International Journal of Nutrition, Pharmacology, Neurological Diseases

: 2020  |  Volume : 10  |  Issue : 3  |  Page : 99--104

The Effect of Bariatric Surgery on the Composition of the Gut Microbiota Community: an Imbalance Associated with Severe Disorders

Elham El Darazi1, Yonna Sacre2, Elias El-Khoury3, Afif M Abdel Nour4,  
1 Faculty of Arts and Sciences, The Holy Spirit University of Kaslik USEK, Lebanon
2 Faculty of Arts and Sciences, The Holy Spirit University of Kaslik, Jounieh, Lebanon
3 Department of surgery, Central Military Hospital, Beirut, Lebanon
4 School of Engineering, The Holy Spirit University of Kaslik, Lebanon

Correspondence Address:
Elham El Darazi
Faculty of Arts and Sciences, The Holy Spirit University of Kaslik USEK


Obesity has become an imminently serious public health problem as nearly 40% of the adult population worldwide is obese. Bariatric surgery is considered the most efficient weight loss treatment by mechanically restricting caloric intake. However, evidence proposes a strong association between weight loss and the alteration of the gut microbial composition after bariatric surgery. Modifications in the concentration of gut microbial bacteria affect human health by either increasing or reducing the likelihood of diseases associated with specific bacteria. In this review, we discuss the effect of bariatric surgery on the gut microbial composition in patients. Additionally, we detail the modifications in the abundance of specific phylum and species in the intestinal microbiota after bariatric surgery. Different disorders associated with the increase or decrease of a particular bacterium are mentioned in order to evaluate the impact of the alteration of post-surgery intestinal microbial structure on individuals’ health.

How to cite this article:
Darazi EE, Sacre Y, El-Khoury E, Abdel Nour AM. The Effect of Bariatric Surgery on the Composition of the Gut Microbiota Community: an Imbalance Associated with Severe Disorders.Int J Nutr Pharmacol Neurol Dis 2020;10:99-104

How to cite this URL:
Darazi EE, Sacre Y, El-Khoury E, Abdel Nour AM. The Effect of Bariatric Surgery on the Composition of the Gut Microbiota Community: an Imbalance Associated with Severe Disorders. Int J Nutr Pharmacol Neurol Dis [serial online] 2020 [cited 2022 Aug 7 ];10:99-104
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Full Text


Obesity is one of the leading public health concerns worldwide.[1] Bariatric surgery is the most efficient method for weight loss, as it results in dramatic decrease in food intake.[2] Besides the complications and comorbidities developed post-surgery, functional and taxonomic modifications were reported in the gut microbiota. Gut microbiota, which are the microorganisms colonizing the digestive tract, have a major role in the storage of energy and mechanism of obesity.[3],[4],[5] Major modifications in the concentration of particular gut microbiota were described after bariatric surgery.[6]


In recent years, the increasing incidence of overweight adults and obesity was described as an epidemic problem.[1],[7] In 2015 the World Health Organization (WHO) estimated 2.3 billion adults were overweight and 700 million obese. WHO defines obesity as an excessive or abnormal fat accumulation with a serious impact on personal health.[8] Body mass index (BMI) was invented as a simple and effective way to define levels of obesity. A healthy BMI ranges from 18.5 to 24.9 kg/m2, overweight from 25 to 29.9 kg/m2, and obese is 30 kg/m2 or greater.[9]

Bariatric surgery

Bariatric surgery is a highly effective therapy for obesity, yielding sustained weight loss, improvement of quality of life and reduction of obesity-related comorbidities and mortality.[10] Three classes of bariatric surgery exist: first, restrictive methods, such as laparoscopic adjustable gastric banding (LAGB), sleeve gastrectomy (SG), and vertical banded gastroplasty (VBG); second, malabsorptive techniques including biliopancreatic diversion (BPD) and jejunoileal bypass (JIB); third, roux-en-Y gastric bypass (RYGB) and duodenal switch (DS) with BPD, which combines restrictive and malabsorptive interventions.[11] Bariatric surgery not only reduces food intake and digestion but also alters food preferences, bile acids (BAs) metabolism and gastric emptying time, allowing an efficient weight loss.[11],[12] Adversely, patients often develop multiple post-surgery complications in the short and long term, ranging from pulmonary embolism and deep venous thrombosis to nutritional deficiencies, marginal ulcers, stromal stenosis, internal hernias, and dumping syndrome.[13]

