Streptococcus Isolate Antibiotic Susceptibility Across 7 Classes
No, you should not expect a Streptococcus isolate to be sensitive to all 7 classes of antibiotics tested, as resistance patterns vary significantly by species and geographic location, with documented resistance to multiple antibiotic classes being common.
Expected Resistance Patterns by Streptococcus Species
Group B Streptococcus (S. agalactiae)
- Penicillins and cephalosporins: 100% susceptibility remains universal with no documented resistance worldwide 1, 2
- Macrolides (erythromycin): Variable resistance rates exist, requiring susceptibility testing before use 1
- Clindamycin: Resistance occurs and requires testing, including D-zone testing for inducible resistance 1
- Vancomycin: Universal susceptibility as a backup option 2
S. pneumoniae (Pneumococcus)
- Beta-lactams: 31-37% of isolates show penicillin nonsusceptibility (intermediate or resistant) 1, 3
- Macrolides: 20-40% resistance rate in most studies 3
- Clindamycin: Approximately 22% resistance 3
- Trimethoprim-sulfamethoxazole: 35-37% resistance 1, 3
- Tetracyclines: 21% resistance 1
- Fluoroquinolones: <2% resistance (lowest resistance rate) 1
- Vancomycin: Universal susceptibility 4
Group A Streptococcus (S. pyogenes)
- Penicillins: 100% susceptibility maintained over decades 5, 6
- Macrolides: 5-8% resistance in the United States, though variable by region 5, 6
- Clindamycin: Resistance documented but less common 6
- Tetracycline: Resistance documented 6
- Trimethoprim-sulfamethoxazole: Not effective due to intrinsic resistance mechanisms 5
Key Clinical Implications
Cross-Resistance Patterns
- Penicillin-resistant S. pneumoniae typically shows resistance to multiple other classes, with 26% resistant to penicillin plus two other classes, and 16% resistant to four or more classes 1
- As penicillin resistance increases, resistance to erythromycin, clindamycin, TMP-SMX, and tetracycline also increases proportionally 1
Testing Requirements
- Susceptibility testing is mandatory when considering alternatives to penicillin for GBS in penicillin-allergic patients 1
- The D-zone test must be performed for clindamycin when erythromycin resistance is present to detect inducible resistance 1
- All pneumococcal isolates with proven pathologic significance require susceptibility testing due to high incidence of multidrug resistance 7
Common Pitfalls to Avoid
- Never assume universal susceptibility across all classes, even for historically susceptible organisms like Group A Streptococcus, as geographic variation exists 6
- Do not use trimethoprim-sulfamethoxazole empirically for streptococcal infections due to high resistance rates (35-37% for S. pneumoniae) and intrinsic resistance in Group A Streptococcus 1, 5, 3
- Avoid empiric clindamycin use without susceptibility testing, particularly for GBS where inducible resistance may not be detected by standard testing 1
- Recognize that older stored isolates show similar resistance patterns to recent isolates, indicating resistance is not a new phenomenon but requires ongoing vigilance 8
Practical Algorithm for Interpretation
When reviewing susceptibility results for 7 antibiotic classes:
Expect universal susceptibility to: Penicillin/ampicillin (for GBS and Group A Strep), vancomycin (all species) 1, 2, 4
Expect variable resistance requiring testing: Macrolides, clindamycin, tetracyclines, TMP-SMX 1, 3, 6
If penicillin resistance is present (S. pneumoniae): Anticipate resistance to 2-4 additional classes 1
Consider fluoroquinolones as most reliably active against resistant S. pneumoniae (<2% resistance) 1