AUG 27, 2017 1:09 PM PDT

The amoeba's microbiome

WRITTEN BY: Kerry Evans

With all the talk of the human microbiome, it’s worth pointing out that other organisms- even teeny tiny ones - have microbiomes of their own.

Even amoebas, you ask? Yep, even amoebas.

Amoebas have bacterial endosymbionts.

Image: Alamy/Melba Photo Agency

While free-living amoebas feed on bacteria, some bacteria actually live inside amoebas - sometimes as endosymbionts and sometimes as pathogens. When an amoeba “eats” a bacterium, it is usually digested inside of a phagolysosome. Some bacteria can actually resist this digestion, and that may be how they became endosymbionts in the first place.

Nineteen different bacterial endosymbionts have been identified in amoebas with rRNA sequencing technology, and these comprise 5 different lineages. The number of bacteria in one amoeba can range from between 5 and 100. Some of these bacteria live freely in the cytoplasm, but some hang out in host-derived vacuoles - phagolysosomes modified by the bacteria.

Some of these bacteria aren’t so friendly, however. Some endosymbionts actually lyse their amoeba hosts (in which case, it’s not really correct to call them endosymbionts). For example, O. thessalonicensis lives happily inside an amoeba at 22oC. Turn the heat up to 30oC, however, and these bacteria lyse their amoebas! The change in temperature may signal danger, prompting the bacteria to produce virulence factors.

Interestingly, most of the endosymbionts of amoebas belong to the order Chlamydiales. Some familiar human pathogens also fall into this lineage. For example, Chlamydophila pneumoniae causes community-acquired pneumonia, especially in people with chronic health conditions. Chlamydia trachomatis, of course, causes the STI chlamydia.

Could these chlamydia-related endosymbionts cause disease in humans? Some of these strains have actually been identified (through rRNA and antibody titers) in patients with pneumonia and other respiratory diseases, suggesting that the amoebas may help to spread these potential pathogens.

Indeed, amoebas appear to act as reservoirs and vehicles (tiny Trojan horses?) for Legionella pneumophila. Amoebas and Legionella both live in similar environments (like hot water tanks) and amoebas are often transient hosts of Legionella.

Legionella causes legionellosis, a respiratory illness that occurs when the bacteria are inhaled with aerosolized contaminated water. In 1980, Rowbotham and colleagues proposed that people don’t get legionellosis by inhaling free bacteria. Instead, he reasoned, the disease is transmitted when people inhale amoebas (or vesicles) that contain Legionella! Consistent with this, free-living amoeba are required for Legionella to reproduce in aquatic biofilms.

Some pathogenic species of Mycobacterium can also live inside amoebas. In some cases, endosymbiosis may actually make these bacteria more virulent! M. avium (a human respiratory pathogen) is more virulent when it is grown in amoebas than in broth culture - they invade intestinal epithelial cells and macrophages more readily. M. avium’s intracellular lifestyle also makes it resistant to various drugs, such as the antimicrobials used to treat M. avium in people with AIDS.

As I mentioned earlier, “endosymbiosis” may not be the best term to describe every relationship between amoebas and intracellular bacteria - endosymbiosis implies that both parties benefit from the relationship. In some cases, however, it seems that only the bacteria benefit. To reflect this, some reports describe “amoeba-resistant” microorganisms. These organisms survive phagocytosis by an amoeba and are often capable of multiplying within their host.

As far as amoeba-resistant bacteria go, Listeria monocytogenes, Burkholderia cepacia, and Pseudomonas aeruginosa will all live inside an amoeba if given the chance, as will some human pathogens. These include Chlamydophila pneumoniae (mentioned earlier), Simkania negevensis, Bradyrhizobium japonicum, and Coxiella burnetii.

Sources: The Journal of Eukaryotic Microbiology, Clinical Microbiology Reviews, Journal of Bacteriology

 

About the Author
  • Kerry received a doctorate in microbiology from the University of Arkansas for Medical Sciences.
You May Also Like
MAY 04, 2020
Genetics & Genomics
Molecular Tools Reveal More About the Impacts of the Slave Trade
MAY 04, 2020
Molecular Tools Reveal More About the Impacts of the Slave Trade
Scientists still have a lot to learn about the numerous and varied consequences of the transatlantic slave trade, which ...
MAY 09, 2020
Clinical & Molecular DX
Rapid, Efficient COVID-19 Diagnostic Method Relies on Magnetic Beads
MAY 09, 2020
Rapid, Efficient COVID-19 Diagnostic Method Relies on Magnetic Beads
The pandemic virus SARS-CoV-2 has spread widely, and a lack of testing has been a serious problem.
MAY 23, 2020
Microbiology
The FDA Yanks Some COVID-19 Antibody Tests From the Market
MAY 23, 2020
The FDA Yanks Some COVID-19 Antibody Tests From the Market
The massive demand for diagnostic testing led the FDA to open a short window for many testing products to go to market w ...
JUN 10, 2020
Immunology
Natural Killer Cells with "Memory" to Target Hepatitis B
JUN 10, 2020
Natural Killer Cells with "Memory" to Target Hepatitis B
An immune cell type thought to be restricted to the general, first response to pathogenic invaders may actually have som ...
JUL 19, 2020
Microbiology
A Hybrid Fungus Is Linked to Lung Infections
JUL 19, 2020
A Hybrid Fungus Is Linked to Lung Infections
A type of fungus that's been found in soil or plants has now been identified in a hospital environment and in people for ...
JUL 22, 2020
Infographics
The Science of Sourdough
JUL 22, 2020
The Science of Sourdough
During the coronavirus stay-at-home order, many people have taken up the art of making sourdough bread and have learned ...
Loading Comments...