Aggregation by fragilis and non-fragilis Bacteroides strains in vitro

Bacteroides fragilis is associated with the formation of intra-abdominal abscesses, whereas other Bacteroides species are rarely involved. Since bacterial clumping may contribute to the survival of bacteria in the face of host defence mechanisms, the hypothesis has been put forward that differences in aggregation between fragilis and non-fragilis strains of Bacteroides may account for their differences in survival in vivo. All seven B. fragilis strains tested formed aggregates within 4 h, but strains not associated with intra-abdominal sepsis--B. vulgatus, B. thetaiotaomicron and B. distasonis--did not form aggregates in vitro. Aggregation occurred at 37 degrees C, but not at 4 degrees C or 20 degrees C. Treatment with pronase partially inhibited aggregation. Periodate treatment killed the cells and caused them to form clumps which were distinguishable from the control aggregates. Heat-killed B. fragilis cells formed similar distinct clumps, but cells killed by glutaraldehyde and formaldehyde did so to a lesser degree. No inhibition was found upon addition of carbohydrates, ethylenediaminetetraacetic acid or after treatment with trypsin. These results demonstrate that aggregate formation occurs with B. fragilis strains alone, and that surface proteins probably mediate this interaction.