Turbidimetric microbiological assays

Figure 4. Comparison of the vitamin B12 content of 22 varieties of food by the radiometric and turbidimetric microbiological assays correlation coefficient, r = 0.988. Foods include various mil baby food, fish, poultry, meat, and cereal. Each point represents the mean of five experiments.

Assay Methods. The primary assay for the streptovaricins is the microbiological assay using the agar diffusion method or a turbidimetric procedure (60). The streptovaricins can also be identified by paper (60,88) or thin-layer chromatography (3). 

Microbiological assays are the primary assay method for antibiotics. They provide sensitive but non-selective methods. A variety of microbiological methods have been described for the assay of gramicidin. The official method in the United States is the turbidimetric method described in the Code of Federal Regulations127 12 . 

Folic Acid (folate). Chemically, folic acid is a pteryl-glutamic acid. The several forms that occur in nature depend on the numbers of glutamic acid units and methyl groups in the molecules. Because of its usual low concentration, folic acid is generally determined in food materials by the microbiological assay with lactobacillus casei and measured turbidimetrically or titrimetrically. Deficiency of this vitamin could result in... 

Kavanagh and Dennen20 report microbiological turbidimetric and plate assays for erythromycin base in Analytical Microbiology,... 

Food and Drug Administration (FDA) replaced the above mentioned method, " Microbiological agar diffusion assay ", with a microbiological turbidimetric assay because the modified method are more accurate and reliable than agar diffusion assay (43). 

When the red cell folate levels from 57 normal volunteers were determined by both the radiometric and the standard microbiological methods, the values obtained had a correlation coeflBcient of r = 0.98 (Figure 6). The mean red cell folate value obtained by the RMA was 270.0 ng/mL with a range of 149.4-769.2 ng/mL, using 5-methyl-H4Pte-Glu as the standard. By the turbidimetric assay, the mean red cell folate value was 281.7 ng/mL with a range of 142.7-707.1 ng/mL. 

The microbiological method using Lactobacillus phntarum, originally described by Snell and Wright (49), has been widely used for the determination of niacin in urine, serum, blood, foods, and tissues (16, 48,50,51,52). This organism is able to detect minute amounts of niacin (1-30 ng/mL) using a turbidimetric or titrimetric assay. 

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