While this information will be new for most readers, this concept has been in use with farm animals for some time. When certain bacteria strains dominate or other strains are absent, animals gain weight faster. Knowing this aspect, Scientists decided to measure and see if the bacteria mix in obese versus slender people was different too. Yes, it was. ref
In studies on mice, Scientists isolated out the thin or obese type bacteria from humans, and then added them into mice with no intestinal bacteria. The mice which had the obese bacteria types gained more weight.
The theory goes like this: Some bacteria types are better at digesting the indigestible fibers in whole foods better than other bacteria types. Thus, more energy as calories would become available to the body to turn into fat and increase weight. ref
CAUTION: ref Which came first, the bacteria different strains, or did the diet (or antibiotics) modify intestinal chemistry and this change allowed different bacteria types to dominate? Plus, there is also the possibility that other functions of bacteria are involved, like inflammation reduction, or other immune activities. The ratio described here is vital too and may really be the deciding factor. ref
More studies are building the case. Here is one. This next one shows it works mostly in animals, with just a small effect on humans from bypass surgery. This will be a long process to determine since so many factors probably influence, like diet, genetics, exposure to bacteria, etc. But generally, as the body ages, it is bifidobacteria that is reduced while lactobacillus bacteria counts remain fairly constant. These two types are bacteria represent most of the probiotic supplements available, but there are many other bacteria types that influence human intestinal health as well. ref
The major takeaway point here is not mainly about bacteria and weight gain or loss, but that some probiotic bacteria have scientific research, usually the ones with strains codes listed after name, while others do not. One should use probiotics wisely. And that takes knowing the research about functions and dosages.
Limit intake to 20 billion and under. And then not for long periods without breaks. The idea is to use more PREBIOTICS to feed and establish your own indigenous bacteria strains. ref Some supplement strains can go to war with your native strains, but they generally are used to increase lactic acid and build up a favorable intestinal chemistry. It is this change that then allows for your native cultures to again gain dominance..
One bacteria study showed a 20% cold reduction for one billion and only a 10% reduction at 10 billion. Supplemented bacteria are just visitors and do not take up permanent residence. The clinically studied strains have identified functions they excel at minimizing any destructive actions.
One way that shows this effect is when antibiotics are given long term. Certain bacteria tend to survive antibiotics longer and thus gain acceptance as a weight gaining bacteria type, such as lactobacillus reuteri.
Lactobacillus gasseri tends to have the opposite effect.