Gas may be caused by an overgrowth of the unfriendly types of gut bacteria, often referred to as pathogenic or bad bacteria. Examples of this type of bacteria include Helicobacter pylori (H. pylori) and Escherichia coli (E. coli)
Most of us will have these strains of bacteria in our gastrointestinal (GI) tract, but the key is to maintain a balance of good to bad bacteria.
If the unfriendly bacteria are able to grow they can become more dominant and start to ferment some of the undigested foods that enter our intestines (particularly sugar) and cause gas and other gut symptoms.
The Lactobacillus and Bifidus species of bacteria are not gas-forming, so do not cause gas or flatulence. Sometimes, when people start taking additional friendly Lactobacillus and/or Bifidus bacteria, either via supplements or by eating more fermented foods, they experience gas initially and assume the friendly bacteria (probiotics) are causing the gas.
However, one of the ways these friendly bacteria work is by crowding out the pathogenic species and as they die-off, these bad bacteria cause fermentation and gas. As a result, symptoms of extra gas should start to diminish after a week or two of taking additional good bacteria, as gut bacterial balance begins to be restored.