*4.1.1 Metatranscriptomics*

In fact, transcriptomics is the analysis of the whole gamut of RNA molecules expressed by a particular cell. There are many RNA molecules including mRNA, rRNA, tRNA, and other noncoding RNA transcribed in a microbial ecosystem which play an important role in the gene expression or metagenome expression in the case of the microbial community. Traditionally, the transcriptomics analysis is carried out by measuring the level of RNA expression by using cDNA-based microarray chip. To study microbial communities, thousands of fluorescent probes were required to be immobilized on the microarray chip surface. Actually, metatranscriptomics is the studies of RNA molecules encoded by a metagenome present in a local ecosystem, for example, gut microbiota. Recently, metatranscriptome is studied with the help of the RNA-seq method; this technique is extremely suitable to confirm the gene expression of complete metagenome in the sample which provides the basic data for proteomics and metabolomics [55]. Metatranscriptomics is highly sensitive methods which can even differentiate between dead and live bacterial cell present in a sample. The major drawback of the method is its high cost and it requires great care during the design and execution of experiments because of the momentary stability of mRNA and its contaminations. There are several demerits associated with this method, for example, less amount of mRNA in bacteria, and hence, it creates an experimental problem. Recently, metatranscriptomics methods have been used to identify the pathway of carbohydrate metabolism and energy extraction and physiological functions regulated by a metagenome [56].
