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 microelectrophoresis as a method to quantitate subpopulations with different cell surface properties in pure, single cultures of microbial cells. To this end, two primary isolates of urogenital lactobacilli were serially passaged in liquid medium up to 50 times. In one strain, serial passaging was accompanied by the development of a structural cell surface heterogeneity, in which about half of the cells possessed a thick ruthenium red/uranyl acetate stained layer observed by transmission electron microscopy on sectioned cells, where the other half of the cells was devoid of such a layer. Particulate microelectrophoresis with an automated 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 single 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 uncharged fraction. The other strain developed a less clear structural cell surface heterogeneity after serial passaging and consequently only a small shift in zeta potentials, albeit that the standard deviation over the zeta potential distribution increased from 2.3 to 3.4 mV after serial passaging. It is concluded that particulate microelectrophoresis is a good method to quantitate subpopulations with different cell surface properties and that is less time consuming than electron microscopy. Depending on the nature of the cell surface heterogeneity, either two zeta potential distributions are measured for one culture or the standard deviation over a single zeta potential distribution is enlarged.