Lecture 13 - antibiotics Flashcards
define Antibiotic
substance produced by a microorganism that is effective in killing or inhibiting growth of other microorganisms.
define an Antimicrobial agent
any chemical drug used to treat an infectious disease
describe the types of Antimicrobial agent
- antibacterial agent
- antifungal agent
- antiprotozoal agent
- antiviral agent
what are the three origins of antibiotics?
Organic
Semi-synthetic
Synthetic
Describe the organic origin of antibiotics
isolated from bacteria or fungi (e.g., penicillin, erythromycin)
Describe the semi-synthetic origin of antibiotics
urther derivatisation of organic compound for
greater efficacy, e.g. ampicillin
Describe the synthethic origin of antibiotics
generated in the lab (e.g. sulfonamides
What are the Ideal qualities of an antibiotic
- kill or inhibit growth of pathogens
- cause no damage to the host
- cause no allergic reaction to the host
- be stable for storage as solid or liquid
- remain in specific tissue long enough to be effective • kill the pathogens before they mutate and become resistant
Define antibacterial spectrum
range of activity against bacteria, e.g. broad or narrow
define narrow spectrum
only against a limited variety of bacteria e.g. vancomycin (only against gram-positives)
define broad spectrum
against a variety of Gram+/- bacteria e.g. ampicillin
Define bacteriostatic activity
level of antimicrobial activity that inhibits growth of organism.
define bacteriocidal activity
level of antimicrobial activity that kills organism.
define antibiotic combination
combining different antibiotics for:
- broaden the antimicrobial spectrum
- treatment of polymicrobial infections
- prevent emergence of resistant organism - achieve a synergistic killing effect
define antibiotic antagonism
activity of one antibiotic interferes with activity of other, e.g.
the sum of activity is less then the most active individual drug (penicillin-chloramphenicol)
define antibiotic synergism
enhanced antimicrobial activity of combined drugs compared to activity of each individual antibiotic (penicillin-streptomycin)
what is an antibiogram
result of lab testing for sensitivity of an isolated strain to different antibiotics
name three types of antibiograms
- Semi-quantitative based on diffusion (Kirby-Bauer method)
- Quantitative based on dilution series
- Determining the Minimum Bacteriocidal Concentration (MBC)
define Semi-quantitative based on diffusion (Kirby-Bauer method)
Antibiotic concentration highest at centre of disc and lowest at edges. The Minimum Inhibitory Concentration (MIC) can be calculated from the diameter.
define Quantitative based on dilution series
The last vial in which no bacteria grow contains antibiotic at Minimum Inhibitory Concentration (MIC)
define Determining the Minimum Bacteriocidal Concentration (MBC)
subculturing the clear broth onto antibiotic-free solid media
MBC = reduction of colony forming units (CFU) by 99.9% below control.
What are the basic sites of antibiotic activity?
- cell wall synthesis
- Rna synthesis
- DNA rep
- antimetabolites
- protein synthesis - 50s ribosome
- protein synthesis - 30s ribosome
Explaim how beta lactam antibiotics work?
a. β-lactam antibiotics: - penicillins (from Penicillium) - cephalosporins (from Acremonium) - carbapenems
- inhibit peptidoglycan synthesis by irreversible inhibition of peptidyl transferase (=penicillin binding protein, PBP)
weak cell wall and osmotic lysis - bactericidal
- PG is unique to bacteria !!
What are gram nagative bacteria reseistant to? how does penicillin work?
Penicillin does not pass through outer membrane porins - extended spectrum penicillins (amoxicillin, ampicillin, …)
- semi-synthetic
What types of antibiotics affect beta lactamases? describe beta lactamases
Penicillinase Cephalosporinase
β-lactamases are plasmid encoded enzymes that can be transferred between bacterial strains and species.
what are β-lactamase resistant penicillins
- methicillin, oxacillin, flucloxacillinthese are unaffected by β-lactamase
Clavulanic acid: β-lactamase inhibitor Augmentin = ampicillin + clavulanic acid
what are Drugs that block bacterial cell wall synthesis
b. Glycopeptides: Vancomycin
- inhibits PG synthesis by binding to the peptides of the peptidoglycan monomers.