Gut microbiota

Gut microbiota are enteric microorganisms, numbering almost 15000 species of bacteria in variety, which colonize the intestines.[14] Bacteria are divided into six primary phyla that are the following Proteobacteria, Firmicutes, Bacteroidetes, Verrucomicrobia, Fusobacteria, and Actinobacteria.[15] Bacteriodetes and Firmicutes phyla are the main constituents of the gut microbiota implicated in metabolic, immunological and physiological activity in the human body.[16] Gut microbes are normally beneficial to the host and ensure immune functions,[17] the protection against enteropathogens[18],[19] and the extraction of energy and nutrients from the host’s diet.[20],[21] Multiple factors can alter the symbiotic balance between the host and the gut microbiota, such as malnutrition,[22] neurological diseases,[23] cancer[24] and, most particularly, obesity.[25],[26] Modifications in the intestinal microbiota are responsible for a number of diseases discovered so far such as allergies, type 2 diabetes, and inflammatory bowel diseases.[13],[27]

Gut microbiota after bariatric surgery

Obese patients subjected to bariatric surgery often develop various physiological and behavioral abnormalities. One of those abnormalities is the significant alteration of the complex intestinal microbial composition, which subsequently adapts in order to maintain weight loss.[3],[28] Reduction of food intake, food types, and nutrient malabsorption are the main causes of gut microbial modifications.[2] Moreover, alterations in pH,[29] bile acids,[30],[31] hormones,[32],[33] and serum leptin levels are responsible for modifications of post-surgery gut microbiota diversity.[34],[35],[36] Gut microbiota diversity after bariatric surgery has always been a controversial subject as research results have been limited. Four consistent findings were reported showing that post-surgery gut microbiota benefit from a high diversity based on the Shanon index.[37],[38],[39],[40] Differential intestinal microbiota signatures were identified by comparing the microbiota profile between subjects before and after bariatric surgery. These studies evaluate the impact of weight-loss surgery on the composition of gut microbiota. Most studies to date have focused on RYGB’s influence on the gut microbiota composition and function as it is the most common bariatric surgery, whereas little research has been done concerning other types of bariatric surgery.[4],[37],[40] Due to their role in causing infections or worsening inflammatory diseases, a link between the increased and reduced bacteria in the intestinal microbiota of patients subjected to bariatric surgery and their associated diseases needs to be established. Studies that clinically prove the correlation between post-surgery gut microbial composition, and inflammatory and metabolic diseases are lacking currently.

Primary species of bacteria changes after bariatric surgery

In the late decade, many studies have been conducted in order to profile the gut microbiota after bariatric surgery with the aim of evaluating the impact of bariatric surgery on patients’ health. Research has shown wide changes in microbial composition after bariatric surgery. A number of phyla have been shown to increase after surgery while other types of bacteria decreased. Metagenomic data evaluating the changes before and after RYGB identify 1061 species, 729 genera, and 44 phyla.[41] Palleja et al.[37] also demonstrated the increase in gut microbial diversity after bariatric surgery with an increase in the abundance of 31 species.


Proteobacteria known as the “microbial signature” of disease was highly increased after bariatric surgery.[37],[42] While a minor population in healthy gut flora, Proteobacteria populations were shown to be increased post-surgery, especially Enterobacter hormaechei, Salmonella enterica, Enterobacter cancerogenus and Scedosporium boydii.[41],[43] High concentration of Proteobacteria reflects an imbalance in the microbiota representing a diagnostic signature for dysbiosis, which is widely implicated in metabolic disorders.[42] The high concentration of Proteobacteria is also associated with a high risk of diseases such as asthma,[44] inflammatory bowel diseases (IBD),[45] and colorectal carcinomas.[41] Moreover, cystic fibrosis and scedosporiosis can be caused by Scedosporium boydii. These results suggest that a high post-operative concentration of Scedosporium boydii can lead to fatal infections.[46] A common association between Salmonella enterica and a set of invasive and noninvasive infections was reported.[47],[48] Enterobacter cancerogenus can also cause pneumonia, wounds, and bronchial asthma according to a clinical study.[49] Further studies are needed in order to identify the impact of high concentrations of Scedosporium boydii and Enterobacter cancerogenus on post-operative patients’ health.

