Do probiotics actually reach the large intestine?
In my journal search over the weekend, I came across an article with the intriguing title “Lactobacillus plantarum passage through an oro-gastro-intestinal tract simulator,” which was very recently published in the journal Microbiological Research. Given our recent bonus submission from Sandra, I thought this article might help to address whether or not orally ingested organisms actually survive passage through the stomach in any significant numbers. The digestive tract has a number of very potent defenses against growth of microbes, including the very low pH of the stomach, the action of bile salts secreted into the small intestine, plus the action of digestive enzymes mainly by the exocrine pancreas. Any microorganisms that survive the action of these defenses must be significantly resistant to them. Some of these defenses include bacterial proteins which repair proteins that become damaged, enzymes such as catalase that help to detoxify reactive oxygen compounds used to destroy bacteria, and membrane protein pumps that counteract decreases in pH. Additionally, ingested compounds known as prebiotics can also protect probiotic microorganism during their transit through the gastrointestinal tract to the colon.
This current study reports the viability of one probiotic bacterium, Lactobacillus plantarum, after being exposed to conditions that mimic the oral cavity, the stomach, and the small intestine. L. plantarum is a commonly found inhabitant of the mammalian colon, and is also frequently found in many fermented foods. A number of lactic acid bacteria also are found in a variety of foods marketed as probiotics. Bacteria in this study were grown to mid-exponential phase, and resuspended into a carrier matrix. Various carrier matrices included: 10% reconstituted skim milk, saline solution, ordinary pasta and barley glucan-enriched pasta. Simulation of the various oro-gastro-intestinal environments was by mixing matrix embedded bacteria as follows:
- oral cavity–5 minutes in the presence of lysozyme, pH 6.5
- gastric environment–25 minutes in the presence of lysozyme plus pepsin, pH 5.0, followed by duodenal stress with 85 minutes in the presence of pancreatin plus bile salts, pH 6.5
- gastric compartment–65 minutes in the presence of lysozyme plus pepsin, pH 3.0, followed by duodenal stress with 125 minutes in the presence of pancreatin plus bile salts, pH 6.5
Following each of the treatments, bacteria were recovered, and gene expression levels were measured by polymerase chain reaction amplification of messenger RNA. The researchers found that many of the genes involved in stress responses were rapidly turned on by exposure to the simulated gastric environment, further supporting their role in protecting cells from a harmful environment. The most potent stimulator of the stress response genes was the low pH of the gastric compartment. Furthermore, increased survival of L. plantarum was observed when organisms were exposed to this environment in the presence of complex or nutrient-rich matrices, also supporting their role in facilitating the successful transit of the bacteria to the colon.
The take home message from this study is that L. plantarum possesses important defenses that enable it to the environmentally hostile areas of the mammalian digestive tract, which can enable it to reach the more amenable region of the lower colon. Poorly digestible food additives (complex plant carbohydrates) can aid in the ability of the organism to more effectively colonize the colon. The study also opens up the potential for using the gene sequences identified here as markers for the screening of other bacterial species that might have potential suitability in probiotics.