Bacteria
bacteria One of two prokaryotic (no nucleus) domains, the other being the archaea. Bacteria are microscopic, simple, single-cell organisms. Some bacteria are harmless and often beneficial, playing a major
| Photomicrograph of Streptococcus (Diplococcus) pneumoniae bacteria, using Gram’s stain technique. Streptococcus pneumoniae is one of the most common organisms causing respiratory infections such as pneumonia and sinusitis, as well as bacteremia, otitis media, meningitis, peritonitis, and arthritis. (Courtesyof Centers for Disease Control and Prevention, 1979) |
role in the cycling of nutrients in ecosystems via aerobic and anaerobic decomposition (saprophytic), while others are pathogenic, causing disease and even death. Some species form symbiotic relationships with other organisms, such as legumes, and help them survive in the environment by fixing atmospheric nitrogen. Many different species exist as single cells or colonies, and they fall into four shapes based on the shape of their rigid cell wall: coccal (spherical), bacillary (rod-shaped), spirochetal (spiral/helical or corkscrew), and vibro (comma-shaped). Bacteria are also classified on the basis of oxygen requirement (aerobic vs. anaerobic).
In the laboratory, bacteria are classified as grampositive (blue) or gram-negative (pink) following a laboratory procedure called a Gram’s stain. Gram-negative bacteria, such as those that cause the plague, cholera, typhoid fever, and salmonella, for example, have two outer membranes, which make them more resistant to conventional treatment. They can also easily mutate and transfer these genetic changes to other strains, making them more resistant to antibiotics. Gram-positive bacteria, such as those that cause anthrax and listeriosis, are more rare and are treatable with penicillin but can cause severe damage by either releasing toxic chemicals (e.g., clostridium botulinum) or by penetrating deep into tissue (e.g., streptococci). Bacteria are often called germs.
