Surface density of major outer membrane proteins in Salmonella typhimurium in different growth conditions

The amount of major proteins per unit of surface area in the outer membrane (OM) of Salmonella typhimurium LT2 remained constant during steady-state growth in different media. Between growth rates of 2.40 doublings h-1 and 0.31 doublings h-1, the surface density of major OM proteins varied between 0.9 X 105 molecules per micrometer2, while surface area per cell more than halved. The accumulation of molecules of the major OM proteins was not affected by addition of cyclic AMP to the growth medium. When exponentially growing cells were subjected to shift-up transitions, cell dimensions began to increase after a lag period of 20 min. Accumulation of major OM proteins followed the same pattern as total protein; this created a transitory imbalance of major OM protein density in the shift from acetate minimal medium to LB medium, before the steady situation was reached. After shift-down transitions, cell dimensions began to decrease immediately, cells eventually becoming shorter than in steady-state conditions. No fluctuations in major OM protein density were observed during the shift-down, although final stable levels differed from those in steady-state conditions. All these results indicate that bacteria adapt the accumulation of major proteins into the OM according to the amount of surface. Thus, no large differences exist at different cell sizes, although transitions between media can lead to transitory or stable changes in the composition of the OM.