The number of Gamma Proteobacteria increase after SG,[38] RYGB,[50] and DJB[51] surgeries, and facilitate stabilized postoperative weight loss . Escherichia coli among them, the main cause of childhood diarrhea,[52] could give rise to different diseases including meningitis, wound infections, atherosclerosis, and immunological diseases (rheumatoid arthritis).[37],[53] A high concentration of Escherichia coli not only promotes diarrheal diseases but also threatens patients’ health by adhering to intestinal cells, altering neurological electrolyte transport.[53] Klebsiella pneumoniae, a photogenic strain detrimental to public health, is a potential cause for multiple infectious diseases, such as those of the blood and respiratory and urinary systems. For this reason, a high concentration of Klebsiella pneumonia in obese patients after bariatric surgery is a negative prognostic affecting a patient’s quality of life.[54] Moreover, Shigella Boydii, the main cause of shigellosis or bacillary dysentery,[55],[56] was shown increase after surgery.[4] Citrobacter braakii, which is capable of causing infections as well as bacteremia, was reported as increased after surgery [Table 1].[4],[57]{Table 1}


A low concentration of Firmicutes was reported to increase the incidence of inflammation and colorectal cancers.[41] Firmicutes’ abundance was reported to decrease after bariatric surgery, especially for Dorea, Anaerostipes caccae, Coprococcus comes, Blautia, and F. prausnitzii.[39],[58] Interestingly, Blautias’ concentration was correlated with a severe clinical stage and histoprognostic grade in subjects with breast cancer, suggesting a role in estrogen metabolism.[59] A low concentration of Blautia in the post-intervention state can be a good prognostic. A case study by Workneh et al.[60] demonstrated Anaerostipes caccae, for the first time identified in 2002, as a cause of infection; the exact effects of a low post-surgery concentration of A. caccae remains unknown. Moreover, the decreased concentration of Coprococcus comes post-bariatric surgery can be a preventive factor since it was defined as one of the causes of Crohn’s disease. A study showed Coprococcus comes may have long-term effects due to its capacity to bind IgG through the Fab portion, which suggests a possible role in Crohn’s disease.[61] The relationship between increases in these bacteria and the progression of inflammatory and metabolic disorders is yet to be studied. It should be noted that a high Firmicutes/Bacteroidetes ratio was detected in obese patients compared to healthy patients, which explains the reduced concentration of Firmicutes after weight-loss surgery.[62],[63]

On the other hand, a significant post-surgery increase of Veillonella parvula (V. parvula) and Veillonella dispar (V. dispar) was demonstrated.[37],[41] V. parvula may contribute to multiples infections such as meningitis, osteomyelitis, endocarditis, peritonitis and sepsis.[64],[65],[66],[67],[68],[69],[70] An increased abundance of V. parvula after surgery may be a serious risk not only because of strong infections, but also its crucial role in causing Epidural abscess, a potentially life-threatening disease.[71] Moreover increased concentration of V. dispar was associated with an increase chance of intestinal infections especially for Crohns’ disease.[72] Likewise, Streptococcus gordonii and Enterococcus faecalis concentrations were found to increase after RYGB significantly impacting patients’ health.[37] As the main cause of thoracic empyema, high level of Streptococcus gordonii can be a risk factor in worsening various infections.[73] Enterococcus faecalis is linked to a set of infections such as meningitis, endocarditis, urinary tract infections, bacteremia, and root canal infections.[74] The post-surgery change of Enterococcus faecalis is indicative of a serious life-threatening risk. In addition, Faecalibacterium prausnitzii, with reduced post-surgeryconcentration, especially after RYBG, was seen as a major factor of inflammation and glucose homeostasis in obese persons.[75] The post-surgery concentration of Clostridia decreased, which is a good prognostic for patients, as it produces a high number of toxins related to dangerous diseases.[76],[77],[78] A reduced concentration of Ruminococcus, which correlates with inflammatory bowel disease, was shown in patients after bariatric surgery.[39],[79] Out of these two phyla, Akkermansia, a microbe with probiotic properties, was increased after bariatric surgery.[77],[80]


Bariatric surgery is the most effective weight loss method for severe obesity. Multiple changes occur in the gut microbiota after surgery. Association between modifications of gut microbial composition and the outcome or disappearance of metabolic and inflammatory illness needs to be empirically established. Specific disease diagnostics and prognostics, along with novel therapeutic strategies, can be developed after such correlations are proven. More clinical studies about the health effect of changes in gut bacteria after bariatric surgery would be helpful.


All authors contributed equally to the writing of this article.

Financial support and sponsorship


Conflicts of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.


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