Does Zosyn Cover Gram-Positive Bacilli?
Yes, piperacillin/tazobactam (Zosyn) provides effective coverage against most clinically relevant gram-positive bacilli, including Bacillus species, though some resistant organisms like Corynebacterium jeikeium require vancomycin. 1
Spectrum Against Gram-Positive Bacilli
Piperacillin/tazobactam demonstrates broad activity against gram-positive organisms, including bacilli:
The FDA label confirms activity against gram-positive bacteria including Streptococcus species and methicillin-susceptible Staphylococcus species, with in vitro data showing 91.7% susceptibility for Bacillus species at MIC50 of 0.5 mcg/mL. 2, 3
Clinical guidelines specifically note that piperacillin (the active component of Zosyn) has "excellent activity against most strains" of gram-positive bacteria, contrasting favorably with ceftazidime which has poor gram-positive coverage. 1
In vitro surveillance data from 5,889 clinical isolates demonstrated that piperacillin/tazobactam possessed a "superior spectrum against Gram-positive cocci" with 92.2% susceptibility, outperforming both ticarcillin/clavulanic acid and ceftazidime. 4
Important Clinical Distinctions
The combination provides reliable coverage for most gram-positive bacilli, but specific exceptions exist:
Bacillus species show good susceptibility (91.7%), though these infections are typically not severe and may not require vancomycin unless the organism proves resistant. 1, 3
Corynebacterium jeikeium is "susceptible only to vancomycin" and represents a notable exception where piperacillin/tazobactam will not provide adequate coverage. 1
The drug shows 81.8% susceptibility against Corynebacterium species overall (excluding C. jeikeium) with MIC50 of 2 mcg/mL. 3
Comparison to Other Agents
When considering empiric therapy for infections potentially involving gram-positive bacilli:
Piperacillin/tazobactam offers significantly broader gram-positive coverage than ceftazidime, which has "poor activity against gram-positive cocci" and would be inadequate for gram-positive bacilli. 5
The combination demonstrates activity encompassing "most Gram-positive and Gram-negative aerobic bacteria and anaerobic bacteria," making it suitable for polymicrobial infections. 6, 7
For serious infections where C. jeikeium or vancomycin-only-susceptible organisms are suspected (particularly in immunocompromised patients with catheter-related infections), vancomycin should be added to the initial regimen. 1
Clinical Caveats
Key considerations when relying on piperacillin/tazobactam for gram-positive coverage:
While the drug covers most gram-positive bacilli, it does NOT cover methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant enterococci (VRE). 2, 3
In neutropenic patients at institutions where resistant gram-positive organisms are common, consider adding vancomycin empirically until cultures exclude resistant pathogens, then discontinue within 24-48 hours if not identified. 1
The broad spectrum allows for monotherapy in most polymicrobial infections without requiring additional gram-positive coverage, unlike ceftazidime which would necessitate combination therapy. 5, 8