Virtual Medical Centre

Anti-bacterials (Antibiotic Medicine)

• Introduction to Antibiotics
• When Should Antibotics be used?
• How Do Antibiotics Work?
• Classes of Antibiotics
• Beta Lactam Antibiotics

An Introduction to Antibiotics

Antibiotics (Anti-Bacterials) are defined as medications which work to kill or inhibit the growth of bacteria. Bacteria are single-cell organisms which can invade our bodies and cause disease. Antibiotics work to kill or inhibit the growth of bacteria to eliminate disease. Once the growth of bacteria is stopped, our immune system can mount a response to get rid of them. For example - if you get a chest infection such as pneumonia, the bacteria have gotten into your lungs. They act to produce chemicals and multiply in the lung cavities, causing inflammation and discomfort. Antibiotics act to get rid of the bacteria and help your body fight the infection. It is important to understand that antibiotics do not work against viruses. There is a difference between bacteria and viruses - a bacterium is a single-celled, living organism whereas a virus is a piece of RNA or DNA. A virus acts to affect living cells and reproduces through the affected cell. There is no living organism to kill, thus antibiotics are ineffective.  

When Should Antibiotics be used?

Antibiotics act to either:

• directly kill bacteria (bactericidal)
• inhibit their growth and ability to reproduce (bacteriostatic)

Antibiotics should be used to help fight off a bacterial infection, when your immune system may need extra help to overcome the invading organisms.

• We should only use antibiotics where the benefits have been scientifically proven and demonstrated.
• The correct dose of a single antibiotic should be used, to treat the bacteria you are infected with. The dose should be enough to ensure that the antibiotic works, but is balanced against side effects and tolerability.
• Your choice of antibiotic should be based on the results of investigations that you have had, such as blood tests.
• Antibiotic treatment should only be continued for the required period of time and not any longer.

How Do Antibiotics Work?

An antibiotic is a selective medication, which acts to kill unwanted bacteria, but not the other cells in your body. Each class of antibiotic is characterized by it's own defined mechanism of action. It can affect bacteria in many different ways. For example, an antibiotic may act to stop a bacterium from producing it's own cell wall - this stops the bacteria from reproducing properly and it dies off. To interfere with bacteria growing and reproducing in the body, antibiotics interfere with certain processes that bacteria use to grow and survive. These include:

• Preventing the bacteria from producing necessary proteins by binding to the building blocks that build the proteins
• Interfering with the production of metabolic processes such as production of folic acid, an essential vitamin that bacteria depend on to survive.
• Obstructing production of the bacteria's cell wall, which results in an ineffective wall that does not protect the bacterium and allow it to function properly.
• Blocking both RNA and DNA synthesis.

Classes of Antibiotics

Beta Lactam Antibiotics

The group of Beta Lactam antibiotics includes: penicillins, cephalosporins, carbapenems and monobactams. They are characterized by a special beta lactam ring structure made of four major components. Beta Lactams antibiotics inhibit the growth of bacteria by targeting the bacterial cell wall. Molecules in the bacterial cell wall are linked by bridges. This whole process is controlled by certain enzymes, which are inactivated by beta lactam antibiotics.

Penicillins

These are divided into groups depending on how many classes of bacteria they are active against.  

Narrow spectrum penicillins

Narrow spectrum penicillins are only active against certain bacteria, including: Gram-positive bacteria such as Staphylococcus Aureus and Streptococci. Examples include:

• Benzylpenicillin (penicillin G) is administered through the veins and remains the treatment of choice for susceptible infections.
• Procaine penicillin is a form of penicillin available to inject into the muscles of affected patient. This penicillin is combined with a compound to delay it's absorption into the bloodstream.
• Benzathine penicillin is also given as an injection into the muscles.
• Phenoxymethylpenicillin (penicillin V) can be given orally, but should be taken at least an hour before food, because food impairs it's absorption.

Widespread resistance to penicillins has developed over the many years since it's discovery. However, it is still the antibiotic choice for susceptible organisms. There are some penicillins in this group that have activity against a strain of bacteria called Staphylococcus Aureus, which produce enzymes called beta lactamases, to destroy other penicillins. These include: Dicloxacillin, Flucloxacillin and methicillin. They can be taken orally, but best absorbed when the stomach is empty.

Moderate spectrum penicillins

Moderate spectrum penicillins Amoxycillin and Ampicillin , have a similar range of activity as narrow spectrum penicillin, but are more active against extra bacteria such as those responsible for chest infections and urinary tract infections.

Broad spectrum penicillins

Broad spectrum penicillins consist of a special compound combined with penicillin that protects the medication against destruction by the beta lactamase enzyme that can be produced by bacteria. A commonly used medication that belongs in this group is called Augmentin Duo Forte, which can be used to treat chest infections. However, these medications should be reserved for treatment of infections due to bacteria that are resistant to other antibiotics due to production of beta lactamases.

Cephalosporins

These antibiotics are divided into four major groups. In general, the first group of cephalosporins are quite active against bacteria that belong to the 'Gram positive' group. These bacteria commonly cause skin infections and possible wound infections. The third to fourth group of agents have better activity against bacteria that belong to the 'Gram negative' class - ie those commonly causing urinary tract infections, and some chest infections. However, these agents must be used carefully, to avoid producing resistant strains of bacteria.

First generation cephalosporins:

These include: Cephalexin, Cephalothin and Cefaclor.

Second generation cephalosporins

Second generation antibiotics aren't as commonly used as compared to the first or third generation antibiotics. These include: Cefamandole and Cefuroxime.

Third generation cephalosporins

Third generation cephalosporins include Cefotaxime, Ceftriaxone and Ceftazidime and Cefpodoxime. These antibiotics are effective in infections such as meningitis, because they penetrate the fluid around the brain and spinal cord.

Fourth generation cephalosporins

Antibiotics such as Cefepime fall under the fourth and broadest generation of Cephalosporins. This is a very broad spectrum antibiotic, with activity against a wide range of bacteria. Use of Cefepime is often limited to whilst you are in the hospital, and it may be used in treatment of serious infections that have failed to respond to other therapies.

Carbapenems

These include: Meropenem, Imipenem and Ertapenem. This class of antibiotics has a broad spectrum of activity and are effective against many bacteria. However, they should be reserved for resistant infections. They are expensive antibiotics to use. Carbapenems are another class of beta lactam antibiotics, with a chemical structure which makes them highly resistant to enzymes produced by bacteria to counteract them, such as beta lactamases. However, increasing use of Carbapenems has led to increased resistance and development of infections due to organisms such as methicillin resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE).

Monobactams

This is another class of the beta lactam antibiotics, which has good activity against Gram negative bacteria.