Fermentation HiVeg™ Medium for Neisseriae

Principle And Interpretation

Neisseria species are oxidative i.e. they produce acid from carbohydrate by oxidation. Because these species are oxidative and produce less acid from carbohydrates than do fermentative organisms and because they also produce ammonia from peptones which may neutralize any acid produced from carbohydrates, acid production is determined in a medium with a low protein/carbohydrate ratio and a sensitive indicator such as phenol red (1,2). Fermentation Medium for Neisseriae is recommended for studying the fermentation reactions of fastidious organisms such as Neisseria (3). This medium is the modification of the medium originally formulated by Vera (4). Neisseria species oxidize the added carbohydrates to yield acids. The acids thus formed change the colour of the pH indicator, phenol red form orange to yellow. The organism also degrades the peptone source to yield ammonia. The alkalinity thus formed causes the phenol red to change to pink. However, if the acidity formed by carbohydrate metabolism is greater than the alkalinity formed by peptone degradation, the medium remains yellow in colour. Fermentation HiVeg™ Medium for Neisseriae is prepared by using vegetable peptones in place of animal based peptones which make the media free of BSE/TSE risks.
HiVeg™ hydrolysate supplies the necessary nitrogenous nutrients to the organisms. L-Cystine acts as an amino acid source as well as a reducing agent, which can remove (bind) molecular oxygen thereby preventing the accumulation of peroxides which are lethal to certain microorganisms (5). Small amount of agar in the medium reduces convection currents in the medium and hence contributes to maintaining anaerobic conditions in the depth of the tubes. Sodium chloride maintains the osmotic equilibrium in the medium. Phenol red is the pH indicator, which turns yellow at acidic pH. Observe the inoculated tubes after every 4 hours. Development of yellow colour throughout the medium indicates that the carbohydrate has been oxidized leading to the production of acids. Development of pink colour indicates that carbohydrates have not been oxidized and only the peptones have been degraded. Neisseria species tend to produce acids from carbohydrate in the vicinity of inoculated (stab) area. If accompanying contaminating organisms are present the entire medium may turn yellow.