Iron Sulphite HiVeg™ Agar

Principle and Interpretation :

This medium is prepared by using Hiveg hydrolysate which is free from BSE/TSE risks associated with animal based peptones. Iron Sulphite HiVeg Agar is the modification of Iron Sulphite Agar which is based on formula of Cameron Sulphite Agar developed by the National Canners Association of America (1). 0.1% sulphite concentration in the original formula was reported by Beerens to be inhibitory to some strains of Clostridium sporogenes (2). This observation was later on confirmed by Mossel et al (3), who consequently showed that 0.05% sulphite concentration was not inhibitory to the organisms. For the detection of organisms causing sulphide spoilage either Deep-Shake Culture method or Attenborough and Scarr (4) method may be followed:

In Deep-Shake Culture method, the medium is dispensed in 10 ml amounts in sterile tubes. Sample is inoculated when the medium is at about 50°C and allowed to set. Further incubation is carried out at 55°C for 24-48 hours. Typical thermophilic species of Desulfotomaculum nigrificans, produces distinct black spherical colonies in the depth of the medium.

In Attenborough and Scarr (4) method, diluted samples of sugar or any other food to be tested are filtered through membrane filters. These filters are then rolled up and placed in tubes containing just sufficient Iron Sulphite HiVeg Agar (at 50°C) to cover them. The medium is allowed to set and then incubated at 55 - 56°C for 24-48 hours. After incubation, the number of black colonies on the membrane filter are counted. This membrane filter technique is quicker, of comparable accuracy and permits the examination of larger samples.

Note: The blackening reaction is only presumptive evidence of Clostridial growth. Confirmation tests must be carried out to identify the organisms growing in the medium.