XLT4 HiVeg™ Agar Base

Principle And Interpretation

Salmonella is a genus of gram-negative enterobacteria commonly implicated in foodborne illness and is the causative agent of typhoid and paratyphoid fever. Although most Salmonella cannot be distinguished by biochemical characteristics, one serotype, namely S. Typhi produce only a trace amount of hydrogen sulphide and is less active biochemically than the more common serotypes (1). XLT4 Agar Base is formulated as described by Miller and Tate (2) for isolating Salmonella from faecally contaminated farm samples, which contains other bacteria as well. XLT4 Agar Base enhances the recovery of Salmonella species other than Salmonella Typhi (3-7). XLT4 HiVeg™™ Agar Base is prepared by using vegetable peptones in place of animal based peptones which make the media free of BSE/TSE risks.

HiVeg™™ peptone No. 3 is a source of carbon, nitrogen and other essential amino acids and growth factors. Yeast extract supplies nitrogenous requirements and vitamins required for growth. The sugars namely lactose, saccharose and xylose are the fermentable carbohydrates. Salmonella rapidly utilize xylose, producing acidity. Subsequently they decarboxylate lysine and revert to alkalinity. To add to the differentiating ability of the formulation, an H2S indicator system, consisting of sodium thiosulphate and ferric ammonium citrate is included for the visualization of the hydrogen sulphide produced, resulting in the formation of colonies with black centers. The non-pathogenic H2S producers do not decarboxylate lysine; therefore, the acid reaction generated by them prevents the blackening of the colonies (8). XLT4 Agar is both selective and differential. Tergitol 4 (FD152) inhibits growth of non-Salmonella organisms. Presumptive Salmonella colonies should be confirmed by performing biochemical tests.