THE MCFARLAND NEPHELOMETRIC METHOD TO ESTIMATE NUMBERS OF BACTERIA

In this method, suspensions of barium sulphae are made with different densities; these have been calculated to be comparable to the densities of suspensions of bacteria with known numbers of bacteria. In order to estimate the numbers of bacteria in suspensions from cultures, the density of the experimental suspension (absorbance or transmission) is compared to that of the set of reference tubes in the series. The reference tubes are prepared by adding the following amounts of stock solutions of 1% aqueous barium chloride and 1% aqeous sulfuric acid to tubes of a size appropriate for the nephelometer. Different proportions of bariumchloride and sulfuric acid are used to prepare a range of barium sulphate concentrations for the McFarland nephelometry standards.

The amounts of each stock and the corresponding densities of bacteria are given in the following table:

Parts of BaCl2 (1%) in 1 ml | Parts of H2SO4 (1%) in 1 ml | Corresponding number of bacteria (million/ml)
0.1 | 9.9 | 300
0.2 | 9.8 | 600
0.3 | 9.7 | 900
0.4 | 9.6 | 1200
0.5 | 9.5 | 1500
0.6 | 9.4 | 1800
0.7 | 9.3 | 2100
0.8 | 9.2 | 2400
0.9 | 9.1 | 2700
1.0 | 9.0 | 3000