Viability of amoebae, fungal conidia, and yeasts: rapid assessment by flow cytometry

Conventional methods for the evaluation of antimicrobials and disinfecting solutions with microorganisms involve culture-based techniques, which are time-consuming and underestimate the number of viable organisms. Rapid detection and viability measurements of microorganisms in homogenous and heterogenous microbial populations have been greatly enhanced by recent advances in the use of fluorescent stains in flow cytometry (FCM). FCM has been applied to enumerate, differentiate, and identify microorganisms, determine protein and DNA content of cells, analyze the physiological state of individual cells, and analyze the interaction of drugs, antibiotics, and antimicrobials with microbial cells. Four physiological states of cells can be distinguished by FCM: (1) reproductively viable, (2) metabolically active, (3) intact, and (4) permeabilized.FCM permits a rapid and quantitative measurement of the optical characteristics of cells as they pass through, in a single file, a focused beam of light. As cells are carried within a fast-flowing fluid stream and through the focus of exciting light, three parameters are measured: forward angle light scatter, side angle light scatter, and fluorescence emitted by dyes that have specific interaction with intracellular components of individual cells. FCM data that are presented in histogram and dot plots can be generated to give information on a variety of properties of interest among cells in the population as a whole.FCM offers major advantages in multiparameter data acquisition and multivariate data analysis, high-speed analysis, and cell-sorting capabilities. Disadvantages may be associated with the cost, which is usually over 100,000 (US Dollars) for a typical laser-based flow cytometer with just analyzing capabilities. Another disadvantage is that skilled personnel are usually required to operate these complex instruments so as to get optimum performance. A schematic overview of flow cytometry is presented in Fig. 1.