As featured in Waterline Winter 2024-25
5 Things About Legionella That Might Surprise You
by Richard Danielson, M.W.MSoc (Snr)
Second Element Ltd
1) They Are Talking To Each Other!
It’s not quite whispering behind your back, but it is a fascinating process called quorum sensing. This intriguing communication method used by Legionella and other bacteria enables them to ‘talk’ with one another to monitor their population size and then adjust their behaviour to enhance their chances of survival. Here’s how it works: Legionella bacteria release signalling molecules known as autoinducers into their environment.
This process allows bacteria to monitor the environment for other bacteria and to alter behaviour on a population-wide scale (Waters & Bassler, 2005).
Many bacterial processes are mediated by quorum sensing, including biofilm formation and regulating the production of virulence factors (Li & Zhao, 2020). Virulence factors are molecules that significantly influence a pathogen’s ability to cause disease.
The type and quantity of these virulence factors can affect disease severity, the host’s immune response, and the overall outcome of the infection. Communication between cells allows a collective timing in the expression of these factors – in other words Legionella checks with its comrades to coordinate an attack on the host’s defences – it’s good to talk!
2) They Make the Baby Boomers Look Like They Weren’t Trying
When it comes to increasing their population, Legionella bacteria have a remarkable talent for rapid growth. They replicate through an asexual process called binary fission. During binary fission a single bacterium divides into two identical daughter cells. When things are cosy — Legionella pneumophila can double its population in just two hours (Steinert et al. 2002). Factors such as temperature, nutrients and oxygen levels play a critical role in determining how quickly they can multiply. In environments where conditions aren’t ideal, their doubling time can be much longer or even stop. While two hours is impressive, Escherichia coli has a doubling time of every twenty minutes under ideal settings (Tuttle et al. 2021).
3) They Can Turn Invisible
When it comes to longevity, Legionella bacteria have some survival skills to rival the X-Men! While the lifespan of a single Legionella bacterium can vary based on environmental conditions from minutes to months, they can tough it out for even longer periods in a dormant state. As we know, Legionella can thrive and replicate through binary fission, but what happens when the environment isn’t a lot of fun for them?
Legionella can enter a state known as the viable but non-culturable (VBNC) state. In this state, they are alive but cannot easily be cultured in a lab setting effectively making them ‘invisible’ to traditional methods. This clever adaptation allows bacteria to ride out harsh environments until more favourable conditions return, at which point they can reactivate and proliferate. (Oliver et al., 2005). This ability to remain dormant yet viable plays a crucial role in their survival and persistence in water systems – and makes them really good at hide and seek!
4) They Could Give Shackleton a Run for His Money
Any Water Hygiene Technician worth their salt will tell you that Legionella is dormant below 20°C, but what happens if you freeze it? You might think that freezing could be a good method for eliminating these bacteria, but you’d be wrong! Legionella can actually survive exposure to freezing temperatures. HTM 04-01 has plenty to say on the microbial hazards of ice for patient consumption and studies have shown that freezing Legionella bacteria at temperatures of minus 20°C doesn’t immediately kill them – that pesky VBNC state again!
Eventually, freezing will breakdown the cell membrane leading to death, but you’re going to need it extremely cold to significantly reduce viability!
5) Not All Legionella Bacteria Are Bad
There are over 60 known species, with Legionella pneumophila getting the spotlight as the notorious troublemaker; further divided into 18 different serogroups with serogroup 1 being the main culprit for Legionnaires’ disease.
Did you know that not all Legionella bacteria are out to get you? Approximately 20 species can cause legionellosis (Kanarek et al., 2022) which means many Legionella species are not associated with disease in humans.
These non-pathogenic species may play important roles in their natural aquatic ecosystems contributing to the microbial diversity and structure of freshwater habitats. Basically, enjoying the great outdoors and hanging out with their microorganism buddies like amoebas, without causing too much fuss and not posing any threat to us.
Unfortunately, there is no evidence to suggest that Legionella species offers us any health benefits, so don’t expect to see a Legionella Yakult in the supermarket anytime soon!
References
1. Danielson, R. E. M. (2024). Dad jokes: A good idea for technical writing? (Actually, I made that one up).
2. Falkinham, J. O. III. (2015). Common features of opportunistic premise plumbing pathogens. International Journal of Environmental Research and Public Health.
3. Kanarek, P., Bogiel, T., & Breza-Boruta, B. (2022). Legionellosis risk: An overview of Legionella spp. habitats in Europe. Environmental Science and Pollution Research International.
4. Li, J., & Zhao, X. (2020). Effects of quorum sensing on the biofilm formation and viable but nonculturable state. Food Research International, 137.
5. Oliver, J. D., & D. P. E. (2005). Viable but nonculturable bacteria. Annual Review of Microbiology.
6. Steinert,M., Hentschel, U. Hacker, J (2002) Legionella pneumophila: an aquatic microbe goes astray, FEMS Microbiology Reviews, Volume 26, Issue 2
7. Tuttle et al. (2021). Growth and Maintenance of Escherichia coli Laboratory Strains.
8. Waters, C. M., & Bassler, B. L. (2005). Quorum sensing: Cell-to-cell communication in bacteria. Annual Review of Cell and Developmental Biology.
