The Assay of the Antibiotic Nisin by Means of a Reductase (Resazurin) test

SUMMARY: The familiar resazurin test used in milk testing has been adapted to the estimation of nisin, using a fast-growing strain of Streptococcus cremoris as test organism. The reproducibility and accuracy of this test compares favourably with the many other assay methods for nisin which use Strep. agalactiae as test organism. Results may be obtained in less than an hour as compared with 15-48 hr. in the other tests and are a measure of bactericidal power only. Aseptic technique during the actual assay procedure is unnecessary. Methods were devised to ensure a suitable standard inoculum, which is imperative for this test. As little as 0.5 unit nish/ml. may be estimated. Inhibition of reductase activity and acid production by bacteria in milk have been used for the assay of penicillin and streptomycin. Reid & Brewer (1946) used Bacillus subtilis as test organism and added methylene blue as indicator system at the beginning of incubation. Approximate readings were possible after 4 hr. and final readings were taken after 18 hr. Sanchez & Lamensans (1947) used Lactobacillus bulguricus as test organism. Their indicator syste