Bacteria form biofilms like settlers form cities
Microbiologists have long adopted the language of human settlement to describe how bacteria live and grow: They “invade” and “colonize.” Relations dwelling in close proximity are “colonies.”
By pairing super-resolution imaging technology with a computational algorithm, a new study in Nature Communications confirms that this metaphor is more apt than scientists may have realized, phys.org reported.
The findings show that, as individual bacteria multiply and grow into a dense and sticky biofilm, such as the community that forms dental plaque, their growth patterns and dynamics mirror those seen in the growth of cities.
“We take this ‘satellite-level’ view, following hundreds of bacteria distributed on a surface from their initial colonization to biofilm formation,” said Hyun (Michel) Koo, a professor in Penn’s School of Dental Medicine and senior author on the work.
“And what we see is that, remarkably, the spatial and structural features of their growth are analogous to what we see in urbanization.”
This new perspective on how biofilms grow could help inform efforts to either promote the growth of beneficial microbes or break up and kill undesirable biofilms with therapeutics.
The idea for the research emerged from conversations among Koo; Geelsu Hwang, a Penn Dental Medicine assistant professor who applies engineering to problems of oral health; and Amauri Paula, a physicist who worked as a visiting professor with Koo’s lab.
“Usually when people study biofilms, they analyze a single cell in a narrow field of view as it multiplies, becomes a cluster, and starts to build up,” said Koo.
“But we wondered if we followed multiple individual cells simultaneously whether we could identify some patterns at large length-scales.”
Hwang developed powerful time-lapse imaging tools, employing confocal laser scanning microscopy capable of analyzing surface topography and tracking bacteria populating a surface down to the individual cell in three dimensions over time. Meanwhile, Paula worked to build an algorithm that could analyze the behavior of this growth over time.
For their study, they used the microbe Streptococcus mutans, an oral pathogen responsible for causing cavities when it forms a biofilm known more commonly as dental plaque and releases acids that decay tooth enamel.
They distributed the bacteria on a tooth enamel-like material and followed hundreds of individual microbes during several hours as they divided and grew.
Overall, the growth patterns were reminiscent of the
formation of urban areas, the team found. Some individual “settlers” grew, expanding into small bacteria “villages.” Then, as the boundaries of the villages grew and, in some cases met, they joined to form larger villages and eventually “cities.” Some of these cities then merged to form larger “megacities.”
Surprising the researchers, their results showed that only a subset of the bacteria grew. “We thought that the majority of the individual bacteria would end up growing,” said Koo. “But the actual number was less than 40 percent, with the rest either dying off or being engulfed by the growth of other microcolonies.”
They also didn’t expect a lack of inhibition when this engulfment took place. They thought that, as different microcolonies met, they might compete with one another, causing the two edges to perhaps repel.
“Instead they merge and begin to grow as a single unit,” said Koo.
On both the individual bacteria and biofilm-wide scale, the researchers confirmed that the glue-like secretion known as extracellular polymeric substances (EPS) enabled bacteria to pack together closely and firmly in the biofilm. When they introduced an enzyme that digested EPS, the communities dissolved and returned to a collection of individual bacteria.
“Without EPS, they lose the ability to densely pack and form these ‘cities,’” said Koo.
Finally, the researchers experimented to see how the addition of a microbial “friend” or “foe” would influence the original bacteria’s growth. The “foe” was Streptococcus oralis, a bacteria that can inhibit the growth of S. mutans. This addition dramatically impaired the ability of S. mutans to form larger “cities,” like disruptive neighbors that can affect the collective growth of the community.
The “friend” – the fungus Candida albicans, which Koo and others have found to interact with S. mutans in biofilms and to contribute to tooth decay – did not affect the biofilm’s growth rate but did help bridge adjacent microcolonies, enabling the development of larger “cities.”
Koo cautions about taking the urbanization metaphor of biofilm growth too far but underscores the useful lessons that can result from studying the system holistically and by looking at the events under both “close-up” and “bird’s eye” views.
