Nonopsonic phagocytosis of Pseudomonas aeruginosa by macrophages and polymorphonuclear leukocytes requires the presence of the bacterial flagellum. 
Whereas the mechanism of nonopsonic phagocytosis of Pseudomonas aeruginosa has been described, the bacterial ligands required are poorly understood. To identify the requisite bacterial ligands, studies with isogenic mutants of P. aeruginosa PAK lacking pili, flagella, and the RpoN sigma factor were undertaken. The RpoN mutant, lacking pili, flagella, and nonpilus adhesins, bound poorly and was resistant to ingestion by both macrophages and neutrophils. Pili were not absolutely required for binding or phagocytosis of P. aeruginosa. The presence of a flagellum was not required for binding of P. aeruginosa to macrophages but was critical for the subsequent internalization of the bacterium, suggesting that this factor or a surface ligand associated with its assembly was responsible for stimulation of nonopsonic phagocytosis.