Animals and Microbes: Who Is Influencing Who?

Do we choose our microbes by our behavior? Or do the microbes choose and manipulate us by their behavior?

Our own eukaryote cells are the result of two microbes creating a working relationship (our cell and mitochondria). There is increasing evidence that these symbioses could be vital at every step of evolution. This is particularly true since gene transfer from microbes to other microbes and to larger creatures is ubiquitous. Bacteria are noted to use virus like particles to inject DNA sequences to others. In fact, it is hard to distinguish virus like particles from real viruses. Most recently, it was found that bacteria might create these viruses as a vector for this necessary transfer of material. The phage virus in the picture at left also transfers genes from bacteria to humans. 

When two creatures live together there is a gradual adjustment. At first, there are immune like responses to the alien molecules and toxins. In the case of our cells the two microbes gradually re-apportioned which DNA each will use going forward. The general rule of microbes is to give away any functions it can, to make existence easier with less baggage. Interestingly, a new symbiosis between microbes is just now occurring and is being observed in the laboratory.

A much more complex story exists between large animals and their symbiotic microbes.  All animals, including humans, are inundated, and completely surrounded, by microbe communities. Microbes fill any empty cavities. Usually, they are friendly but can become pathological.

How are the microbe communities for each animal determined? Is it because of food or behavior? Do the microbes choose? Do the animals choose? Or is it both?

Once established how do microbe communities affect the animals behavior.

Previous posts have shown surprising cognitive abilities in all levels of animals, and even in microbes. In plants, fungus has been shown to be vital to gather nutrients. A two-way dependence occurs where the fungus will supply nutrients but can be punished if not providing enough. How much choice is involved in microbe animal relationships?

Animal Behavior Affecting Choice of Microbe Communities

In elderly humans, perhaps the most important determinant of microbe communities is the diet. Also, one very recent study found that dietary lack of one amino inflammation acid from malnutrition greatly altered the microbe communities stimulating.

In the elderly gut there is a greater variation in the types of microbe colonies in those who are healthy and independent. Elderly nursing home patients had a limited repertoire of microbe communities that correlated with inferior health.

Other animals, also, first attract the microbes that will set up colonies inside them often through eating, but also through contact, such as the mandatory touching among hives of insects. Then the animal must deal with monitoring its immune reaction to the strangers onboard. Often the microbe communities release toxins that must be neutralized.

It is not clear how much an animal determines which microbes it will harbor.


One case that is suggestive of choice by the animal is the squid.  The squid’s use of bioluminescent microbes might be deliberate.

The squid use light to surprise other animals in the very dark regions of the ocean. One way they generate light is through using luminescent bacteria vibrio fisheri. Surprisingly, the squid accumulate intermittently glowing bacteria, and store it in a light organ. Somehow the light organ is able to coax the bacteria to light up at particular times, especially when camouflage is needed. They will not light up if not in the light organ. In one circumstance the squid adjust the blue luminescence based upon how much moon or starlight is present allowing a complete camouflage of the squid even near the surface. The bacteria benefit from this arrangement by getting abundant food.

Stink Bug or Kudzu Bug

The stinkbug (Megacopta punctatissima) ordinarily carries a specific symbiotic bacteria in a special cavity in the gut. Without the cadidatus ishikawaella capsulata’ the stinkbug would not grow and soon die. When the mother lays eggs she leaves a small capsule of the all-important bacteria next to the egg because the babies are both without any bacteria.  When hatched, the infant immediately eats the microbes in the capsule.

This behavior is very ingrained since, in experiments, if the sack is hidden, the bug will go in search for it and steal some from other nearby eggs.  Without the symbiotic bacteria the babies behavior changes and will soon die.

What factors are responsible for the behavioral alteration is unknown. But, somehow the stinkbugs know that they need to have the microbes.


Social contact is a major way that non-pathological beneficial microbes are shared among insects. Bumblebees must contact each other or some bee feces to establish the best microbe communities in their guts. Those bees who do not are attacked by a dangerous microbe and often die.

In another post it was discussed that bees use propolis for its antibiotic properties against fungus and other microbes.  Bees expend considerable time and energy to gather and prepare propolis, consisting of plant and tree sap and their own secreted wax. When fungi infect the hive, the bees somehow know this and bring in significantly more propolis. They can also distinquish harmful fungi from harmless types and bring in even more. They, also, specifically remove larvae that is infected with fungal spores

Other animals, such as Iguanas, also eat soil and feces to establish the best microbe communities.

Microbes Influencing Animal Behavior

Microbes are very clever at entering the bodies and brains of animals.

