Terms and Concepts:

1. Agents:

Chemotherapeutic agents:

• These are group of drugs or substances which are used to interfere with functioning of any type of foreign cells.
• Includes antibiotics and synthetic antimicrobials and covers all antibacterial, antifungal, antiviral, antiprotozoal, anthelmintic and antineoplastic drugs.

Antibiotics:

• Subgroup of chemotherapeutic agents which are produced by various species of microorganisms.
• They are used to kill or supress the growth of other microorganism or foreign cell.

Antimicrobials:

• Chemical substances which are used to kill or supress the growth of microorganism.
• They may be natural, synthetic or semi-synthetic.

2. Selective toxicity:

• Ability of antimicrobial agents to kill an invading microorganism without harming the cells of its host.
• In most cases, it is relative rather than absolute requiring that concentration of antimicrobial be carefully controlled to attack invading organism while still being tolerated by host.

3. Antimicrobial activity:

• Ability of compound to react with microbial cell molecules in a way that interferes with the growth and multiplication of microorganism (static effect) or causes the killing of microorganisms (cidal effect).
• Depending on the type of activity it could be of following types:

I. Bacteriostatic  II. Bacteriocidal

I. Bacteriostatic:

• Ability of drugs to inhibit growth and multiplication of bacteria.
• Inhibited growth in time results in death of microorganism and removal of organism by hosts defence cells.

II. Bacteriocidal:

• Ability of drugs to cause death of bacteria.
• These drugs may be static and cidal according to situation. Some drugs are bacteriocidal at higher concentration and some bacteriocidal drugs are only bacteriostatic under certain circumstances.
• It is also possible for an antimicrobial agent to be static for one organism and cidal for another. For ex; chloramphenicol is bacteriostatic against gram -ve rods and bactericidal for pneumococci.

4. Antibacterial/antimicrobial spectrum:

• Refers to the range of pathogenic organism against which an antimicrobial agent is active.
• Classified as:

a. Narrow spectrum: Active against a few or class/type of organism

b. Broad spectrum: Active against a wide variety of organism

5. Potency:

• Antimicrobial activity per milligram of chemotherapeutic agents
• Usually expressed on the basis of minimum inhibitory concentration, minimum bacterial concentration or minimum antibiotic concentration
• Minimum inhibitory concentration (MIC):
• Lowest concentration of antimicrobial drug that prevents visible growth of bacteria when grown against sequentially diminishing drug concentration in vitro.
• Affected by host factors, methodology used to determine MIC, different bacterial strains prevalent in a region etc.
• Minimum bactericidal concentration (MBC):
• Lowest concentration of antimicrobial drug that kills the bacteria
• Also determined in vitro similar to MIC.
• Minimum antibiotic concentration (MAC):
• Concentration of drug that reduces the growth of an microorganism in vitro by a factor of 10.
• May be one-quarter or one tenth of MIC depending on drug and organism
• Used occasionally to express the activity of some drugs which exert antimicrobial action in vivo at concentration below MIC.

6. Other terms:

# Post antibiotic effect (PAE):

• Persistence of antimicrobial effect for a longer period after brief exposure to or in absence of detectable concentration of antimicrobial drug.
• Can affect the dosing interval. Ex; Aminoglycosides are given at 12-24 hours intervals, although their half-lives are much shorter.

# Biphasic (Eagle) effect:

• Phenomenon in which low doses of antibacterial in vitro against certain bacteria produce lysis whereas high doses donot.
• Associated primarily with β-lactam antibiotics and believed to be due to differential sensitivity of the penicillin binding proteins to high dose of β-lactams which inhibit autolysins.

# Chemotherapeutic effect:

• Ratio of toxic dosage level to the therapeutic dosage level is termed as chemotherapeutic index.
• It is defined as maximum tolerable dose per kg body weight divided by minimum dose per kg body weight which will cure the disease.
• Higher this number, the safer is drug.

# Evasion:

• Phenomenon in which organisms may enter or be present in anti-microbial resistant state such that all members of population are destroyed by antimicrobial except those that happen to be in the resistant state.