A study demonstrated Fecal Microbiota Transplantation (FMT) alone can change a skinny mouse into a fat one is detailed in the research published by the journal Science. In this study, researchers transplanted gut bacteria from human twins discordant for obesity into germ-free mice. The mice that received gut bacteria from obese twins grew fat, while those that received bacteria from lean twins remained lean. This experiment provided compelling evidence that gut microbiota can influence body weight and adiposity independently of diet or other factors. Visual below of mice feed identical diet.

My own experience is loosing 30 pounds from the addition of a specific probiotic (Akkermansia) over a year without a change of diet. IMHO, a change of diet along may not do it. Often it takes two things: changing the diet AND changing the bacteria in the gut.

Literature

Probiotics that have the best actual evidence

Note that some probiotics can result in weight gain, so taking random ones is not the way to do it. Many of the studies found effective using probiotics mixtures included Lactobacillus plantarum

• Use of probiotics in preventing and treating excess weight and obesity. A systematic review “the use of probiotics in the absence of dieting produced a significant reduction in body weight and body mass index in 66.6% of the reviewed studies”
• Akkermansia muciniphila Suppresses High-Fat Diet-Induced Obesity and Related Metabolic Disorders in Beagles.
• Effects of oral Akkermansia muciniphila supplementation in healthy dogs following antimicrobial administration.
• Akkermansia and Microbial Degradation of Mucus in Cats and Dogs: Implications to the Growing Worldwide Epidemic of Pet Obesity.
• The Role of Lactobacillus plantarum in Reducing Obesity and Inflammation: A Meta-Analysis. “The analysis of nine studies revealed significant weight reduction and BMI decreases with L. plantarum supplementation compared to a placebo”
• The Effect of the Lacticaseibacillus paracasei BEPC22 and Lactiplantibacillus plantarum BELP53 Combination (BN-202M) on Body Fat Percentage Loss in Overweight Individuals: A Randomized, Double-Blind, Placebo-Controlled Study. “Akkermansia showed a statistically significant with these probiotics”
• Lactobacillus paracaseiN1115 attenuates obesity in high-fat diet-induced obese mice.

Diabetes

While many studies show promise, the evidence is still mixed, and more long-term research is needed to determine the most effective probiotic strains and protocols for diabetes management [Probiotics Contribute to Glycemic Control in Patients with Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis] – study was pre-Akkermansia availability.

• A Critical Perspective on the Supplementation of Akkermansia muciniphila: Benefits and Harms.
  “Accumulating evidence indicated Akkermansia as a promising therapeutic probiotic against metabolic disorders such as obesity, type 2 diabetes and cardiovascular diseases.”

• Akkermansia muciniphila and Gut Immune System: A Good Friendship That Attenuates Inflammatory Bowel Disease, Obesity, and Diabetes.
• Review of Literature on Akkermansia muciniphila and its Possible Role in the Etiopathogenesis and Therapy of Type 2 Diabetes Mellitus.
• Molecular Mechanism of Pasteurized Akkermansia muciniphila in Alleviating Type 2 Diabetes Symptoms.
• Live and pasteurized Akkermansia muciniphila ameliorates diabetic cognitive impairment by modulating gut microbiota and metabolites in db/db mice.
• Akkermansia muciniphila: a potential candidate for ameliorating metabolic diseases.
• Function of Akkermansia muciniphila in type 2 diabetes and related diseases.
• Health and Disease: Akkermansia muciniphila, the Shining Star of the Gut Flora.
• [Research Progress on the Role of Akkermansia Muciniphila in Prevention and Treatment of Diabetes Mellitus].

And

• Ameliorative Effects of Lactobacillus paracasei L14 on Oxidative Stress and Gut Microbiota in Type 2 Diabetes Mellitus Rats.
• Anti-Diabetic Effect of Lactobacillus Paracasei Isolated from Malaysian Water Kefir Grains.
• Screening of Potential Probiotic Lactobacillaceae and Their Improvement of Type 2 Diabetes Mellitus by Promoting PI3K/AKT Signaling Pathway in db/db Mice.
  •  Lactiplantibacillus plantarum
  • Limosilactobacillus fermentum
• Heat-Killed Lactiplantibacillus plantarum LRCC5314 Mitigates the Effects of Stress-Related Type 2 Diabetes in Mice via Gut Microbiome Modulation.
• Effects of Lactobacillus plantarum supplementation on glucose and lipid metabolism in type 2 diabetes mellitus and prediabetes: A systematic review and meta-analysis of randomized controlled trials.
• Effect of postbiotic Lactiplantibacillus plantarum LRCC5314 supplemented in powdered milk on type 2 diabetes in mice.
• Lactobacillus paracaseiN1115 attenuates obesity in high-fat diet-induced obese mice.
• A Potential Synbiotic Strategy for the Prevention of Type 2 Diabetes: Lactobacillus paracasei JY062 and Exopolysaccharide Isolated from Lactobacillus plantarum JY039.

Bottom Line

My (incomplete) review of the many many studies suggests that four species of probiotics are the most likely to help with Obesity and Diabetes. Below are links to manufacturers that directly sell my preferred single strain probiotics. This means that you will get the probiotics within weeks (or days) of being manufactured (i.e. Very fresh and live probiotics).

• Akkermansia muciniphila
• Lactiplantibacillus plantarum aka Lactobacilllus plantarum
• Limosilactobacillus fermentum aka Lactobacilllus fermentum
• Lactobacillus Paracasei

Suggested daily dosage is a therapeutic 50 BCFU/day for the Lactobacillus, and likely a capsule of Akkermansia muciniphila (Pendulum is the established provider).

For the Lactobacillus, I find using powder dissolved in a glass of warm water works very well. It often has a side effect of inhibiting wanting to have a treat and gives a satisfied feeling in the gut. Taking them as capsules do not seem to have that effect.