* weak cell wall and osmotic lysis
- mainly against gram-positives
- drug of “last resort” , e.g. for MRSA
- bactericidal
what are Inhibitors of RNA transcription
a. Rifamycins
- inhibit bacterial RNA polymerase, e.g. Rifampin (Rifampicin)
- typically used against Mycobacteria (e.g. tuberculosis, leprosy), MRSA, bactericidal
what are Inhibitors of nucleic acid synthesis
Quinolones and Fluoroquinolones (synthetic)
- bind DNA topoisomerases (enzymes needed for supercoiling, replication and separation of circular bacterial DNA), e.g. gyrase, topoisomerase IV, helicase
- prevent DNA supercoiling, broad spectrum, bactericidal - commonly used against urinary tract infections.
Nalidixic acid (Q) Ciprofloxacin (FQ)
what are Antimetabolites
a. Sulfonamides
- analogues of PABA, competitively inhibits pteridine synthase
b. Trimethoprim
- inhibits bacterial dihydrofolate reductase (DHFR)
Both antibiotics block pathway required for the synthesis of tetrahydrofolic acid, a cofactor needed to make nucleic acids.
mammalian cells depend on external folate (B9)
- bacteriostatic,
- used against e.g. urinary tract infections
What are Inhibition of protein synthesis
a) by binding to the 30S subunit
Aminoglycosides: e.g. neomycin, gentamycin, kanamycin
- freeze the 30S initiation complex (30S-mRNA-tRNA)
- misreading of DNA due to frame-shifts
- mostly effective against aerobic, gram-negative bacteria, bactericidal
- synergistic with penicillins
What are Inhibition of protein synthesis
a) by binding to the 30S subunit - Tetracyclines (tetracycline, doxycycline)
- inhibit binding of aminoacyl-t-RNA to the acceptor site on the ribosome.
- broad-spectrum, bacteriostatic, used against many bacterial infections
What are Inhibition of protein synthesis
b) by binding to the 50S subunit
Macrolides, e.g. erythromycin
- inhibit translocation of the peptidyl tRNA from the A to the P site - against gram-positives, Mycoplasma, bacteriostatic

Chloramphenicol
- broad-spectrum, bacteriostatic
- toxic, used against bacterial meningitis
Describe Antibiotic resistance
1. Non-genetic (intrinsic) drug resistance
a. Metabolic inactivity
most drugs only effect metabolically active cells, not dormant cells
b. Lack of target structure
Mycoplasmas lack cell wall (resistance to penicillins, cephalosporin, vancomycin)
c. Exclusion
antibiotic can’t enter cell (e.g. penicillin/Gram-negative bacteria)
Describe Antibiotic resistance - Genetic resistance
a. Chromosomal resistance
- spontaneous mutation in a gene encoding a target receptor: mutation of RNA- polymerase gene results in rifamycin resistance.
b. acquired resistance
What is Selection for antibiotic resistant strains
survival of the fittest
Describe 6 ways of Antibiotic resistance mechanisms
- Exclusion of antibiotic from site of action
- Efflux pump for the antibiotic removal from site of action
- Enzymatic modification
- Destruction
-β-lactamase - Modified antibiotic insensitive target
- New antibiotic-insensitive target
alternative PBP
what are the two types of drug resistance?
Multidrug-resistant strains: resistant to more than one class of drugs Pandrug-resistant strains: resistant to all clinically safe drugs
What is Methicillin-resistant S. aureus (MRSA) resistant to?
resistant to all antistaphylococcal drugs, except Vancomycin
what is Vancomycin-intermediate S. aureus (VISA) resistant to?
resistant to usual dosage of vancomycin
what is Vancomycin-resistant S. aureus (VRSA) resistant to?
resistant against all antistaphylococcal drugs
what is the full form of VREn and MRTB
- Vancomycin-resistant Enterococcus (VRE)
* Multidrug-resistant Mycobacterium tuberculosis (MRTB)
