for MD pharmacology

1. ANTIMICROBIAL
RESISTANCE MECHANISMS
- Dr Mangala Nischal

2. CONTENTS :-
• Introduction
• Natural Resistance
• Acquired Resistance
• Resistance Mechanisms
- Biochemical
- Mutation
- Gene Transfer
• Cross Resistance
• Prevention of Drug Resistance

3. INTRODUCTION :-
• WHO - Defines as micro-organisms that are not inhibited by usually
achievable systemic concentration of an antimicrobial agent(AMA)
with normal dosage schedule and / or fall in the minimum inhibitory
concentration (MIC) range.
• when a species is subjected to chemical warfare, that threatens its
extinction it often evolves mechanisms to survive under that stress
development of resistance.

4. • Two major factors are associated with emergence of antibiotic
resistance:
- Evolution
- Clinical/ Environmental practices.

5. NATURAL RESISTANCE :-
• Some microbes lack the metabolic process or the target site for
particular drug.
• e.g: - Gram-negative bacilli are normally unaffected by penicillin G
- M. tuberculosis is insensitive to tetracyclines.
• This resistance does not pose a significant clinical problem.

6. ACQUIRED RESISTANCE :-
• It is the development of resistance by an organism (which was
sensitive before) due to the prolonged use of an AMA.
• Some bacteria are notorious for rapid acquisition of resistance
e.g. staphylococci, coliforms, tubercle bacilli.

7. RESISTANCE MECHANISMS :-
BIOCHEMICAL MECHANISMS -
• Antimicrobial resistance can develop at any one or more of steps in
the process
• Reduced entry of antibiotic into pathogen
• Enhanced export of antibiotic by efflux pumps
• Release of microbial enzymes that destroy the antibiotic

8. • Alteration of microbial proteins that transform pro-drugs to the
effective moieties
• Alteration of target proteins
• Development of alternative pathways to those inhibited by the
antibiotic

10.  Reduced Entry of Drug into Pathogen :-
• Small polar molecules & antibiotics, enter the cell through protein
channels called Porins.
• Absence of, mutation in, or loss of a favored porin channel can slow
the rate of drug entry into a cell or prevent entry altogether 
reducing drug concentration at the target site.
• If target is intracellular  mutation or phenotypic change that slows
or abolishes this transport mechanism  resistance.

11.  Resistance Due to Reduced Affinity of Drug to Altered Target Structure :-
• A reduced affinity of drug for its target or the enzyme that converts the
prodrug to active drug. Such alterations may be due to
 Mutation of the natural target (e.g., fluoroquinolone resistance)
 Target modification (e.g., ribosomal protection type of resistance to
macrolides and tetracyclines)
Acquisition of a resistant form of the native, susceptible target (e.g.,
staphylococcal methicillin resistance caused by production of a low-affinity
penicillin-binding protein)

12. • Examples :-
• The penicillin-resistant gonococci are less permeable to penicillin G.
• Chloroquine-resistant P. Falciparum accumulates less chloroquine.

13.  Resistance Due to Drug Efflux :-
• Microorganisms can overexpress efflux pumps and then expel
antibiotics to which their susceptible.
• Five major systems of efflux pumps
- The multidrug and toxic compound extruder (MATE)
- The major facilitator superfamily (MFS) transporters
- The small multidrug resistance (SMR) system
- The resistance nodulation division (RND) exporters
- ATP binding cassette (ABC) transporters

15. • Drug resistance to erythromycin, fluoroquinolones & Anti-malarial
drugs are mediated through these Efflux pumps.

16.  Resistance Due to Destruction of Antibiotic :-
• Drug inactivation is a common mechanism of drug resistance.
• Bacterial resistance to aminoglycosides  aminoglycoside-modifying
enzyme
• β -lactam antibiotics  β -lactamase

18.  Hetero-resistance and Viral Quasi Species :-
• It is said to be present when only a subset of the total microbial
population is resistant.
• Increased therapeutic failures and mortality is seen.
• Viral evolution due to drug and immune pressure  Quasi species.
• Quasi species are resistant to antiretroviral agents  failure of
antiretroviral therapy.

