2. Dr Sonal Saxena, MD
Director Professor and Head of the Department of Microbiology
Maulana Azad Medical College,
New Delhi
and
Dr Amala A Andrews, MD
Maulana Azad Medical College,
New Delhi
UNIVERSITIES PRESS PVT LTD.
3. ANTIBIOTICS
Substances that kill or inhibit bacterial growth
Selectively inhibit the growth of microorganisms
There are four major sites in the bacterial cell that
antibiotics act on:
Cell wall
Cell membrane
Nucleic acid
Ribosome
UNIVERSITIES PRESS PVT LTD.
4. ANTIBIOTICS
Bactericidal: An antibiotic, which in the optimum
dose, kills the target bacteria
Bacteriostatic: An antibiotic, which in the
appropriate dose, inhibits the growth of bacteria but
does not kill them
UNIVERSITIES PRESS PVT LTD.
5. MECHANISMS
OF ACTION OF
ANTIBIOTICS ON
BACTERIAL CELL
Fig. 10.1 Sites and targets of
antimicrobial action on bacterial cells
(Source: Image 14.9.
https://openstax.org/books/microbiolo
gy/ pages/14-3-mechanisms-of-
antibacterial-drugs)
UNIVERSITIES PRESS PVT LTD.
6. 1. Inhibition of cell wall synthesis
2. Inhibition of protein synthesis
3. Inhibition of nucleic acid synthesis
i) Inhibition of precursor synthesis
ii) Inhibition of DNA synthesis
iii) Inhibition of mRNA synthesis
4. Action on bacterial cell membrane
MECHANISMS OF ACTION OF ANTIBIOTICS ON BACTERIAL CELL
UNIVERSITIES PRESS PVT LTD.
8. ANTIMICROBIAL
RESISTANCE
1. Intrinsic resistance
2. Acquired resistance: By mutation/acquisition of resistance
genes from other organisms
Production of enzymes that destroy the antibacterial drug
Expression of efflux systems
Reduction of permeability of drug through mutation of
porin proteins
Modification of the drug’s target site
Production of an alternative metabolic pathway
UNIVERSITIES PRESS PVT LTD.
9. ANTIMICROBIAL
RESISTANCE
Fig. 10.2 Mechanisms of antibiotic resistance by a
bacterium (Source: image 14.18:
https://openstax.org/books/microbiology/pages/1
4-5-drug-resistance)
UNIVERSITIES PRESS PVT LTD.
10. GENETIC MECHANISMS OF DRUG RESISTANCE IN BACTERIA
1. Mutation
2. Genetic transfer
3. Physical mechanisms
Decreased permeability to the drug
Development of alternative metabolic
pathways
Production of enzymes that inactivate the
drugs
Table 10.2 Comparison of mutational and transferable drug
resistance
UNIVERSITIES PRESS PVT LTD.
11. GENETIC
MECHANISMS
OF DRUG
RESISTANCE IN
BACTERIA
Mutational resistance
i) Stepwise mutation, as seen with penicillin
ii) ‘One-step’ mutation, as seen with streptomycin
Clinical implication: Of great importance in tuberculosis
Genetic transfer
1. Vertical gene transfer
2. Horizontal gene transfer:
i) Conjugation ii) Transformation
iii) Transduction iv) Transposons v) Plasmids
UNIVERSITIES PRESS PVT LTD.
12. GENETIC MECHANISMS OF
DRUG RESISTANCE IN
BACTERIA
Plasmid-mediated transferable drug resistance
- Mediated by the R factor
- Due to the production of degrading enzymes
- Level of resistance is usually high
- Multiple drug resistance
This Photo by Unknown Author is licensed under CC BY
UNIVERSITIES PRESS PVT LTD.
14. ANTIBIOTIC
SENSITIVITY TESTS
DIFFUSION TESTS
Antibiotic is allowed to diffuse through a solid medium
Disc diffusion method uses filter paper discs, 6.0 mm in diameter, impregnated with appropriate
concentrations of the antibiotic
UNIVERSITIES PRESS PVT LTD.
Fig. 10.3 Antibiotic sensitivity tests
15. ANTIBIOTIC SENSITIVITY
TESTS
Kirby–Bauer method
Most used
Zones of inhibition around the discs are recorded and interpreted
Guidelines issued by:
- Clinical and Laboratory Standards Institute (CLSI)
- European Committee on Antimicrobial Susceptibility
Testing (EUCAST)
UNIVERSITIES PRESS PVT LTD.
Fig. 10.4 Zones of inhibition around antibiotic discs on the lawn culture of the test bacterium: (a)
sensitive strain (b) resistant strain (Source: Dept of Microbiology, PIMS, Puducherry) and (c)
schematic diagram of the Kirby–Bauer disc diffusion test (Source:
https://commons.wikimedia.org/wiki/File:Zones_of_Inhibition.png)
16. ANTIBIOTIC
SENSITIVITY
TESTS
Stokes method
• Comparison of zone of inhibition of control with that of
test bacteria
• Based on zone of inhibition:
- Susceptible bacteria
- Intermediately susceptible
- Resistant bacteria
E-test
• Quantitative diffusion gradient
• Minimum inhibitory concentration (MIC) is the lowest
concentration of the gradient at which bacterial growth
is inhibited
UNIVERSITIES PRESS PVT LTD.
