Bacitracin Susceptibility Test for Group A Streptococcus Identification

The bacitracin susceptibility test is a rapid, cost-effective disk diffusion method used to presumptively identify Group A streptococci (Streptococcus pyogenes) from clinical or environmental isolates. This test exploits the inherent sensitivity of Group A streptococci to low concentrations of bacitracin, a polypeptide antibiotic that inhibits cell wall synthesis by interfering with the dephosphorylation of the lipid carrier involved in peptidoglycan assembly. When a filter paper disk impregnated with 0.04 units of bacitracin is placed on a lawn of beta-hemolytic streptococci, a zone of growth inhibition around the disk indicates susceptibility, providing strong presumptive evidence for S. pyogenes identification. The test is most useful in routine microbiology laboratories as a primary screening tool for throat swab cultures and other clinical specimens where beta-hemolytic streptococci are suspected, particularly in pediatric populations where Group A streptococcal pharyngitis is a common concern [1][2].

At a Glance

Scientific Principle

Bacitracin is a cyclic polypeptide antibiotic produced by Bacillus licheniformis that exerts its bactericidal effect by binding to undecaprenyl pyrophosphate, a lipid carrier essential for transporting peptidoglycan precursors across the bacterial cell membrane. By preventing the dephosphorylation of this carrier, bacitracin blocks the recycling of the lipid carrier and halts cell wall synthesis. Group A streptococci (S. pyogenes) exhibit a uniquely high susceptibility to bacitracin compared to other beta-hemolytic streptococci, a characteristic that has been exploited for diagnostic identification since the 1940s.

The differential sensitivity arises from structural and functional differences in the cell wall biosynthesis machinery among streptococcal species. While the exact molecular basis remains incompletely understood, it is well established that S. pyogenes consistently produces a zone of inhibition around a 0.04-unit bacitracin disk, whereas other beta-hemolytic streptococci (Groups B, C, F, and G) are typically resistant at this concentration. This phenotypic distinction provides a reliable, low-cost screening tool that can be performed in any basic microbiology laboratory equipped with standard culture materials.

The test is most reliable when performed on pure cultures of beta-hemolytic streptococci isolated on sheep blood agar. Direct testing from primary culture plates is not recommended due to potential mixed cultures and variable inoculum density. The 0.04-unit disk concentration is critical; higher concentrations (e.g., 10-unit disks used for antibiotic susceptibility testing) will inhibit most streptococci and eliminate the differential diagnostic value.

Materials and Instrumentation Choices

Essential Materials

• Bacitracin disks (0.04 units): Commercially available from multiple manufacturers (e.g., BD BBL, Oxoid, HiMedia). Disks must be stored at 2–8°C in sealed containers with desiccant. Allow disks to reach room temperature for 30–60 minutes before use to prevent condensation. Never use expired disks or disks from containers left open.
• Sheep blood agar plates: 5% defibrinated sheep blood in Tryptic Soy Agar base. Plates should be prepared fresh or stored at 2–8°C for no more than 7–10 days. Avoid using human blood agar, as it may contain inhibitory substances or support different growth characteristics.
• Sterile saline or Mueller-Hinton broth: For preparing the 0.5 McFarland standard suspension.
• 0.5 McFarland turbidity standard: Commercial standard or prepared by adding 0.5 mL of 1% barium chloride to 99.5 mL of 1% sulfuric acid. Vortex before each use. Alternatively, use a spectrophotometer set to 625 nm (target absorbance 0.08–0.10).
• Sterile cotton swabs: For inoculum application.
• Forceps or disk dispenser: For aseptic placement of disks.
• Incubator: Set to 35–37°C. A CO₂ incubator (5% CO₂) is preferred for optimal hemolysis and growth, but ambient air incubation is acceptable for most isolates.
• Quality control organisms: S. pyogenes ATCC 19615 (susceptible) and S. agalactiae ATCC 13813 (resistant).

