How does the microscopic amoeba track down prey in the vastness of the forest floor? Ample research on the soil amoeba Dicytostelium discoideum, affectionately called “Dicty” by researchers, has provided some clues. But the mechanisms behind long-distance sensing of bacterial signals are still largely a mystery. A recent study takes a simple behavioral biology approach to uncover crucial clues. Further insights could elucidate other cell-sensing mysteries, such as how immune cells pick up signals from invading pathogens.
Previous research suggested that Dicty, a popular model system for studying phagocytosis, chemotaxis, and host-pathogen interactions, can tell the difference between the Gram-negative and Gram-positive bacteria that it eats. It appears to selectively turn on certain cellular machinery depending on the type of prey it ingests. But few studies have investigated whether Dicty can distinguish between different bacteria during the hunt.
Now, a low-tech behavioral study, published in January in Biology Letters, shows that Dicty prefers to hunt Gram-negative bacteria over Gram-positives. “For an organism that’s been studied extensively for many decades, including its ability to sense bacteria, the fact that it discriminates or responds differentially to bacteria at a distance hadn’t been shown,” says paper coauthor Elizabeth Ostrowski, an evolutionary biologist at Massey University in Auckland, New Zealand.
Researchers know that Dicty can phagocytize bacteria, but much is still unknown about the mechanisms underlying bacterial recognition, explains Michelle Snyder, a cell biologist at Towson University in Maryland who was not involved in the research.
Ostrowski and her coauthor set up paired assays in which Dicty had to choose whether to crawl toward one of four Gram-negative bacterial species or one of four Gram-positive species. In 21 of the 24 assays, Dicty chose the Gram-negative bacteria. “This is a classic test in behavioral biology,” Ostrowski says. “We used the tools of behavioral biology and learned something new about its behavior that had been overlooked.”
Three different Dicty strains collected from different geographical regions of the US all behaved surprisingly similarly in the Gram-negative/Gram-positive paired assays. “To me, this suggests that [the Dicty strains] have a very basic mechanism of sensing bacteria that is not a rapidly evolving trait,” Ostrowski says. “It’s something that’s fairly evolutionarily conserved.”
In a follow up experiment, the researchers tested whether Dicty might be homing in on the large amounts of cyclic adenosine monophosphate (cAMP) that Gram-negative bacteria regularly spill from their cells, whereas Gram-positives excrete little to no cAMP. They observed that Dicty preferentially migrated toward a mutant Gram-negative E. coli strain that overproduced cAMP versus a wild type E. coli and another mutant strain that produced no cAMP.
The experiments suggest that cAMP might play a role in Dicty preference for Gram-negatives. However, cAMP is likely not the only chemotactic molecule at work. In a control experiment, Dicty preferred the cAMP non-producing E. coli mutant over no bacteria at all.
“It would be nice to see if you could somehow manipulate cAMP levels so that you had a Gram-negative and a Gram-positive that secreted the same levels of cAMP,” Snyder suggests. She also would like to see experiments that take a Dicty cell missing the receptor that allows for chemotaxis toward cAMP and investigate whether it still discriminates between Gram-negatives and Gram-positives.
The finding that a phagocyte like Dicty can identify and track particular prey long-distance not only helps explain how the amoeba survives in bacteria-poor soil environments, but may also have implications in the mammalian immune system. Neutrophils and macrophages are immune system phagocytes that provide an important first line of defense against bacterial infection. “I think it’s very likely that you would find this type of discrimination in a standard immune system if you actually went and looked for it,” Ostrowski says.
“We need to understand the molecular events if we’re going to be able to manipulate systems or design drugs,” Snyder adds. “But sometimes I think we miss bigger pictures by getting so wrapped up in trying to pinpoint particular molecular events. And so I think what’s really interesting about this paper is it opens up a whole new field of questions.” From a molecular perspective, Snyder highlights questions about how extensively immune system phagocytes might respond to chemical signals emitted by pathogens, and what other chemoattractants besides cAMP play a role in environmental systems and in the mammalian body. She also would like to know if Dicty cells communicate with each other once they’ve found food, similar to how immune cells communicate in the body.
And to what extent do phagocytes actively seek out the bacteria that they want to associate with and avoid those that they don’t? “It’s a really big question,” Ostrowski says. “But I think the first step was to show that there is some discrimination at all.”