Should I routinely select antibiotics with activity against anaerobes in my patients with presumed aspiration pneumonia?

Anaerobes have been considered a major cause of aspiration pneumonia (AP) based on studies published in 1970’s (1-3). More recent data, however, suggest that anaerobes no longer play an important role in most cases of AP (4-7) and routine inclusion of specific anti-anaerobic drugs in their treatment is no longer necessary.

An important reason for anaerobes not playing an important role in AP in the current era is the change in the demographics of patients who may be affected. Patients reported in older studies often suffered from alcohol use disorder, drug ingestion, seizure disorders and acute cerebrovascular accident. In contrast, more recent data show that AP often occurs in nursing home residents, the elderly with cognitive impairment, and those with dysphagia, gastrointestinal dysmotility or tube feeding (8,9).

In addition, many cases of AP reported in older studies involved delay of 4 or more days before seeking medical attention and, not surprisingly, often presented with lung abscess, necrotizing pneumonia, empyema, or putrid sputum, features that are relatively rare in the current era.

Further supporting the diminishing role of anaerobes in AP, are recent microbiological studies of the respiratory tract in AP revealing the infrequent isolation of anaerobes and, even when isolated, often coexisting with aerobic bacteria. The latter observation is important because, due to the alteration in the redox potential (9,10), treatment of aerobic bacteria alone may lead to less oxygenation consumption and less favorable environment for survival of anaerobes in the respiratory tract.

We should also always consider the potential adverse effects of unnecessary antibiotics with anaerobic activity in our frequently debilitated patients, including gastrointestinal dysbiosis (associated with Clostridiodes difficile infections and overgrowth of antibiotic-resistant pathogens such as vancomycin-resistant enterococci (VRE), hypersensitivity reactions, drug interactions, and central nervous system toxicity (11,12).

Thus, the weight of the evidence does not justify routine anaerobic coverage of AP in today’s patients.


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1. Bartlett JG, Gorbach SL, Finegold SM. The bacteriology of aspiration pneumonia. Am J Med. 1974;56(2):202-7.
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3. Bartlett JG, Gorbach SL. The triple threat of aspiration pneumonia. Chest. 1975;68(4):560-6.
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8. Bowerman TJ, Zhang J, Waite LM. Antibacterial treatment of aspiration pneumonia in older people: a systematic review. Clin Interv Aging. 2018;13:2201-13.
9. Mandell LA, Niederman MS. Aspiration Pneumonia. N Engl J Med. 2019 Feb 14;380(7):651-663. doi: 10.1056/NEJMra1714562.
10. Walden, W. C., & Hentges, D. J. (1975). Differential effects of oxygen and oxidation-reduction potential on the multiplication of three species of anaerobic intestinal bacteria. Applied microbiology, 30(5), 781–785.
11. Sullivan A, Edlund C, Nord CE. Effect of antimicrobial agents on the ecological balance of human microflora. Lancet Infect Dis. 2001;1(2):101-14.
12. Bhalla A, Pultz NJ, Ray AJ, Hoyen CK, Eckstein EC, Donskey CJ. Antianaerobic antibiotic therapy promotes overgrowth of antibiotic-resistant, gram-negative bacilli and vancomycin-resistant enterococci in the stool of colonized patients. Infect Control Hosp Epidemiol. 2003;24(9):644-9.


Contributed by Amar Vedamurthy, MD, MPH, Mass General Hospital, Boston, MA

Should I routinely select antibiotics with activity against anaerobes in my patients with presumed aspiration pneumonia?

Is my hospitalized patient with possible pneumonia at risk of Clostridium difficile-associated disease after only 1-3 days of empiric antibiotic therapy?

Yes! Even relatively brief duration of antibiotic therapy may increase the risk of Clostridium difficile-associated disease (CDAD) in a susceptible host.
In a study of hospitalized patients with new-onset diarrhea, prior exposure to levofloxacin and cefazolin was significantly associated with CDAD with the median duration of therapy for levofloxacin of 3 days (range 1-18 days), and for cefazolin 2 days (range 1-3 days) (1). Similarly, a study in hospitalized patients during a CDAD epidemic found a significantly increased risk of CDAD among patients who received fluoroquinolones for only 1-3 days (hazard ratio 2.4) with a 95% confidence interval (1.6-3.6) that overlapped 4-6 days and ≥ 7 days treatment groups (2). Yet another study found no significant difference in the risk of CDAD between those on antibiotic for < 4 days vs 4-7 days of antibiotics (3). CDAD following a single dose of cefazolin has also been reported (4).
Of interest, laboratory studies in mice have shown a profound alteration of intestinal microbiota following a single dose of clindamycin, resulting in increased susceptibility to C. difficile colitis (5).
So although duration of antibiotic therapy is an important factor in CDAD (3, 6) and we should minimize the duration of antibiotic therapy whenever possible, not starting antibiotics in the absence of clear indication is even better!

1. Manian FA, Aradhyula S, Greisnauer S, et al. Is it Clostridium difficile infection or something else? A case-control study of 352 hospitalized patients with new-onset diarrhea. S Med J 2007;100:782-786.
2. Pepin J, Saheb N, Coulombe MA, et al. Emergence of fluoroquinolones as the predominant risk factor for Clostridium difficile-associated diarrhea: a cohort study during an epidemic in Quebec. Clin Infect Dis 2005;41:1254-60.
3. Stevens V, Dumyati G, Fine LS, et al. Cumulative antibiotic exposures over time and the risk of Clostridium difficile infection. Clin Infect Dis 2011;53:42-48.
4. Mcneeley SG, Anderson GD, Sibai BM. Clostridium difficile colitis associated with single dose cefazolin prophylaxis. Ob Gynecol 1985;66:737-8.
5. Buffie CG, Jarchum I, Equinda M, et al. Profound alterations of intestinal microbiota following a single dose of clindamycin results in sustained susceptibility to Clostridium difficile-induced colitis. Infect Immun 2011;80: 62-73.
6. Chalmers JD, Akram AR, Sinanayagam A, et al. Risk factors for Clostridium difficile infection in hospitalized patients with community-acquired pneumonia. J Infect 2016;73:45-53.

Disclosure: The contributor of this post was a coinvestigator of a cited study (ref. 1).

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Is my hospitalized patient with possible pneumonia at risk of Clostridium difficile-associated disease after only 1-3 days of empiric antibiotic therapy?