Small Intestinal Bacterial Overgrowth (SIBO) – Nature, Symptoms, Diagnosis, and Treatment

Small intestinal bacterial overgrowth (SIBO) represents a **disordered microbial ecology** within the small bowel, characterized by **excessive bacterial colonization**—both in terms of quantity and atypical species—within a segment where microbial presence is normally minimal. The gastrointestinal microbiota can be categorized into three primary groups: **commensal beneficial strains** (e.g., *Lactobacillus plantarum*, *Bifidobacterium breve*), **opportunistic pathogens with conditional virulence** (such as *Escherichia coli* or *Streptococcus salivarius*), and **frankly pathogenic microorganisms** (including *Clostridium botulinum*). This delicate balance is maintained through multiple physiological safeguards, including the **acidic gastric barrier** (pH range of 1.5–3.5), **digestive enzyme activity** (pepsin, pancreatic lipase), and the **bacteriostatic effects of bile acids**. Disruptions to these mechanisms—such as **hypochlorhydria**, **impaired intestinal motility**, or **ileocecal valve incompetence**—predispose the small intestine to colonization by colonic-type bacteria. The **core pathophysiological defect in SIBO** involves dysfunction of the **migrating motor complex (MMC)**, a **cyclical, interdigestive peristaltic activity** designed to **clear the small bowel of residual nutrients and bacteria** between meals. The **Phase III MMC**, lasting approximately 5–10 minutes every 90–120 minutes, generates powerful propagating contractions to expel luminal contents. Impairment of this process leads to **intestinal stasis** and subsequent bacterial proliferation. Clinically, SIBO presents with a **heterogeneous spectrum of symptoms**, predominantly **gastrointestinal in nature**, including bloating, diarrhea, constipation, abdominal pain, and food intolerances. Due to **significant overlap with irritable bowel syndrome (IBS)**, diagnostic clarification often requires **hydrogen-methane breath testing (lactulose/glucose challenge)** or **small bowel aspiration with quantitative culture** to confirm microbial overgrowth.