Exacerbations of chronic obstructive pulmonary disease: when are bacteria important?

The progressive course of chronic obstructive pulmonary disease (COPD) is often aggravated by exacerbations, the majority of them produced by bronchial infection. Frequent exacerbations have been demonstrated to have a negative impact on quality of life and pulmonary function in patients with COPD, particularly in active smokers. Furthermore, acute exacerbations are the most frequent cause of medical visits, hospital admissions and death among patients with chronic lung disease. Evidence indicates that the number of patients with pathogenic bacteria in respiratory secretions and the bronchial bacterial load increase during exacerbations. Furthermore, the local inflammatory response of the host parallels the increase in bacterial load. From these observations, it can be speculated that, for symptoms of acute exacerbation to appear, there must be a minimum bacterial load in the airways, i.e. a threshold above which the inflammatory reaction is severe enough to elicit clinical symptoms of exacerbation. This threshold may vary from patient to patient owing to different modifying factors. Some of these factors may be the recognised risk factors for relapse, such as increasing age, impairment of lung function, comorbid conditions or frequent exacerbations in the past. Relapse rates after ambulatory treatment of acute exacerbation of COPD may be as high as 20-25% of cases. Relapses are associated with significant mordibity and increased costs. A number of unanswered questions remain regarding exacerbations of chronic obstructive pulmonary disease. These include the role of viral infection, the importance of residual bacterial colonisation and the impact of new antibiotics on the treatment of exacerbations.