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C4478 Sigma-Aldrich

S-Gal®/LB Agar Blend

reagent for selection of recombinant bacterial clones




Ingredients (g/L)
Tryptone, 10
Yeast extract, 5
Sodium chloride, 10
Agar, 12
S-Gal, 0.3
Ferric ammonium citrate, 0.5
IPTG, 0.03

Features and Benefits

• More intense color contrast than X-gal
• Water-soluble and autoclavable for easiest use
• Convenient, pre-mixed media

General description

S-Gal® (sodium salt) is an autoclavable, water-soluble, chromogenic substrate for β-galactosidase, used to determine the presence or absence of a cloned DNA insert in bacteria growing on agar plates. S-gal® is designed to replace X-Gal in blue-white selection of recombinant bacterial colonies with the lac+ phenotype.

Other Notes

The ferric or Fe3+ ion is required for color development and must be added to any S-Gal®
formulation. A medium prepared with S-Gal® is moderately dark due to the presence of ferric ammonium citrate. This darker background often provides enhanced contrast for automated colony counting or isolation.


When S-Gal® is cleaved by β-galactosidase, the resulting product will chelate ferric ion to create a black, insoluble precipitate. Lac+ colonies grown in the presence of S-Gal® and ferric ion turn an intense black color, allowing for easy differentiation between lac+ and lac- colonies.


Suspend contents of one packet in 500 ml distilled or deionized water. Sterilize by autoclaving for 15 to 20 minutes at 121-124 °C. For microwaving, heat suspended mix until initial boiling. Mix well. Heat for short intervals with mixing until agar component is in solution. Do not allow boiling for extended periods of time. Antibiotics should be added following autoclaving or microwaving, after cooling to 48-52 °C.


S-Gal®/LB Agar Blend has been used for the identification of recombinant bacterial colonies.

Legal Information

S-GAL is a registered trademark of Sigma-Aldrich Co. LLC

Safety & Documentation

Safety Information

NONH for all modes of transport
WGK Germany 
Protocols & Articles


Blue-White Screening & Protocols for Colony Selection

Blue-white screening is a rapid and efficient technique for the identification of recombinant bacteria. It relies on the activity of β-galactosidase, an enzyme occurring in E. coli, which cleaves lac...
Keywords: Antibiotics, Cloning, Culture media, Gene expression, Immunocytochemistry, Molecular biology, Peptide synthesis, Polymerase chain reaction, Recombination, Sterilizations, transformation

Microbiology Introduction

An initial aim of all microbiologists is the reproducible growth of their microbial cultures, no matter whether the microorganisms are of natural origin or have been genetically engineered by man. Re...
Keywords: Absorption, Aerobic, Anaerobic, Antibiotics, Antimicrobials, Buffers, Culture media, Diffusion, Environmental, Filtration, Genetic, Growth factors, Metabolism, Phase transitions, Precipitation, Reductions, Respiration, Sample preparations, Vitamins

Microbiology Theory: Media Preparation

Microorganisms need nutrients, a source of energy and certain environmental conditions in order to grow and reproduce. In the natural environment, microbes have adapted to the habitats most suitable ...
Keywords: Culture media, Digestions, Environmental, Filtration, Growth factors, Indicators, Microbiology, Phase transitions, Precipitation, Vitamins

Peer-Reviewed Papers


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