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
JAN 30, 2020
JAN 30, 2020
25% of Antibiotic-Resistant Bacteria Can Spread Resistance Directly to Other Microbes
This research also suggests that antibiotics do not increase the rate at which bacteria acquire drug resistance genes....
FEB 18, 2020
FEB 18, 2020
Coronavirus Illness COVID-19 Has Now Caused Over 2,000 Deaths
The outbreak of COVID-19 disease caused by a novel coronavirus called SARS-CoV-2 has now claimed 2,005 lives and caused at least 75,079 cases....
FEB 23, 2020
FEB 23, 2020
New discovery could help preterm babies breathe easy
Babies born more than three weeks before full term are met with complicated and often life-threatening health challenges. Most notably, these tiny preterm...
MAR 02, 2020
Drug Discovery & Development
MAR 02, 2020
DIY Fecal Transplants Improve Symptoms in 82% of People
Fecal transplants (FMT), the process of putting a healthy person’s fecal matter into another person’s colon, has been approved as a procedure t...
MAR 09, 2020
Genetics & Genomics
MAR 09, 2020
Researchers Alter How Bacteria Communicate
The bacterium Escherichia coli comes in many forms, and researchers have used a harmless strain of it to redesign how the microbes communicate....
MAR 22, 2020
MAR 22, 2020
Men Are More Susceptible to COVID-19 Than Women
More data is being gathered from an unfortunate and dramatic rise in the number of COVID-19 cases around the world....
Loading Comments...