What are bacteria?

This colorized image (a scanning-electron micrograph) shows four spherical methicillin-resistant Staphylococcus aureus (MRSA) bacteria (purple) in the process of being “ingested” by a human neutrophil white blood cell (blue). (Image credit: Callista Images via Getty Images)

Bacteria are single-celled organisms that are pretty much everywhere: in the ground, in the ocean, on your hands and in your gut. While some are harmful, most are not — and some are even beneficial to human health. In many cases, humans live in symbiosis with bacteria, maintaining a mutually beneficial relationship without even knowing it.

So let’s demystify this diverse group of single-celled organisms. Here is an overview of what bacteria are, what they do and which ones to watch out for. 

What are bacteria?

(Image credit: Shutterstock)

Bacteria are single-celled organisms with a unique internal structure. Humans and other multicellular organisms are eukaryotes, which means our cells have distinct nuclei bound with a membrane. Bacteria are prokaryotes, meaning they don’t have organized nuclei or any other membrane-bound organelles. 


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