A Rapid Method for the Turbidirnetric Assay of Antibiotics

SUMMARY: By using a culture of Streptococcus agalactiae in its logarithmic phase of growth as test organism, it is possible to assay nisin satisfactorily with an incuba-tion period of approximately half an hour. Penicillin, streptomycin, aureomycin and gramicidin cause a qualitatively similar response in the test organism. A similar technique could therefore be used to assay these antibiotics. A considerable number of turbidimetric assay methods have been described (see, for example, Florey, Chain, Heatley, Jennings, Sanders, Abraham & Florey, 1949). One of the most rapid of these was due to Lee, Foley, Epstein & Wallace (1944), who used a streptococcus of group B and incubated until a sufficient turbidity had. developed, ‘usually for 90 min. or less’. For the sake of rapidity Friedmann & Beach (1951) used Streptococcus cremoris in nisin assays; resazurin was employed as an indicator of bacterial growth, enabling assays to be read after 20 min. A standard error ‘of the order of 10 yo ’ was claimed for five estimations. In the work to be described Strep. agaluctiue was preferred, since previous experience had shown Strep. cremoris to be sensitive to antibiotics which had little effect against pathogens. The properties of nisin which affect its assay have been discussed by Hirsch (1950), who has also described two other methods of assay, one based on the bactericidal properties of nisin, the other on its property of prolonging the lag phase of growth of the test organism (Hirsch, 1950). The former method is also of low reproducibility, but the latter has an accuracy varying from f i7 to +15%, and is therefore far more accurate than any of the earlier methods. However, a rapid assay was desired and a comparison of Fig.