Mc farland standards

1. AGGREH ERHOVWON PETER
U2011/5555010
MICROBIOLOGY
MCB 410.1
McFarland Standards
In microbiology McFarland standards are used as reference to adjust the turbidity of
bacterial suspension so that the number of bacteria will be within a given range to
standardized microbial testing.
McFarland standards involve mixing specified amounts of barium chloride and sulfuric acid
together. Mixing the two compounds forms a barium sulfate precipitate which causes
turbidity in the solution. A 0.5 McFarland standard is prepared by mixing 0.05l of 1.175%
barium chloride dehydrate (Bacl2 .2H20) with 0.95ml of 1% sulfuric acid (H2S04).
Presently, McFarland standards are prepared from suspensions of latex particles which
lengthens the shelf life and stability of the suspensions. The standard is compared usually to
a suspension of bacteria in sterile saline or nutrient broth.