GENERAL BACKGROUND​

• Anaerobes encompass a collection of G(+) and G(-) organisms that require a reduced oxygen environment for growth
• Anaerobes that cause infection in humans can survive in environments with more oxygen but cannot replicate in such conditions
• Some produce spores, while others do not; some have factors that increase their virulence compared to other anaerobes
• More than 100 species of anaerobes comprise normal human flora and are found in a number of areas, including skin, nose, mouth, throat, intestines, and vagina
• Infections are due to breakdown of relationship with the host and are often polymicrobial
• About 5% of all bacteremias are due to anaerobes
  • Bacteroides is #1 cause, Clostridium #2 cause (perfringens and septicum species, not difficile)
  • Source is abdominal in 50-70% of cases, female genital tract 5-20% of cases, and soft tissue 5-20% of cases
• ​Resistance is increasing to clindamycin, cefotetan, cefoxitin, cefotaxime, penicillin, and fluoroquinolones
• Resistance to carbapenems and beta lactamase/beta lactamase inhibitor combinations remains low

LABORATORY

• ​Gram (-), non-spore forming
• B. fragilis most common species, also most common anaerobe found in intra-abdominal infections

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​TREATMENT OPTIONS

• Susceptibilities within the Bacteroides group vary by species
  • ​B. fragilis is most susceptible, B. thetaiotaomicron most resistant
• Most are resistant to penicillin, ampicillin, and increasingly resistant to cephalosporins (cefoxitin, cefotetan, and cefmetazole resistance increasing)
• Over 90% are susceptible to beta lactam/beta lactamase inhibitors (amox/clav, ampicillin/sulbactam, piperacillin/tazobactam) and carbapenems
• Clindamycin resistance varies by area but is considered significant
• Moxifloxacin resistance is variable, or may develop during treatment in susceptible strains