How Antibiotics Work

Antibiotics halt bacterial growth and cure infection through two main mechanisms. Antibiotics are either bactericidal or bacteriostatic. Bactericidal antibiotics kill the bacteria causing the infection through direct action, usually by causing the cells to split open, or lyse. Bacteriostatic antibiotics act on the internal workings of the bacterial cell to stop it dividing and so slow down the advance of the infection. A bacterial population that divides more slowly, or that cannot divide at all is much more easily dealt with by the body’s immune system.

Bactericidal Antibiotics

Many bactericidal antibiotics work by altering the biochemical pathway through which bacteria make the cell wall. As the antibiotic is taken into the cell, it stops the biochemical machinery of the cell producing or attaching one the major components of cell wall structure.

The cell wall produced is thinner than usual. As the cell divides, the two daughter cells then also have weaker cell walls and they cannot strengthen them because they are also prevented from making all of the necessary components. As they try to divide subsequently, the cell walls of these daughter bacteria fail. Lysis of the cell follows and the bacterium dies. Penicillin antibiotics work in this way, as do the cephalosporins.

Other antibiotics in the aminoglycoside class are also bactericidal in some infections (they are bacteriostatic in others). They bind to part of an intracellular structure - the ribosome. This usually assembles amino acids together to form complete proteins. When the antibiotic is bound to the ribosome, it cannot make proteins efficiently, and fewer proteins, or proteins that contain mistakes, are made. Vital proteins that are required by the bacterium are therefore in short supply and the cell dies.

The quinolones disable bacterial enzymes that normally replicate bacterial DNA – making it impossible for the bacterium to divide. This happens quickly, and the affected bacteria die within a few hours. Quinolone antibiotics enter human cells very easily, so are useful for treating bacteria that penetrate and invade cells. The antibiotic has no toxic effects on the human cell itself because its own enzymes for copying DNA are completely different.

Bacteriostatic Antibiotics

Bacteriostatic antibiotics do not kill bacteria directly, but slow their growth so that antibodies and white blood cells of the immune system destroy them more quickly. Antibiotics that are predominantly bacteriostatic include the tetracyclins, the macrolides, chloramphenicol and trimethoprim. These antibiotics all tend to work in a similar way, by binding to part of the ribosome structure that controls the synthesis of proteins in the bacterial cell. Most tend to act more slowly than the quinolones, which have a similar mechanism of action, but that can be bactericidal at some doses.

Narrow Spectrum And Broad Spectrum

Antibiotics are classed as bactericidal or bacteriostatic according whether they kill bacterial cells directly or indirectly. They are also divided into classes such as cephalosporins or macrolides depending in their chemical structure and action. All antibiotics can also be described as either narrow spectrum or broad spectrum. Those with a narrow spectrum of action can kill only a small number of species of bacteria, maybe even just one. Broad spectrum antibiotics are active against a wide range of bacterial species.

Narrow spectrum antibiotics tend to be very specific and act on a molecule in the metabolism of one particular type of bacteria that is special to that species. Broad spectrum antibiotics act on structures or processes that are common to many different bacteria, such as the components of the cell wall.

Choosing Which Antibiotic

Doctors are sometimes faced with difficult choices. So many different bacteria cause infections, and there are many different antibiotics to choose from. The problem of antibiotic resistance is always an important consideration. Widespread use of broad spectrum antibiotics is one factor that is thought to have helped the spread of antibiotic resistance and doctors aim to prescribe antibiotics sensibly so that antibiotic resistance does not increase further.

The ideal treatment involves an antibiotic that is effective against the organism causing the infection and few others. Unfortunately, although taking samples of infected material from each patient and culturing the bacteria to make a positive identification would make this possible, this would not be a cost-effective practice because of the huge numbers of patients who go to their GP with an infection.

For most doctors, the choice ultimately depends on the seriousness of the infection, the antibiotics that are readily available, the most cost effective treatment, and the drug that will cause the fewest side effects.