Phenotypic susceptibility testing means: judge whether a drug will work by how the living organism behaves around that drug — not only by reading a gene list. Genes can hint at resistance. Phenotype asks the practical question: does this cell’s response look susceptible or resistant under the test conditions?

One modern lab path takes material directly from a positive blood culture, exposes cells to antibiotics, and images single cells over time. Computer vision then helps score that response into categorical calls. The living response is the evidence; vision is the reader.

Direct-from-positive means the susceptibility step can start from the flagged bottle contents (still after positivity), skipping a slow plate grow-out for that step. It does not mean skipping the blood culture bottle.

The output is a categorical map — Susceptible / Intermediate / Resistant (and related label categories where they apply). That is a lab decision aid for clinicians and pharmacists, not an automatic prescription, and not organism identification by itself.