Define the term 'mutagen'.

A mutagen is defined as an agent that causes changes in DNA. Mutagens can be physical agents, such as radiation, or chemical agents that interact with DNA and induce mutations, which are changes in the nucleotide sequence. These mutations can lead to various outcomes, including modifications in protein coding, disruptions in gene regulation, or even the onset of diseases like cancer.

Understanding the role of mutagens is crucial in genetics and microbiology because they can affect cellular processes, lead to genetic diversity, and influence evolutionary mechanisms. Additionally, research into mutagens is vital for assessing risks, developing treatments, and understanding mechanisms of mutations in pathogens.

Other options refer to different biological concepts: one pertains to viral activity, another describes a bacterial structure, and the last one is related to antibiotic classification. None of these correctly encapsulate the definition or function of a mutagen.