Lactobacillus sanfranciscensis (formerly L. sanfrancisco) is a species of lactic acid bacteria

Lactic acid bacteria

The lactic acid bacteria comprise a clade of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and lactic products, produce...

 that helps give San Francisco sourdough bread its characteristic taste.

Sourdough starters are leavened by a mixture of yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 and lactobacilli in a ratio of about 1:100. The yeast is most commonly Candida milleri

Candida milleri

Candida milleri is a species of yeast in the genus Candida. It is used, along with the bacterium Lactobacillus sanfranciscensis, in the production of sourdough bread.-See also:*Candida *Sourdough*Lactobacillus sanfranciscensis...

. This yeast cannot metabolize the maltose

Maltose

Maltose , or malt sugar, is a disaccharide formed from two units of glucose joined with an αbond, formed from a condensation reaction. The isomer "isomaltose" has two glucose molecules linked through an α bond. Maltose is the second member of an important biochemical series of glucose chains....

 found in the dough, while the Lactobacillus needs maltose. They therefore act without conflict for substrate

Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...

, with the Lactobacillus utilizing maltose and the yeast utilizing the other sugars, including the glucose produced by the Lactobacillus. The lactobacilli produce an antibiotic

Antibiotic

An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

, cycloheximide

Cycloheximide

Cycloheximide is an inhibitor of protein biosynthesis in eukaryotic organisms, produced by the bacterium Streptomyces griseus. Cycloheximide exerts its effect by interfering with the translocation step in protein synthesis thus blocking translational elongation...

, which kills many organisms but not Candida spp.; Candida tolerates the lactic acid produced by the lactobacilli.

For commercial use, specific strains of the L. sanfranciscensis are grown on defined media

Growth medium

A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....

, freeze-dried, and shipped to bakeries worldwide.