Bacteria are examples of single-celled organisms. More specifically, a bacterium is a single-celled organism. Bacteria is the plural of bacterium. There are a large number of bacteria everywhere, including our own body. They are called single-celled organisms or prokaryotes because they contain only one cell, as compared to higher life forms like plants, animals, and human beings (all of which are multi-cellular life forms or eukaryotes). There are a large number of differences between prokaryotes and eukaryotes, including the presence in eukaryotes and absence in prokaryotes of complex organelles such as nucleus, mitochondria, etc. Prokaryotes have simple structures and were the first to develop. They evolved, over time, into eukaryotes. Bacteria are a major constituent of prokaryotes and can be both useful and harmful to us. We make use of lactobacilli (a bacterium) for making yogurt (or curd), while some bacterium like E. coli can cause us urinary tract infections, etc.
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