Microbiome Monday: The Ecology of a Killer Microbe

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It's Microbiome Monday again! Before the Museum’s upcoming exhibition The Secret World Inside You opens November 7, we’re offering weekly primers on the microbiome and the research surrounding it from Curators Rob DeSalle and Susan Perkins, as well as from other scientists who are working in this exciting field.

This week, Dr. DeSalle and Dr. Perkins explain the deadly microbe behind this summer's outbreak of Legionnaire's disease.

The Microbe Behind Legionnaire's Disease

The recent outbreak of Legionnaire’s disease in New York City came as a devastating surprise to health officials, killing 15 and sending dozens more to the hospital. This outbreak, however, was not quite as terrible as the one that took place when the disease debuted in 1976. That instance, which took place at a Philadelphia convention of the American Legion, from which the disease takes its name, claimed 25 lives and left more than 150 individuals hospitalized. 

in 1976, the microbial agent of Legionnaire’s disease, which causes symptoms akin to acute pneumonia, was unknown. This spurred a massive microbe hunt as doctors from the Centers for Disease Control (CDC) worked to pinpoint the cause. It only took a month to determine that the microbial killer was an unnamed, little-known bacterium, making the microbe one of only a few infectious agents to “debut” in the 20th century. Since this initial discovery, about 40 different kinds of Legionellahave been identified, all of them with similar attributes and all of them able to infect and kill humans. 

Bellevue Stratford Hotel
The Bellevue Stratford Hotel, where Legionnaire's disease was first identified in 1976.
Courtesy of Library of Congress

Like many other microbes of all kinds and sizes, bacteria in the genus Legionella live mainly in bodies of freshwater like rivers, ponds, and lakes. What is strange about Legionella and apparently a couple of other genera of bacteria (Brucella and Coxiella) is that they spend part of their lives inside single-celled organisms like amoeba, where they replicate themselves.

While these bacteria are at home in amoebas, they also like to live inside of and replicate in human macrophages, the workhorse cells of the human immune system. That makes Legionella and similar microbes a double whammy as diseases: they incapacitate an element of the human immune system, which evolved to stop infections, while at the same time multiplying.

Making the trip from amoeba to immune cell is no small journey for Legionella, though. This small organism has two ways it can survive other than inside of an amoeba: It can hibernate in what is called a viable but non-cultivable state (VNBC) or as a free organism as part of a biofilm, a colony of bacteria that join together for survival.

Legionella microbes viewed under a scanning electron microscope.
Courtesy of CDC

Biofilms are an especially relevant lifestyle for these microbes, because they can form in watery man-made devices like cooling systems. Once there are a large number of Legionella in one place, the probability of some making their way into water droplets that are released by air conditioning systems and then breathed in by unsuspecting humans increases, which can result in an outbreak of the disease.

There are no known cases of human-to-human infection by Legionella, so the only way to cut off the progression of the infectious agent is to clear out the water source containing the biofilms and amoeba these bacteria call home. That’s why the Opera House Hotel, where the disease broke out in July 2015, was closed and its water sources cleansed in response to the outbreak, which was traced to a contaminated water cooling tower.

The exterior of the Opera House Hotel in The Bronx, New York, the site of a Legionnaires' disease outbreak in 2015.
The  Opera House Hotel, where a recent outbreak of Legionairre's disease took place in the summer of 2015.
Courtesy of Wikimedia Commons

The good news is that as we learn more about this disease, we’re learning new ways to use Legionella’s own ecology to control it. It turns out that certain microbial members of biofilms and freshwater ecologies slow its growth. Once we humans understand it better, its own ecology may very well be its demise.