Bacitracin Susceptibility (A Disk)
An original bench-card visual demonstrating bacitracin susceptibility for presumptive Group A strep identification.
Bench card visual
Bacitracin plates A-B
Read first
Compare the zone of inhibition around the 0.04-unit bacitracin (A) disk.
A
Susceptible (Positive)Streptococcus pyogenesClear zone of inhibition >10 mm around disk A; presumptive identification for Group A Strep.
B
Resistant (Negative)Streptococcus agalactiaeNo zone of inhibition; growth right up to the 6 mm disk edge.
Susceptible (Positive)Streptococcus pyogenes
Clear zone of inhibition >10 mm around disk A; presumptive identification for Group A Strep.
Resistant (Negative)Streptococcus agalactiae
No zone of inhibition; growth right up to the 6 mm disk edge.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Zone of Inhibition | Clear area around the disk where bacterial growth is inhibited | Shorthand: Zone (S) |
| No Zone of Inhibition | Bacterial growth right up to the edge of the 6 mm disk | Shorthand: No Zone (R) |
| Beta-Hemolysis | Complete lysis of red blood cells in agar, creating a cleared/yellowish appearance | Look for transparency around lawn |
| 0.04 U Disk (Taxo A) | Paper disk impregnated with 0.04 units of bacitracin | Labeled with "A" |
Bacitracin study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Susceptible (Positive) | Any zone of inhibition (>10 mm) | Presumptive Streptococcus pyogenes (Group A streptococci) or Micrococcus luteus. |
| Resistant (Negative) | No zone of inhibition (growth up to disk) | Presumptive Streptococcus agalactiae (Group B streptococci) or Staphylococcus aureus. |
| Beta-Hemolytic Context | Must pair with morphology and catalase | Catalase-negative, Gram-positive cocci in chains; confirms it is streptococci first. |
Key takeaways
- Presumptively differentiates S. pyogenes (susceptible) from S. agalactiae (resistant).
- Requires a lawn of beta-hemolytic streptococci on sheep blood agar (SBA).
- The PYR test is more specific and has largely replaced bacitracin in modern labs, but bacitracin remains high-yield for exams (ASCP/ASCPi).
Remember
PYR is positive for Group A Strep and Enterococci; use PYR alongside Bacitracin to secure your exam prep.