The pneumococcus: why a commensal misbehaves

Abstract

Several characteristics of Streptococcus pneumoniae (pneumococcus) combine to make it a particularly problematic pathogen. Firstly, the pneumococcus has the capacity to cause disease through the expression of virulence factors such as its polysaccharide capsule and pore-forming toxin. In addition, the pneumococcus is highly adaptable as demonstrated by its ability to acquire and disseminate resistance to multiple antibiotics. Although the pneumococcus is a major cause of disease, the organism is most commonly an "asymptomatic" colonizer of its human host (the carrier state), with transmission occurring exclusively from this reservoir of commensal organisms. Thus, it is unclear how the organism's virulence and adaptability promote its persistence or host to host spread during its carrier state. This review summarizes current understanding of how these characteristics may contribute to the commensal lifestyle of the pneumococcus.

Fig. 1

Nasal colonization of C57Bl/6 mice with a serotype 4 pneumococcal isolate. Forty-eight hours following intranasal inoculation bacteria are seen in association with neutrophils that have migrated into the nasal spaces. Bacteria (red) were detected using type-specific antisera. Mouse tissue was stained using DAPI (4′,6-diamidino-2-phenylindole; blue). Neutrophils are stained (green) with a mAb to mouse Ly6G. Magnification 200×. Images courtesy of A. Roche, University of Pennsylvania. (figure published in Infection and Immunity, American Society for Microbiology Publications, January 2007, volume 75. The copyright remains with ASM. ASM grants a nonexclusive license number 2252091449539 to use this material.) 149 × 200 mm (300 × 300 DPI)