PURPOSE: Viable bacteria are often isolated from airway secretions in clinically stable patients with chronic bronchitis. We hypothesized that the number of organisms and bacterial species might be important modulators of airway inflammation.

SUBJECTS AND METHODS: We performed quantitative sputum cultures in 160 stable patients [55 with chronic obstructive pulmonary disease (COPD) and normal serum alpha₁-antitrypsin levels, 62 with COPD and severe alpha₁-antitrypsin deficiency (PiZ), and 43 with idiopathic bronchiectasis]. The results were related to several indicators of the mechanisms and severity of airway inflammation.

RESULTS: Airway bacterial load correlated with sputum myeloperoxidase level, an indirect measure of neutrophil activation and number (r = 0.50, P <0.001); sputum neutrophil chemoattractants [interleukin-8 level (r = 0.68, P <0.001) and leukotriene B4 level (r = 0.53, P <0.001)]; sputum leukocyte elastase activity (r = 0.55, P <0.001); and albumin leakage from serum to sputum (r = 0.26, P <0.01). Markers of inflammation increased at bacterial loads of 10⁶ to 10⁷ colony-forming units per milliliter, and increased progressively with increasing bacterial load. For example, the median (interquartile range) sputum myeloperoxidase level was 0.3 U/mL (0.1 to 0.5 U/mL) for patients who were not colonized or who had mixed normal oropharyngeal flora alone; 0.5 U/mL (0.2 to 0.7 U/mL) for patients with 10⁵ to 10⁶ colony-forming units per milliliter (P = 0.07); 0.5 U/mL (0.3 to 1.2 U/mL) for patients with 10⁶ to 10⁷ colony-forming units per milliliter (P <0.01); 0.7 U/mL (0.3 to 1.2 U/mL) for patients with 10⁷ to 10⁸ colony-forming units per milliliter (P <0.005); and 2.4 U/mL (0.7 to 4.8 U/mL) for patients with 10⁸ or greater colony-forming units per milliliter (P <0.0001). The bacterial species influenced airway inflammation; for example, sputum myeloperoxidase activity was greater (P <0.005) in patients colonized with Pseudomonas aeruginosa [median 32 U/mL (interquartile range, 20 to 65 U/mL)] than those colonized with nontypeable Hemophilus influenzae [4 U/mL (2 to 31 U/mL)], which in turn was greater (P = 0.01) than among those colonized with Moraxella catarrhalis [1.1 U/mL (0.6 to 1.8 U/mL)]. We did not find a relation between bacterial load and lung function.

CONCLUSIONS: The bacterial load and species contribute to airway inflammation in patients with stable chronic bronchitis. Further studies are required to determine the consequences of bacterial colonization on patient morbidity and decline in lung function.