Instrumentation Considerations

• Automated disk dispensers: Useful for high-throughput laboratories but require regular calibration and cleaning to prevent disk jamming or misplacement. Manual forceps are acceptable for low-volume testing.
• Zone measurement tools: Calipers or a ruler with 0.5 mm increments. Automated zone readers (e.g., BioMic, Oxoid Aura) improve reproducibility but are not essential.
• MALDI-TOF MS: While not required for the bacitracin test, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry can provide definitive species identification and is increasingly used in reference laboratories [3].

Reagent System Choices

• Disk source: Different manufacturers may produce disks with slightly different bacitracin concentrations or binding substrates. Always verify performance with quality control organisms before using a new lot number.
• Blood agar base: Tryptic Soy Agar is standard, but Columbia Agar base is also acceptable. Avoid media containing antibiotics or selective supplements that might affect bacitracin activity.
• Incubation atmosphere: CO₂ enhances hemolysis and growth of S. pyogenes but may slightly alter zone sizes. Laboratories should establish their own zone diameter ranges for quality control organisms under their specific incubation conditions.

Controls

Positive Control

• Organism: Streptococcus pyogenes ATCC 19615
• Expected result: Zone of inhibition ≥ 10 mm (typically 15–25 mm)
• Frequency: Each day of testing, with each new lot of disks, and whenever test conditions change

Negative Control

• Organism: Streptococcus agalactiae ATCC 13813 (Group B streptococcus)
• Expected result: No zone of inhibition or zone < 10 mm
• Frequency: Same as positive control

Additional Controls

• Sterility control: Incubate an uninoculated blood agar plate to verify medium sterility.
• Disk potency control: Test a known susceptible S. pyogenes isolate weekly to monitor disk degradation over time.
• Inoculum density control: Visually compare the prepared suspension to the 0.5 McFarland standard. Overly dense inoculum may produce false resistance; under-inoculum may produce false susceptibility.

Control Documentation

Record the following for each control run:

• Organism name and ATCC number
• Disk lot number and expiration date
• Medium lot number and preparation date
• Incubation conditions (temperature, atmosphere, duration)
• Zone diameter (mm)
• Pass/fail assessment
• Technician initials and date

Conceptual Workflow

Step 1: Isolate Selection

Select a pure, well-isolated colony of beta-hemolytic streptococci from a sheep blood agar plate. The colony should be 18–24 hours old. Avoid mucoid or mixed colonies. Confirm Gram stain morphology (Gram-positive cocci in chains) and catalase negativity (no bubble formation with 3% hydrogen peroxide).

Step 2: Inoculum Preparation

Using a sterile loop, transfer 3–5 morphologically identical colonies into 2–3 mL of sterile saline or Mueller-Hinton broth. Vortex thoroughly. Adjust the turbidity to match a 0.5 McFarland standard. This corresponds to approximately 1.5 × 10⁸ CFU/mL. If the suspension is too turbid, add sterile diluent; if too light, add more colonies.

Step 3: Inoculation

Within 15 minutes of preparing the suspension, dip a sterile cotton swab into the suspension. Rotate the swab against the tube wall above the liquid level to remove excess fluid. Streak the swab evenly across the entire surface of a sheep blood agar plate in three directions (rotating the plate approximately 60° between each streaking) to ensure confluent growth. Allow the plate surface to dry for 3–5 minutes with the lid slightly ajar.

Step 4: Disk Application

Using sterile forceps or a disk dispenser, aseptically place a 0.04-unit bacitracin disk onto the inoculated agar surface. Gently press the disk to ensure complete contact with the agar. For routine testing, one disk per plate is sufficient. If performing multiple disk tests on the same plate (e.g., bacitracin and optochin), space disks at least 24 mm apart (center to center) to prevent overlapping zones.

Step 5: Incubation

Invert the plate and incubate at 35–37°C in 5% CO₂ for 18–24 hours. CO₂ incubation enhances hemolysis and growth but is not strictly required. If using ambient air, extend incubation to 24 hours and carefully examine for hemolysis.

Step 6: Reading and Interpretation

After incubation, examine the plate for a zone of inhibition around the bacitracin disk. Measure the zone diameter (including the disk) in millimeters using a ruler or caliper. Hold the plate against a dark background with transmitted light for optimal visualization. Any zone of inhibition (≥ 10 mm) is interpreted as susceptible, providing presumptive identification of Group A streptococcus.

