Antibiotics
š Antibiotics are drugs used to treat infections caused by pathogenic (disease causing) bacteria.
š They do not harm the cells of the infected organism.
š Derived from living organisms, they are made more effective by chemical processes.
š Antibiotics are not effective against viruses such as the common cold or influenza; drugs which inhibit viruses are termed antiviral drugs or antivirals rather than antibiotics.
š Antibiotics either kill bacteria or slow its growth.
How Antibiotics Work
š Antibiotics interfere with some aspect of growth or metabolism of its target cell. The include:
š Synthesis of bacterial cell walls (penicillin, cephalosporin, vancomycin)
š Activities of proteins in the cell surface membranes (polymycin)
š Enzyme action (sulfa drugs)
š DNA synthesis (quinolones, rifampicin)
š Protein synthesis (chloramphenicol, erythromycin, tetracycline, streptomycin)
š They are grouped into three main categories of action; macrolides, beta-lactam and quinolones.
š Macrolides
š Antibiotics in the macrolide group affect ribosomes, the cell’s protein-building machines.
š Ribosomes build proteins in both bacteria and human cells, but there are differences between bacterial
š and human ribosomes. Macrolides block only bacterial ribosomes and prevent them from building proteins.
š Since proteins do all the cell’s work, a bacterium that cannot build proteins cannot survive.
š Erythromycin, which is commonly used to treat respiratory tract and skin infections, is a macrolide.
š Beta-Lactam
š Beta-lactam antibiotics kill bacteria that are surrounded by a cell wall. Bacteria build cell walls by linking
š molecules together—beta-lactams block this process. Without support from a cell wall, pressure inside the
š cell becomes too much and the membrane bursts. Examples of beta-lactams include penicillin and
š cephalosporin, which are used to treat many types of bacterial infections.
š Quinolones
š Quinolones include antibiotics like ciprofloxacin and levofloxacin, which are used to treat infections like
š bronchitis and pneumonia. When bacteria begin to copy their DNA, quinolones cause the strands to break
š and then prevent the breaks from being repaired. Without intact DNA, bacteria cannot live or reproduce.
š Using the specific example of penicillin, a beta-lactam, penicillin prevents the synthesis of the crosslinks between the peptidoglycan polymers in the cell walls of bacteria by inhibiting the enzymes that build them.
š While a newly formed bacterial cell is growing, it secretes autolysins, which are enzymes that make holes in its cell wall to allow the wall to stretch so new peptidoglycan chains can link together.
š Penicillin prevents the formation of these chains, but the autolysin keeps making new holes so the cell membrane eventually weakens. The bacterial cell takes water in by osmosis from its watery environment. When the cell walls are weakened they cannot withstand the pressure potential exerted on them by the cell contents and the cell bursts.
š Penicillin can thus only affect bacterial cells while they are growing, and can’t affect cells without cell walls, which is why it does not adversely affect human cells.
Antibiotic Resistance
š Bacteria mutate randomly, and often these mutations are of no use to the bacteria. They do not mutate as a result of an antibiotic being present. However, a mutation may make a bacterium resistant to an antibiotic. When this happens, the non resistant bacteria around it die in the presence of that antibiotic, but the resistant survive and multiply, effectively making the entire strain resistant.
š Vertical gene transmission occurs when antibiotic resistance is passed from one generation of bacteria to the next, because the resistant form is selected for over the non resistant form, for example in the presence of penicillin.
š Resistance for an antibiotic in bacteria is carried on a section of DNA in a small ring called a plasmid. These can be transferred from cell to cell by a process called conjugation. When this happens, it’s known as horizontal gene transmission.
š Horizontal gene transmission can lead to some strains of bacteria developing resistance to many types
š of antibiotics. These are known as superbugs.
š Mutations occur very randomly all the time in bacteria, but the more we use antibiotics, the higher the
š chances that superbugs will gain an advantage over the usual variety of bacteria, and may become populous.
š Antibiotics are now being chemically modified to increase effectiveness against resistant bacteria.
Why is Antibiotic Resistance on the Rise?
š Antibiotics are used to treat minor ailments whose symptoms are trivial.
š Antibiotics are sometimes used for viral diseases against which they’re ineffective.
š Patients do not always complete the course of antibiotics as prescribed.
š Patients stockpile unused antibiotics from previous prescriptions and then later use them in smaller doses.
š Doctors accept patients demands for antibiotic treatments when they are not absolutely necessary.
š Antibiotics are used in the treatment of minor ailments in domesticated animals.
š They are used in preventing disease among intensively reared animals such as chickens.
š They are used by farmers and companies to reduce disease and increase the productivity of animals.
Experiment to Show Bacterial Resistance
š Different antibiotics are placed in agar jelly containing bacteria and left for 48 hours. The circles in the sample shown on the right depict the amount by which the antibiotic is effective against the bacteria. 
