A standardized, full thickness, dermal burn in rabbits was used to study interference between strains of Staph. aureus inoculated on the wound surface. Several strains appeared equally capable of colonizing lesions and of preventing superinfection by other staphylococci inoculated at a later time. In addition, cross-infection between rabbits colonized by different strains (502A and Q461) and placed together in cages was prevented, presumably by the same mechanism. Interference appeared to be a strictly local phenomenon, since it did not occur when an animal was colonized by strain 502A at one burn site and subsequently challenged with strain Q461 at a separate lesion.

For interference to occur, a minimal time interval (9 hr) was required between inoculation of the interfering strain and inoculation of the challenge strain. In vivo growth rates indicated rapid growth in the first 24 hr by the interfering strain but no detectable multiplication by the challenge strain. Heat-killed staphylococci, even in large numbers, were incapable of producing interference. Penicillin treatment of animals colonized by strain 502A (penicillin-sensitive) abolished interference with strain Q461 (penicillin-resistant). These findings indicate that bacterial multiplication by the interfering strain is an essential feature of this phenomenon.

The mechanism of interference between strains of Staph. aureus remains obscure. There was no evidence in these studies for direct bacterial antagonism in vitro or in vivo between most of the strains examined; yet, all were capable of producing interference. Attempts to identify antistaphylococcal activity in passively transferred tissue homogenates and serum collected from infected animals were also negative. The ability of large inocula of staphylococci grown in broth to superinfect colonized lesions indicates that the numerical superiority of the interfering strain over the challenge strain is an important aspect of interference. The observation that in vivo-grown organisms may superinfect in significantly smaller quantities is suggestive of a qualitative advantage as well.

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