WASHINGTON – Antibiotics given in advance of gastric bypass surgery preferentially alter the microbiome, nudging it toward a more “lean” physiologic profile.
Given before a sleeve gastrectomy, vancomycin, which has little gut penetration, barely shifted the high ratio of Firmicutes to Bacteroidetes, a profile typically associated with obesity and insulin resistance. But cefazolin, which has much higher gut penetration, suppressed the presence of Firmicutes, which metabolize fat, and allowed the expansion of carbohydrate-loving Bacteroidetes – a profile generally seen in lean people.
Cyrus Jahansouz, MD, of the University of Minnesota, Minneapolis, and his colleagues wanted to examine whether a shift in preoperative antibiotics might affect the way the microbiome re-establishes itself in the wake of vertical sleeve gastrectomy. They enrolled 32 patients who were candidates for the procedure. None had undergone prior gastrointestinal surgery, and none had been exposed to antibiotics in the 3 months prior to bariatric surgery. They were similar in age, weight, body mass index, and fasting glucose. The mean HbA1c was about 6%.
Patients were randomized to three groups: maximal diet therapy (800 calories per day) without surgery; vertical sleeve gastrectomy with the usual preoperative antibiotic cefazolin and the postsurgical diet; and vertical sleeve gastrectomy with preoperative vancomycin and the postsurgical diet. All patients gave a fecal sample immediately before surgery and another one 6 days after surgery.
Preoperative cluster analysis of bacterial DNA showed that all of the samples had a similar composition, predominated by Firmicutes species (60%-70%). Bacteroidetes species made up about 20%-30%, with Proteobacteriae, Actinobacteriae, Verrucomicrobia, and other phyla comprising the remainder of the microbiome.
At the second sampling, the diet-only group showed no microbiome changes at all. The vancomycin group showed a very small but not significant expansion of Bacteroidetes and reduction of Firmicutes.
Patients in the cefazolin group showed a significant shift in the ratio – and it was quite striking, Dr. Jahansouz said. Among these patients, Firmicutes had decreased from 70% to 40% of the community. Bacteroidetes showed a corresponding shift, increasing from 20% of the community to 45%. The findings are quite surprising, he noted, considering that only one dose of antibiotic was associated with the changes and that they were evident within just a few days.
Although “a little hard to interpret” because of its small size and short follow-up, the study suggests that antibiotic choice might contribute to the success of weight-loss surgery, Dr. Jahansouz said at the annual clinical congress of the American College of Surgeons.
“There are still several factors in the perioperative period that we have to study to be able to identify what other things might have also influenced the shift,” he said in an interview. “But I do think that, in the future, these changes can be manipulated to benefit metabolic outcomes.”
Two phyla – Bacteroidetes and Firmicutes – dominate the human gut microbiome in a dynamic ratio that is highly associated with the way energy is extracted from food. Bacteroidetes species specialize in carbohydrate digestion and Firmicutes in fat digestion. “In a lean, insulin-sensitive state, Bacteroidetes dominates the human gut microbiome,” Dr. Jahansouz said. “With the progression of obesity and insulin resistance, there is a subsequent shift in the microbiome phenotype, favoring the growth of Firmicutes at the expense and reduction of Bacteroidetes. This is a significant change, because this obesity-associated phenotype has an increased capacity to harvest energy. It’s not the same for a lean person to consume 1,000 calories as it is for an obese person to consume them.”
Bariatric surgery has been shown to alter the gut microbiome, shifting it toward this more “lean” profile (Cell Metab. 2015 Aug 4;22[2]:228-38). This shift may be an important component of the still not fully elucidated mechanisms by which bariatric surgery causes weight loss and normalizes insulin signaling, Dr. Jahansouz said.
Dr. Jahansouz is following this group of patients to explore whether there are differences in weight loss and insulin signaling. He also will track whether the microbiome stabilizes at its early postsurgical profile, or continues to shift, either toward an even higher Bacteroidetes to Firmicutes ratio, or back to a more “obese” profile.
He and his colleagues are also investigating the effect of antibiotics and gastric bypass surgery in mouse models. “I can say that antibiotics seem to have a remarkable impact on the effect of mouse sleeve gastrectomy. We’re not quite there yet with humans,” but the data are compelling.
Dr. Jahansouz said that he had no financial disclosures.
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