Brilliant Green Agar (BGA) ISO

Description

Since it has a high brilliant green concentration, it inhibits the growth of most bacteria, except Salmonella. However, S. typhi and S. paratyphi are also inhibited. Therefore, when their presence or Shigella is suspected, it is recommended to use other media in parallel, such as Deoxycholate Lactose Agar, MacConkey Agar, Salmonella-Shigella Agar, Xylose Lysine Deoxycholate Agar or Endo Agar Base, which are less inhibitory.

The presence of lactose and sucrose allows a good differentiation between Salmonella, which produce pink or colourless colonies with a red halo or zone, and the companion microbiota, which produce smaller and green yellowish colonies with a yellow halo, due to acid created by lactose and/or sucrose fermentation.

Osborn and Stokes suggested the addition of 0.08 g/L of sulfadiazine or 1 g/L of sulfapyridine in order to make this medium more selective for Salmonella and therefore making the medium more suitable for the testing of food and eggs and their derivatives.

Technique

Collect, dilute and prepare samples and volumes as required according to specifications, directives, official standard regulations and/or expected results.

Spread the plates by streaking methodology or by spiral method. Incubate the plates right side up aerobically at 30-35 °C for 24-48 hours (according to methods).

(Incubation times greater than those above or different incubation temperatures may be required depending on the sample, on the specifications, etc. This medium can be inoculated directly or after enrichment broth like MKTTn broth.)

After incubation, enumerate all the colonies that have appeared onto the surface of the agar.

Salmonella produce pink to colourless colonies with a red halo; the accompanying flora produce smaller colonies with greenish to yellowish haloes.

Each laboratory must evaluate the results according to their specifications.

Presumptive isolation of Salmonella sp. must be confirmed by further microbiological and biochemical tests.

Calculate total microbial count per mL of sample by multiplying the average number of colonies per plate by the inverse dilution factor if a diluted sample was streaked. Report results as colony forming units (CFU) per mL or g with incubation time and temperature.

Physical/chemical control

Microbiological control

Spiral spreading: practical range 100 ± 20 CFU; min. 50 CFU (productivity) / 10⁴-10⁶ CFU (selectivity).

Aerobiosis. Incubation at 35 ± 2 °C, reading at 24-48 hours.

Sterility control

Incubation 48 hours at 30-35 °C and 48 hours at 20-25 °C: NO GROWTH.

Check at 7 days after incubation in same conditions.

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