Streptococcus pneumoniae Biofilm Formation Is Strain Dependent, Multifactorial, and Associated with Reduced Invasiveness and Immunoreactivity during Colonization

<jats:title>ABSTRACT</jats:title> <jats:p> Biofilms are thought to play an important role during colonization of the nasopharynx by <jats:named-content content-type="genus-species">Streptococcus pneumoniae</jats:named-content> , yet how they form <jats:italic>in vivo</jats:italic> and the determinants responsible remain unknown. Using scanning electron microscopy, we show that biofilm aggregates of increasing complexity form on murine nasal septa following intranasal inoculation. These biofilms were highly distinct from <jats:italic>in vitro</jats:italic> biofilms, as they were discontiguous and appeared to incorporate nonbacterial components such as intact host cells. Biofilms initially formed on the surface of ciliated epithelial cells and, as cells were sloughed off, were found on the basement membrane. The size and number of biofilm aggregates within nasal lavage fluid were digitally quantitated and revealed strain-specific capabilities that loosely correlated with the ability to form robust <jats:italic>in vitro</jats:italic> biofilms. We tested the ability of isogenic mutants deficient in CbpA, pneumolysin, hydrogen peroxide, LytA, LuxS, CiaR/H, and PsrP to form biofilms within the nasopharynx. This analysis revealed that CiaR/H was absolutely required for colonization, that PsrP and SpxB strongly impacted aggregate formation, and that other determinants affected aggregate morphology in a modest fashion. We determined that mice colonized with Δ <jats:italic>psrP</jats:italic> mutants had greater levels of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), IL-1β, and KC in nasal lavage fluid than did mice colonized with wild-type controls. This phenotype correlated with a diminished capacity of biofilm pneumococci to invade host cells <jats:italic>in vitro</jats:italic> despite enhanced attachment. Our results show that biofilms form during colonization and suggest that they may contribute to persistence through a hyperadhesive, noninvasive state that elicits a dampened cytokine response. </jats:p> <jats:p> <jats:bold>IMPORTANCE</jats:bold> This work demonstrates the first temporal characterization of <jats:named-content content-type="genus-species">Streptococcus pneumoniae</jats:named-content> biofilm formation <jats:italic>in vivo</jats:italic> . Our results show that the morphology of biofilms formed by both invasive and noninvasive clinical isolates <jats:italic>in vivo</jats:italic> is distinct from that of formed biofilms <jats:italic>in vitro</jats:italic> , yet propensity to form biofilms <jats:italic>in vivo</jats:italic> loosely correlates with the degree of <jats:italic>in vitro</jats:italic> biofilm formation on a microtiter plate. We show that host components, including intact host cells, influence the formation of <jats:italic>in vivo</jats:italic> structures. We also found that efficient biofilm formation <jats:italic>in vivo</jats:italic> requires multiple bacterial determinants. While some factors are essential for <jats:italic>in vivo</jats:italic> biofilm formation (CiaRH, PsrP, and SpxB), other factors are less critical (CbpA, LytA, LuxS, and pneumolysin). In comparison to their planktonic counterparts, biofilm pneumococci are hyperadhesive but less invasive and elicit a weaker proinflammatory cytokine response. These findings give insight into the requirements for and potential role of biofilms during prolonged asymptomatic colonization. </jats:p>