Granada medium

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Gran Granada medium is a special culture medium used to selectively isolate Streptococcus agalactiae (Group B Streptococcus, GBS) and help tell it apart from other microbes. It was developed by Manuel Rosa-Fraile and colleagues in Granada, Spain. Identification relies on a red pigment called granadaene produced by GBS; this pigment, along with beta-hemolysis, is linked to a gene cluster (cyl) in GBS and may be related to virulence.

Granada agar is made from proteose peptone starch agar buffered with MOPS and phosphate, and it includes methotrexate and antibiotics to suppress other bacteria. It also relies on the peptide Ile-Ala-Arg-Arg-His-Pro-Tyr-Phe (produced when mammal albumin is digested) and starch to support pigment production and stabilize the red colonies.

On Granada medium, beta-hemolytic GBS colonies appear pink-red after 18–48 hours at 35–37°C, with better growth in low-oxygen conditions. Non-hemolytic GBS colonies are white. The appearance of an orange color is a strong indicator of GBS, though non-hemolytic strains may need additional tests.

The medium is used for primary isolation and screening, including testing pregnant women for vaginal/rectal GBS carriage to prevent neonatal infection. Some other microbes may grow as colorless or white colonies despite the selective agents.

Specimens can be plated directly or after enrichment in a selective broth (such as Todd-Hewitt with gentamicin or colistin and nalidixic acid). For faster results, swabs can be rolled onto the plate. Incubation is typically 35–37°C; plates can be checked overnight and again at about 48 hours. Granada can also be used in liquid form (granada broth) and can be incubated aerobically if needed.

Note: About 1–5% of GBS strains are non-hemolytic and non-pigmented, and are generally less virulent.