Morphologic study of Staphylococcus aureus L-form, reverting, and intermediate colonies in situ

Staphylococcus aureus strains of bovine origin were induced to L-form by exposure to 100 U of penicillin in brain heart infusion broth supplemented with 5% NaCl, 5% sucrose, and 10% horse serum. L-forms were cultured on similarly supplemented brain heart infusion agar containing no antibiotic. Light and electron microscopic examination of plastic-embedded L-form colonies revealed a variety of morphologic types. The primary site of growth appeared to be the core area below the agar surface, consisting mainly of pleomorphic budding forms. At the surface, these forms gave rise to large spherules with a gradation from smaller to larger spherules toward the periphery of the colony. Some colonies progressed to reverting forms with the growth of bacterial cells containing cell wall. In addition to L-forms, intermediate colony forms were observed that lacked typical L-form morphology and progressed rapidly to the parent cell form on subculture to bovine blood agar. Description of these forms will be used in the search for similar morphologic types in vivo during antibiotic treatment of chronic S. aureus bovine mastitis.