What reagent is primarily used in the String test?

The String test primarily utilizes 0.5% sodium deoxycholate as its reagent. This test is particularly valuable in microbiology for the differentiation of bacteria, especially in identifying certain strains of Enterobacteriaceae, like Vibrio cholerae. When a bacterial culture is mixed with the sodium deoxycholate solution, the reagent disrupts the bacterial cell wall and membrane, leading to the release of the bacterial DNA into the solution. This results in the formation of a viscous string when the sample is stretched between the applicator or loop, which is an indication of a positive test.

Using sodium deoxycholate is essential because its detergent properties effectively solubilize lipid membranes, allowing for this unique reaction. Other reagents mentioned, such as methyl red, bromthymol blue, and carbohydrates, are linked to different types of biochemical tests rather than the string test itself. Methyl red and bromthymol blue serve as pH indicators in various acid-base tests, while carbohydrates are used primarily to assess fermentation capabilities. Thus, each of these options plays a role in microbiology but not specifically within the context of the string test, highlighting the importance of 0.5% sodium deoxycholate for this particular procedure