This document discusses antibiotics, including their definitions, classifications, mechanisms of action, and resistance. Antibiotics are substances produced by microorganisms that kill or inhibit the growth of other microorganisms. They are classified based on their mechanism of action (such as inhibiting cell wall synthesis or protein synthesis), spectrum of activity (narrow or broad), and mode of action (bacteriostatic or bactericidal). Antibiotic resistance can occur through several mechanisms such as bacteria producing enzymes to inactivate antibiotics or reducing permeability to drugs. Combining antibiotics can result in synergistic effects against certain organisms.

1. Antibiotics
Wala Hassan Mustafa
M.SC microbiology
U of G

2. What is an antibiotic?
 “Antibiotic” is from antibiosis, meaning against life.
 Antibiotic” Substances derived from a microorganism to kill or suppress the
growth of other microorganisms.
 “Antimicrobial” is abrader term referring to any substance that can affect
microbial life including synthetic and semi synthetic without selective toxicity

3. Terminologies
 Selective toxicity :
A drug that kills harmful microbes without damaging the host
 Antibacterial spectrum :
Range of activity of an antibiotic
 Minimum bactericidal concentration (MBC) :
The lowest concentration that kills 99.9% of the bacterial population
 Minimum inhibitory concentration (MIC) :
The lowest concentration that inhibits the growth of bacterial population

4.  Antibiotic resistance
is the phenomenon that susceptibility of pathogenic microorganisms to
antibiotic becomes lower or even loses after the microorganisms contact with
antibiotic
 Cross antibiotic resistance
When the bacteria show resistance to one antibiotic, they are also resistant
to some other antibiotics .

5. Antibiotic sources :
1. Natural :
A: fungi_penicillin
B :Bacteria_bacillus sp (polymixin)
2. synthetic

6. Classification of Antibiotics
 Antibiotics are classified by several ways:
1. On the basis of mechanism of action
2. On the basis of spectrum of activity
3. On the basis of mode of action

7. 1 :On the basis of mechanism of action
1. Inhibition of cell wall synthesis:
• Penicillins, Cephalosporins, Bacitracin and Vancomycin .
2. Inhibition of functions of cellular membrane:
• Polymyxins
3. Inhibition of protein synthesis:
• Chloramphenicol, Macrolides and Clindamycin
• Tetracyclines and Aminoglycosides
4. Inhibition of nucleic acid synthesis:
• Quinolones
• Rifampin
5. Inhibition of folic acid synthesis:
• Sulfonamides & trimethoprim

9. 2: On the basis of spectrum of activity
 A: narrow spectrum :
Antibiotic that is active only against a limited number of bacteria e.g.
penicillin G, 1 -2nd generation cephalosporins, oxazolidone
 B :broad spectrum :
Antibiotic that can inhibit wide range of bacteria ( G- positive and Gnegative
bacteria e.g. Carbapenems, 3-4th generation cephalosporins, quinolones)

11. 3:On the basis of mode of action
 Bacteriostatic antibiotics:
Level of antimicrobial activity that inhibits the growth of bacteria
• Tetracycline
• Chloramphenicol
• Erythromycin
• Lincomycin
 Bacteriocidal antibiotics:
Level of antimicrobial activity that kill bacteria
• Cephalosporin
• Penicillin
• Erythromycin
• Aminoglycosides
• Cotrimoxazole

12. Measuring Antimicrobial
Sensitivity
E Test
• MIC: Minimal
inhibitory concentration

13. Ideal Antimicrobial Agent
1. Have highly selective toxicity to the pathogenic microorganisms in host body
2. Have no or less toxicity to the host
3. Low propensity for development of resistance
4. Not induce hypersensitive reactions in the host
5. Have rapid and extensive tissue distribution
6. Be free of interactions with other drugs
7. Be relatively inexpensive

14. Selection of Antimicrobial Agent
• Empiric therapy - prior to identification of Organism
- critically ill patients
• Organism’s susceptibility to the antibiotic
• Patient factors - immune system, renal/hepatic function
• Effect of site of infection on therapy - eg CAN CROSS blood brain barrier
• Safety of the agent
• Cost of therapy

15. Effects of Combinations of Drugs
 Synergism occurs when the effect of two drugs together is greater than the
effect of either alone.
 Antagonism occurs when the effect of two drugs together is less than the
effect of either alone.
 Additive (indifferent) effect: the activity of two drugs in combination is
equal to the sum (or a partial sum) of either alone.

16. USE OF ANTIBIOTIC COMBINATIONS
1. To treat serious infections .
2. To achieve a synergistic inhibitory effect against certain organisms.
3. To prevent the emergence of resistant organisms.

17. Classes of Antibiotics
 The main classes of antibiotics are:
1) Beta-Lactams
Penicillins
Cephalosporins
Carbapenems
Monobactams
2) Macrolides and Ketolides
3) • Aminoglycosides
4) • Fluoroquinolones
5) • Tetracyclines, Amphenicols

18. ANTIBIOTIC RESISTANCE
PRINCIPLES:
There are four major mechanisms that mediate bacterial resistance to drugs:
(1) Bacteria produce enzymes that inactivate the drug
(2) Bacteria synthesize modified targets against which the drug has a reduced effect
(3) Bacteria reduce permeability to the drug such that an effective intracellular
concentration of the drug is not achieved
(4) Bacteriaactively export drugs using a “multidrug resistance pump”

20. The end