Abstract
The plating efficiency on salmonella-shigella (SS) agar of many R+ derivatives of Enterobacteriaceae that received R factors by in-vitro transfer was found to be markedly lower than that of respective R strains. This reduction of the plating efficiency was most marked in the presence of all of four components of the medium, bile salts, hyposulfite, neutral red, and sodium citrate. This observation has clinical and epidemiologic implications in that resistant bacteria from a specimen containing both resistant and sensitive organisms may not be detected if only 88 agar is used. This could lead to improper antibiotic therapy, to an underestimate of R+ bacteria, or to in-vivo transmission of R factors to pathogenic bacteria.

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