Can you guess what organisms are pictured here? Are they fat green worms on a red leaf? Here’s a clue: There are more organisms like these than any other on Earth. Here’s another clue: Each organisms consists of a single cell without a nucleus.
The organisms are bacteria called Salmonella. If the word Salmonella rings a bell, that’s probably because Salmonella causes human diseases such as food poisoning. Many other types of bacteria also cause human diseases. But not all bacteria are harmful to people. In fact, we could not survive without many of the trillions of bacteria that live in or on the human body. You will learn why when you read this chapter.