Iron Agar (Lyngby)

Bacteriological peptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is a source of vitamins, particularly of the B-group. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Sodium thiosulfate is reduced to hydrogen sulfide, which reacts with the iron salt to give the black iron sulfide. L-Cysteine is added to detect the production of H₂S of bacteria that do not produce hydrogen sulfide from thiosulfate but only from the sulfur-containing amino acid. Bacteriological agar is the solidifying agent. Bacteria capable of forming H₂S from either source of sulfur would appear as black colonies.

Preparation

Suspend 46.2 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into tubes and sterilize in autoclave at 121 °C for 15 minutes.

Instructions for use

Agar stab method:

1. Pick up the sample with a sterile needle.
2. Stab the medium with the sterile needle until it is 1 cm from the bottom of the tube.
3. Carefully remove the inoculation needle.
4. Incubate at 30 ± 2 °C for 3-4 days.

• Gram, L., Trolle, G. and Huss, H.H. (1987). Detection of specific spoilage bacteria from fish stored at low (0 °C) and high (20 °C) temperatures. Int. J. Food Microbiol., 4:65-72.
• Popovic, N. T. et al. (2010) Microbiological quality of marketed fresh and frozen seafood caught off the Adriatic coast of Croatia.