SOURCES AND CLASSES/TYPES OF ANTIBIOTICS

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Before the advent of conventional medicine used in clinical medicine today for the treatment of infectious diseases, people in ancient times and even now have been using some herbal plants traditionally for centuries to combat microbes that cause diseases in man. Recently, scientists have studied and reported results that support the use of these herbal plants. Herbal antibiotics have been found to be milder than pharmaceutical antibiotics, and according to the Center for Disease Control and Prevention (CDC), the overuse of pharmaceutical antibiotics cause pathogens to mutate and grow stronger, giving them the chance to mount resistance to antibiotics. Some of the herbal plants used for the treatment of infectious diseases in most rural parts of the developing world (especially in places where conventional drugs may not readily be available) include but not limited to: Neem plant (Azadirachta indica), Ginger, Garlic, Tea Tree Plant and Moringa oleifera plant.

MAJOR SOURCES OF ANTIBIOTICS

  1. MICROORGANISMS: Microorganisms are the primary source of antibiotics. Though not all antibiotics used today in clinical medicine are produced completely (wholly) from microorganisms, microorganisms still provide the parent root from which antibiotics are developed. Bacitracin and polymyxins are obtained from Bacillus species; streptomycin and tetracycline are obtained from Streptomyces species; gentamicin from Micromonospora purpurea; penicillins and cephalosporins are obtained from Penicillium and Cephalosporium species respectively.
  2. Synthetic antibiotics: Synthetic antibiotics are antibiotics produced from natural drugs by making a prototype (model) of the natural drug in the laboratory through chemical reactions without having to do anything with the original source of the drug. Example of a synthetic antibiotic is chloramphenicol.
  3. Semi-synthetic antibiotics: Semi – synthetic antibiotics are natural antibiotics that are modified by the removal or addition of a particular chemical group in order to increase the therapeutic effect of the drug. Here, compounds isolated from natural sources (e.g. plants or microorganisms) are used as starting materials and a fermentation process is involved in the production of such antibiotics. After which, the antibiotic is further modified by a chemical process in the laboratory. Examples of drugs produced this way include: penicillins, cephalosporins, and the anti-malarial drug, artemether.

CLASSES/TYPES OF ANTIBIOTICS

There are several classification/types of antibiotics today, which is based on bacterial spectrum of activity (whether broad or narrow) or type of activity exhibited by the agent (whether bactericidal or bacteriostatic). Some antibiotics are also classified based on their chemical structure. And this leaves antibiotics within a particular structural class to have similar patterns of effectiveness, toxicity, and allergic potential. The types of antibiotics expanded here are not exhaustive of the different classes or types of antibiotics.

  1. BETA – LACTAM ANTIBIOTICS: The beta – lactam antibiotics are a broad class of antibiotics that consist of all antibiotic agents that contains a beta – lactam ring/nucleus in its molecular structure. They are the oldest class of antibiotics especially the penicillins and they are produced from Penicillium and Cephalosporium bacteria.Examples of antibiotics in this class include: penicillins, cephalosporins, monobactams and carbapenems. The beta – lactam antibiotics work by inhibiting the synthesis of cell wall in bacteria. They are the most widely used group of antibiotics in clinical medicine, and they are active on both Gram-positive and Gram-negative bacteria. Beta – lactam antibiotics have no antibacterial activity on bacterial cells that lack cell wall e.g. Mycoplasmas. They are only effective on bacterial species that have cell wall, and are actively growing.
  2. MACROLIDES: The macrolides are a group of antibiotics that are characterized by possessing molecular structures that contain large (12-16 membered) lactone rings linked through glycosidic bonds with amino sugars. They are derived from Streptomyces bacteria and they are bacteriostatic, binding with bacterial ribosomes to inhibit protein synthesis. Macrolides are active against most Gram-positive bacteria but not against the Enterobacteriaceae. Examples of antibiotics in this category include: erythromycin, azithromycin, and clarithromycin.
  3. FLUOROQUINOLONES: The fluoroquinolones (fluorinated – quinolones) are second – generationquinolones that are produced by the addition of a fluorine atom (molecule) on the carbon-6 (C-6) of quinolones.  They are synthetic antibiotics and are not sourced from microorganisms. Nalidixic acid is the first quinolone while ciprofloxacin, ofloxacin and norfloxacin are examples of fluoroquinolones. They are active on both Gram-positive and Gram-negative bacteria, and they are mostly used in the treatment of urinary tract infections (UTIs). The fluoroquinolones target the DNA gyrase and topoisomerase IV enzymes of bacterial cell, leading to the inhibition of DNA synthesis or replication. Thus, the fluoroquinolones inhibit bacteria by interfering with their ability to make DNA. This activity makes it difficult for bacteria to multiply and cause havoc in vivo. They are used to treat most UTIs, skin infections, and respiratory infections because of their excellent absorption in vivo.
  4. TETRACYCLINES: The tetracyclines are a group of antibiotics that is characterized by a four cyclic ring. They are derived from a group of Streptomyces bacteria and they inhibit bacterial protein synthesis. Examples include oxytetracycline, doxycycline and chlortetracycline. Tetracyclines have a wide range of activity on both Gram-positive and Gram-negative bacteria. They are broad spectrum bacteriostatic agents.
  5. AMINOGLYCOSIDES: Aminoglycoside antibiotics contain amino sugars in their structures and they possess a cyclohexane ring. They are derived from Streptomyces bacteria, and they are bactericidal in action. Aminoglycosides inhibit the synthesis of protein in bacterial cell. Examples of antibiotic in this category include gentamicin, kanamycin, tobramycin and streptomycin.

FURTHER READING

Ashutosh Kar (2008). Pharmaceutical Microbiology, 1st edition. New Age International Publishers: New Delhi, India. 

Block S.S (2001). Disinfection, sterilization and preservation. 5th edition. Lippincott Williams & Wilkins, Philadelphia and London.

Courvalin P, Leclercq R and Rice L.B (2010). Antibiogram. ESKA Publishing, ASM Press, Canada.

Denyer S.P., Hodges N.A and Gorman S.P (2004). Hugo & Russell’s Pharmaceutical Microbiology. 7th ed. Blackwell Publishing Company, USA. Pp.152-172.

Ejikeugwu Chika, Iroha Ifeanyichukwu, Adikwu Michael and Esimone Charles (2013). Susceptibility and Detection of Extended Spectrum β-Lactamase Enzymes from Otitis Media Pathogens. American Journal of Infectious Diseases. 9(1):24-29.

Finch R.G, Greenwood D, Norrby R and Whitley R (2002). Antibiotic and chemotherapy, 8th edition. Churchill Livingstone, London and Edinburg.

Russell A.D and Chopra I (1996). Understanding antibacterial action and resistance. 2nd edition. Ellis Horwood Publishers, New York, USA.

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