Antimicrobials used in dentistry
Chemotherapy is the use of chemicals to destroy or inhibit the growth of cells. Two broad classes of chemotherapeutic agents are used in pharmacology:
- antimicrobials and
- anticancer drugs.
The basis of antimicrobial chemotherapy is a differential sensitivity of the patient and microbe cells to the action of the drug. The drug may affect a structural component of the target cell which is not found in the patient, for example, the bacterial cell wall. Alternatively, a chemotherapeutic agent may inhibit a metabolic pathway peculiar to the microbe cells, for example, synthesis of folate.
Classification of antimicrobials
Antimicrobials can be classified as:
- anti bacterial
- anti fungal
- anti viral
- anti protozoal
- anti helminthic
- anti trematodal
The most commonly prescribed agents are the antibacterials, although
antiviral and antifungal agents are also widely used in the practice of oral
medicine. The antiprotozoal drug metronidazole is too used extensively as it possesses antibacterial activity.
Strictly speaking, the term antibiotic refers to an antimicrobial agent produced by a micro-organism. However, this term nowadays, is used as a synonym for an antibacterial agent
Routes of administration
Antimicrobial drugs can be administered via the following routes
- topically
- orally
- by intramuscular injection
- by intravenous injection or infusion
Antibacterial drugs
There are many different antibacterials and they have no common structure. Some, for example, penicillins are produced by microorganisms to destroy other microbe. Others, for example, sulfonamides are entirely synthetic.
Antibacterial drugs can be classified as:
- bactericidal that kill bacteria
- bacteriostatic that prevent growth of the bacterial population.
Combination therapy with antimicrobials
Antimicrobial agents may be used in combination, for example in the treatment of polymicrobial infections or in the empirical management of serious infections. Synergism can occur by a variety of mechanisms such as the sequential inhibition of a metabolic process. However, not all combinations are helpful as the bacteriostatic agents can inhibit the action of bactericidal drugs.
Mechanism of action of antibacterial drugs
Antibiotics can affect bacteria in following ways:
- by inhibiting bacterial cell-wall synthesis, for example, penicillins
- by inhibiting protein synthesis, for example, tetracyclines
- by interfering with bacterial nucleic acid, for example, metronidazole
- by antimetabolic actions, for example, sulfonamides
Antifungal drugs affect membrane permeability and most antiviral agents affect nucleic acid or protein synthesis.
Resistance
Some species of bacteria are protected against the actions of certain
antibiotics by virtue of the fact that the drug will not reach the target site.
On the contrary, bacteria that would normally be expected to be susceptible to
a particular antibiotic can develop resistance.
Resistance develops:
- by the decreased permeability of the cell membrane to the drug, for example, resistance to ampicillin)
- by inactivation of the drug by enzymes , for example, beta lactamase inactivates some penicillins
- by alteration of the drug receptor site , for example, the mechanism of erythromycin resistance is a change in the ribosomal binding site
- by the development of an alternative metabolic pathway unaffected by the drug , for example, in sulfonamide resistance
- by increased elimination of the drug from the cell, for example, fluoroquinolones may be actively expelled from the cell
Resistance can develop by
- Vertical transfer
- Horizontal transfer
To read more on general principles of antimicrobial therapy, click the link given below:
General Principles of Antimicrobial Therapy