The wetting agent required for swarming in Salmonella enterica serovar typhimurium is not a surfactant

Abstract

We compared the abilities of media from agar plates surrounding swarming and nonswarming cells of Salmonella enterica serovar Typhimurium to wet a nonpolar surface by measuring the contact angles of small drops. The swarming cells were wild type for chemotaxis, and the nonswarming cells were nonchemotactic mutants with motor biases that were counterclockwise (cheY) or clockwise (cheZ). The latter strains have been shown to be defective for swarming because the agar remains dry (Q. Wang, A. Suzuki, S. Mariconda, S. Porwollik, and R. M. Harshey, EMBO J. 24:2034-2042, 2005). We found no differences in the abilities of the media surrounding these cells, either wild type or mutant, to wet a low-energy surface (freshly prepared polydimethylsiloxane); although, their contact angles were smaller than that of the medium harvested from the underlying agar. So the agent that promotes wetness produced by wild-type cells is not a surfactant; it is an osmotic agent.

FIG. 1.

Three images of a drop in the cheY sequence of Fig. 2A, together with results for different drop parameters. The lower half of each image is a reflection from the PDMS surface. The indentations at the left and right edges of the first image at the level of the PDMS surface indicate contact angles of >90°. Eventually, these indentations disappear, as the contact angles fall below 90° (not shown).

FIG. 2.

(A) Contact angles for wild-type cells, cheY and cheZ mutants, LB broth, and agar removed either from the bottle or from a wild-type plate (or from a cheZ plate, which gave identical results [data not shown]). Averages are for 4 to 6 drops from each of three series of experiments. The error bars are standard deviations, with each drop weighted equally. (B) Contact angles for water and various solutions of the detergent Triton X-100. As expected, the contact angles decrease with the concentrations of the detergent. They also decrease with time. The interfacial tensions for water and these solutions, measured at 23.7° in triplicate with the Fisher tensiometer, were 70.7 ± 1.0, 53.9 ± 1.6, 48.7 ± 0.2, 41.7 ± 0.4, and 37.3 ± 0.7 dynes/cm, respectively.