Double Anaerobic Coverage: What is the role in clinical practice?
Anaerobic pathogens are normal flora of the oral cavity and the gastrointestinal tract. While oral anaerobic flora are mostly gram-positive organisms such as Peptococcus and Peptostreptococcus spp., the principal anaerobic intestinal flora are gram-negative bacil i such as Bacteroides fragilis, Prevotel a melaninogenica, and Fusobacterium spp. Gram-positive oral anaerobes are widely covered by most of the oral y-available agents, including penicil in. However, antibiotic activity against the most common intestinal anaerobic bacteria, Bacteroides spp., is variable. Anaerobic coverage is indicated in a variety of infectious processes, including but not limited to aspiration pneumonia, intra-abdominal infection, gynecologic infection, and diabetic foot ulcer infection. Antimicrobial agents with appreciable anaerobic activity include the fol owing: Double anaerobic coverage is the use of any combination of the above agents, which is prevalent at The Nebraska Medical Center. Redundant anaerobic coverage is the third most common problem intervened upon by the Antimicrobial Stewardship Program, accounting for approximately 20% of the interventions. Available susceptibility and clinical data do not support this practice. The fol owing susceptibility data from 2005-2007 were observed for the B. fragilis group, the most common pathogenic gram-negative anaerobes:1 RESISTANCE RATES OF VARIOUS ANTIBIOTIC AGENTS AMONG B. FRAGILIS GROUP ISOLATES
Antibiotic Agent (No. of Isolates Tested)
breakpoint (mg/L)
% Resistanta
aIsolates categorized according to CLSI breakpoints. Adapted and modified from Snydman DR, Jacobus NV, McDermott LA, et al. Lessons learned from the anaerobe survey: historical perspective and review of the most recent data (2005-2007). Clin Infect Dis. 2010;50 Suppl 1:S26-33.
With regard to gram-positive anaerobes, al the agents listed above maintain excel ent activity.2 For example, moxifloxacin was shown to have excel ent activity against gram-positive anaerobic cocci such as Peptostreptococcus spp with MICs as low as 0.25mg/L (range 0.25-1mg/L).3-5 None of the available treatment guidelines published by the Infectious Diseases Society of America (IDSA) recommend the use of double anaerobic coverage. CLINICAL SYNDROMES
Aspiration PneumoniaAspiration pneumonia and pneumonitis are common clinical syndromes. In the case of aspiration pneumonia, oral gram-positive anaerobic flora and gram-negative enterics are the pathogens of interest as opposed to those traditional y associated with intra-abdominal infections. Amoxicil in/clavulanate, clindamycin, or moxifloxacin provide excel ent anaerobic coverage for aspiration pneumonia. Aspiration pneumonitis fol ows the aspiration of gastric contents, and often no organism is implicated.
Intra-abdominal InfectionThe recent intra-abdominal guidelines published by the Infectious Diseases Society of America and the Surgical Infection Society recommend metronidazole as the anaerobic agent of choice for combination therapy with agents devoid of clinical y-significant anaerobic activity (i.e., agents other than those listed above), whereas beta-lactam monotherapy such as piperacil in/tazobactam or a carbapenem is reserved for complicated cases of intra-abdominal infection.6 Table 1 summarizes the guideline recommendations for community-acquired intra-abdominal infections.
Table 1 Empiric antibiotic therapy for community-acquired intra-abdominal infection
Healthcare-associated intra-abdominal infection includes a spectrum of adult patients who have close association with acute care hospitals or reside in chronic care settings. These patients are typical y at risk for infection with multidrug resistant (MDR) flora, P. aeruginosa and Acinetobacter species, extended-spectrum beta-lactamase (ESBL)–producing Klebsiel a and E. coli, Enterobacter species, Proteus species, methicillin-resistant Staphylococcus aureus (MRSA), Enterococci, and Candida species. Some identified risk factors for healthcare-associated intra-abdominal infection include: (1) presence of an invasive device at time of admission; (2) history of MRSA infection or colonization; or (3) history of surgery, hospitalization, dialysis, or residence in a long-term care facility in the 12 months preceding the culture date. The decision regarding an appropriate empiric regimen in these cases should be guided by local susceptibility data. Reasonable empiric therapy options for healthcare-associated intra-abdominal infections include piperacil in/tazobactam, meropenem, or a combination of cefepime plus metronidazole. Vancomycin may be added if MRSA is a concern. Pelvic Inflammatory Disease
In pelvic inflammatory disease (PID), the most common pathogens are Neisseria gonorrhoeae and
Chlamydia trachomatis, but other pathogens such as anaerobes, G. vaginalis, Haemophilus influenzae,
enteric Gram-negative bacilli, Streptococcus agalactiae, mycoplasmal bacteria (M. hominis and M.
