Small Intestinal Bacterial Overgrowth (SIBO) is defined by an abnormal increase in the population of bacteria within the small intestine. While a dense concentration of bacteria is normal in the large intestine, the small intestine should maintain a low bacterial count. When this microbial balance shifts, it causes digestive symptoms such as bloating, abdominal pain, and chronic diarrhea. Because SIBO involves bacteria, many people mistakenly wonder if it can be spread. SIBO is not a communicable disease and is not transmissible between individuals.
Why SIBO Is Not Contagious
SIBO is an endogenous condition, meaning the bacteria responsible for the overgrowth are already natural inhabitants of the human body. The issue is not the introduction of a foreign pathogen but a disruption in the location and quantity of the body’s own microbes. These bacteria, which primarily belong in the large intestine, colonize the small intestine due to a failure of the body’s internal protective mechanisms. SIBO is a consequence of physiological dysfunction, not an infection caught from an external source.
The digestive tract has several built-in defenses that prevent bacteria from migrating upstream or multiplying excessively. The first is the strong acidity of stomach acid, which serves as a chemical barrier, killing most microbes before they reach the intestines. Another element is the Migrating Motor Complex (MMC), a wave-like muscular contraction that sweeps undigested food and bacteria from the small intestine into the colon during fasting.
A third defense is the ileocecal valve, a muscular sphincter that acts as a one-way barrier between the small and large intestines. This valve prevents the substantial bacterial load of the large intestine from refluxing backward. Since SIBO develops when these individualized mechanisms fail, the condition cannot be passed on through casual contact or any other form of person-to-person exposure.
Internal Triggers and Risk Factors
The development of SIBO results from failures in the digestive system’s protective functions. Impaired motility is a frequent trigger, often stemming from damage to the Migrating Motor Complex (MMC). This damage can result from chronic diabetes or post-infectious events, such as food poisoning that alters nerve function in the gut. When the MMC is compromised, the small intestine fails to clear food and bacteria efficiently, leading to stasis.
Structural or anatomical issues within the digestive tract can also predispose an individual to SIBO. Past abdominal surgeries, such as gastric bypass or procedures that create blind loops, alter the normal flow of intestinal contents. These changes form pockets where bacteria can collect and multiply unchecked. Conditions causing intestinal strictures, diverticula, or fistulas also impede the natural downward movement of matter.
A compromise in chemical or mechanical protective barriers increases the risk of SIBO. Low stomach acid (hypochlorhydria) reduces the initial line of defense against ingested bacteria. This can be caused by age-related changes or the long-term use of acid-suppressing medications, allowing more microbes to survive and reach the small intestine. When combined with a malfunctioning ileocecal valve, these factors allow large intestine bacteria to colonize the small bowel.
SIBO Versus Contagious GI Illnesses
The confusion about SIBO’s communicability likely arises from its symptoms, which can resemble those of genuine, transmissible gastrointestinal illnesses. It is important to distinguish SIBO, which is a chronic internal imbalance, from acute infectious gastroenteritis. Contagious illnesses are caused by exogenous pathogens, meaning they are acquired from an external source like contaminated food, water, or contact with an infected person.
Examples of contagious conditions include Norovirus, Salmonella, or Clostridium difficile infection. These diseases are characterized by the rapid onset of acute symptoms like vomiting and fever, and they spread through mechanisms such as the fecal-oral route. The disease process involves an external microbe invading the body and overcoming its defenses.
SIBO, by contrast, is not an invasion but an over-colonization by bacteria already present in the gut. The chronic, relapsing nature of SIBO differs from the acute, self-limiting course typical of infectious gastroenteritis. While SIBO involves bacteria, the mechanism of disease is localized to an individual’s internal physiological failure and is non-communicable.