What is the drug of choice for treating an infection caused by Parabacteroides distasonis?

Drug of Choice for Parabacteroides distasonis Infections

Metronidazole is the drug of choice for treating Parabacteroides distasonis infections, as it demonstrates universal susceptibility and is specifically FDA-approved for treating infections caused by Bacteroides species including B. distasonis. 1

Primary Treatment Recommendation

• Metronidazole (IV or oral) remains the most potent agent against P. distasonis, with only one documented metronidazole-resistant B. fragilis strain reported in the USA through 2008, and no resistance documented among P. distasonis isolates 2
• All P. distasonis bacteremia isolates in recent case series were susceptible to metronidazole, with successful outcomes in patients treated with this agent 3
• The FDA label specifically indicates metronidazole for intra-abdominal infections caused by Bacteroides species including B. distasonis 1

Alternative Agents (When Metronidazole Cannot Be Used)

Beta-lactam/beta-lactamase inhibitor combinations are the preferred alternatives:

• Piperacillin-tazobactam demonstrated 100% susceptibility across all Bacteroides and Parabacteroides isolates in multicenter surveillance 4
• This agent is particularly important because P. distasonis exhibits notably high MICs to carbapenems compared to other Bacteroides fragilis group members 2

Carbapenems have limited utility:

• P. distasonis shows high MICs to all beta-lactam agents including carbapenems (imipenem, meropenem, ertapenem) 2
• Resistance rates to carbapenems among B. distasonis specifically were documented, unlike the broader B. fragilis group 4
• Avoid carbapenems as monotherapy for confirmed P. distasonis infections unless susceptibility is proven 2

Important Clinical Pitfalls

Do not assume susceptibility patterns from other Bacteroides species:

• P. distasonis demonstrates the highest resistance rates within the B. fragilis group to multiple agents including clindamycin (19-29% resistance), cefoxitin, and ampicillin-sulbactam (11% resistance) 2, 4
• Among non-B. fragilis species, P. distasonis specifically shows elevated MICs to tigecycline compared to B. fragilis 2

Clindamycin should be avoided:

• Resistance rates to clindamycin among B. distasonis are among the highest in the B. fragilis group 4
• Significant increases in clindamycin resistance have been documented over surveillance periods 2

Dosing Recommendations

For metronidazole (from FDA labeling): 1

• Loading dose: 15 mg/kg IV infused over 1 hour
• Maintenance: 7.5 mg/kg IV or PO every 6 hours (maximum 4 g/24 hours)
• Duration: Typically 7-10 days for intra-abdominal infections, guided by clinical response 2

For piperacillin-tazobactam (if used):

• 3.375-4.5 g IV every 6-8 hours depending on infection severity 2

Source Control Considerations

• Surgical drainage of abscesses is paramount when present, as antimicrobial therapy alone is insufficient 5
• P. distasonis bacteremia typically occurs in patients with underlying gastrointestinal pathology or recent abdominal procedures requiring source control 3