In microbiology, viable describes cells or organisms that are alive and capable of growth or metabolic activity when provided with appropriate conditions.
Assessing viability and influencing factors
The viability of microorganisms depends on their ability to maintain metabolic functions, repair damage and reproduce. Environmental factors such as nutrient availability, temperature, pH, osmotic pressure and exposure to antimicrobial agents can promote or impair viability. Viability is commonly assessed by culturing organisms on nutrient media and counting colony-forming units (CFUs), which represent viable cells capable of division. However, some microorganisms enter a viable but non-culturable state under stress, remaining metabolically active without forming colonies. To detect these cells, alternative methods are used, including fluorescent viability stains that differentiate cells with intact membranes from those with compromised membranes, ATP bioluminescence assays that measure metabolic activity, and flow cytometry. For viruses, viability is measured in terms of infectivity, often via plaque assays or tissue culture infectious dose assays that determine the number of infectious units. The choice of viability assay depends on the organism and context.
Examples and applications of viability determination
Viability counts are used in water quality testing to assess coliform contamination, in food microbiology to determine spoilage organisms, and in clinical laboratories to quantify pathogenic bacteria in patient samples. Probiotic products list viable cell counts to ensure adequate numbers of live microbes at consumption. Viability assays guide sterilisation and disinfection procedures by confirming the absence of live organisms after treatment. Assessing viability is essential in vaccine production, where live attenuated vaccines require a defined number of viable viral particles. In research, live/dead staining and flow cytometric viability analyses monitor the health of cell cultures and biofilms. Cryopreservation techniques aim to maximise viability of stored cells by controlling cooling rates and using cryoprotectants to prevent ice crystal formation.
Understanding and measuring viability enables researchers and clinicians to evaluate microbial health, ensure safety of products and optimize conditions for growth and preservation.
Related Terms: Viability, Colony-forming unit, VBNC, Vital stain, Infectivity