CLASSIFICATION OF ANTIBACTERIAL AGENTS BASED ON THEIR TARGET SITE

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  • CELL WALL INHIBITORS

Antibiotics in this category disrupt the cell wall of bacteria (inclusive of Gram-positive and Gram-negative bacteria). Examples of antibiotics that are cell wall inhibitors include: penicillins, cephalosporins, vancomycin, monobactams such as aztreonam and other anti-mycobacterial agents that specifically target the cell wall of mycobacteria (whose cell wall contain mycolic acid). Antibiotics that are cell wall inhibitors are only effective against bacteria with cell wall. They have no activity on bacteria that do not have cell wall (e.g. mycoplasmas). Cell wall inhibitors basically stop the synthesis of peptidoglycan in bacteria, and this ultimately prevents their cell wall from forming. Bacterial cell wall is a natural, physical barrier that controls the flow of molecules in- and out- of the cell. The disruption of this physical barrier disrupts the integrity of the cell; and this may expose the interior environment of the organism to harsh environmental conditions. In most cases, fluids from the external environment enter the cell; and the cell swells and burst. Cell wall inhibitors are active on bacterial cells that are actively growing.  

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  • PROTEIN SYNTHESIS INHIBITORS

Protein synthesis inhibitors interact with the ribosomes of bacteria (especially the 50S and 30S ribosomes); and this interactions prevents translation in the target bacterial cells. Translation is vital for protein synthesis because it is at this stage that the genetic information encoded by the DNA is decoded by the messenger ribonucleic acid (mRNA) for the formation of a specific protein molecule. Bacterial ribosomes are usually 30S, 50S and 70S while the ribosomes of eukaryotic cells particularly humans is 80S. The “S” used in describing the ribosomal subunit of organisms is generally referred to Svedberg units; and it shows the sedimentation coefficients of ribosomal subunits (e.g. 30S, 50S and 70S) when they are subjected to centrifugal force in an ultracentrifuge. Streptomycin, chloramphenicol, tetracyclines and erythromycins are typical examples of antibiotics that are protein synthesis inhibitors. Each of these antibacterial agents is specific in action in that they target specific ribosomal subunit of the bacterial cell either 50S or 30S ribosomal subunit. 

  • ANTIBIOTICS THAT DAMAGE BACTERIAL CELL MEMBRANE

While some antibiotics target only the bacterial cell wall, others target the cell membrane of the organism. Some antibiotics target only the cell membranes of bacteria cells. Examples include the polymyxins. The integrity of the cytoplasmic or plasma membrane is vital for the normal functioning of all bacterial cells. Cytoplasmic membranes act as diffusion barriers to some molecules including water, antibiotics and nutrients. But antibiotics that target bacterial cell membranes destabilizes the plasma membrane, and makes it more permeable to harmful substances including antibiotics that destroy the cell.  

  • NUCLEIC ACID (DNA) INHIBITORS

Nucleic acid inhibitors are group of antibiotics that target the DNA of bacterial cells. Typical examples of nucleic acid inhibitors include rifampicin, nalidixic acid, ofloxacin, ciprofloxacin and a range of other antimicrobial drugs. Nucleic acid inhibitors target the DNA and RNA synthesis of bacterial cells especially by disrupting the activities of key enzymes required for these processes such as DNA gyrase (topoisomerase II and IV) and RNA polymerase. DNA gyrase and/or topoisomerases are crucial for the synthesis of nucleic acids (particularly DNA) in bacterial cells. When their natural activity is interfered with, DNA synthesis will be interrupted. Because the activities of the cell is mainly directed by the DNA (which is the actual genetic material), the bacterial cell will die once its ability to synthesize nucleic acid molecules have been disrupted. 

  • ANTI-METABOLITES

Microbes are metabolic entities that synthesize different metabolites from time to time in order to remain active in their environment. While most of these metabolites are directly beneficial to the organism, other metabolic products of microbes are utilized by man to solve several economic problems. Thus, targeting the metabolic pathways responsible for the synthesis of a key metabolite in a pathogenic bacterium leads to the death of the organism. And this is what drugs known as anti-metabolites are expected to do when used for therapeutic purposes. Anti-metabolites are antibiotics that inhibit the metabolic pathways of pathogenic bacteria especially as it relates to the biosynthesis of important molecules in the microbes. They include sulpha drugs that interfere with the metabolic pathways responsible for synthesizing important growth molecules (e.g. folic acid) in bacterial cells. Trimethoprim, pyrimethamine and sulphamethoxazole are typical examples of antimetabolites. Generally, anti-metabolites compete for essential metabolites, nutrient molecules or growth factors that are considered necessary in bacterial metabolism especially as it relates to their unperturbed development. Antibiotics in this category are chemically synthesized drugs i.e. they are synthetic agents.        

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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