This large group of natural and semi-synthetic all contain a Beta-lactam ring fused to a thiazolidine ring. Substitutions confer varying affinities for penicillin binding proteins of various microorganisms. Cell wall synthesis is inhibited by the binding of these proteins.

Penicillin G and Penicillin V

Penicillin G is the “original” and still a very useful agent. For bacteria that have remained susceptible, penicillin is a potent and effective therapy. Parenteral penicillin G is commonly used in the therapy of meningitis caused by Neisseria meningitidis (all strains of which are susceptible) and susceptible strains of Streptococcus pneumoniae (resistance is rapidly increasing). It is also employed in the therapy of syphilis and subacute bacterial endocarditis caused by susceptible strains of viridans Streptococci.

Penicillin V is the phenoxymethyl ester that confers acid stability making it the preferred oral preparation. It is the drug of choice for Group A streptococcal pharyngitis.

Ampicillin and Amoxicillin

These are aminopenicillins that are sometimes referred to as extended-spectrum penicillins. Ampicillin is most commonly used in parenteral form whereas amoxicillin is the preferred oral form. They are commonly used to treat infections caused by susceptible Enterococcus spp.

Amoxicillin is the most commonly prescribed antibiotic in Canada, much of which is inappropriate. It “took hold” as a respiratory tract infection therapy because of its activity against Streptococcus pneumoniae and Haemophilus influenzae. Resistance is now common amongst H. influenzae and increasing amongst S. pneumoniae limiting usefulness.


In Canada this is the commonly used member of the isoxazolyl class of penicillins that are active against Staphylococcus aureus. They are also active against penicillin-susceptible streptococci making them excellent agents for the therapy of skin and soft-tissue infections. IV and oral forms are available.

Piperacillin-tazobactam and Ticarcillin-clavulanate

These two agents are sometimes referred to as “antipseudomonal” penicillins. They tend to be reserved for circumstances where P. aeruginosa is a potential or proven pathogen. They are now only available in fixed combination with beta-latamase inhibitors (see below) Piperacillin-tazobactam is more commonly used in adults while ticarcillin-clavulanate has historically been used in pediatrics in Canada.

Clavulanic Acid and Tazobactam

These two agents are beta-lactamase inhibitors. When combined with penicillins, they expand their spectrum of activity. There are three commercially-available preparations in Canada. Amoxicillin-clavulanic acid (Clavulin® in Canada) is an oral preparation used as a second line agent for several types of infections. Piperacillin-tazobactam (Tazocin®) is a very broad spectrum agent that is reserved for serious nosocomial infections and Ticarcillin-clavulanic acid (Timentin®) has become one of the most common agents used to treat pediatric patients with febrile neutropenia.