Cigarette smoke exposure induces expression of the pneumococcal erm(B) macrolide resistance gene

Cigarette smoking is a well-recognized risk factor for development of severe, invasive pneumococcal disease. However, little is known about the direct effects of exposure to cigarette smoke on the virulence mechanisms of the pathogen, particularly in respect of resistance to macrolide antibiotics, which are widely used in the treatment of pneumococcal infection. This study aimed to investigate the effects of exposure to cigarette smoke condensate (CSC, 80 and 160 mg/L) and clarithromycin (2 and 8 mg/L), alone and in combination in vitro, on expression of the erm(B) and mef(A) macrolide resistance genes of strains 2507 and 521 (both serotype 23F), respectively, of the pneumococcus.

Following exposure to CSC or clarithromycin, individually and in combination, erm(B) and mef(A) gene expression were measured by sequential extraction of RNA, conversion to and amplification of cDNA, and detection by qRT-PCR.

As expected, exposure of both test strains of the pneumococcus to clarithromycin resulted in substantial upregulation of both macrolide resistance genes, which was significantly (p<0.001) augmented by prior exposure to CSC in the case of erm(B), but not mef(A). Somewhat unexpectedly, exposure of strain 2507 to CSC (160 mg/L) alone (in the absence of clarithromycin) also resulted in significant (p<0.05) expression of the erm(B) gene.

Although the possible clinical significance remains to be established, these findings suggest that smoking may impede the efficacy of macrolide-based antimicrobial therapy by accelerating the onset and magnitude of erm(B)-mediated resistance, representing a novel pro-infective mechanism of smoking.