Microbial physiology is defined because the consideration of how microbial cell structures, growth and metabolism function in living organisms. It covers the study of viruses, bacteria, fungi and parasites. it's also bring because the study of microbial cell functions which incorporates the study of microbial growth, microbial metabolism and microbial cell structure. Microbial physiology is decisive within the field of metabolic engineering and also genomics .Multiple dyes indicative of microbial physiology are available, but we've chosen to focus here on four dyes (two indicative of metabolic activity and two of cell damage; Fig. 5.1A) that are extensively utilized in bacterial cultures, aquatic ecosystems, biofilms, and even the human gut microbiota (Del Giorgio and Gasol, 2008; Maurice, Haiser, and Turnbaugh, 2013; Novo, Perlmutter, Hunt, and Shapiro, 2000). Most of the opposite available dyes remain to be validated before their application to the human microbiota (Maurice and Turnbaugh, 2013).
Numerous studies indicate that cell growth and transcriptional activity are often determined using nucleic acid-binding fluorescent dyes, like SybrGreen I. These dyes enter all microbial cells regardless of the membrane status, with strong affinity to double- and single-stranded nucleic acids (Del Giorgio and Gasol, 2008; Martens-Habbena and Sass, 2006; Maurice and Turnbaugh, 2013).
Extensive data from aquatic microbial communities show that microbial cells tend to cluster into two groups, consistent with their relative green fluorescence: cells with low (LNA) and high macromolecule (HNA) content (Bouvier, del Giorgio, and Gasol, 2007; Del Giorgio and Gasol, 2008). Relative macromolecule content can function a proxy of cellular activity, with several studies indicating that HNA cells exhibit higher rates of cellular division and metabolic activity than LNA cells (Bouvier et al., 2007; Wang, Hammes, Boon, Chami, and Egli, 2009; Zubkov, Fuchs, Burkill, and Amann, 2001). Both populations have also been observed within the human gut microbiota, where the fluorescence differences couldn't be explained by increased cell or genome size, and most likely reflected differences in metabolic activity.