Antimicrobial Susceptibility Testing
Antimicrobial susceptibility testing estimates whether an organism is likely to respond to an antimicrobial under defined testing conditions.
Core idea
AST results guide therapy, but they are only valid when the organism, method, inoculum, medium, incubation, and breakpoint rules are appropriate.
Principle
Standardized methods compare organism growth against antimicrobial exposure and interpret the result using accepted breakpoints.
Common AST Methods
| Method | What is measured | Student note |
|---|---|---|
| Disk diffusion | Zone diameter around antibiotic disks | Medium depth, inoculum, and incubation are critical |
| Gradient diffusion | MIC where the ellipse intersects the strip scale | Useful for selected organism-drug questions |
| Broth microdilution | MIC across antimicrobial concentrations | Reference-style method for many drugs |
| Automated AST | Instrument-derived growth patterns and MIC/category output | Review unusual phenotypes and suppression rules |
Setup Variables That Change Results
| Variable | If too high or too low | Possible effect |
|---|---|---|
| Inoculum density | Too heavy or too light | Can shift zones or MICs |
| Agar depth | Too thick or too thin | Can change diffusion and zone size |
| Incubation time | Too short or too long | Can hide or exaggerate resistance |
| Medium chemistry | Wrong cation or supplement conditions | Can affect specific drug classes |
Basic workflow
- Start with a pure, clinically meaningful isolate.
- Prepare the inoculum to the required density.
- Use the correct medium, incubation atmosphere, and incubation time.
- Read zones, MICs, or instrument output according to method rules.
- Apply organism-specific breakpoints and reporting rules.
How to read it
- Disk diffusion reports zone sizes that map to interpretive categories.
- MIC methods report the lowest concentration that inhibits visible growth under test conditions.
- Some organism-drug combinations require special tests, suppression rules, or confirmatory methods.
High-yield trapCommon mistakes
- Testing contaminants or colonizers without clinical value.
- Using the wrong medium or incubation condition.
- Reading a result without checking whether a breakpoint exists.
ShortcutStudent shortcut
AST is standardized on purpose. Small setup errors can become wrong therapy signals.