FLOW-CYTOMETRY AND CELL SORTING OF HETEROGENEOUS MICROBIAL-POPULATIONS - THE IMPORTANCE OF SINGLE-CELL ANALYSES

The most fundamental questions such as whether a cell is alive, in the sense of being able to divide or to form a colony, may sometimes be v ery hard to answer, since even axenic microbial cultures are extremely herterogeneous. Analyses that seek to correlate such things as viabil ity, which is a property of an individual cell, with macroscopic measu rements of culture variables such as ATP content respiratory activity, and so on, must inevitably fail. It is therefore necessary to make ph ysiological measurements on individual cells. Flow cytometry is such a technique, which allows one to analyze cells rapidly and individually and permits the quantitative analysis of microbial heterogeneity. It therefore offers many advantages over conventional measurements for bo th routine and more exploratory analyses of microbial properties. Whil e the technique has been widely applied to the study of mammalian cell s, its use in microbiology has until recently been much more limited, largely because of the smaller size of microbes and the consequently s maller optical signals obtainable from them. Since these technical bar riers no longer hold, flow cytometry with appropriate stains has been used for the rapid discrimination and identification of microbial cell s, for the rapid assessment of viability and of the heterogeneous dist ributions of a wealth of other more detailed physiological properties, for the analysis of antimicrobial drug-cell interactions, and for the isolation of high-yielding strains of biotechnological interest. Flow cytometric analyses provide an abundance of multivariate data, and sp ecial methods have been devised to exploit these. Ongoing advances mea n that modem flow cytometers may now be used by nonspecialists to effe ct a renaissance in our understanding of microbial heterogeneity.