Step 7: Confirmation

Report the result as "Presumptive Group A streptococcus (bacitracin susceptible)" and proceed to confirmatory testing. Recommended confirmatory tests include:

• PYR test (pyrrolidonyl arylamidase): S. pyogenes is PYR-positive; other beta-hemolytic streptococci are PYR-negative.
• Lancefield grouping: Commercial latex agglutination kits detect Group A carbohydrate antigen.
• Molecular methods: PCR targeting the speB or emm genes provides definitive identification.

Quality Checks

Pre-Analytical Quality Checks

• Verify disk integrity: Disks should be intact, not discolored, and within expiration date.
• Verify medium quality: Blood agar should show no contamination, no hemolysis before use, and appropriate color (cherry red).
• Verify McFarland standard: Replace barium sulfate standards every 6 months; spectrophotometric standards should be calibrated weekly.
• Document all reagent lot numbers and expiration dates.

Analytical Quality Checks

• Confirm pure culture: Examine the inoculum source for mixed morphology.
• Verify inoculum density: Compare to 0.5 McFarland standard visually or spectrophotometrically.
• Monitor incubation conditions: Record temperature and CO₂ levels daily.
• Include controls with each test batch.

Post-Analytical Quality Checks

• Measure zones accurately: Read at the point of complete inhibition, ignoring faint growth or pinpoint colonies within the zone.
• Record results promptly: Do not delay reading beyond 24 hours, as zones may shrink or become obscured by overgrowth.
• Compare with expected patterns: A bacitracin-susceptible, beta-hemolytic, catalase-negative, Gram-positive coccus is highly suggestive of S. pyogenes.

Proficiency Testing

Participate in external quality assessment schemes (e.g., from CAP, UK NEQAS, or national reference laboratories) at least twice yearly. Maintain records of all proficiency test results and corrective actions taken.

Result Interpretation

Susceptible (Presumptive Group A Streptococcus)

• Zone diameter: ≥ 10 mm (any visible zone of inhibition)
• Interpretation: The isolate is susceptible to bacitracin, providing strong presumptive evidence for S. pyogenes (Group A streptococcus).
• Reporting: "Presumptive identification: Streptococcus pyogenes (Group A streptococcus). Confirm with PYR test or Lancefield grouping."

Resistant (Not Group A Streptococcus)

• Zone diameter: < 10 mm or no zone
• Interpretation: The isolate is resistant to bacitracin, indicating it is likely not S. pyogenes. Common alternatives include Group B (S. agalactiae), Group C (S. equi subsp. zooepidemicus, S. dysgalactiae subsp. equisimilis), Group F (S. anginosus group), or Group G streptococci.
• Reporting: "Not Group A streptococcus (bacitracin resistant). Perform additional testing for species identification."

Borderline Results

Occasionally, isolates produce a zone of exactly 10 mm or a very narrow zone (8–9 mm). In such cases:

• Repeat the test with fresh inoculum and a new disk.
• Examine the colony morphology and hemolysis pattern carefully.
• Proceed directly to confirmatory testing (PYR, Lancefield grouping) without relying solely on the bacitracin result.

Important Caveats

• False susceptibility: Some non-Group A streptococci (particularly some Group C and G strains) may occasionally show bacitracin susceptibility. This is rare but underscores the need for confirmatory testing.
• False resistance: Overly dense inoculum, expired disks, or improper storage can produce false resistance. Always verify with controls.
• Mixed cultures: Testing from mixed cultures can yield misleading results. Always test pure isolates.

Troubleshooting

Limitations

Presumptive Nature

The bacitracin susceptibility test is a screening tool, not a definitive identification method. False-positive results (non-Group A streptococci that are bacitracin-susceptible) and false-negative results (S. pyogenes strains that are bacitracin-resistant) occur, albeit rarely. All presumptive identifications must be confirmed by at least one additional method (PYR test, Lancefield grouping, or molecular identification).

Species Variability

Some Group C and Group G streptococci, particularly S. dysgalactiae subsp. equisimilis, may occasionally exhibit bacitracin susceptibility. Additionally, S. pneumoniae (which is optochin-susceptible) is inherently resistant to bacitracin, but this is rarely a diagnostic confusion because S. pneumoniae is alpha-hemolytic and optochin-susceptible.

