Virulence Studies, in Mice, of Transposon-Induced Mutants of Staphylococcus aureus Differing in Capsule Size

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Abstract
We used three related strains of Staphylococcus aureus to determine whether capsule size influenced bacterial virulence. Strain SA1 mucoid elaborated a large capsule demonstrable by transmission electron microscopy (TEM). Nonmucoid isolates were derived from strain SA1 mucoid by Th551 insertional mutagenesis. By TEM, strain JL24 produced a “microcapsule,” whereas strain JL25 was unencapsulated. Strain SA1 mucoid had a 50% lethal dose for mice S. aureus was more virulent in mice than two related nonmucoid strains. The microencapsulated mutant was not more virulent than the unencapsulated mutant.