Antibacterial agents are specifically chemical agents that kill or inhibit the growth of pathogenic bacteria. They are generally known as antibiotics. Antibiotics are complex chemical and natural substances that are secreted by some group of microorganisms (especially fungi and bacteria), and which have the ability to either kill or inhibit the growth of other microbes. Prior to the manufacture of synthetic and semi-synthetic antibiotics, antibiotics were formerly defined as substances produced by microorganisms and which killed or inhibited the growth of other microbes. Though there is no consensus to the actual definition of antibiotics; but to be all encompassing, an “antibiotic” can best be defined as a substance produced by a microorganism (wholly or partly by chemical synthesis) which in low or defined concentrations have the ability to kill or inhibit the growth of other susceptible microbes. The term antibiotics can synonymously be used with antimicrobial agents. Penicillin and tetracycline are examples of some naturally synthesized antibiotics produced by Penicillium species and Streptomyces species respectively. However, some of the available antibiotics are now synthesized by chemical processes using derivatives from their natural or original sources. Antibiotics are either used orally, topically or systemically. Their primary function is to kill or inhibit the growth of invading pathogenic bacteria in humans. While some antibacterial agents inhibit bacterial cell wall synthesis, others inhibit protein synthesis and DNA synthesis. Other antibacterial agents affect the cell membrane of the cell while some act as anti-metabolites (Table 1).
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Table 1 Some selected antibiotics used as inhibitory substances in the preparation of selective culture media
|Antibiotics||Spectrum of activity||Mechanism of action|
|Penicillin||Active on Gram-positive and Gram-negative bacteria, but mostly active on the former||Inhibits bacterial cell wall synthesis by preventing the formation of peptidoglycan, a key cell wall component|
|Tetracycline||Broad spectrum agent||Inhibits protein synthesis in bacteria by binding to the 30S ribosomal subunit|
|Novobiocin||Active on Gram-negative bacteria||Inhibits bacterial DNA synthesis|
|Chloramphenicol||Broad spectrum agent||Inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit|
|Streptomycin||Broad spectrum agent||Inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit|
|Erythromycin||Mostly active on Gram-positive bacteria||Inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit|
|Polymyxin||Active on Gram-negative bacteria especially Pseudomonas species||Disrupts cell membrane of bacteria|
|Rifampicin||Active on Gram-positive bacteria||Inhibit bacterial RNA synthesis by inhibiting bacterial DNA-dependent RNA polymerase enzyme|
|Nalidixic acid||Active on Gram-negative bacteria||Inhibits bacterial DNA synthesis by interfering with the activity of DNA gyrase|
|Trimethoprim||Broad spectrum agent||Inhibit purine synthesis in bacteria|
|Cycloheximide||Active on saprophytic fungi||Inhibit protein biosynthesis in eukaryotic organisms such as fungi by interfering with the translocation step in protein synthesis. It inhibits translation elongation through binding to the E-site of the 60S ribosomal unit and interfering with deacetylated tRNA|
Antibiotics are secondary metabolites produced by bacteria during their growth/development. When used for therapeutic purposes, they destabilize certain metabolic process of their target organisms, and in the process the health of the affected host is restored. It is noteworthy that the bacteria that produce these antimicrobial agents remain resistant to their own lethal substance, but they are susceptible to the antibiotics secreted by other bacterial species or microbes. Antibiotics can be classified as cell wall inhibitors; protein synthesis inhibitors; anti-metabolites or nucleic acid synthesis inhibitors depending on the mechanism of action of the drug. Other classes of antibiotics also target the cytoplasmic or plasma membrane of the bacterial cell. Different antibiotics have different modes or mechanisms of action, owing to the nature of their structure and degree of affinity to certain target sites within bacterial cells. Antimicrobial agents and/or antibiotics are critical for the selective isolation of microbes from both clinical and environmental samples. Antibiotics are incorporated into culture media including bacteriological and mycological media to act as inhibitory substances to the growth of some unwanted organisms (Table 1). They are mainly used in culture media generally known as selective culture media. Selective colure media are culture media that allow some organisms to grow while inhibiting the growth of unwanted microbes. Such culture media usually contains growth inhibitory substances such as antibiotics that prevent the growth of unwanted bacteria while allowing the bacteria of interest to grow.
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