“It’s a useful analogy, but it should be taken with a grain of salt,” Koo said.
“We’re not saying these bacteria are anthropomorphic. But taking this perspective of biofilm growth gives us a multiscale, multidimensional picture of how they grow that we’ve not seen before.”
These ants have a revolutionary escape strategy
Ants are bristling with defense weaponry. Different speciesmight sting their enemies, bite them with powerful jaws or shoot them with jets of formic acid. Some even explode.
But Myrmecina graminicola – an ant about the size of a sesame seed – doesn’t want to get into all that. According to research published last week in Scientific Reports, if one of these ants encounters danger while it’s on a slope, it makes a practical choice: It tucks itself into a little ball and rolls away, The New York Times reported.
It is the only ant known to move in this way, and one of few rollers in the animal kingdom over all, said Donato Grasso, the paper’s lead author and an ant ethologist at the University of Parma in Italy.
Grasso and his colleagues first spotted this unique behavior while scanning the forest floor during a trip to one of their field sites in Fornoli, Italy. (Many entomology discoveries are made this way: “When you are a biologist interested in insects, it is impossible not to look at the ground,” Grasso said.)
The team found a few colonies of M. graminicola, which are so small and elusive they often go unnoticed. When the insects were menaced by spiders and other ants, “they curled their bodies and disappeared” into the leaf litter, Grasso said. “They rolled away.”
The research team decided to take some of the ants back to the lab. It was difficult to find them, and when the researchers picked the ants up, they would sometimes somersault out of their hands. But eventually, they caught some living inside fallen tree galls.
In the lab, the researchers used slow motion video to tease out the ants’ choreography. Roughly: A ready-to-roll ant tucks in its head and pulls its abdomen forward to form a ball. It then lifts its legs up and tips itself forward to rest on its mandibles and antennae, which balance it like arms, Grasso said. A final push with the hind legs, and it’s off.
On smoother surfaces like stones and leaves, the ants traveled at about 15 inches per second — about 80 times faster than their average walking speed. They could move themselves about six inches, or about 50 body-lengths.
Such a distance is “pretty impressive,” and shows that the ants’ rolling form must be very efficient, said Nicholas Gravish, an engineer who studies ant locomotion at the University of California, San Diego and was not involved in the new research.
Grasso’s team also set out to learn exactly what prompts the ants to make this unique exit. They placed the ants on increasingly steep gradations, from zero degree all the way up to 90.
As the ants walked the slopes, the researchers exposed them to stressful stimuli. The ants modulated their reactions depending on the context. If they were on a relatively flat surface when the researchers bothered them, they simply froze.
But once the slope reached 10 degrees, some of the ants started to tumble away. And at inclines above 25 degrees — about as steep as a beginner-level ski trail — all the ants tucked and rolled.
These and other tests show that “these ants behave this way only in specific circumstances,” and that rolling wasn’t a byproduct of other defenses like curling into a ball, Grasso said.
While humans enjoy assisted rolling, it is rare that other animals take part. There are spiders that cartwheel across the desert, and some salamanders back-flip down boulders. If you give a mother-of-pearl moth caterpillar a good poke, it will spin away like a little green coin.
But in nature, rolling is not nearly as popular a tactic as, say, running away.
It’s also high-impact, which could be “catastrophic to larger animals,” said Glenna Clifton, a scientist in Dr. Gravish’s lab.
But if you can pull it off, it is effective. In a later experiment, the researchers exposed M. graminicola to one of its enemies, a different ant species. On flat ground, the M. graminicola casualty rate was 63 percent. But when they tussled on a slope, it dropped to 10 percent.
“Fighting is not always the best way to survive,” Dr. Grasso said. “Sometimes the best way is to escape.”
Physicists get most accurate estimate for true size of neutron stars
How big is a neutron star? These extreme, ultra-dense collapsed stars are fairly small, as far as stellar objects are concerned. Even though they pack the mass of a full-sized star, their size is often compared to the width of a medium-to-large-sized city.
