This group of antibiotics is composed of various penicillins, some occurring naturally and others produced by a semi-synthetic process. All are bactericidal and the range of activity of the group is now wide as both Gram-positive and certain Gram-negative organisms are sensitive to individual penicillins. The outstanding adverse effect is the risk of inducing a hypersensitivity reaction in some persons. Even so the penicillins are the most useful antibiotics at present available.
The penicillins can be subdivided into four sub-groups for descriptive purposes: