Antibiotics can be classified in a number of ways. The most straightforward way is to class them as either: bacteriostatic, halting the multiplication of bacteria, and enabling the body’s natural defences to overcome the remaining infection; or bactericidal, as in penicillin, which destroys the bacteria’s ability to build its defensive cell wall or, as with most other antibiotics, act inside the bacteria interfering with the chemicals essential to its life cycle.
An important fact to always have in mind is that there is no single antibiotic that is effective against all the many strains or species of bacteria; some are effective against many (although not all) and others are more effective against specific bacteria. In most cases initially, without specific tests, it is not possible to be certain which bacteria species are causing the infection, and so an educated guess is made and a broad-spectrum antibiotic that is effective against many bacterial strains or species is prescribed. This works well in the majority of cases of suspected bacterial conjunctivitis/keratitis. However, it does have risks that we should not forget (see later).