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Did you ever wonder why there were so many different types of antibiotics? In this lesson you will learn about one group of antibiotics – the cephalosporins.

Cephalosporin Versus Penicillin

You’ve probably heard of penicillin: the first antibiotic discovered that could kill bacteria. Discovered in 1928, penicillin was used to treat bacterial infections starting in 1930.

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However, penicillin couldn’t kill all bacteria; some species of bacteria had an enzyme called beta-lactamase that could deactivate or destroy penicillin. This made the bacteria naturally resistant to the antibiotic.For over 20 years, there was no treatment for infections caused by these bacteria, including the dreaded disease, typhoid fever. Then in 1945, an Italian professor named Giuseppe Brotzu discovered a culture in a Sardinian sewer that couldn’t be deactivated by beta-lactamase. It was called cephalosporin, after its fungal source, and first used clinically in 1964. Most cephalosporins have ceph-, cef- or kef- in their name. This makes it easy to distinguish them from relatives of penicillin, which end in -cillin.

How Cephalosporins Work

Penicillins and cephalosporins have similar structures so they act in similar ways. These compounds bind to and block the activity of enzymes that make peptidoglycan, the major component of most bacterial cell walls. Peptidoglycan is a polymer made up of long parallel strands of sugar molecules (‘glycan’ = sugar) that are attached together by very small proteins (‘peptido’ = protein).

To understand how the molecules work, imagine a large train yard. The strands of sugar are like long freight trains running parallel to each other. Except in this train yard, there are metal beams that are cross-linking one train to another train. The very small proteins act as the metal cross-linking beams.

As a result, you get an enormous structure resembling chain mail armor from the Middle Ages. This enormous polymer wraps around the bacterial cell, supporting it like a girdle.If a bacterial cell can’t make peptidoglycan, their cell walls become very weak. Eventually the bacteria will die by bursting. Imagine a balloon where the latex is a little thin and worn in places. Over time, pressure from the inside will become too much, and the balloon bursts.

This is what happens when a bacterial cell encounters a molecule of cephalosporin or penicillin.

Peptidoglycan (PG) is composed of strands of sugar molecules (green/blue strips) and very small proteins (colored circles). An enzyme (gray) is needed to crosslink the PG subunits, which makes the cell wall strong. Cephalosporins (orange P) bind to this enzyme and deactivate it.
Cephalosporins bind to and inhibit and enzyme required for cell wall construction

Gram-positive Versus Gram-negative Bacteria

Most species of bacteria have cell walls made of peptidoglycan. They can be divided into two groups.

The first group is called Gram-positive bacteria, which have very thick peptidoglycan cell walls. Imagine hundreds of chain mail armor suits stacked on top of each other: that’s a Gram-positive cell wall.The second group is Gram-negative bacteria, which have a thin peptidoglycan cell wall. In addition, Gram-negative bacteria have a membrane on the outside of the thin cell wall that helps shield the bacteria.

The greasy outer membrane makes it pretty difficult to kill the bacteria.

Generations and Types

Since the discovery of the first cephalosporin in 1945, medicinal chemists have been tinkering with its structure in order to change its properties. Medicinal chemists do this to antibiotics to see if they can make them more effective against different bacterial species or have fewer side effects. Over time, medicinal chemists have added different chemical groups to the core cephalosporin structure.

Each time they add a different type of chemical group, they make a new ‘generation’ of cephalosporin compounds.To date there are five generations of cephalosporins. Examples from each generation and what types of bacteria they are effective against are described below.

In general, each subsequent generation of cephalosporins is more effective against Gram-negative species than the previous one and less effective against Gram-positive species than the previous one. For example, first-generation cephalosporins can kill only a few Gram-negative species, but are very effective at killing Gram-positive species. This is a general trend and there are a few exceptions.

Generation Examples Bacterial Targets
First Generation Cephazolin (Kefzol) Cephalexin (Keflex) Gram-positive cocci bacteria with susceptibility to methicillin, including Staphylococcus aureus // A few Gram-negative bacteria like Proteus mirabilis and Klebsiella pneumoniae
Second Generation Cefaclor (Keflor) Cefonicid (Monocid) Less effective against Gram-positive than first generation More effective against Gram-negative like Enterobacter and Haemophilus
Third Generation Cefotaxime (Claforan) Less effective against Gram-positive than second generation Broad-spectrum against Gram-negative, especially GI tract infections
Fourth Generation Cefpirome (Cefrom) Cefepime (Maxipeme) Most beta-lactamase producing Gram-positive bacteria Broad-spectrum against Gram-negative species similar to third generation
Fifth Generation Ceftobiprole Methicillin-resistant Staphylococcus (MRSA)

Side Effects

One reason that some people are prescribed cephalosporins is that they are allergic to penicillin.

Although 10% of people who are allergic to penicillin are also allergic to first-generation cephalosporins, the later generations are safer for those with a penicillin allergy.Like all drugs, cephalosporins have side effects. The most common side effects include:

  • Nausea and diarrhea
  • Rash
  • Electrolyte imbalances
  • Pain at the injection site if the drug was administered through a muscle or vein

An unintended consequence of any antibiotic treatment is killing good bacteria along with the ‘bad’ bacteria.

You have millions of good bacteria that reside in your gastrointestinal (GI tract), which keep disease-causing bacteria from proliferating. When you take antibiotics like cephalosporins, you kill the bacteria that caused the initial infection, but you also kill the good bacteria in your body. If the bad bacteria are resistant to the antibiotic you are taking, and there are no good bacteria keeping them at bay, you can get a superinfection, or second infection.

Lesson Summary

Cephalosporin was first discovered in 1945, and since then, five generations of cephalosporins have been developed. Generations of cephalosporin differ from each other in their structure and effectiveness at killing Gram-positive and Gram-negative bacteria. However, all generations of cephalosporins act by preventing bacteria from making their peptidoglycan cell wall.

Common side effects of cephalosporins are generally mild. However, overuse of cephalosporins can lead to superinfections.Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.

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