WJEC Biology for A2: Student Bk

Key Term Antibiotic resistance : Situation in which a micro-organism that has previously been susceptible to an antibiotic is no longer affected by it. St u dy point Vancomycin is an antibiotic of last resort, used to treat highly resistant infections. But some resistance to vancomycin now occurs. A newer antibiotic, linezolid, is available but must be used sparingly. Going further The M in MRSA is methicillin in the USA but its internationally approved name (IAN) is meticillin. It is no longer manufactured but the name has remained in use in MRSA. Knowledge check Fill in the gaps. Antibiotics are produced by ............................. They affect .............................. but not viruses. Those that prevent bacteria reproducing are described as ............................... Their overuse has provided a selection pressure for bacteria that are ..................... to antibiotics, e.g. MRSA. 45 YOU SHOULD KNOW › › › ››› The natural barriers of the body to infection ››› The details of the humoral and cell-mediated response ››› The characteristics of the primary and secondary immune response The overuse of antibiotics Antibiotics have been used for thousands of years, and their presence is recognised in historical treatments of disease, such as the use of mouldy bread by the ancient Greeks to prevent wounds becoming infected. A major use has been to treat war wounds and they have saved huge numbers of lives, for example, in the Second World War. Since then, they have been used extensively to treat infection in people, but also to prevent infection in farmed animals. Freedom from infection allowed the animals’ energy to go into growth, rather into fighting infection, so farms were more productive. In many farms now, antibiotics are not used in response to infection, but are used continually, to prevent infection. Antibiotics in the environment kill susceptible individuals but any that have a mutation that makes them resistant will survive. They have a selective advantage in the presence of antibiotics. They reproduce, passing on the allele that confers antibiotic resistance , and build a resistant population. There are two sources of antibiotic resistance alleles: ▪ Every time bacterial DNA replicates, a mutation conferring resistance may arise. Bacteria divide rapidly when conditions are suitable, so they have a high mutation rate. ▪ Bacteria may acquire plasmids that carry an allele conferring resistance from their environment. The plasmids replicate inside the bacterium and are passed on to the daughter cells when the bacterium replicates. The mutated genes code for proteins that prevent antibiotics from working in ways that depend on the mechanism of the antibiotic: Penicillin resistance Tetracycline resistance Secrete ß-lactamase, an enzyme that degrades penicillin Pump tetracycline out of the cell Altered PBP, so penicillin cannot bind Dislodge bound tetracycline Reduce penicillin entry with fewer or smaller porins Prevent tetracycline attaching to a ribosome If there were no antibiotics in the environment, using energy to synthesise these proteins would put the bacteria at a selective disadvantage and they would die. They only have a selective advantage while the antibiotic is present. So it is the continuous use of high levels of antibiotics that generates the resistance problem. Several clinically important bacteria show antibiotic resistance, including those causing leprosy, TB and gonorrhea. Some show resistance to several antibiotics, including: ▪ MRSA – meticillin-resistant Staphylococcus aureus . It is resistant to penicillin and all its derivatives, and is treated with vancomycin. ▪ Clostridium difficile – severe cases are treated with vancomycin. Development of new antibiotics is a major priority. Without them, the control of infection following surgery will not be possible. Bacterial infection will become life threatening once again. The immune response The immune system enables the body to resist disease. There are physical barriers to protect against the entry of pathogens. If they fail, there are cellular and chemical responses. So, even though you will meet and be invaded by huge numbers of micro- organisms every day, most of the time, you remain healthy. The body must detect foreign, ‘non-self’ antigens and distinguish them from the ‘self’ antigens in its own tissues. WJEC A2 Biology: Chapter 14 270