Small colony variants of Pseudomonas aeruginosa in chronic bacterial infection of the lung in cystic fibrosis

Pseudomonas aeruginosa is the most common pathogen that colonizes the lungs of patients with cystic fibrosis. Isolates from sputum are typically all derived from the same strain of bacterium but show extensive phenotypic heterogeneity. One of these variants is the so-called small colony variant, which also shows increased ability to form a biofilm and is frequently resistant to multiple antibiotics. The presence of small colony variants in the sputum of patients with cystic fibrosis is associated with a worse clinical condition. The underlying mechanism responsible for generation of the small colony phenotype remains unclear, but a final common pathway would appear to be elevation of intracellular levels of cyclic di-GMP. This phenotypic variant is thus not just a laboratory curiosity, but a significant bacterial adaptation that favors survival within the lung of patients with cystic fibrosis and contributes to the pulmonary damage caused by P. aeruginosa.