Penicillin belongs to a family of drugs called beta-lactam antibiotics, which are effective at treating many common bacterial infections including ear, skin, sinus and upper respiratory tract infections. There are several types of penicillin and each targets different infections in different parts of the body. These include:
- Amoxicillin
- Ampicillin
- Co-amoxiclav
- Dicloxacillin
- Nafcillin
- Penicillin V (usually given orally)
- Penicillin G (usually given intravenously)
Antibiotics such as penicillin are bacteriocidal, which means that they kill bacteria. Other antiobiotics are called bacteriostatic because they stop the bacteria from growing and multiplying and keep them at a reduced level without completely destroying them. The mode of action of penicillin against bacteria is important in assessing how safe they are to use.
What is Selective Toxicity?
Effective and safe antimicrobial therapy involves selective toxicity. This means that the antibiotic is toxic to prokaryotic bacterial cells but not to animal cells. Penicillins work by interfering with bacterial cell wall synthesis and repair. Since animal cells do not have a cell wall, penicillins have extremely low toxicity to humans. Most bacteria which cause infections have a peptidoglycan cell wall which provides strength and structure to the bacteria cell and allows the bacteria to withstand unfavorable conditions and osmotic pressure. Mycoplasma and mycobacteria do not have a peptidoglycan cell wall and they are therefore not susceptible to penicillins and other cell wall inhibitor drugs.
How Does Penicillin Destroy Bacteria?
Penicillin is bactericidal because it destroys the cell wall of microorganisms by inactivating an enzyme called transpeptidase, required for the cross linking of bacterial cell walls. The enzyme mistakenly accepts the penicillin as a substrate (or activation component) and is then rendered ineffective. This prevents synthesis and repair of the bacterial cell wall, causing the bacteria to burst. Bacterial cell wall inhibitors such as pencillins are some of the most powerful and broad-spectrum antibiotics. However, they can only destroy actively growing bacteria, not dormant colonies or spores. Thus taking penicillin cannot prevent illness and the correct duration and dosage of medication is important to fight an existing infection.
In some cases, penicillins may be prescribed to work in synergy with another drug. For instance when penicillin is taken with antibiotics called aminoglycosides which only work intracellularly and must gain entry into the bacteria, it makes them more effective by weakening and opening the bacterial cell wall.
The chemical structure of penicillins give them their antibacterial properties. All penicillins have a thiazolidine ring attached to a beta-lactam ring. The beta-lactam ring carries a secondary amino group (R-NH). The substitution of different radicals or side groups on the amino group determines the essential pharmacologic properties of the antibiotic. The beta-lactam ring shape is important for antibiotic activity and if it is cleaved (as occurs in resistant bacteria) the penicillin will lose its antimicrobial activity.
Is Penicillin Safe?
Though penicillins are relatively safe compounds with high selective toxicity, all antibiotics have the potential to cause harm if misused. Understanding the likely toxicity associated with each antibiotic class is important for responsible antibiotic use. Discuss drug allergies with your doctor and always carefully follow instructions when taking a course of antibiotics.
