Facts About Penicillins

The penicillins are the oldest class of antibiotics. Penicillin, the world’s first antibiotic, was discovered by British scientist Alexander Fleming in 1928 on accident.

Over the years, this family of antibiotics has grown, and the antimicrobial activity has expanded. However, despite the discovery of many new antibiotics, the penicillins have remained a primary choice for treatment of a wide variety of bacterial infections. They are used to treat ear, nose and throat infections, respiratory and urinary tract infections, and certain sexually transmitted diseases.

There are four types of penicillins:

* The natural pencillins are based on the original penicillin-G structure. Penicillin-G types are effective against gram-positive strains of streptococci, staphylococci, and some gram-negative bacteria such as meningococcus.
* Penicillinase-resistant penicillins are active even in the presence of the bacterial enzyme that inactivates most natural penicillins.
* Extended spectrum penicillins which are effective against a wider range of bacteria.
* Aminopenicillins such as ampicillin and amoxicillin have an extended spectrum of action compared with the natural penicillins.

Disadvantages:

* Many of these drugs are destroyed by gastric acid
* Short duration of action - because of the short half-life, the penicillins must be administered at short intervals, usually every 4 hours
* Not effective against most Gram-negative organisms
* Penicillin allergy - about 10% of population has allergy to penicillin
* High rate of gastrointestinal side effects, particularly diarrhea

Advantages:

* Bactericidal against sensitive bacteria
* Relatively safe
* Suitable for use in children and pregnant women
* Excellent tissue penetration
* Inexpensive in comparison with other antibiotics.

Newer penicillins, such as penicillin V, are resistant to stomach acid or, like ampicillin and amoxicillin, have a broader spectrum of action.

Mode of action

Penicillin antibiotics are bactericidal, and act by inhibiting the synthesis of the peptidoglycan layer of bacterial cell walls.

Penicillins have a bacteriocidal effect on Gram-positive bacteria. In Gram-positive cells, peptidoglycan makes up as much as 90% of the thick, compact cell wall, and is the outermost layer.
Penicillins are not effective against Gram-negative bacteria, which have cell walls in which peptidoglycan is not the outermost layer, but that lies between the plasma membrane and a lipopolysaccharide (LPS) outer membrane. Penicillin cannot access the peptidoglycan of Gram-negative cells.

Penicillin resistance

Many pathogenic Gram-positive bacteria produce enzymes called Penicillinases (or beta-lactamases) that can inactivate penicillin. The capability of bacteria to synthesize this enzyme is an adaptation that was selected for in the presence of substances that inhibit bacterial growth. This is an example of a type of antibiotic resistance. Penicillins are sometimes combined with beta-lactamase inhibitors (e.g. amoxicillin and clavulanic acid), agents that protect the penicillin from these bacterial enzymes.

Yury Bayarski is the contributing author of eMedExpert.com.

More information about commonly prescribed antibiotics such as amoxicillin and penicillin VK is available on author’s website.

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