Competitive mixed cultures were grown from inocula of a large number of bacteria of a genotypically nonfimbriate (fim⁻) strain of Salmonella typhimurium and a small number of a genotypically fimbriate (fim⁺) variant strain that formed type 1 fimbriae and had been derived from the fim⁻ strain by phage transduction. The fim⁺ strain differed from the fim⁻ strain in fermenting l-rhamnose (rha⁺), and the viable fim⁺ and fim⁻ bacteria present in the cultures after different periods at 37 C were counted differentially in platings on rhamnose media. When the cultures were grown under aerobic static conditions in tubes of nutrient broth, the fim⁺ bacteria rapidly outgrew the fim⁻ bacteria, so that, although starting as a small minority (e.g., 1 in 10⁷), they approached or surpassed the number of the fim⁻ in 48 hr. A pellicle consisting of fimbriate bacteria was formed on the surface of the broth between 6 and 24 hr, and it is thought that the advantage of access to atmospheric oxygen enjoyed by these bacteria in the pellicle enabled them to outgrow the fim⁻ bacteria confined in the oxygen-depleted broth. The fim⁺ bacteria did not show selective outgrowth in mixed cultures grown in broth aerated by continuous shaking, in static broth incubated anaerobically in hydrogen, and on aerobic agar plates, i.e., under conditions not allowing an advantage from pellicle formation. The outgrowth of fim⁺ bacteria in aerobic static broth was prevented by the addition of α-methylmannoside, a substance that inhibits the adhesive and early pellicle-forming properties of bacteria with type 1 fimbriae. A motile flagellate (fla⁺) variant of a fim⁻fla⁻ strain of S. typhimurium outgrew its parent strain in mixed cultures in aerobic static broth, but the selective advantage conferred by motility was weaker than that conferred by fimbriation.

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