Bacteria inhabit various environments throughout the earth. They live virtually everywhere, including within our bodies. Most bacteria do not cause humans harm, but some can infect humans and cause disease. In fact, bacteria have caused some of the most devastating diseases in human history, such as the bubonic plague and dysentery.

Bacteria are unicellular and prokaryotic, meaning they do not have a nucleus and are much simpler than eukaryotic cells. Also, unlike eukaryotic cells, most bacteria have a cell wall. The composition of the cell wall varies, and this variation helps scientists tell bacteria apart. The Gram stain helps scientists distinguish between bacteria types based on components of their cell walls. It is often used as a diagnostic test to determine what kind of bacteria is causing an infection. Although bacteria are diverse, they come in three major shapes: rod, sphere, and curved.

Bacterial infection can occur through ingestion, inhalation, or contact with an open wound. Bacteria can infect any part of the body. Some bacteria are highly specific as to which parts of the body they infect. However, others can spread throughout the body via the bloodstream. Toxins produced by the bacteria are often responsible for causing illness because they adhere to cellular structures and inhibit function.

Improved sanitary conditions and antibiotics have helped decrease incidences of bacterial infections. The immune system typically fights off harmful bacteria, but in some cases antibiotics are needed to treat bacterial infections. Antibiotics can be broad spectrum, acting on a wide range of bacteria, or narrow spectrum, targeting specific bacteria. These drugs kill bacteria through several methods depending on the antibiotic. Antibiotics work by destroying the bacteria’s cell wall, DNA, or ribosomes (the organelles that make proteins).

However, overuse of antibiotics can cause problems. Over time, bacteria can become resistance to antibiotics, making it difficult to treat infections caused by new, resistant strains. One such example is the bacteria methicillin-resistant Staphylococcus aureus—or MRSA. Antibiotics can also kill helpful bacteria that reside inside an organism when taken over long periods of time.

Although bacteria can invade human bodies and cause disease, most bacteria are not harmful. Many bacteria live on our skin and in our digestive tract and make up our microbiome, or the populations of microbes coexisting in and on our bodies. This collection of bacteria keeps us healthy by synthesizing vitamins, helping us break down food, and preventing the growth of harmful bacteria.