A bacterial suspension of 1.5 ? 108 colony-forming units (CFUs)/mL is comparable with which McFarland standard?

Question

a. 0.5
b. 1.0
c. 1.5
d. 2.0

Answer 1

A
Accurate and reliable antimicrobial susceptibility testing results require standardized conditions. Properly prepared inocula are key to any antimicrobial susceptibility testing method. Two im-portant requirements for correct inoculum preparation are use of a pure culture and use of a standard-sized inoculum. Pure inocula are obtained by selecting four or five colonies of the same morphology, inoculating them into broth medium or 0.9% saline solution to achieve a turbid suspension. A standard inoculum size is accomplished by comparing the turbidity of the organism suspension with a turbidity standard. The 0.5 McFarland standard, which is commercially available, provides an optical density comparable to the density of a bacterial suspension of 1.5 ? 108 CFU/mL. If the bacterial suspension does not match the standard's turbidity, the suspension must be further diluted or supplemented with more organisms as needed.