The Role of Bile Salts in Microbial Culture Media

News 8 5 月, 2025

In microbiology, especially when cultivating intestinal pathogenic bacteria, many culture media include an ingredient called bile salts. But how exactly are bile salts used in microbial culture, and what is their role?

Bile salts refer to the sodium or potassium salts of bile acids. Bile acids are secondary bile acids (derived from primary bile acids) conjugated with amino acids (glycine or taurine) to form glycocholic acid or taurocholic acid, which are types of conjugated bile acids. Bile salts, along with bile acids, cholesterol, and phospholipids, are components of bile.

Structurally, bile salts contain both hydrophilic groups (hydroxyl and carboxyl) and hydrophobic groups (methyl groups and hydrocarbon nucleus), giving them amphipathic properties. This dual-sided structure makes bile salts surface-active agents, allowing them to reduce surface tension between hydrophilic and hydrophobic phases.

Because bacterial cell surfaces typically carry a negative charge, bile salts (acting as cationic surfactants) adsorb onto the bacterial surface, causing damage to the cell membrane, promoting leakage of intracellular contents, and inducing enzyme and protein denaturation. Bile salts can even dissolve bacterial membranes, leading to cell death. This inhibitory effect is particularly significant against Gram-positive bacteria, while for Gram-negative bacteria, bile salts may induce remodeling of membrane polysaccharides.

Since bile salts naturally exist in the intestines of humans and animals (such as pigs and cattle), and because they inhibit Gram-positive bacteria but promote the growth of many Gram-negative bacteria, intestinal pathogens can tolerate bile salts. Therefore, bile salts are commonly added to selective media for detecting intestinal pathogens.

Examples include:

• Ox bile in Enteric Enrichment Broth (EE Broth)
• Bile salts No. 3 in Brilliant Green Lactose Bile Broth (BGLB)
• Bile salts in VRBA (Violet Red Bile Agar) for detecting coliforms in food testing
• Sodium deoxycholate in XLD Agar for isolating Salmonella and Shigella

Many other substances similar to bile salts are used in microbial culture to either promote or inhibit growth. By adjusting such components according to the target microorganism’s characteristics (e.g., acid-loving, salt-loving), microbiologists can create selective environments to isolate specific strains.