Does amoxicillin (amoxicillin) cover Bacteroides fragilis?

From the Guidelines

No, amoxicillin does not effectively cover Bacteroides fragilis. Bacteroides fragilis is an anaerobic gram-negative bacterium that typically produces beta-lactamase enzymes, which can inactivate amoxicillin 1. For infections where B. fragilis is suspected or confirmed, more appropriate antibiotic choices would include metronidazole, amoxicillin-clavulanate (Augmentin), piperacillin-tazobactam, carbapenems (like meropenem or imipenem), or clindamycin.

Key Points to Consider

• B. fragilis is commonly found in intra-abdominal, pelvic, and anaerobic soft tissue infections, so proper coverage is essential when treating these conditions.
• The emergence of resistance in B. fragilis group isolates, especially against metronidazole, has important implications in the treatment of surgical infections 1.
• According to a US national survey, the rates of resistance to clindamycin and moxifloxacin were high for B. fragilis isolates, at 19% and 27%, respectively 1.
• In contrast, the rates of resistance to carbapenems and beta-lactam–beta-lactamase inhibitor combinations were low 1. If you're dealing with a polymicrobial infection that might include B. fragilis, combination therapy or a broader-spectrum agent would be necessary rather than amoxicillin alone.

Treatment Options

• Metronidazole: a potent agent against B. fragilis, with only one metronidazole-resistant B. fragilis strain documented in the study 1.
• Amoxicillin-clavulanate (Augmentin): a beta-lactam antibiotic that resists beta-lactamase degradation.
• Piperacillin-tazobactam: a beta-lactam antibiotic that inhibits beta-lactamase enzymes.
• Carbapenems (like meropenem or imipenem): broad-spectrum antibiotics that are effective against B. fragilis.
• Clindamycin: an antibiotic that is effective against B. fragilis, but with increasing resistance rates 1.

From the FDA Drug Label

Anaerobic Bacteria Bacteroides species, including Bacteroides fragilis (β-lactamase and non-β-lactamase- producing) The combination of amoxicillin and clavulanic acid is effective against Bacteroides fragilis.

• The FDA drug label for amoxicillin and clavulanate potassium indicates that it has been shown to be active against Bacteroides species, including Bacteroides fragilis 2.
• This suggests that amoxicillin, when combined with clavulanic acid, is effective against Bacteroides fragilis.

From the Research

Amoxicillin Coverage for Bacteroides Fragilis

• Amoxicillin alone has limited coverage for Bacteroides fragilis, as most strains are beta-lactamase positive and resistant to amoxicillin 3, 4, 5.
• The addition of clavulanate to amoxicillin (amoxicillin-clavulanate) enhances its coverage for Bacteroides fragilis, as clavulanate inhibits beta-lactamase production 6, 3, 4.
• Studies have shown that amoxicillin-clavulanate is effective against Bacteroides fragilis, with MIC90s ranging from 1-16 micrograms/ml 3, 4.
• Other antibiotics, such as metronidazole, imipenem, and cefoxitin, also have good activity against Bacteroides fragilis 3, 4, 5.

Susceptibility of Bacteroides Fragilis to Amoxicillin

• The susceptibility of Bacteroides fragilis to amoxicillin varies, with some studies reporting resistance rates of up to 93% 4.
• The addition of clavulanate to amoxicillin improves its susceptibility profile, with resistance rates decreasing to around 2-11% 3, 4.
• Beta-lactamase production is a major factor contributing to amoxicillin resistance in Bacteroides fragilis, with most strains being beta-lactamase positive 3, 4, 5.

Clinical Implications

• Amoxicillin-clavulanate may be a suitable option for the treatment of infections caused by Bacteroides fragilis, particularly in cases where beta-lactamase production is a concern 6, 3, 4.
• However, the use of amoxicillin-clavulanate should be guided by susceptibility testing and clinical judgment, as resistance rates can vary depending on the population and geographic location 7, 4.