The Mystery of Infection

Natural Sciences4 MIN READ

Suegene Noh’s new study seeks to uncover how and why amoebas react differently to the same bacteria

Assistant Professor of Biology Suegene Noh (back) with Emily Larson ’21 in the lab.
By Kardelen Koldas ’15
September 18, 2020

Assistant Professor of Biology Suegene Noh has been awarded a Maine INBRE (IDeA Network of Biomedical Research Excellence) grant, sponsored by the National Institutes of Health, to study how and why bacterial infections impact amoebas differently.

The research aims to identify genes that impact infection between amoebas (the host) and bacteria (the symbiont). Noh’s goal, she explained, is to better understand how and why fitness—a general term describing health—outcomes of amoebas vary when they encounter the same bacteria.

In the study, Noh will infect single-cell Dictyostelium discoideum amoebas with Burkholderia bacteria. Species of this bacteria infect about one-third of these amoebas in the wild, she noted. But in the lab environment, exposing any of the amoebas to the bacteria results in a 100-percent infection rate.

“I’m really curious why that is,” said Noh, who also wants to see what differs among hosts when they become infected, either they behave as before or suffer consequences when interacting with other amoebas that are uninfected. As for the bacterial symbiont, she’ll examine whether there is any variation in its ability to infect different hosts and how easily they transmit from host to host.

Amoebas in a petri dish from Noh's lab.
Amoebas in a petri dish from Noh’s lab.

Noh uses Dictyostelium discoideum because of its unique lifecycle. These amoebas live in the soil and eat bacteria. But when they starve, they can aggregate to form a multicellular body in the form of a slug. This slug, searching for light, migrates upward toward the surface of the soil. Finally, it becomes a fruiting body, resembling a tiny mushroom that’s not quite the size of a hair on your arm, she explained. The amoebas that end up on the top (called the sorus) of the fruiting body become spores while the ones at the bottom (or the stalk) die. This is how some amoebas escape starvation.

“I really like having this mix of hands-on bench work as well as computational analytical work so that you can get the whole range of experience from a project. I think that’s a really nice combination to be exposed to as an undergrad.”

Assistant Professor of Biology Suegene Noh

“It’s really exciting to me that you can try to take advantage of that [lifecycle] to look at things like the spread of infections among hosts that form social groups, and why not all hosts are infected,” said Noh.

Noh will measure fitness by looking at how it affects competitive ability. In the amoebas, she’ll determine that by analyzing what proportion of the infected hosts end up in the sorus, at the top of the fruiting body structure. “If you mix the same number of infected and uninfected amoebas but find more infected ones in the sorus than uninfected ones, then infected amoebas will have higher competitive fitness,” she said, “because they’ve managed to survive the process of fruiting body formation at a higher rate.”

For the bacteria, she will measure fitness through what she’s calling “infectivity”—the rate at which they transmit from host to host. “My theory here is that the symbionts that are most successful might be the ones that most readily spread into new hosts,” she said.

“It’s a big project,” Noh said. “We are planning on doing quite a bit of different mixes of infection and competition.” This complex approach not only makes her study unique but also provides her with a wealth of data to conduct further analysis.

Results from flow cytometry.
Here, the plots show how amoebas in spore form look in flow cytometry when they aren’t infected. Noh uses flow cytometry to distinguish amoebas that were already infected from amoebas that become infected through symbiont transmission.

“I’m excited to have this much data at once,” said Noh, whose experiments will ultimately compare different responses of amoebas to infections by using functional genomics. Through this, she intends to find regulatory changes, meaning genetic changes that are outside of the regions of a genome that directly code for proteins.

In her Colby lab, she’ll process both amoeba and bacterial samples for high throughput sequencing. Using these data, she will identify host and symbiont genes that have correlated responses to infection outcomes, that change expression to potentially cause some hosts to experience benign effects of infection while others suffer more.

The three-year grant of $270,000 will enable Noh to hire two student researchers per academic year. These students will be involved with all aspects of research, ranging from mixing microbes to conducting the experiments to doing genomics.

“I really like having this mix of hands-on bench work as well as computational analytical work so that you can get the whole range of experience from a project,” said Noh. “I think that’s a really nice combination to be exposed to as an undergrad.”