For years, astronomers have pegged neutron stars at somewhere between 19 to 27 kilometers (12 to 17 miles) across. This is quite actually quite precise, given the distances and characteristics of neutrons stars. But astronomers have been working to narrow that down to an even more precise measurement, sciencealert.com reported.
An international team of researchers has now done just that. Using data from several different telescopes and observatories, members of the Max Planck Institute for Gravitational Physics, the Albert Einstein Institute (AEI) have narrowed the size estimates for neutron stars by a factor of two.
“We find that the typical neutron star, which is about 1.4 times as heavy as our Sun, has a radius of about 11 kilometers,” said Badri Krishnan, who led the research team at the AEI Hannover.
“Our results limit the radius to likely be somewhere between 10.4 and 11.9 kilometers.”
The object of this team’s study is rather famous: The binary neutron star merger GW170817 which created the gravitational waves detected in 2017 by the LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo consortium.
This object has been studied numerous times by multiple telescopes, including the Fermi satellite, the Hubble Space Telescope and other telescopes and observatories around the world. All those observations gave the Max Planck team a boatload of data to work with.
“Binary neutron star mergers are a gold mine of information!” said Collin Capano, researcher at the AEI Hannover and lead author of a paper published in Nature Astronomy.
“Neutron stars contain the densest matter in the observable universe. … By measuring these objects’ properties, we learn about the fundamental physics that governs matter at the sub-atomic level.”
Neutron stars are formed when a massive star runs out of fuel and collapses. The very central region of the star – the core – collapses, crushing together every proton and electron into a neutron.
If the core of the collapsing star is between about one and three solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star.
Stars with even higher masses will continue to collapse into stellar-mass black holes.
But the collapse into a neutron star creates the densest object known – again, an object with the mass of a sun crushed down to the size of a city. And you’ve probably heard this other comparison before, but it’s worth a repeat because of how dramatic it is: One sugar cube of neutron star material would weigh about one trillion kilograms (or one billion tons) on Earth – about as much as Mount Everest.
But since the size of other stars can vary widely, couldn’t the size of neutron stars also vary?
First, to clarify, the radius quoted in this study is for a neutron star that has a mass 1.4 times that of our Sun.
“This is a fiducial mass that’s typically used in the literature because nearly all neutron stars that have been observed in a binary have a mass close to this value,” Capano told Universe Today in an email.
“The reason we can use GW170817 to estimate the radius of 1.4 solar-mass neutron star is that we expect nearly all neutron stars to be made of the same stuff.”
For other “regular” stars, the relationship between their mass and radius depends on a number of variables, such as the element that the star is fusing in its core, Capano explained.
“Neutron stars, on the other hand, are so compact and dense, that there are not really separate atoms in them – the entire star is basically a giant single atomic nucleus, consisting almost entirely of neutrons packed tightly together,” he said.
“For that reason, you cannot think of neutron stars as being comprised of possibly different elements. Indeed, ‘element’ doesn’t really have any meaning at these densities, since what defines an element is the number of protons it has in its constituent atoms.”
Capano said that since all neutrons are made of the same things (quarks, held together by gluons), astronomers expect there to be a universal mapping between the mass and radius that applies to all neutron stars.
“So, when we quote the possible size of 1.4 solar mass neutron star, what we’re actually doing is constraining the possible physical laws that describe the sub-atomic world,” he said.
As the team describes in their paper, their results and processes can also be applied to the study of other astronomical objects, like pulsars, magnetars, and even the way gravitational waves are emitted to provide details of what is creating these waves.
“These results are exciting, not just because we have been able to vastly improve neutron star radii measurements, but because it gives us a window into the ultimate fate of neutron stars in merging binaries,” said Stephanie Brown, coauthor of the publication and a Ph.D. student at the AEI Hannover.
This article was first published by Universe Today.
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