What is the term used to describe the zone diameter below which all resistant strains fall?

The term used to describe the zone diameter below which all resistant strains fall is known as the resistance breakpoint. This concept is crucial in microbiology and antimicrobial susceptibility testing, as it helps define the threshold of resistance for a particular antibiotic against specific bacterial strains. The resistance breakpoint is determined by evaluating the zone of inhibition produced by an antibiotic on a bacterial lawn during susceptibility testing; if the measured zone diameter is smaller than this breakpoint, the strain is classified as resistant.

In contrast, the susceptibility breakpoint refers to the threshold above which bacteria are considered susceptible to an antibiotic. Hemagglutination is a phenomenon related to the clumping of red blood cells and is not relevant in the context of resistance testing. Similarly, antigen presence relates to the immune response and identification of pathogens and does not pertain to the discussion of resistance breakpoints. Understanding these terms is essential for accurately interpreting susceptibility tests and guiding appropriate treatment options.