17. ANTIBIOTIC SENSITIVITY TESTS
Fig. 10.5 Stokes method of testing for antibiotic
susceptibility
Fig. 10.6 MIC of colistin by E-test for Pseudomonas aeruginosa—
the ellipsoidal zone of inhibition has a cut-off where the zone meets
the strip incorporated with gradient concentration of the antibiotic
(MIC is 1.5 μg/mL) (Source: Dept of Microbiology, PIMS,
Puducherry)
UNIVERSITIES PRESS PVT LTD.
18. ANTIBIOTIC
SENSITIVITY
TESTS
DILUTION TESTS
Serial dilutions of the antibiotic are prepared and inoculated
along with the test bacterium
Broth dilution
The minimum bactericidal concentration (MBC), the
lowest concentration of the drug that kills the bacterium, is
estimated
Agar dilution
Several strains can be tested at the same time
UNIVERSITIES PRESS PVT LTD.
19. ANTIBIOTIC
SENSITIVITY TESTS
AUTOMATED METHODS TO
DETECT ANTIBIOTIC
SUSCEPTIBILITY
i) Automated microtitre plates
ii) VITEK system
Fig. 10.7 VITEK system for rapid identification of bacteria and determining
the MIC levels of a set of antibiotics for an organism (Source: Centre for
Virology and Molecular Biology, Pushpagiri Institute of Medical Sciences
and Research Centre, Thiruvalla, Kerala)
UNIVERSITIES PRESS PVT LTD.
20. ANTIBIOTIC POLICY
Guides the treating physician on the most appropriate
antibiotic to be used empirically in a particular infection
Policy should be based on the following parameters:
Data on the susceptibility patterns of pathogens
causing infection in the hospital
Periodic antibiogram pattern
Information regarding the emergence of resistance to
a particular antibiotic
UNIVERSITIES PRESS PVT LTD.
21. ANTIBIOTIC
STEWARDSHIP
The optimal selection, dosage, and duration of
antimicrobial treatment to produce the best clinical
outcome for the treatment or prevention of infection, with
minimal toxicity to the patient and minimal impact on
subsequent resistance
To reduce inappropriate treatment
Core elements of the antibiotic stewardship
programme
Management support
Physician
Pharmacist
UNIVERSITIES PRESS PVT LTD.
22. ANTIBIOTIC STEWARDSHIP
‘Antibiotic time-out’ after 48 hours
Tracking—monitoring antibiotic prescription and
resistance patterns
Calculating the antibiotic utilisation/consumption
Reporting
Educating clinicians regarding resistance and optimal
prescription
Strategies of antibiotic stewardship
1. The front-end or pre-prescription approach
2. Back-end or post-prescription approach
UNIVERSITIES PRESS PVT LTD.
24. ANTIVIRAL AGENTS
TARGETS FOR DRUG ACTION
Attachment of virus to host cells
Uncoating of the viral genome
Viral nucleic acid synthesis
Synthesis of viral proteins
Assembly of the virus
Release of virus particles
Fig. 10.8 Target sites of action of antiviral drugs on the
viral cell (Source: https://openstax.org/books/microbiology/
pages/14-4-mechanisms-of-other-antimicrobial-drugs)
UNIVERSITIES PRESS PVT LTD.
26. ANTIVIRAL
AGENTS
MECHANISMS OF ACTION
1. Inhibition of nucleic acid synthesis
i) Viral nucleoside and nucleotide analogues—inhibit
nucleic acid synthesis
ii) Reverse transcriptase
2. Inhibition of protein synthesis by protease inhibitors
Other mechanisms include:
i) Prevention of entry of virus into the cell
ii) Inhibition via interferons, which prevent viral replication
UNIVERSITIES PRESS PVT LTD.
28. INTERFERONS
Proteins—members of the cytokine family—that are coded
by the host and that inhibit viral replication
Produced very quickly (within hours) in response to viral
infection
Non-specific defence
Acts protective to other cells (not synthesising cell)
Host species-specific
Virus mechanisms to counteract interferons
Block the induction of expression of IFN
Block IFN-included signal transduction
Neutralise IFN-l
UNIVERSITIES PRESS PVT LTD.
29. RESISTANCE TO ANTIVIRAL
AGENTS
An emerging problem
E.g., resistance of HIV to antiretroviral therapy
Necessitates the use of multiple drugs and the
maintenance of a first line and a second line of
antiretroviral therapy (ART)
If the viral load remains static or goes up despite
treatment, it is inferred that the antiviral agent is
ineffective against that virus
UNIVERSITIES PRESS PVT LTD.
30. ANTIFUNGAL AGENTS Table 10.5 Common antifungal agents used to treat fungal
infections
UNIVERSITIES PRESS PVT LTD.
31. ANTIFUNGAL
AGENTS
Fig. 10.9 Antifungal agents and their
modes of action (Source:
https://openstax.org/books/microbiolo
gy/ pages/14-4-mechanisms-of-other-
antimicrobial-drugs)
UNIVERSITIES PRESS PVT LTD.
32. ANTIFUNGAL
AGENTS
Susceptibility testing is done for some yeasts and
moulds by the following methods:
Disc diffusion tests
Microbroth dilution methods
E-test (gradient MIC)
UNIVERSITIES PRESS PVT LTD.
