, mainly biotype 9, is an important nosocomial opportunist pathogen in Chile and other countries. The biological basis of its virulence and prevalence is still unknown. As lipopolysaccharide (LPS) is often associated with virulence, some biological properties of purified LPS from seven nosocomial isolates, comprising four isolates of biotype 9, two isolates of biotype 8 and one isolate of biotype 1, were investigated. LPS was extracted and purified from each isolate by the hot phenol-water method, and its ability to elicit a mitogenic response and to induce the synthesis of tumour necrosis factor (TNF-) in mouse spleen cells was determined. Activity was evaluated by determining the splenic index in comparison with LPS from Typhimurium. All seven LPS samples were mitogenic on the basis of cellular proliferation experiments and six induced synthesis of TNF-. Similar results were obtained in in-vivo experiments in which LPS induced spleen cell growth, as shown by determination of the splenic index. These results suggest that the LPS of might contribute to the pathogenic properties of this species.


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