May 09, 2020 16:00:00

Why are there animals that do not have intestinal bacteria that live in the body?

In recent years, interest in the

bacterial flora (microbiome) that exists in the body of animals has increased, and it is also known that the intestinal bacteria that live in the human intestine influence the mood and diet of people. On the other hand, not all animals seem to coexist with abundant bacteria, and it is known that more animals have few intestinal bacteria than scientists imagine.

Why Is the Microbiome Important in Some Animals but Not Others? | Quanta Magazine
https://www.quantamagazine.org/why-is-the-microbiome-important-in-some-animals-but-not-others-20200414/

The microbiome is an important supporter of human health, including food digestion, nutrient absorption, and immune system. In recent years, tools for measuring and analyzing microbial counties have evolved, and it has become clear that microbiomes exist everywhere in the animal kingdom and coexist with animals. As a result, some researchers are proposing to extend the range of 'animals' to the microbiomes they carry.

In the summer of 2011, biology researcher

John Sanders entered the tropical forests of Peru and conducted a survey of ants in the vicinity using the fluorescent microscope he brought. After catching various types of ants, Mr. Sanders immediately observed with a fluorescent microscope and found a microbiome dense enough to be described as 'a galaxy of microorganisms' in the intestines of some ants.

However, among the ants caught by Mr. Sanders, about two-thirds of them have almost no microbiome in their intestines. Although there were foods, debris, and cell walls of the intestines in the intestine, almost no intestinal bacteria that had been thought to coexist in the intestines of animals were found.

The image below shows the types of ants captured and analyzed by Mr. Sanders and the bacteria present in their intestines. The shining part shows bacteria, but it can be seen that many kinds of bacteria are shining like a galaxy, and some kinds of ants have almost no intestinal bacteria.

After bringing back the ants collected by Mr. Sanders to the laboratory and quantifying the amount of bacteria present in the body, the type of ants rich in bacteria in the body is 10,000 times more than the type of ants with a low amount of bacteria It was found to have a quantity of intestinal bacteria. Expanding these ants to human size means that some varieties carry one pound of bacteria, while others harbor as little as coffee beans. I will. “This is a huge difference,” says Sanders.

In 2017, Mr. Sanders

published a paper on this discovery, saying that herbivorous ants tend to have more intestinal bacteria, and carnivorous and omnivorous ants tend to have less intestinal bacteria. I will. However, even some herbivorous ants had few intestinal bacteria, and the relationship between diet and intestinal bacterial content was not consistent.

In recent years, many researchers believe that animals have a symbiotic relationship with bacteria in the body, but it was in the early 20th century that people became aware of the symbiotic relationship between animals and bacteria. Initially, there was little thought that 'every animal has abundant intestinal bacteria,' but eventually, 'all animals have a symbiotic relationship with bacteria, and if they do not have a symbiotic relationship, they cannot live well.' The idea became a majority. Opposing this trend, in 1953,

Paul Buchner , a leading researcher on symbiosis between animals and bacteria, repeatedly said that authors claim that symbiosis is the basic principle of all living things. doing.

'The human flora has put our ideas into the work of microorganisms,' said Tobin Hammer , an ecological and evolutionary biology researcher at the University of Texas at Austin . We tend to project our own examples to the outside. ' The human example does not necessarily apply to other animals, and Sarah Hurd , a microbiologist at the University of Connecticut , argued, 'The story is more complex and vague.'

Almost at the same time Sanders was investigating ants in Peru, Hummer was investigating the caterpillar flora in Costa Rica. However, Hamer, who was not able to detect the intestinal bacteria of the caterpillar, said, `` Finally, the caterpillars have stable intestinal bacteria, rather than failing to detect the intestinal bacteria after several months. It seems that there is no possibility. ” “This was a totally unexpected shift in thinking for me,” Hummer commented.

Eventually, a much smaller amount of bacteria was found in the intestines of caterpillars than expected, but these were only eaten with plants at the time of eating and digested in the intestines. And Hummer's research team conducted an experiment to eliminate bacteria inside the body of caterpillars with antibiotics. If the caterpillar had a close symbiotic relationship with the intestinal bacteria, eliminating the bacteria was expected to cause various problems, but it seems that the caterpillar had no effect, It has been shown that caterpillars do not have a symbiotic relationship with intestinal bacteria.

Deepa Agashe , a researcher of evolutionary biology at the National Center for Biological Sciences in Bengaluru , India, said that intestinal bacteria of butterflies and dragonflies collected in several places are strongly correlated with diet. I have found 'Most bacteria were in the intestine just because they were included in the diet.' 'These insects do not appear to be selecting a particular type of bacteria,' Agashe points out.

Similar to Hammer, Agashe repeated experiments to destroy the bacterial flora in the butterfly's body, but it did not affect the growth or development of the butterfly. For Agashe, who commented that 'the butterfly doesn't seem to care about its own flora at all,' it was a blind spot that there was no intestinal flora that co-exists with the host in the insect's body. It was a headache. This was an amazing result and it took me a while to understand. '

The symbiotic relationship between bacteria and animals is extremely complex, and bacteria do not always provide only benefits to the host animal. So, if they can produce the nutrients and enzymes they need, they may avoid the potential risk of animals coexisting with bacteria and not form an intestinal flora. I am. In addition, Agashe pointed out that it may be difficult for stable gut microbiota to form because butterflies and dragonflies depend on foods that vary according to location and season. There is also an opinion that there is an anatomical reason that bacteria cannot stably inhabit if the intestine is short.

Sanders pointed out that there is no single rule that governs the evolution of intestinal flora in animals, 'evolution is incredibly specific, and many organisms follow completely different pathways. It will evolve. ' Mr Hurd agreed, 'Most of our assumptions about the flora are based on studies in mammals. Mammals may be a rare case, and fish, birds, caterpillars, etc. You may not have it. ' There is a diversity in the gut flora even among mammals, Sanders said of the intestinal flora is not consistent with the bat research results have been announced. Regarding this point, Mr. Sanders thinks that it may be a disadvantage for bats flying in the sky to carry extra bacteria and increase weight.

Recent studies on gut microbiota suggest much can be learned by comparing animal species and problems with making premature assumptions about the symbiotic relationship between animals and bacteria. .. Since differences in intestinal microbiota are related to absorption and reaction of drugs, researchers may be more cautious about applying the results of experimenting specific animals as model organisms to humans. Points out. 'We need to keep our eyes and ears open. There's a lot to learn from the diversity of nature,' Sanders said.