QUANTITATION OF MICROBIAL CELL-SURFACE HETEROGENEITY BY MICROELECTROPHORESIS AND ELECTRON-MICROSCOPY - APPLICATION TO LACTOBACILLI AFTER SERIAL PASSAGING

This paper attempts to demonstrate the use of particulate microelectro phoresis as a method to quantitate subpopulations with different cell surface properties in pure, single cultures of microbial cells. To thi s end, two primary isolates of urogenital lactobacilli were serially p assaged in liquid medium up to 50 times. In one strain, serial passagi ng was accompanied by the development of a structural cell surface het erogeneity, in which about half of the cells possessed a thick rutheni um red/uranyl acetate stained layer observed by transmission electron microscopy on sectioned cells, where the other half of the cells was d evoid of such a layer. Particulate microelectrophoresis with an automa ted image analysis system enabled determination of the zeta potentials of individual cells in a suspension, showing that in 10 mM potassium phosphate solution, pH 5.0, the primary isolate of this strain had a s ingle zeta potential of +2.8 mV. In both p=20 and p=50 cultures a more negatively charged subpopulation (zeta potentials -19.7 and -18.3 mV for p=20 and p=50, respectively) existed next to the virtually uncharg ed fraction. The other strain developed a less clear structural cell s urface heterogeneity after serial passaging and consequently only a sm all shift in zeta potentials, albeit that the standard deviation over the zeta potential distribution increased from 2.3 to 3.4 mV after ser ial passaging. It is concluded that particulate microelectrophoresis i s a good method to quantitate subpopulations with different cell surfa ce properties and that is less time consuming than electron microscopy . Depending on the nature of the cell surface heterogeneity, either tw o zeta potential distributions are measured for one culture or the sta ndard deviation over a single zeta potential distribution is enlarged.