Rapid detection of hyaluronic acid capsules on group A streptococci by buoyant density centrifugation

One of the virulence factors of group A streptococci is the hyaluronic acid polysaccharide capsule. A rapid method for ascertaining the status of the capsule phenotype in Streptococcus pyogenes is described. Bacteria with a capsule have a lower buoyant density than acapsular or hyaluronidase-treated cells. Early log phase cultures were underlaid with 65% Percoll and centrifuged at 500-1000 g for 5 min. Upon visual examination, encapsulated cells were observed at the interface, whereas acapsular cells appeared in the pellet. Cultures that produced at least 7 micrograms/ml of hyaluronic acid per A600 unit of cells were detected at the interface; this level of polysaccharide is only about 0.5%-4% of that found for most mucoid strains. Therefore, this procedure can detect capsules around strains that do not appear to be encapsulated by light microscopy or do not possess mucoid colony morphology. Furthermore, this method reduces dependence on other expensive assays that use labile radioactive reagents to detect hyaluronic acid.