Quantitative Variability in Requirements for Opsonization of Strains Within the Bacteroides Iragiiis Group

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Abstract
Requirements for opsonization of various strains within the speciesBacteroidesfragilis, Bacteroides thetaiotaomicron, Bacteroides vulgatus, and Bacteroides distasonis were investigated. The minimal concentration of normal human serum that facilitated maximal uptake by human peripheral leukocytes of 14C-labeled strains ranged from 5% to 80%. Heated (56 C at 30 min) serum produced less uptake than untreated serum, suggesting that complement was required for opsonization. Using human serum depleted of C4 by immunoadsorption as the source of opsonins, predominant utilization of the alternative complement pathway during opsonization was established. Acidic polyanionic exopolymer on the strains was also assessed by ruthenium red staining, electron microscopy, and India ink wet mounts. Strains of the speciesB thetaiotaomicron, B vulgatus, and B fragilis possessed exopolymer, and a unique configuration of this material was observed on two strains that resisted opsonization. Utilization of classical and/or alternative complement pathway activity during opsonization was not related to the presence of exopolymer.

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