Technical Limitations

• Inoculum density: The test is sensitive to inoculum variation. Over-inoculation can mask susceptibility.
• Disk storage: Bacitracin disks are labile and must be stored properly. Degraded disks produce unreliable results.
• Medium effects: Blood agar composition and age can affect zone sizes. Always use fresh, high-quality media.
• Incubation atmosphere: CO₂ incubation enhances growth but may slightly alter zone diameters. Laboratories must establish their own QC ranges.

Clinical Context

The bacitracin test does not provide information about antibiotic resistance or treatment options. It is purely a diagnostic identification tool. For antibiotic susceptibility testing of S. pyogenes, standard methods (e.g., broth microdilution or disk diffusion with therapeutic antibiotics) must be performed separately.

Population-Specific Considerations

While S. pyogenes is consistently bacitracin-susceptible in most geographic regions, rare resistant strains have been reported. Laboratories serving populations with high antibiotic exposure should maintain a low threshold for confirmatory testing.

Documentation

Required Records

For each bacitracin susceptibility test performed, document:

• Patient/sample identifier: Unique laboratory accession number
• Source of isolate: Specimen type (e.g., throat swab, wound)
• Date and time of testing: Including date of primary isolation
• Medium used: Type, lot number, expiration date, preparation date
• Disk information: Manufacturer, lot number, concentration, expiration date
• Inoculum preparation: Method (visual McFarland or spectrophotometer)
• Incubation conditions: Temperature, atmosphere, duration
• Result: Zone diameter (mm) and interpretation (susceptible/resistant)
• Controls: Results for positive and negative controls
• Technician identification: Name or initials
• Confirmatory test results: If performed (e.g., PYR, Lancefield grouping)
• Final report: Definitive identification and any relevant comments

Reporting Format

Example report entry:

Specimen: Throat swab (Accession #24-5678)
Isolate: Beta-hemolytic, catalase-negative, Gram-positive cocci
Bacitracin test (0.04 U): Zone = 18 mm (Susceptible)
Interpretation: Presumptive Group A streptococcus (Streptococcus pyogenes)
Confirmation: PYR test positive; Lancefield Group A positive
Final identification: Streptococcus pyogenes (Group A streptococcus)

Quality Assurance Documentation

Maintain a quality control log that includes:

• Daily control results with pass/fail assessment
• Corrective actions for any out-of-range results
• Disk lot change records
• Proficiency testing results
• Annual review of test performance

Biosafety Considerations

Risk Assessment

Streptococcus pyogenes is classified as a Risk Group 2 pathogen by the CDC and NIH [6]. While the bacitracin test itself is performed on pure cultures, the initial processing of clinical specimens (throat swabs, wound cultures) may involve exposure to potentially infectious material. Laboratories should conduct a site-specific risk assessment considering the prevalence of S. pyogenes in their patient population and the procedures performed.

BSL-1 vs. BSL-2 Requirements

• Teaching laboratories: When using well-characterized, non-pathogenic control strains (e.g., ATCC strains) and no clinical specimens, BSL-1 practices are appropriate. Standard microbiological practices (hand washing, no eating/drinking, decontamination of work surfaces) are sufficient.
• Clinical laboratories: When processing throat swabs or other clinical specimens, BSL-2 practices are required. This includes:
  • Restricted access to the laboratory
  • Use of personal protective equipment (lab coat, gloves, eye protection)
  • Biosafety cabinet (Class II) for specimen processing and culture manipulation
  • Proper sharps disposal
  • Decontamination of all waste before disposal

Specific Precautions

• Aerosol generation: Avoid vigorous vortexing of open tubes. Use sealed tubes or vortex in a biosafety cabinet.
• Sharps: Use plastic rather than glass Pasteur pipettes. Dispose of all sharps in puncture-resistant containers.
• Decontamination: Disinfect work surfaces daily and after any spill with 10% bleach (0.5% sodium hypochlorite) or an EPA-registered disinfectant effective against Gram-positive bacteria.
• Waste disposal: All cultures and contaminated materials must be autoclaved before disposal. Follow institutional guidelines for biohazard waste.

