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tissues to prevent the growth and spread of microorganisms and thereby prevent
infection. Antibiotics are low-molecular-weight substances of natural origin that at
low concentrations inhibit the growth of microorganisms. They are the most important and widely used group of antimicrobial. Although by definition antibiotics are
natural compounds, now the term has come to include semisynthetic and synthetic
variants too. It is important to note that all antibiotics are antimicrobials but all
antimicrobials are not antibiotics. Antimicrobials produce the inhibitory effect on
the microorganism either transiently or permanently. When the inhibition due to an
antimicrobial persists only for the duration it exists in the medium, the effect is
called ‘static’ (e.g., bacteriostatic antimicrobials). If the antimicrobial inhibition is
irreversible, the effect is called ‘cidal’ (e.g., bactericidal and fungicidal antimicrobials). Antimicrobial agents act against a wide range of microorganisms, including
bacteria (antibacterial antimicrobial), fungus (antifungal antimicrobial), protozoa
(antiprotozoal antimicrobial), and virus (antiviral antimicrobials).
Antimicrobials can be classified based on their spectrum of activity, chemical
structure or mode of action. Antimicrobials are said to have a narrow spectrum of
activity when they inhibit only a limited number of organisms. For example, glycopeptides and bacitracin inhibit only gram-positive bacteria, whereas polymyxins act
against gram-negative bacteria. While amino glycosides and sulfonamides act primarily against aerobic organisms, nitroimidazoles inhibit only anaerobic microbes.
Antimicrobials are said to have a broad spectrum of activity when they are able to
inhibit a wide range of organism across groups. For instance, tetracycline and fluoroquinolones are effective against both gram-positive and gram-negative bacteria.
12.2.1 Classes of Antimicrobials
12.2.1.1 Disinfectants and Antiseptics
These are biocidal antimicrobials that are extensively used for surface sterilization
in hospitals, healthcare and household settings and play an important role in preventing infection. In general, these agents have a wider spectrum of activity compared to antibiotics and are nonspecific in terms of the intracellular targets that they
attack to kill the microorganism. Disinfectants are agents used to prevent, control or
destroy microorganisms present on nonliving objects or surfaces, while antiseptics
are antimicrobial agents used for inhibition of microorganisms present on living
surfaces. Certain chemical agents like tincture of iodine, iodophore and alcohol are
used as both disinfectants and antiseptics. Three levels of disinfectants are normally
identified. High-level disinfectants kill some bacterial spores, fungi, many viruses,
vegetative bacteria and mycobacteria. Examples of this group of disinfectants are
8% formaldehyde, 2% gluteraldehyde, 100% ethylene oxide gas, etc. Intermediate
disinfectants kill all of the above agents except bacterial spores. Examples are
70–90% alcohol and 0.5% chlorine. Low-level disinfectants like quaternary ammonium compounds can only act on vegetative bacteria, fungi and lipid viruses.
12 Antimicrobials from Microbes
tissues to prevent the growth and spread of microorganisms and thereby prevent
infection. Antibiotics are low-molecular-weight substances of natural origin that at
low concentrations inhibit the growth of microorganisms. They are the most important and widely used group of antimicrobial. Although by definition antibiotics are
natural compounds, now the term has come to include semisynthetic and synthetic
variants too. It is important to note that all antibiotics are antimicrobials but all
antimicrobials are not antibiotics. Antimicrobials produce the inhibitory effect on
the microorganism either transiently or permanently. When the inhibition due to an
antimicrobial persists only for the duration it exists in the medium, the effect is
called ‘static’ (e.g., bacteriostatic antimicrobials). If the antimicrobial inhibition is
irreversible, the effect is called ‘cidal’ (e.g., bactericidal and fungicidal antimicrobials). Antimicrobial agents act against a wide range of microorganisms, including
bacteria (antibacterial antimicrobial), fungus (antifungal antimicrobial), protozoa
(antiprotozoal antimicrobial), and virus (antiviral antimicrobials).
Antimicrobials can be classified based on their spectrum of activity, chemical
structure or mode of action. Antimicrobials are said to have a narrow spectrum of
activity when they inhibit only a limited number of organisms. For example, glycopeptides and bacitracin inhibit only gram-positive bacteria, whereas polymyxins act
against gram-negative bacteria. While amino glycosides and sulfonamides act primarily against aerobic organisms, nitroimidazoles inhibit only anaerobic microbes.
Antimicrobials are said to have a broad spectrum of activity when they are able to
inhibit a wide range of organism across groups. For instance, tetracycline and fluoroquinolones are effective against both gram-positive and gram-negative bacteria.
12.2.1 Classes of Antimicrobials
12.2.1.1 Disinfectants and Antiseptics
These are biocidal antimicrobials that are extensively used for surface sterilization
in hospitals, healthcare and household settings and play an important role in preventing infection. In general, these agents have a wider spectrum of activity compared to antibiotics and are nonspecific in terms of the intracellular targets that they
attack to kill the microorganism. Disinfectants are agents used to prevent, control or
destroy microorganisms present on nonliving objects or surfaces, while antiseptics
are antimicrobial agents used for inhibition of microorganisms present on living
surfaces. Certain chemical agents like tincture of iodine, iodophore and alcohol are
used as both disinfectants and antiseptics. Three levels of disinfectants are normally
identified. High-level disinfectants kill some bacterial spores, fungi, many viruses,
vegetative bacteria and mycobacteria. Examples of this group of disinfectants are
8% formaldehyde, 2% gluteraldehyde, 100% ethylene oxide gas, etc. Intermediate
disinfectants kill all of the above agents except bacterial spores. Examples are
70–90% alcohol and 0.5% chlorine. Low-level disinfectants like quaternary ammonium compounds can only act on vegetative bacteria, fungi and lipid viruses.
12 Antimicrobials from Microbes