<i>Pseudomonas aeruginosa</i> interacts with epithelial cells rapidly forming aggregates that are internalized by a Lyn-dependent mechanism

Growing evidence is pointing to the importance of multicellular bacterial structures in the interaction of pathogenic bacteria with their host. Transition from planktonic to host cell-associated multicellular structures is an essential infection step that has not been described for the opportunistic human pathogen &lt;i&gt;Pseudomonas aeruginosa&lt;/i&gt;. In this study we show that &lt;i&gt;P. aeruginosa&lt;/i&gt; interacts with the surface of epithelial cells mainly forming aggregates. Dynamics of aggregate formation typically follow a sigmoidal curve. First, a single bacterium attaches at cell-cell junctions. This is followed by rapid recruitment of free-swimming bacteria and association of bacterial cells resulting in the formation of an aggregate on the order of minutes. Aggregates are associated with phosphatidylinositol 3,4,5-trisphosphate (PIP3)-enriched host cell membrane protrusions. We further show that aggregates can be rapidly internalized into epithelial cells. Lyn, a member of the Src family tyrosine kinases previously implicated in &lt;i&gt;P. aeruginosa&lt;/i&gt; infection, mediates both PIP3-enriched protrusion formation and aggregate internalization. Our results establish the first framework of principles that define &lt;i&gt;P. aeruginosa&lt;/i&gt; transition to multicellular structures during interaction with host cells