Dicloxacillin Does NOT Adequately Cover Group C Streptococcus
Dicloxacillin is NOT recommended for Group C streptococcal infections—penicillin G or penicillin V remains the treatment of choice. While dicloxacillin is a penicillinase-resistant penicillin with excellent activity against staphylococci, it is specifically indicated for penicillinase-producing staphylococcal infections, not streptococcal infections 1.
Why Penicillin, Not Dicloxacillin
The FDA label for dicloxacillin explicitly states it is "indicated in the treatment of infections caused by penicillinase-producing staphylococci" 1. Group C streptococci do not produce penicillinase and remain exquisitely sensitive to standard penicillin.
For Group C streptococcal infections, use:
- Penicillin G: 2-4 million units IV every 4-6 hours 2
- Penicillin V: 250-500 mg orally 4 times daily for 10 days (for less severe infections)
Evidence on Group C Streptococcus Susceptibility
Research demonstrates that Group C streptococci maintain excellent susceptibility to penicillin 3, 4. A comprehensive susceptibility study of 312 beta-hemolytic streptococci (including 50 Group C isolates) showed all Group C strains were susceptible to penicillin G and other beta-lactam antibiotics 3. The study specifically noted that only 2 Group C strains showed resistance to oxacillin (a penicillinase-resistant penicillin similar to dicloxacillin), while remaining susceptible to standard penicillin 3.
Key point: Penicillin tolerance can occur with Group C streptococci 5, 4, meaning the organism may require higher concentrations or longer exposure for bactericidal effect. This makes choosing the right antibiotic even more critical—penicillin G achieves much higher serum levels than dicloxacillin and is the proven agent.
Clinical Context for Group C Streptococcus
Group C streptococci (particularly Streptococcus dysgalactiae subsp. equisimilis) cause:
- Pharyngitis
- Skin and soft tissue infections (cellulitis, erysipelas)
- Invasive infections (bacteremia, endocarditis, toxic shock syndrome) 6
For skin and soft tissue infections where Group C streptococcus is suspected or confirmed, guidelines recommend agents active against streptococci 2:
- Penicillin G 2-4 million units IV every 4-6 hours PLUS clindamycin 600-900 mg IV every 8 hours for necrotizing infections 2
- Dicloxacillin, cephalexin, or clindamycin for empiric cellulitis treatment 2, 7—but note this is empiric therapy before culture results, targeting both staphylococci and streptococci
When Dicloxacillin Might Be Used
Dicloxacillin may be appropriate for empiric treatment of cellulitis when the causative organism is unknown 2, 7. However, once Group C streptococcus is identified by culture, therapy should be changed to penicillin 1. The FDA label explicitly states: "If the susceptibility tests indicate that the infection is due to an organism other than a resistant staphylococcus, therapy should not be continued with a penicillinase-resistant penicillin" 1.
Common Pitfalls
Assuming all penicillins are equivalent: Dicloxacillin is optimized for staphylococci, not streptococci. It achieves lower serum levels and has a narrower spectrum against streptococci compared to penicillin G or V.
Continuing empiric therapy after culture results: Once Group C streptococcus is identified, de-escalate from broad-spectrum or anti-staphylococcal agents to penicillin 1.
Ignoring penicillin tolerance: If clinical response is slow despite appropriate penicillin therapy, check MIC/MBC ratios and consider adding clindamycin for its bactericidal effect and toxin suppression 2, 4.