How should I interpret the growth of “normal respiratory flora” from sputum of my patient with community-acquired pneumonia (CAP)?

Since the primary reason for obtaining a sputum culture in a patient with pneumonia is to sample the lower respiratory tract, you should first verify that the sputum was “adequate” by reviewing the gram stain. Absence of neutrophils (unless the patient is neutropenic) with or without epithelial cells on gram stain of sputum suggests that it may not be an adequate sample (ie, likely saliva)1, and therefore growth of normal respiratory flora (NRF) should not be surprising in this setting.  

Other potential explanations for NRF on sputum culture in patients with CAP include:2-5

  • Delay in sputum processing with possible overgrowth of oropharyngeal flora.
  • Pneumonia caused by pathogens that do not grow on standard sputum culture media (eg, atypical organisms, viruses, anaerobes).
  • Pneumonia caused by potential pathogens such as as Streptococcus mitis and Streptococcus anginosus group that may be part of the NRF.
  • Initiation of antibiotics prior to cultures (eg, in pneumococcal pneumonia).

Of note, since 2010, several studies have shown that over 50% of patients with CAP do not have an identifiable cause.3 So, growing NRF from sputum of patients with CAP appears to be common.

References

  1. Wong LK, Barry AL, Horgan SM. Comparison of six different criteria for judging the acceptability of sputum specimens. J Clin Microbiol 1982;16:627-631. https://www.ncbi.nlm.nih.gov/pubmed/7153311
  2. Donowitz GR. Acute pneumonia. In Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases (2010). Churchill Livingstone, pp 891-916.
  3. Musher DM, Abers MS, Bartlett JG. Evolving understanding of the causes of pneumonia in adults, with special attention to the role of pneumococcus. Clin Infect Dis 2017;65: 1736-44. https://www.ncbi.nlm.nih.gov/pubmed/29028977
  4. Abers MS, Musher DM. The yield of sputum culture in bacteremic pneumococcal pneumonia after initiation of antibiotics. Clin Infect Dis 2014; 58:1782. https://www.ncbi.nlm.nih.gov/pubmed/24604901
  5. Bartlett JG, Gorbach SL, Finegold SM. The bacteriology of aspiration pneumonia. Bartlett JG, Gorbach SL, Finegold SM. Am J Med 1974;56:202-7. https://www.ncbi.nlm.nih.gov/pubmed/4812076
How should I interpret the growth of “normal respiratory flora” from sputum of my patient with community-acquired pneumonia (CAP)?

Is cefpodoxime an appropriate oral antibiotic substitute for ceftriaxone when treating patients with respiratory tract infections caused by penicillin-resistant Streptococcus pneumoniae (PRSP)?

Short answer: No!

Although cefpodoxime is also a 3rd generation cephalosporin, its invitro activity against PRSP is not comparable to that of ceftriaxone.  In a study of 21,605 strains of S. pneumoniae collected internationally, whereas 89.1% of PRSP isolates were susceptible to ceftriaxone, only 35% were susceptible to cefpodoxime (1).  Among isolates resistant to penicillin and erythromycin, the susceptibility to ceftriaxone was 86.9% compared to that of 22.7% for cefpodoxime.

This information is important since 32%, and 17.6% of all S. pneumoniae isolates tested in this study  were either penicillin-resistant or penicillin- and erythromycin-resistant, respectively.  So, when it comes to the coverage of PRSP, there is no oral cephalosporin “equivalent” to ceftriaxone and that includes cefpodoxime.  In fact, the package insert of cefpodoxime states that cefpodoxime is active against S. pneumoniae “excluding penicillin-resistant strains” (2).

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References

  1. Pottumarthy S. Fritsche TR, Jones RN. Comparative activity of oral and parenteral cephalosporins tested against multidrug-resistant Streptococcus pneumonia: report from SENTRY Antimicrobial Surveillance Program (1997-2003). Diag Microbiol Infect Dis 2005;51:147-150. https://www.sciencedirect.com/science/article/pii/S0732889304002081    
  2. http://www.accessdata.fda.gov/drugsatfda_docs/label/2007/050674s014,050675s017lbl.pdf; accessed June 20, 2016.
Is cefpodoxime an appropriate oral antibiotic substitute for ceftriaxone when treating patients with respiratory tract infections caused by penicillin-resistant Streptococcus pneumoniae (PRSP)?