The survival of bacteria containing r-plasmids

Abstract

The quantitative minimal requirements of Escherichia coli K12 W3110, with (R+) and without (R-) the R- plasmid RP1l were determined in anaerobic batch culture using a chemically defined medium (CDM). Under these condition the R+ strain had agreater requirement for K* than the R-, but a smaller requirement for SO4² with other requirements being similar. In general, the nutritional requirements under anaerobic conditions for both strains were quantitatively greater than those under aerobic conditions. The ability of the R- and R+ strains to survive in competition with each other, at D~0.1 h7! was examined in the chemostat. When R- cells were added to R+ cultures (in a ratio of 1:10), the R- strain took over under glucose (G-lim) and phosphate (P-lim) limited conditions. In the reverse experiments, the level of the R+ strain fluctuated about the original inoculum level. The R+ strain did not disappear completely in these mixed culture experiments. There was evidence that its persistence was possibly due to preferential attachment to the vessel walls. ,In K -limited conditions (K-lim), the R+ strain took over at low K concentrations, but not when the Kt concentration was increased. Under the same condition as used for competition experiments, the apparent Ky values of G-lim and P-]im R- cultures were lower than those of R+ ones. With K -lim cells. R+ cultures had lower K, values than R- at low K+ concentrations, but similar valuesat higher concentrations. However, in all limitations the apparent K, value increaseads the concentration of the growth limiting nutrient in the inflowing medium increased. In K+-lim experiments R- and R+ small colony variants were detected on NA plates. These had similar biochemical properties to those of the parent cultures. The experimental conditions did not affect the stability of the plasmid. The effect of various growth environments on the cell envelope was investigated. Aerobic cultures had higher 2-keto-3 deoxyoctonic acid contents, with wider variation between depletions and R- and R+ strains, than had the anaerobic cultures. A protein with an apparent M. of 34,000 present under most growth conditions was absent from the R- strain grown aerobically (K+-lim) and all iron-depleted cultures. The R- plasmid decreased the resistance of E. coli to drying, but had no effect on the sensitivity of cells to Tween 80 or to sodium deoxycholate. Sensitivity to the latter was increased by anaerobiosis. Sensitivity to NaCl depended upon the depleting nutrient. The relevance of anaerobic competition and survival under adverse conditions to persistence of R- plasmids in the wild is discussed.