Evaluation and use of the drug inhibition method of measuring intracellular killing in differentiating between staphylococci grown in vivo and in vitro.

A Staphylococcus aureus strain grown once in vivo in rabbit pleural cavities was appreciably more resistant to killing by soluble polymorph bactericidins than its broth-grown counterpart. Parallel results were obtained using an adaptation of Solberg's drug-inhibition method of measuring intracellular killing after critical aspects of it had been evaluated and modified. Individual in vivo grown organisms were surrounded by a layer of less dense material which was not capsular in nature but which made the organisms clump together. This surface coating was lost on subculture in vitro, resulting in a reversion to broth-like susceptibility, thus indicating that the surface coating was largely responsible for the enhanced resistance and that it resulted from interactions between staphylococci and components of the rabbits' natural body fluids. Consequently, growth in plasma produced organisms which mimicked in vivo grown organisms in clumping, surface coating and in resistance to killing. The use of plasma-grown staphylococci in further studies of likely resistance mechanisms in vivo is discussed.