Osteoclasts are the only cells capable of carrying out bone resorption and therefore are responsible for the osteolysis seen in infectious diseases such as chronic otitis media and infected cholesteatoma. Pseudomonas aeruginosa is the most common organism isolated from these infectious middle ear diseases. In this study, we examined the mechanisms by which P. aeruginosa lipopolysaccharide (LPS) stimulates osteoclastogenesis directly from mononuclear osteoclast precursor cells. Osteoclast precursors demonstrated robust, bone-resorbing osteoclast formation when stimulated by P. aeruginosa LPS only if previously primed with permissive, sub-osteoclastogenic doses of receptor activator of NF-kappaB ligand (RANKL), suggesting that LPS is osteoclastogenic only during a specific developmental window. Numerous LPS-elicited cytokines were found to be released by osteoclast precursors undergoing P. aeruginosa LPS-mediated osteoclast formation. Two lines of evidence suggest that several cytokines promote Oc formation in an autocrine/paracrine manner. First, inhibition of several cytokine pathways including TNF-alpha, IL-1, and IL-6 block the osteoclastogenesis induced by LPS. Secondly, increased expression of the receptors for TNF-alpha and IL-1 was demonstrated by real-time quantitative polymerase chain reaction. Such a mechanism has not previously been established and demonstrates the ability of osteoclast precursors to autonomously facilitate bone destruction.