Understanding Group B Streptococcus (GBS)
Group B Streptococcus (Streptococcus agalactiae) is a common type of bacteria that colonizes the human body, particularly the gastrointestinal and genitourinary tracts. While typically harmless in healthy adults, it can cause severe, life-threatening infections in newborns, the elderly, and immunocompromised individuals. In neonates, GBS can cause early-onset disease (EOD), including sepsis, pneumonia, and meningitis, or late-onset disease (LOD), which can occur up to 89 days after birth.
Identifying GBS is crucial for proper clinical management, especially in pregnant women, where screening and antibiotic prophylaxis are used to prevent neonatal infections. Microbiological laboratories use a series of tests to distinguish GBS from other Streptococcus species and other bacteria. One of the most important of these is the bacitracin test, which exploits a key difference in antibiotic susceptibility between streptococcal groups.
The Bacitracin Test: A Differential Tool
Bacitracin is a peptide antibiotic that works by inhibiting cell wall synthesis in actively growing cells. The bacitracin susceptibility test involves placing a small paper disc containing the antibiotic onto an agar plate streaked with a pure bacterial culture. After incubation, the results are interpreted based on whether a zone of inhibition—an area of no bacterial growth—forms around the disc.
- Susceptible Result: A visible zone of inhibition indicates that the bacteria are sensitive to bacitracin, which is characteristic of Group A Streptococcus (Streptococcus pyogenes).
- Resistant Result: No zone of inhibition means the bacteria are resistant to bacitracin, a common trait of GBS.
This test is valuable because it provides a rapid, presumptive identification, helping microbiologists quickly narrow down the possibilities when identifying beta-hemolytic streptococci.
Is Group B Strep Susceptible to Bacitracin? The Definitive Answer
In short, no, Group B Strep is generally not susceptible to bacitracin. This bacitracin resistance is a fundamental characteristic of Streptococcus agalactiae, serving as a key diagnostic marker in clinical microbiology. While bacitracin is effective against Group A strep (S. pyogenes), GBS has innate resistance mechanisms that prevent the antibiotic from disrupting its cell wall synthesis effectively.
Several large-scale studies have confirmed this high rate of resistance in GBS strains. For example, one study found that nearly all GBS isolates (97.9%) were resistant to bacitracin. This consistent resistance pattern makes the bacitracin test a reliable first step in differentiating GBS from the closely related but sensitive Group A Strep.
How the Bacitracin Test is Performed
Microbiologists follow a specific procedure to ensure the reliability of the bacitracin test:
- Preparation: Colonies of the suspected beta-hemolytic streptococcus are isolated from a primary culture plate.
- Inoculation: The isolated colonies are streaked onto a fresh sheep blood agar plate to create a heavy, confluent growth.
- Disc Placement: A bacitracin-impregnated disc is placed aseptically onto the center of the inoculated agar.
- Incubation: The plate is incubated at 35°C in a CO2-enriched environment for 18-24 hours.
- Interpretation: The plate is examined for a zone of inhibition around the disc. The absence of a zone is indicative of GBS.
The Clinical Significance of Bacitracin Resistance
The resistance of GBS to bacitracin has several important clinical implications:
- Diagnostic Tool: The resistance pattern is a critical piece of the puzzle for identifying GBS in a laboratory. It helps microbiologists make a preliminary identification, which is then confirmed with other biochemical tests.
- Clinical Management: Since GBS is resistant to bacitracin, this antibiotic is not an effective treatment option for systemic GBS infections. Instead, clinicians rely on other antibiotics, most commonly penicillin G or ampicillin, for treatment and intrapartum prophylaxis.
- Preventing Misdiagnosis: Understanding bacitracin resistance is crucial for avoiding misdiagnosis. A finding of resistance on a bacitracin test points away from Group A Strep as the cause of infection, guiding further investigation toward GBS or other organisms.
The Limits of the Bacitracin Test
While a useful screening tool, the bacitracin test is not foolproof and has limitations.
- False-Positive Results: A small percentage of non-Group A streptococci, including GBS strains (around 5%), can show some susceptibility to bacitracin, leading to false-positive results.
- Inadequate Specificity: The test is highly sensitive for Group A Strep but lacks complete specificity. Other non-Group A streptococci may also be susceptible.
- Need for Confirmation: Because of these limitations, presumptive identification based on the bacitracin test alone is not sufficient. Laboratories must perform additional tests to confirm the identity of GBS.
For a definitive diagnosis, GBS identification is confirmed by serological methods, such as latex agglutination for group-specific antigens, or through biochemical tests like the CAMP test and hippurate hydrolysis. The CAMP test is particularly useful, as GBS is CAMP-positive while Group A Strep is negative.
The Mechanism of Bacitracin Resistance
The mechanism behind GBS resistance to bacitracin involves specific genetic pathways and transport systems. Bacitracin, a polypeptide antibiotic, normally interferes with peptidoglycan synthesis by binding to a lipid carrier molecule called undecaprenyl pyrophosphate (UPP). This prevents the recycling of the carrier, halting the construction of the bacterial cell wall.
However, resistant bacteria like GBS have developed mechanisms to counteract this process. In some bacterial species, resistance is mediated by ABC-type efflux pump transporters, which actively pump the antibiotic out of the bacterial cell before it can reach its target. These transporters are regulated by two-component systems that sense the presence of bacitracin and activate the resistance genes. While the specific nuances can vary between species, research suggests similar efflux-based mechanisms contribute to bacitracin resistance in GBS.
Comparison of Group A and Group B Streptococcus
| Characteristic | Group A Streptococcus (S. pyogenes) | Group B Streptococcus (S. agalactiae) |
|---|---|---|
| Bacitracin Susceptibility | Susceptible (shows zone of inhibition) | Resistant (no zone of inhibition) |
| CAMP Test | Negative | Positive |
| Hippurate Hydrolysis | Negative | Positive |
| Primary Infections | Strep throat, impetigo, scarlet fever, rheumatic fever, glomerulonephritis | Neonatal sepsis, pneumonia, meningitis, urinary tract infections |
| Typical Hemolysis | Beta-hemolytic (large zone) | Beta-hemolytic (narrow zone), though some strains are non-hemolytic |
| Clinical Importance | Pharyngitis, post-streptococcal sequelae | Neonatal infections, maternal infections |
Conclusion
In conclusion, Group B Strep is not susceptible to bacitracin, a key microbiological fact that helps differentiate it from the sensitive Group A Strep. This resistance is a crucial part of the presumptive identification process in clinical laboratories, where the bacitracin disc test is used alongside other biochemical assays like the CAMP test and hippurate hydrolysis for accurate identification. While the test provides a rapid and valuable indicator, it is not a definitive diagnostic tool due to the possibility of rare false-positive results. The innate resistance of GBS, likely mediated by genetic factors and efflux pump systems, means that bacitracin is not a viable treatment for GBS infections. Instead, clinicians must rely on other antibiotics, such as penicillin or ampicillin, for effective management and prevention, particularly in neonatal cases. Understanding this difference in susceptibility is vital for both laboratory diagnostics and clinical decision-making. For more information on GBS and streptococcal infections, consult authoritative sources like the NCBI Bookshelf and the CDC.
