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Effect of prophylactic antibiotics upon mixed infections with Bacteroides fragilis
1Naval Medical Research Institute, Naval Medical Command National Capital Region, Bethesda, Maryland.
Abstract:
Antimicrobial agents were used alone or in combinations to explore the effect of prophylactic antimicrobial therapy. Subcutaneous abscesses in mice were induced by single and mixed infections of Bacteroides fragilis, Staphylococcus aureus, Group A streptococci and Escherichia coli. The infected mice were treated with three doses of gentamicin, cefoxitin, metronidazole or clindamycin alone or else metronidazole or clindamycin in combination with gentamicin. Mice were sacrificed five days after inoculation and the bacterial contents of the abscesses were determined. Infection induced by a single bacteria always responded to appropriate antimicrobial therapy. However, in infections caused by two organisms, therapy directed at either the Bacteroides fragilis (with metronidazole or clindamycin) or Escherichia coli (with gentamicin) was effective in not only significantly reducing the colony forming units (CFU) of the target organism but also reducing the number of untreated bacteria. Clindamycin alone was effective in reducing the CFU of both components of mixed infections of Bacteroides fragilis with either Staphylococcus aureus or Group A streptococci. Cefoxitin alone and the combination of either clindamycin or metronidazole with gentamicin were effective against all mixed infections. These data support the need to provide coverage for all components of mixed infections with single or combination therapy.
Insights
Prophylactic antimicrobial therapy effectively treated single bacterial infections. For mixed infections, targeting one bacterium significantly reduced both target and non-target bacteria, supporting comprehensive treatment strategies.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Subcutaneous abscesses are often polymicrobial.
- Effective prophylactic antimicrobial therapy is crucial for managing mixed bacterial infections.
- Understanding the efficacy of specific antimicrobial agents against common abscess-causing bacteria is essential.
Purpose of the Study:
- To evaluate the efficacy of prophylactic antimicrobial therapy against single and mixed bacterial subcutaneous abscesses in a mouse model.
- To determine the impact of specific antimicrobial agents (gentamicin, cefoxitin, metronidazole, clindamycin) alone and in combination on bacterial load.
- To assess whether targeting one component of a mixed infection could impact the other.
Main Methods:
- Subcutaneous abscesses were induced in mice using single and mixed infections of Bacteroides fragilis, Staphylococcus aureus, Group A streptococci, and Escherichia coli.
- Mice received prophylactic treatment with gentamicin, cefoxitin, metronidazole, or clindamycin, alone or in combination (metronidazole/clindamycin with gentamicin).
- Bacterial colony-forming units (CFU) in abscesses were quantified five days post-inoculation.
Main Results:
- Single bacterial infections consistently responded to appropriate antimicrobial therapy.
- In mixed infections, therapy targeting Bacteroides fragilis (metronidazole/clindamycin) or Escherichia coli (gentamicin) reduced CFU of both target and non-target bacteria.
- Clindamycin monotherapy effectively reduced CFU in mixed infections of B. fragilis with S. aureus or Group A streptococci.
- Cefoxitin monotherapy and clindamycin/metronidazole-gentamicin combinations were effective against all tested mixed infections.
Conclusions:
- Antimicrobial therapy must provide coverage for all bacterial components in mixed infections.
- Both single-agent and combination therapies can be effective, but strategic selection is key.
- These findings support the development of targeted prophylactic antimicrobial regimens for polymicrobial infections.