Once in the brain microbes can stimulate many different illnesses and can change the behavior of the host, often to their advantage. Aggressive biting behavior in dogs is caused by the rabies virus, which helps to spread the virus to other animals. A new book, Rabid, shows the dramatic behavioral changes that occur in human beings when infected.

Microbe effects in Human Beings

In humans’ a layer of microbes, ten microbes to each human cell, create a protective barrier against other microbes. A very complex communication occurs between the host cells and the microbes including providing absorption for vitamins and digestion for food.

Microbes fight autoimmune diseases, boost the immune system, help maintain proper weight and decrease effects of stress. Microbes on the skin protect through stimulating immune function. Gut microbes alter genes in the brain, which could account for some of the variations in medication effects. Signals from beneficial microbes allow human macrophages to have a better response to interferon, which are signals released when there are dangerous viruses, bacteria and cancer cells. Without these positive microbes the scavenger cells would have a greatly decreased ability to protect against these dangerous microbes.

Toxoplasmosis Gondi

While many microbes probably affect behavior, none has been more studied than Toxoplasmosis Gondi. It has dramatic effects on behavior in a variety of animals. Specifically, in humans, it has been shown to increase suicide and other mental illnesses.  There is research that points to T. Gondi in utero being a possible cause of schizophrenia.

T. Gondi dramatically affects mice, where it stimulates a fatal attraction for cats.  Because of the brain infection, the mice approach cats, which leads to their rapid death and leads to further spread of the T.Gondi into the cats.

BDNF and Depression

Previous posts (new brain cells) have mentioned the important brain trophic factor BDNF (brain derived neurotrophic factor), which has varied effects in many parts of the human brain, but in particular is critical for existing healthy neurons and making new neurons. An important new finding is that microbes in the gut send messages to the brains that influence BDNF.

It is known that when BDNF is low and creation of new brain cells in the hippocampus is diminished depression is triggered. Treatment of depression increases BDNF and increases new brain cells.

Now, a study has shown that microbes in the gut can influence BDNF in the brain and might be implicated in some forms of depression, especially in those with intestinal disorders. Also, the usual ways that the gut affects the brain were not involved in this effect.  These effects of brain and behavior from microbes in the gut were independent of the known signals from gut cells, along neurons, and through inflammation. It appears that the microbes must send specific signals through the blood to the brain to affect the BDNF

Microbe Effects In Other Animals


Fruit flies mating preferences are based on their diet and therefore their microbe communities. When given antibiotics the mating preferences were eliminated. Later re-introducing the microbes brought back the mating preferences.  It was the specific microbe communities of lactobacillus plantarum which create a certain hydrocarbon, not the diets that determined the choices.

In hyenas similar microbes producing specific scents inhabited all of the hyenas in specific tribes.

Spreading Disease


The fungus Cordyceps infects an insect’s brain stimulating the insect to climb to the top of a plant. Once at the higher altitude the fungus destroys the insect’s brain by growing a fungus bloom, or mushroom.  At this high vantage point, the sprouts from this bloom more effectively send spores over a larger area.


African malaria mosquito, anopheles gambiae are attracted to humans with specific microbes, and therefore specific scents. Humans who had a great diversity of microbes were not as attractive to the mosquitos.  Specifically if the humans had Pseudomonas and Variovorax the mosquitos were least attracted to those humans.

Many times there are thousands of different species of microbes on an animal.  Most of these have not been able to be studied until recently because they are not able to be cultured when not on the animal.  But, with the new microbiome wide genetic tests, the specific types of microbes can be determined..

The biofilms on regions in the ocean determine much of the behavior of other animals.  The settlement of different worms can be determined by the types of microbes.


An unusual situation occurs with the wasp Leptopilna boulardi that is a parasite.  When a virus LbFV is present the wasp changes it’s behavior whereby the wasp exhibits behavior of superparasitism which spreads the virus and creates danger for the wasp. A different bacteria Wolbachi has the opposite effect and protects agains the virus.  When the virus is active it uses the wasp to spread much more widely.


Mood is altered in mice when lactobacillus rhamnosus is used. The mice become more tranquil. Gut microbiome can influence stress, anxiety and depression related behavior.  Mice fed this mixture grew greater GABA receptors in the brain.  When the vagus was removed this effect disappeared meaning that the vagus transmitted this gut microbe information to the brain.

Sometimes behavior affects the microbes, sometimes microbes affect behavior, and there are times of feedback and interaction.


Since microbes are quite influential to the host and completely surround the host, it is important how it comes about that particular communities are present. Most of the interactions can be positive but can at any time turn negative.

Animals and microbe communities are two interacting cognitive systems. Do animals choose their microbe communities and manipulate their behavior? Or, do microbes choose their hosts and manipulate their behavior?


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