Acinetobacter baumannii is an opportunist nosocomial pathogen of world-wide importance and produces severe infections in immunocompromised patients. However, the virulence factors contributing to its pathogenic properties are not well known. The effect of normal human serum against 18 clinical isolates of the most prevalent biotypes of A. baumannii in Chile was investigated. The effect of pre-treatment of the cells with ethylene diamine tetraacetic acid (EDTA) or bismuth subsalicylate (BSS), compounds known to decrease the amount of lipopolysaccharide (LPS) and bacterial capsular polysaccharide (CPS), respectively, in other gram-negative bacteria, was evaluated. Most isolates (16 of 18) showed resistance to normal human serum. Prior treatment with EDTA rendered nine of these isolates susceptible to serum, while seven isolates maintained their resistance. Pre-treatment with BSS did not modify the serum-resistant behaviour of the isolates. The results suggest that LPS might be involved in the resistance of A. baumannii to human serum whereas CPS does not seem to contribute to this property.

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Acinetobacter baumannii, 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 A. baumannii 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 a tumour necrosis factor (TNF-a) in mouse spleen cells was determined. Activity was evaluated in vivo by determining the splenic index in comparison with LPS from Salmonella Typhimurium. All seven LPS samples were mitogenic on the basis of cellular proliferation experiments and six induced synthesis of TNF-a. 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 A. baumannii might contribute to the pathogenic properties of this species.

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