Introduction to Antibiotics
1st yr( Respiratory block)
Prof. Mohammad Alhumayyd
Pharmacology Department
Tel.4671350
Definition of Antibiotics
Chemical substances produced by various
microorganisms that have the capacity to inhibit or
destroy other microorganisms.
Now aday they are chemically synthesized.
They either kill bacteria(bactericidal) or keep more
bacteria from growing(bacteriostatic).
Antibiotics will not cure infections caused by
viruses.
CLASSIFICATION OF ANTIBIOTICS
ACCORDING TO MECHANISM OF ACTION
INHIBITION OF CELL WALL
SYNTHESIS e.g. Penicillins,
cephalosporins.
 INHIBITION OF PROTEIN
SYNTHESIS e.g.Macrolides.
 INHIBITION OF NUCLEIC ACID
SYNTHESIS e.g. Quinolones.

According to spectrum

1- Narrow spectrum , e.g.:
penicillin G , aminoglycosides

2- Broad spectrum , e.g.:
ampicillin , amoxicillin
Antibiotic Prescription
Clinical situation
Microbiological
information
Pharmacological
consideration
Choice of Antimicrobial Drugs
1.
Clinical diagnosis eg. Syphilis
2. Bacteriological identification
a) infecting organism is not identified.eg. UTIs ,meningitis.
b) infecting organism is identified but sensitivity to
antibiotic isn’t known e.g.T.B
3. Site of infection
Choice of Antimicrobials ( Cont.)
4. Host factors
a) Immune system e.g. diabetes, HIV, malnutrition, advanced age.
b) Genetic factors
eg. Patients with G-6-PD deficiency treated with sulfonamides
c) Pregnancy and Lactation
Aminoglycosides- hearing loss in the child
Tetracyclines- injury to the developing teeth( child )
d) Age of the patient
eg. Grey baby Syndrome-chloramphenicol
Discolouration of teeth-tetracycline
e) Renal function
eg. Aminoglycosides in renal failure
f)Liver function
eg. Erythromycin in hepatic failure
Choice of Antimicrobials ( Cont.)
5. Drug Allergy
6. Potential Side Effects(Drug safety)
Chloramphenicol ( a plastic anaemia)
Tetracyclines in children (dental discol.)
Flouroquinolone in children &pregnancy
( tendon dammage )
7. The cost of therapy
MISUSES OF ANTIBIOTICS
Treatment of untreatable infections
e.g. viral infections
 Improper dosage.
 Therapy of fever of unknown origin.
 Reliance on chemotherapy with omission of
surgical drainage.
 Excessive use of prophylactic antibiotics in
travellers.
 Lack of adequate bacteriological
information.

BACTERIAL RESISTANCE
Definition
Concentration of antibiotic required to inhibit or
kill the bacteria is greater than the
concentration that can safely be achieved in
the plasma.
General Principles of Chemotherapy
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Administer drug in full dose, at proper interval and by the
best
route
Apparent cure achieved-continue for about 3 days further to avoid
relapse ( exception TB and bacterial endocarditis )
Skipping doses may decrease effectiveness of treatment & increase
the likelihood of bacterial resistance.
Two or more antimicrobials should not be used without good
reason, eg.:
Mixed bacterial (polymicrobial) infections
Desperately ill patient of unknown etiology
To prevent emergence of resistance (eg. TB )
To achieve synergism
eg. piperacillin+ gentamicin (p.aeruginosae)
Aim of chemotherapeutic
combination

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Broaden the spectrum of antibacterial activity e.g:
clindamycin+ gentamycin
Reduce the doses
Reduce the side effects
Overcome drug resistance(delay the rate of drug
resistance) as in treatment of TB or pseudomonal
infections.
Produce a more potent compound
produce a synergistic effect) as in co-trimoxazole
combination or as in penicillin with gentamycin in
treatment of bacterial endocarditis.
Treatment of severe infections of unknownetiology as in
septicaemia.