Edward’s Medium HiVeg™ Base, Modified

Principle And Interpretation

Streptococci are gram-positive facultatively anaerobic bacteria, which constitute normal commensal flora of mouth, skin, intestine and upper respiratory tract of humans. Group B Streptococci are an important cause of systemic infections in infants and occasionally of bacterial endocarditis (1). Mastitis is a disease of cattle caused by the organisms Streptococcus agalactiae. It belongs to the Lancefield group B Streptococci. The most common selective agents used for selective isolation of Streptococci are crystal violet and thallium salts. A selective medium containing crystal violet was used by Haxthausen to isolate skin Streptococci (2). Subsequently it was observed that Streptococci from milk were able to grow on Gentian Violet Blood Agar whereas the other saprophytic milk bacteria were inhibited on this medium (3). An Esculin Blood Agar containing crystal violet was used by Edwards to isolate the causative agent of mastitis (4). A similar medium containing thallous acetate was also used to isolate the causative agent of mastitis (5). Edward's Medium HiVeg™ Base, Modified is prepared by using vegetable peptones in place of animal based peptones which make the media free of BSE/TSE risks. HiVeg™ peptone and HiVeg™ extract serve as sources of carbon, nitrogen and other essential nutrients. Esculin helps to differentiate esculin-positive (group D Streptococci) organisms from esculin- negative (S. agalactiae) organisms. Sodium chloride helps to maintain the osmotic equilibrium of the medium. Crystal violet and thallous sulphate serve as the selective agents for Streptococci. Supplementation with blood provides additional nutrients in addition to serving as an indicator of haemolysis. Mastitis Streptococci show alpha, beta or gamma type of haemolysis. Esculin differentiates esculin- positive group D Streptococci (black colonies) from esculin-negative Streptococcus agalactiae (blue to colourless colonies). Centrifuged test milk sample is directly inoculated on the surface of the medium plate. Esculin-negative (blue to colourless) S. agalactiae organisms are further subcultured for identification tests.