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Bacterial Load and Exacerbations of COPD

The question we attempted to address in our article was whether bacterial load is the primary mechanism for inducing exacerbations (1). It is important to point out that the most rigorous tests of this hypothesis were, first, the change in bacterial load when a preexisting strain caused the exacerbation, and second, comparison of bacterial load with the same strain in the same patient during an exacerbation and stable state. In both these situations, we were unable to show a relationship between bacterial load and the occurrence of exacerbation.

Whether the relative or absolute change is the correct measurement when changes in load are assessed is not clear. We preferred to look at relative change in bacterial numbers while Drs. Abusriwil and Stockley have opted to look at absolute changes. In any case, even among new strains, increase in concentration at exacerbation was only seen with two of the four species examined. Therefore, this increase in load with exacerbation is not a consistent observation.

Drs. Abusriwil and Stockley have stressed the importance of bacterial load as a determinant of airway inflammation in COPD (2, 3). We have made similar observations in the past (4). However, this relationship could mainly reflect a stimulus–response relationship in the lower airway, and does not imply a cause–effect relationship. Though contrary to our hypothesis, the results of this study (1) support the conclusion that bacterial load increase is not a major primary mechanism of exacerbation induction in COPD.

Sanjay Sethi and Timothy F. Murphy

University at Buffalo
State University of New York
Buffalo, New York

FOOTNOTES

Conflict of Interest Statement: Neither author has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

REFERENCES

1. Sethi S, Sethi R, Eschberger K, Lobbins P, Cai X, Grant BJB, Murphy TF. Airway bacterial concentrations and exacerbations of chronic obstructive pulmonary disease. Am J Respir Crit Care Med 2007;176:356–361.[Abstract/Free Full Text]
2. Hill AT, Campbell EJ, Hill SL, Bayley DL, Stockley RA. Association between airway bacterial load and markers of airway inflammation in patients with stable chronic bronchitis. Am J Med 2000;109:288–295.[CrossRef][Medline]
3. Gompertz S, O'Brien C, Bayley DL, Hill SL, Stockley RA. Changes in bronchial inflammation during acute exacerbations of chronic bronchitis. Eur Respir J 2001;17:1112–1119.[Abstract/Free Full Text]
4. Sethi S, Muscarella K, Evans N, Klingman KL, Grant BJB, Murphy TF. Airway inflammation and etiology of acute exacerbations of chronic bronchitis. Chest 2000;118:1557–1565.[CrossRef][Medline]

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