Antimicrobial agents are the agents that are used to prevent infections by killing or preventing the growth of living organisms. They are of various types like antibacterial, antifungal, antiprotozoan, antihelminthic, etc. As per spectrum activity, antimicrobials are of two types namely broad spectrum antimicrobial agents (both for gram-positive and gram-negative bacteria, for example: Tetracyclines) and narrow spectrum antimicrobial agents (acts on either gram-positive or gram-negative organisms, for example: Penicillin G). Based on the mode of action, antimicrobials are mainly of two types either bactericidal or bacteriostatic. Bactericidal agents have lethal action on bacteria whereas bacteriostatic are the drugs that prevent the growth of microbial cells. Hence, the mode of action of antimicrobials is divided into various types that are listed in the given chart.
Flowchart: Mode of action of antimicrobial activity
Factors Affecting Antimicrobial Activity:
Some antimicrobial agents have cidal action under one set of
conditions and static action under others. There are several factors that
influence the activity of antimicrobial agents:
(1) the susceptibility of the microorganism
(2) the concentration or dose of the agent
(3) the length of exposure
(4) the number of microorganisms
(5) environmental conditions
1. Microbial susceptibility: Responses of microbes
vary on different disinfectants. For example: vegetative cells of the
Mycobacteria that cause tuberculosis and leprosy, are covered by a waxy coating
that protects them from many antimicrobial chemicals. The hepatitis B virus and
some fungal spores are resistant to most disinfectants. Bacillus and
Clostridium are especially difficult to eliminate etc.
2. Concentration of the agent: Diluted microbicidal
chemicals have usually weak antimicrobial activity. At lower concentrations,
they become micro-bistatic or lose antimicrobial activity altogether. The
antimicrobial effects of temperature or radiation also depend on the intensity
of the exposure. Low doses may inhibit growth, whereas high doses may result in
sterilization.
3. Length of exposure: Microorganisms die when
physical or chemical conditions irreversibly damage essential cell components.
All organisms do not die rapidly and simultaneously when a critical exposure is
achieved, because microbial death is a function of the longer microbes are
exposed to potentially lethal conditions, the more microbes will be killed.
Microbistatic agents are effective only as long as they are present and must be
used during the entire time that inhibition is to be maintained.
4. Number of microorganisms: Antimicrobial
effectiveness also depends on the initial concentration of the microbial
population. As the number of microbial contaminants increases, either the
exposure period or concentration of the agent must increase to achieve
acceptable levels of decontamination.
5. Environmental conditions: Temperature, pH, and
moisture affect the efficiency of most antimicrobial agents. Some chemical
agents are absorbed by organic materials such as blood, mucus, and tissue are
severely reduce antimicrobial effectiveness and hence they are not used on the
skin. Antibiotics: Antibiotics are chemical drugs (naturally occurring,
semi-synthetic, or synthetic) that kill or inhibit the growth of microorganisms.
Ideal Properties of Antibiotics:
• They have the appropriate spectrum of activity.
• They have no toxicity to the host.
• They should have low development of resistance.
• They should have rapid and extensive tissue distribution.
• They should have a relatively long half-life.
• They should be free of interaction with other drugs.
• They should be easy to administer.
Mode of Action of Antibiotics: The mode of action of
various antibiotics is listed in Table.
Table: Mechanism
of action of antibiotics
|
Mechanisms |
Antibiotics |
|
Inhibition of
synthesis or damage to cell wall |
Penicillins,
Cephalosporins |
|
Inhibition of
synthesis or damage to Cytoplasmic membrane |
Polymyxins |
|
Inhibition of
synthesis or metabolism of nucleic acids |
Rifampin,
Quinolones |
|
Protein
biosynthesis |
Tetracyclines,
Chloramphenicol |
|
Modification
of energy metabolism |
Sulfonamides,
Trimethoprim |