Biofilm formation avoids complement immunity and phagocytosis of Streptococcus pneumoniae

Abstract

Colonization of the nasopharyngeal tract is a first and necessary step in the infectious process and often involves the formation of sessile microbial communities by the human pathogen S. pneumoniae. The ability to grow and persist as biofilms is an advantage for many microorganisms because biofilm-grown bacteria show a reduced susceptibility to antimicrobial agents and hinder the recognition by the immune system. Using pneumococcal strains growing as planktonic cultures or as biofilms, we have investigated the recognition of S. pneumoniae by the complement system and its interactions with human neutrophils. Deposition of C3b, the key complement component, was impaired on S. pneumoniae biofilms. In addition, binding of CRP and the complement component C1q to the pneumococcal surface was reduced in biofilm-growing bacteria demonstrating that pneumococcal biofilms avoid the activation of the classical complement pathway. Besides, recruitment of factor H, the down-regulator of the alternative pathway by a PspC-mediated mechanism was enhanced by S. pneumoniae growing as biofilms. Furthermore, phagocytosis of pneumococcal biofilms was also impaired. The present study confirms that biofilm formation in S. pneumoniae is an efficient way for host immune evasion both from the classical and the PspCdependent alternative complement pathways.