Effect of EDTA on the resistance of clinical isolates of Acinetobacter baumannii to the bactericidal activity of normal human serum

Acinetobacter baumannii is an opportunist nosocomial pathogen of world-wide importance and produces severe infections in immunocompromised patients. H owever, the virulence factors contributing to its pathogenic properties are not well known. The effect of normal human serum against 18 clinical isola tes of the most prevalent biotypes of A, baumannii in Chile was investigate d. The effect of pre-treatment of the cells with ethylene diamine tetraacet ic acid (EDTA) or bismuth subsalicylate (BSS), compounds known to decrease the amount of lipopolysaccharide (LPS) and bacterial capsular polysaccharid e (CPS), respectively, in other gram-negative bacteria, was evaluated. Most isolates (16 of 18) showed resistance to normal human serum. Prior treatme nt with EDTA rendered nine of these isolates susceptible to serum, while se ven isolates maintained their resistance, Pre-treatment with BSS did not mo dify the serum-resistant behaviour of the isolates. The results suggest tha t LPS might be involved in the resistance of A. baumannii to human serum wh ereas CPS does not seem to contribute to this property.