What does the McFarland Standard correlate with?

The McFarland Standard is a reference tool used in microbiology to provide a consistent methodology for estimating the concentration of bacteria in a suspension. It correlates specifically with the turbidity (cloudiness) of a bacterial culture, which directly reflects the number of bacterial cells present per milliliter (mL).

When a bacterial suspension is prepared to match a specific McFarland Standard, it reaches a designated turbidity that equates to a known concentration of bacterial cells. For example, a 0.5 McFarland Standard corresponds to approximately 1.5 x 10^8 CFU/mL (colony-forming units per milliliter). This established correlation allows researchers and laboratory technicians to standardize their microbial counting methods and ensure that experiments, such as antibiotic susceptibility testing, have consistent bacterial loads to work from.

The other options delve into other aspects of microbiology but do not accurately reflect the purpose of the McFarland Standard, which is solely focused on the relationship between turbidity and bacterial concentration.