培养技术是一种有效的way to identify bacterial strains depending on the bacterium’s growth requirements. The growth medium’s composition can be designed to restrict the growth of certain strains while simultaneously allowing specific strains to grow.
The沙门氏菌Shigellaagar (SSA) contains peptone, lactose and all essential micronutrients. Bile salts and brilliant green dye act as selective agents and only allow certain gram-negative strains to grow, therefore killing all of the gram-positive strains. The medium contains the pH indicator neutral red, which is orange in neutral conditions but changes to red when the pH drops below 6.8; the lactose fermenting bacteria,Escherichia coli, can cause this. When lactose is converted into lactic acid, the pH is decreased and the pH indicating molecules in close proximity to the cells turn red.沙门氏菌, however, cannot ferment sugar and uses peptone as a carbon source.
As an additional mechanism for identification, the medium contains ferric acid and sodium thiosulfate, which is a reducible sulfur source. Most沙门氏菌strains can reduce the amount of thiosulfate and produce hydrogen sulfide, which reacts with the ferric acid in the medium and turning it black. Therefore,沙门氏菌growing on a SSA will establish black centers in the colonies.Shigellaspecies, in contrast, lack the enzymes necessary to reduce sulfur; therefore it will maintain its original color.
