Microbiology is the study of microorganisms. Molecular biology was born in the laboratories of microbiologists studying viruses and bacteria. Microbes are molecular models for studying genetics and molecular biology. For example, bacteria such as E. coli (Escherichia coli is one of a number of microorganisms which are normal inhabitants of the colons of virtually all warm-blooded mammals) and its viruses are extensively used in molecular studies.

Experiments with viruses and bacteria provided most of the evidence that genes are made of DNA, and they were important in working out the molecular mechanisms of the fundamental processes like DNA replication, transcription and translation. Genomes from organisms like E. coli and baker's yeast were helpful tools in the effort to define the basic biochemical networks within more complex organisms. Small genome sequences, such as those of viruses and bacteria, have shed light on the basic principles of genomics.