I'm not very sure about this to be honest. I researched earlier and couldn't really find much besides reports talking about commensalist relationships with certain bacteria. Does anyone have any input in this? Generally, I think it's false, but I might be mistaken. Thanks in advance!

Edited March 24, 20232 yr by hey

From the net: "There are about 2 kilograms of bacteria in the human body, forming the so-called physiological flora."

"Because of their small size, however, microorganisms make up only about 1 to 3 percent of the body's mass (in a 200-pound adult, that's 2 to 6 pounds)".

 

If they weigh 2 kg, they have to eat something that the human ate, so the human has to eat more to compensate for the loss.

A person with more mass must expend more energy to accelerate and move.

Microorganisms excrete metabolic products too.

Edited March 24, 20232 yr by Sensei

1 hour ago, Sensei said:

If they weigh 2 kg, they have to eat something that the human ate, so the human has to eat more to compensate for the loss.

That's not necessarily true. With many animals, the microorganisms perform some important functions. Helping with digestion for instance. They can transform foods that the animal can't process into substances they can use. Many animals need to acquire the beneficial bacteria from their parents when they are young in order to thrive. 

Also, in a warm blooded animal, the bacterial action produces heat, which contributes to the overall warming of the animal. You would have to read up on it to find out how much of this applies to humans. 

17 minutes ago, mistermack said:

That's not necessarily true. With many animals, the microorganisms perform some important functions. Helping with digestion for instance. They can transform foods that the animal can't process into substances they can use. Many animals need to acquire the beneficial bacteria from their parents when they are young in order to thrive. 

You are talking about mutualism.

https://en.wikipedia.org/wiki/Mutualism_(biology)

"Mutualism describes the ecological interaction between two or more species where each species has a net benefit."

 

https://en.wikipedia.org/wiki/Gut_microbiota

"The relationship between some gut microbiota and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship."

 

Edited March 24, 20232 yr by Sensei

5 hours ago, Sensei said:

You are talking about mutualism.

True. But it's such a grey area that you would need a study by experts to differentiate accurately for each microorganism. While it looks like something gives no benefit to the host, there might be a bonus that we can't see.

Just by being there, a 'useless' looking bacterium might be keeping more harmful varieties under control. (for example) So the answer to the OP is likely to be different in every case, and quite often non-obvious or extremely subtle. 

4 minutes ago, mistermack said:

Just by being there, a 'useless' looking bacterium might be keeping more harmful varieties under control. (for example) So the answer to the OP is likely to be different in every case, and quite often non-obvious or extremely subtle. 

In microbiology one of the criteria is often the exchange of nutrients. While control of other bacteria is a benefit, it is usually not described as a mutualistic interaction. Rather the interaction between the bacteria would be characterized as amensalism (e.g. one bacterium harms a pathogenic one). I suspect in an ecological research context these interactions could be described as mutualistic, but not sure about that.

Many "stowaways" begin to be harmful when the host's immune system weakens due to age or illness, such as e.g. HIV/AIDS. Normally harmless microbes begin to cause harm to their host..