Bacteria are fairly simple single-celled organisms. They consist of:

• A cell wall (some bacteria don’t have cell walls, but the majority do. The detailed structure of the cell wall does vary between bacteria)
• A cell membrane
• DNA in the form of a circular chromosome. This contains all the bacterium’s genetic information
• Cytoplasm, in which various components can be found, including ribosomes (where proteins are made), proteins and enzymes

1. Cell wall 2. Cell membrane 3. Chromosome 4. Ribosome 5. Protein/enzyme

When bacteria divide, they firstly synthesise the new structural components of the cell wall and membrane, and the DNA replicates. Once this occurs, the cell splits in two, with each new cell containing a complete copy of the “parent’s” DNA.

Antibiotics interfere with certain aspects of bacterial cell growth or replication. Different types of antibiotic are effective against different parts of the bacterium. Generally speaking, there are three areas of the bacterium that are targeted by antibiotics:

• Cell wall synthesis
• Protein synthesis
• DNA synthesis or replication

These will be discussed in more detail when individual antibiotics are discussed.

By either killing the bacteria, or reducing their ability to multiply, the antibiotics give the body’s own immune system a chance to fight more effectively against the organisms. Unfortunately, even with the best antibiotics, the infection may be so severe, or the patient’s immune system so weak, that bacteria are able to carry on multiplying and the patient may die.

Bacteria are commonly divided into two groups – Gram positive and Gram negative. This is nothing to do with weight, it actually refers to the structure of the cell wall, which causes Gram positive organisms to appear purple, while Gram negative organisms appear pink when using a particular staining technique. This technique is Gram’s stain (named after the scientist Gram).

Gram negative bacteria include organisms such as Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Proteus vulgaris, Salmonella, Shigella and Pseudomonas aeruginosa. Some of these organisms are often found as normal flora of the gastro-intestinal tract, and often cause urinary tract infections, gastro-intestinal infections (such as dysentery and typhoid fever) and intra-abdominal infections (such as peritonitis). Other Gram negative bacteria are Neiserria meningitides (causing meningitis), Neisseria gonorrhoeae (unsurprisingly causes gonorrhoea) and Haemophilus influenzae (ear infections, pneumonia, meningitis).

Gram positive bacteria include organisms such as Staphylococcus aureus (as well as all the other staphylococci), Streptococcus pneumoniae, Streptococcus pyogenes, all the other streptococci and Corynebacterium diphtheriae. These organisms cause a variety of infections, including pneumoniae, abscesses, infections of bone (osteitis), throat infections, diphtheria and meningitis.

The above list of organisms and diseases is not exhaustive, and one should remember that some organisms can cause more than one type of infection, and conversely some infections can be caused by more than one specific organism.

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