, bacteria can double in as little as 15 minutes. The time taken by bacteria to double is known as the generation time. Legionella’s generation time is 6 hours. This means that if you start with just 100 Legionella pneumophila, in 6 hours you will have a total of 200. After 6 more hours, you will have a total of 400. You can see the exponential growth happening fairly quickly.

The next phase is the Stationary phasein which bacterial growth declines. This decline is dependent on growth inhibiting factors such as nutrient availability and temperature. A common misconception about this phase is that bacteria do not grow at all. This is not true as bacteria still divide in this phase. The growth rate and death rates are equalwhich means the population size will stay the same and the overall growth is halted by some growth inhibiting factor. Legionella have a mechanism that allows detection of low nutrient conditions so they can escape its host to survive and disperse in the environment.

The last phase, the Death phase, is as the name implies. The bacteria stop dividing and start dying off. This could be due to a lack of nutrients or a change in temperature that the bacteria cannot withstand.

https://www.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/bacterial-binary-fission

Legionella live and divide just like any other bacteria. They attach to a surface and go through the bacterial life cycle mentioned above. As any other bacteria, Legionella divide through Binary Fission. Binary fission is how bacteria reproduce asexually. Anytime an organism undergoes reproduction, DNA is always copied. In bacteria, the copying of DNA begins at the origin of replication. The double stranded DNA in the nucleoid opens up and the two origins move towards opposite ends of the cell until the entire chromosome is copied and the replication enzymes meet at the end. The cell is elongating as replication is taking place. Once replication is completed, Class II topoisomerases come in and unlink the two DNA strands. The cell membrane pinches inwards forming a septum and eventually the cell will split into two cells. Bacteria must regenerate their cell wall after replication so they can continue living as two individual bacterial cells.

https://www.ncbi.nlm.nih.gov/books/NBK7619/figure/A2226/?report=objectonly

Legionella pneumophila is a heterotrophic bacterium found naturally in freshwater environments like lakes and streams. It is a non-spore forming gram-negative bacteria that requires oxygen. Legionella’s nutritional requirements are L-cysteine, L-serine, L-methionine, L-threonine, L-arginine, L-valine, L-leucine, L-tyrosine, L-phenylalanine, and of course- ferric iron. In the picture above, the arrows point to the gram negative bacillus shaped L.pneumophila. Fun fact: These bacteria stain less intensely with safranin than intestinal bacteria.

https://jcm.asm.org/content/jcm/9/5/615.full.pdf

Legionella can be grown in a laboratory setting. Since L.pneumophila did cause a very big outbreak, extensive research was done on the species. A complete defined medium, shown in the picture above, can be used to grow a strain of L.pneumophila. Click on the link below the picture to access the whole article that goes in depth about other factors that may help or decline growth. All the directions are listed in the picture; deletion of any of the amino acids from the defined medium can cause depression or absence of growth.