infections
Overview of Bacteria
Bacterial Defenses
Bacteria have many ways of defending themselves. Some bacteria secrete a substance that helps them attach to other bacteria, cells, or objects. This substance combines with the bacteria to form a sticky layer called biofilm. For example, certain bacteria form a biofilm on teeth (called dental plaque). The biofilm traps food particles, which the bacteria process and use, and in this process, they probably cause tooth decay. Biofilms also help protect bacteria from antibiotics by making them difficult to kill, such as the bacteria that cause prosthetic joint infections. Some bacteria are enclosed in a protective capsule. This capsule helps prevent white blood cells, which fight infection, from ingesting the bacteria. Such bacteria are described as encapsulated. Under the capsule, gram-negative bacteria have an outer membrane that protects them against certain antibiotics. When disrupted, this membrane releases toxic substances called endotoxins. Endotoxins contribute to the severity of symptoms during infections with gram-negative bacteria. Some bacteria produce spores, which are an inactive (dormant) form. Spores can enable bacteria to survive when environmental conditions are difficult (such as being too dry or lacking in nutrients). When conditions are favorable, each spore germinates into an active bacterium. Flagella are long, thin filaments that protrude from the cell surface and enable bacteria to move. Bacteria without flagella cannot move on their own. Some bacteria are naturally resistant to certain antibiotics. Other bacteria develop resistance to drugs because they acquire genes from other bacteria that have become resistant or because their genes mutate. For example, soon after the drug penicillin was introduced in the mid-1940s, a few individualStaphylococcus aureusbacteria acquired genes that made penicillin ineffective against them. The strains that possessed these special genes had a survival advantage when penicillin was commonly used to treat infections. Strains ofStaphylococcus aureusthat lacked these new genes were killed by penicillin, allowing the remaining penicillin-resistant bacteria to reproduce and over time become more common. ( Methicillin-resistant Staphylococcus aureus (MRSA) ). The genes that encode for drug resistance can be passed to following generations of bacteria or sometimes even to other species of bacteria. The more often antibiotics are used, the more likely resistant bacteria are to develop. Therefore, experts recommend that doctors use antibiotics only when necessary and for the shortest amount of time possible. In particular, doctors should prescribe antibiotics only for infections caused by bacteria, not for those caused by viruses such as a cold or the flu. Giving antibiotics to people who probably do not have a bacterial infection, such as those who have cough and cold symptoms, does not make people better but does help create resistant bacteria. Because antibiotics have been so widely used (and misused), many bacteria are resistant to certain antibiotics. Resistant bacteria can spread from person to person. Because international travel is so common, resistant bacteria can spread to many parts of the world in a short time. Spread of these bacteria in hospitals is a particular concern. Resistant bacteria are common in hospitals because antibiotics are so often necessary and because hospital personnel and visitors may spread the bacteria if they do not strictly follow appropriate sanitary procedures. Also, many hospitalized patients have a weakened immune system, making them more susceptible to infection. Resistant bacteria can also spread to people from animals. Resistant bacteria are common among farm animals because antibiotics are often routinely given to healthy animals to prevent infections that can impair growth or cause illness. Many countries have banned the use of antibiotics in animals to reduce the risk of the following: Consuming resistant bacteria in animal food products Being infected with resistant bacteria through contact with animals Being exposed to antibiotics in animal food products 