Personal Protective Equipment (PPE)

• Minimum: Laboratory coat and gloves
• Recommended for clinical work: Disposable gloves, laboratory coat with closed front, eye protection if splashing is possible
• Not required for routine work: N95 respirators (unless performing procedures with high aerosolization risk)

Emergency Procedures

• Spill: Cover with absorbent material, flood with disinfectant, allow 20-minute contact time, then clean up wearing gloves and lab coat.
• Needlestick: Wash wound thoroughly with soap and water, report to supervisor, seek medical evaluation.
• Exposure to mucous membranes: Flush eyes or mouth with copious water for 15 minutes, seek medical evaluation.

Regulatory Compliance

Follow institutional biosafety committee guidelines and any applicable national regulations. In the United States, the NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules [7] may apply if genetic manipulation is involved. For routine diagnostic testing, standard clinical laboratory regulations (CLIA in the US) apply.

Frequently Asked Questions

1. Can I use a 10-unit bacitracin disk instead of the 0.04-unit disk?

No. The 10-unit disk is designed for antibiotic susceptibility testing of therapeutic bacitracin and will inhibit most streptococcal species, eliminating the differential diagnostic value. Always use the 0.04-unit disk for Group A streptococcus identification. Using the wrong concentration is the most common technical error in this test.

2. Why must I confirm a positive bacitracin test with additional methods?

Although S. pyogenes is consistently bacitracin-susceptible, rare strains of Group C and Group G streptococci may also show susceptibility. Additionally, technical errors (e.g., expired disks, over-inoculation) can produce false results. Confirmatory testing (PYR test, Lancefield grouping, or molecular methods) provides definitive identification and is considered the standard of care in clinical microbiology.

3. How long can I store bacitracin disks, and how do I know if they are still good?

Bacitracin disks should be stored at 2–8°C in sealed containers with desiccant. Under proper storage, they are stable until the manufacturer's expiration date. However, disks can degrade if exposed to moisture or temperature fluctuations. Always test with quality control organisms (S. pyogenes ATCC 19615) on each day of use. If the positive control fails to produce a zone, the disks are compromised and must be replaced.

4. Can I perform the bacitracin test directly from a throat swab without isolating the organism?

No. The test requires a pure culture of beta-hemolytic streptococci. Direct testing from a throat swab would involve mixed flora (normal respiratory microbiota) and could produce misleading results. Always isolate the organism on sheep blood agar, confirm beta-hemolysis and Gram stain morphology, and then perform the bacitracin test from a pure subculture.

References and Further Reading

1. Mekuria S, Getu F, Walle M, et al. Throat Swab Culture Positivity Rate, Antibiotic Susceptibility Profile, and Associated Risk Factors of Streptococcus pyogenes Among Children With Acute Pharyngitis Attending Jigjiga University Sheik Hassan Yebere Referral Hospital, Jigjiga, Ethiopia. 2026. PubMed ID: 41798864

2. Mwita JC, Sajini S, Engel K, et al. Prevalence of Group A Streptococcal Pharyngitis and Antibiotic Susceptibility in Paediatric Patients With Sore Throats in Gaborone, Botswana. 2026. PubMed ID: 41101768

3. Wang T, San Juan MR, Choi H, et al. Septic arthritis caused by Streptococcus equi subspecies zooepidemicus: a case report. 2026. PubMed ID: 42094989

4. Sewgoolam B, Jim KK, de Bakker V, et al. Genome-wide antibiotic-CRISPRi profiling identifies LiaR activation as a strategy to resensitize fluoroquinolone-resistant Streptococcus pneumoniae. 2025. PubMed ID: 40659638

5. Piso DYT, Van-Arcken Aguilar YC, Barreto MYP, et al. Identification and antimicrobial susceptibility of bacteria isolated from corneal ulcers and healthy eyes of canines in Ibagué-Tolima, Colombia. 2026. PubMed ID: 42146036

6. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services, 2020. https://www.cdc.gov/labs/bmbl/index.html

7. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules/

8. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. https://www.ncbi.nlm.nih.gov/books/

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