Bacillus subtilis is a species of Gram-positive spore-forming bacteria found in the soil as well as in mammalian gastrointestinal tracts. Because B. subtilis is found naturally in the human gastrointestinal tract and is used to ferment foods, it is considered safe for human consumption. Several studies have shown that oral administration of B. subtilis spores prevents gastrointestinal diseases and there is great interest in identifying the bacterial molecule(s) that are important for protection.

Bacteria secrete and coat themselves in complex carbohydrate moieties called exopolysaccharides (EPSs). EPSs consist of repeating units of 4 to 7 sugar monomers and different bacteria make different types of EPSs. B. subtilis makes a large EPS that is more than 150 kDa in size. This EPS is 90% mannose and 10% glucose. B. subtilis strains that lack EPS do not prevent gastrointestinal inflammation in mouse models of inflammatory bowel disease, suggesting that EPS is the key probiotic molecule. Further, researchers have used flow cytometry to determine that B. subtilis EPS binds to the surface of macrophages.

EpsH is one of fifteen genes present in the operon needed for EPS synthesis and the function of the EpsH protein is unknown. Researchers incubated macrophages with fluorescently-labeled EPSs from either a wild-type or ΔepsH B. subtilis mutant lacking EpsH. The macrophages were then assessed by flow cytometry.
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Figure 1. Fluorescence intensity of macrophages exposed to fluorescent EPS combined with wild type and ΔepsH bacteria. Macrophages exposed to bacteria without any EPS were used as a control.
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In a subsequent experiment, nuclear magnetic resonance spectroscopy was used to determine the structure of EPS isolated from a ΔepsH mutant of B. subtilis as shown in Figure 2.
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Figure 2. EPS structure from a ΔepsH mutant. Man is mannose and Glu is glucose.

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