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Streptococcus: CTA sugar fermentation

The cystine tryptic agar (CTA) sugar fermentation test is used to identify fastidious organisms (especially Streptococcus species), by detecting fermentation reactions. The fermentation of carbohydrates in the media produces acid which turns the pH indicator to a yellow color.

Colonies obtained from a blood agar culture are stabbed into the media in a CTA tube. After incubation, a positive fermentation test will be yellow. A tube of CTA base media is inocculated along with the CTA sugar tube; this base media tube is used to confirm the color change of the CTA sugar tube. If the bacteria grow in the base media tube, that media will remain red.