, and U. urealyticum have also been associated with PID.7,8 Treatments are general y targeted
toward these pathogens.7,8 Anaerobic coverage is indicated if tubo-ovarian abscess is present. The
Centers for Disease Control and Prevention (CDC) guidelines do not recommend double anaerobic
coverage, and no evidence exists to show that double anaerobic coverage in PID results in better clinical
or microbiologic cure rates. The CDC’s treatment recommendations are summarized in table 2.7 Haggerty
et al. have summarized several PID trials in a recent article.9 The therapies and their respective clinical
cure rates were: ofloxacin (95%) vs. cefoxitin plus doxycycline (93%); clindamycin plus ciprofloxacin
(97%) vs. ceftriaxone and doxycycline (95%); moxifloxacin (90%) vs. ofloxacin plus metronidazole (91%);
doxycycline plus metronidazole (91%) or ciprofloxacin plus tinidazole (96%); azithromycin alone (97%) or
azithromycin plus metronidazole (96%) vs. metronidazole plus doxycycline plus cefoxitin plus probenecid
(95%) or doxycycline plus amoxicil in/clavulanate (95%); doxycycline plus metronidazole (35%);
meropenem (88%) vs. clindamycin plus gentamicin (90%). The microbiologic eradication rate was also
high, with a median of over 90% (range 88-100%). Interestingly, regimens with or without an anti-
anaerobic agent produced similar clinical cure rates and microbiologic eradication rates. However, the
CDC stil suggests the optional addition of metronidazole to ofloxacin therapy given higher treatment
failure in non-gonococcal, non-chlyamydial PID in the ofloxacin trial.7,8 The trial that used metronidazole
plus doxycycline plus cefoxitin reported higher rate of adverse events and discontinuations.10 Based on
these trials, the use of double anaerobic coverage in PID is unfounded.
Table 2. Treatment recommendations for PID
Doxycycline ± Metronidazole
Cefoxitin IM plus Probenecid x1 dose
Doxycycline ± metronidazole
Avoid fluoroquinolone-based regimen if N. gonorrheae is suspected and antimicrobial susceptibility data are unavailable. Discontinue parenteral therapy after 24 hours of clinical improvement. Duration of therapy is 14 days and may be completed orally with doxycycline alone or the addition of clindamycin or metronidazole to doxycycline if PID is complicated by tubo-ovarian abscess. CONCLUSIONS
Use of multiple drugs active against anaerobes is not necessary and puts the patients at risk for additional drug toxicities. No data or guidelines support the use of two anti-anaerobic drugs in clinical practice, with two clinical exceptions (see below). 1. Metronidazole can be added to another agent with anaerobic activity when being used to treat 2. Clindamycin can be added to another agent with anaerobic activity when being used for the REFERENCES
1. Snydman DR, Jacobus NV, McDermott LA, et al. Lessons learned from the anaerobe survey: historical perspective and review of the most recent data (2005-2007). Clin Infect Dis. 2010; 50 Suppl 1: S26-332. Kononen E, Bryk A, Niemi P, et al. Antimicrobial susceptibilities of Peptostreptococcus anaerobius and the newly described Peptostreptococcus stomatis isolated from various human sources. Antimicrob Agents Chemother. 2007; 51: 2205-73. Edlund C, Sabouri S, Nord CE. Comparative in vitro activity of BAY 12-8039 and five other antimicrobial agents against anaerobic bacteria. Eur J Clin Microbiol Infect Dis. 1998; 17: 193-54. Stein GE, Schooley S, Tyrrel KL, et al. Bactericidal activities of methoxyfluoroquinolones gatifloxacin and moxifloxacin against aerobic and anaerobic respiratory pathogens in serum. Antimicrob Agents Chemother. 2003; 47: 1308-125. Goldstein EJ, Citron DM, Warren YA, et al. In vitro activity of moxifloxacin against 923 anaerobes isolated from human intra-abdominal infections. Antimicrob Agents Chemother. 2006; 50: 148-556. Solomkin JS, Mazuski JE, Bradley JS, et al. Diagnosis and management of complicated intra-abdominal infection in adults and children: guidelines by the Surgical Infection Society and the Infectious Diseases Society of America. Clin Infect Dis. 2010; 50: 133-647. Centers for Disease Control and Prevention. Updated recommended treatment regimens for gonococcal infections and associated conditions - United States, April 2007. Available at 8. Workowski KA, Berman SM. Sexual y transmitted diseases treatment guidelines, 2006. MMWR Recomm Rep. 2006; 55: 1-949. Haggerty CL, Ness RB. Newest approaches to treatment of pelvic inflammatory disease: a review of recent randomized clinical trials. Clin Infect Dis. 2007; 44: 953-6010. Bevan CD, Ridgway GL, Rothermel CD. Efficacy and safety of azithromycin as monotherapy or combined with metronidazole compared with two standard multidrug regimens for the treatment of acute pelvic inflammatory disease. J Int Med Res. 2003; 31: 45-54 June 2010
Prepared by:
Jessica C. Njoku, Pharm.D., BCPS
Reviewed by: Elizabeth D. Hermsen, Pharm.D., M.B.A., BCPS-ID, Mark E. Rupp, M.D., Trevor C.


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