Normal Respiratory Flora as a Cause of Community-Acquired Pneumonia

Abstract

Background Intensive studies have failed to identify an etiologic agent in >50% cases of community-acquired pneumonia (CAP). Bacterial pneumonia follows aspiration of recognized bacterial pathogens (RBPs) such as Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus after they have colonize the nasopharynx. We hypothesized that aspiration of normal respiratory flora (NRF) might also cause CAP. Methods We studied 120 patients hospitalized for CAP who provided a high-quality sputum specimen at, or soon after admission, using Gram stain, quantitative sputum culture, bacterial speciation by matrix-assisted laser desorption ionization time-of-flight, and viral polymerase chain reaction. Thresholds for diagnosis of bacterial infection were ≥105 colony-forming units (cfu)/mL sputum for RBPs and ≥106 cfu for NRF. Results Recognized bacterial pathogens were found in 68 of 120 (56.7%) patients; 14 (20.1%) of these had a coinfecting respiratory virus. Normal respiratory flora were found in 31 (25.8%) patients; 10 (32.2%) had a coinfecting respiratory virus. Infection by ≥2 RBPs occurred in 10 cases and by NRF together with RBPs in 13 cases. Among NRF, organisms identified as Streptococcus mitis, which share many genetic features of S pneumoniae, predominated. A respiratory virus alone was found in 16 of 120 (13.3%) patients. Overall, an etiologic diagnosis was established in 95.8% of cases. Conclusions Normal respiratory flora, with or without viral coinfection, appear to have caused one quarter of cases of CAP and may have played a contributory role in an additional 10.8% of cases caused by RBPs. An etiology for CAP was identified in >95% of patients who provided a high-quality sputum at, or soon after, the time of admission.

Candida Species in Community-Acquired Pneumonia in Patients With Chronic Aspiration

“…Importantly, these cases were not selected because of a history suggesting aspiration but were identi ed by sputum criteria, after which the historical information at admission was determined from the medical record. Additionally, we observed a high rate of comorbid conditions: the average Charlson Comorbidity Index was 8 for patients with Candida pneumonia compared to 5.5 for patients with CAP due to other respiratory pathogens (t-test, p = 0.02) [10]. In all cases, physical examination disclosed rales, and chest radiographs showed bilateral opacities.…”

“…pneumoniae, H. in uenzae, S. aureus, and/or P. aeruginosa) in 3 cases and > 10 6 CFU/ml NRF (Lactobacillus) in the other 3 cases. When Gram stain identi ed large numbers of bacteria that were not cultured, we regarded these as probable anaerobic organisms [10]. PCR for respiratory viruses was performed in 3 cases and was positive in 2.…”

“…Microbiology. Methods for quantifying bacteria and Candida in sputum were described previously [10]. Brie y, sputum was solubilized with 0.4% N-acetyl cysteine, and ten-fold dilutions were made.…”

“…Our predetermined criterion for determining an etiologic role for a recognized bacterial pathogen, such as Streptococcus pneumoniae, Haemophilus in uenzae, Staphylococcus aureus, or Pseudomonas aeruginosa, was the nding of > 10 5 CFU/ml in a high-quality sputum sample [10,[12][13][14][15]. To designate an etiologic role for Candida, we used more stringent criteria, as we have done for organisms that are usually reported as "normal respiratory ora" such as as viridans streptococci, Corynebacteria, or Lactobacillus [10,16]: (1) Microscopic examination of Gram-stained sputum showed large numbers of yeast forms, including some within polymorphonuclear leukocytes; and (2) quantitative culture demonstrated > 10 6 CFU Candida per ml with or without other bacteria in numbers that met the above-stated criteria. In this observational study, laboratory studies, in addition to blood counts, usual blood chemistries, routine sputum and blood cultures, PCR on a nasopharyngeal swab for respiratory viruses, plasma procalcitonin, sputum WBC counts, and serum 1,3-beta-D glucan were done in a variable number of patients.…”

“…In the course of a prospective observational study that documented an important role for normal respiratory ora as the cause of community acquired pneumonia (CAP) [10], we observed a surprising number of patients in whom Candida appeared to be playing a contributory role. In each case, chronic aspiration had been identi ed as a possible cause in the admitting history.…”