ANTIBACTERIAL AGENTS

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 antibioticcan 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                

AntibioticsSpectrum of activityMechanism of action
   
PenicillinActive on Gram-positive and Gram-negative bacteria, but mostly active on the formerInhibits bacterial cell wall synthesis by preventing the formation of peptidoglycan, a key cell wall component  
TetracyclineBroad spectrum agentInhibits protein synthesis in bacteria by binding to the 30S ribosomal subunit  
NovobiocinActive on Gram-negative bacteriaInhibits bacterial DNA synthesis    
ChloramphenicolBroad spectrum agentInhibits bacterial protein synthesis by binding to the 50S ribosomal subunit  
StreptomycinBroad spectrum agentInhibits bacterial protein synthesis by binding to the 30S ribosomal subunit  
ErythromycinMostly active on Gram-positive bacteriaInhibits bacterial protein synthesis by binding to the 50S ribosomal subunit  
PolymyxinActive on Gram-negative bacteria especially Pseudomonas speciesDisrupts cell membrane of bacteria    
RifampicinActive on Gram-positive bacteriaInhibit bacterial RNA synthesis by inhibiting bacterial DNA-dependent RNA polymerase enzyme  
Nalidixic acidActive on Gram-negative bacteriaInhibits bacterial DNA synthesis by interfering with the activity of DNA gyrase  
TrimethoprimBroad spectrum agentInhibit purine synthesis in bacteria  
CycloheximideActive on saprophytic fungiInhibit 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.   

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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