19.  Resistance due to Enhanced Excision of incorporated drug :-
• These drugs are incorporated into the viral DNA chain and cause
chain termination.
• E.g. Nucleoside reverse transcriptase inhibitors such as zidovudine are
2′-deoxyribonucleoside analogs  5′-triphosphate and compete with
natural nucleotides.

20.  MUTATION –
• Mutation and antibiotic selection of the resistant mutant are the
molecular basis for development of resistance in many bacteria,
viruses, and fungi.
• Mutations are not caused by drug exposure. They occur as a survival
advantage, when drug is present.

21. • Mutations may occur in the gene encoding
(1) The target protein, altering its structure so that it no longer binds
the drug
(2) A protein involved in drug transport
(3) A protein important for drug activation or inactivation
(4) In a regulatory gene or promoter gene affecting expression of the
target, a transport protein, or an inactivating enzyme

22. • Suboptimal dosing strategies  selective kill of the more susceptible
population, which leaves the resistant isolates to flourish.
• A single-step mutation  high degree of resistance.
• The Multi-step mutation  clinically significant resistance.
• E.g : Combination of pyrimethamine and sulfadoxine inhibits
Plasmodium falciparum’s folate biosynthetic pathway via inhibition of
dihydrofolate reductase (DHFR) by pyrimethamine and
dihydropteroate synthetase (DHPS) by sulfadoxine.

23.  Hypermutable Phenotypes :-
• The ability to protect genetic information from disintegrating and also
to be flexible enough to allow genetic changes.
• This is accomplished principally by the
- Insertion of the correct base pair by DNA polymerase III
- Proofreading by the polymerase
- Postreplicative repair.
• Mutator (Mut) phenotypes  antibiotic resistance .

24. Quorum sensing :-
• Microbes communicate with each other and exchange signaling
chemicals (Autoinducers) coordinate gene expression for virulence,
conjugation, apoptosis, mobility and resistance.
• QS signal molecules AHL, AIP, AI-2 & AI-3 have been identified in
GM-ve bacteria
• Gram-positive bacteria use processed oligo-peptides to
communicate.

25. • Several QS inhibitors molecules have been synthesized  AHL, AIP,
and AI-2 analogues  Potent Virulence inhibitors.
• QS controls virulence factor production in Gram-positive human
pathogens including S. aureus, Listeria monocytogenes, Enterococcus
faecalis, and Clostridium perfringens
• V. cholera, P. aeruginosa  Gram negative bacteria .

26.  GENE TRANSFER :-
• Drug resistance may be acquired by passage of the trait vertically to
daughter cells, but more commonly it is acquired by horizontal
transfer of resistance by,
- Transduction
- Transformation
- Conjugation

27. • Horizontal transfer of resistance genes is greatly facilitated by
Mobile genetic elements
Plasmids Transducing Transposable Integrons Gene
phages elements cassettes
Insertion sequences Transposons Transposable phages

28. • Insertion sequences do not encode resistance, but they function as
sites for integration of other resistance-encoding elements.
• Transposons are insertion sequences that also code for drug
resistance & other function.
• Transposon move between chromosome and plasmid thus
“hitchhike” the resistant gene out of the host and into a recipient.

29.  Transduction - Is acquisition of bacterial DNA from a phage that has
incorporated DNA from a previous resistant host bacterium.
e.g. strains of S. aureus.

30. Transformation - Is the uptake and incorporation into the
host genome by free DNA released into the environment by
other bacterial cells. E.g. Penicillin resistance in
Pneumococci and Neisseria.

31. Conjugation - Is gene transfer by direct cell-to-cell contact through a
sex pilus or bridge.
• Multiple resistance genes can be transferred in a single event.
• Genetic transfer by conjugation is common among gram-negative
bacilli, and Enterococci.

33. CROSS RESISTANCE :-
• Acquisition of resistance to one AMA conferring resistance to
another AMA to which the organism has not been exposed
e.g. - resistance to one sulfonamide means resistance to all others,
-resistance to one tetracycline means insensitivity to all others
• Partial cross resistance is sometimes seen in unrelated drugs
e.g. - between tetracyclines and chloramphenicol
- between erythromycin and lincomycin.

34. • Cross resistance may be
 Two-way, e.g. between erythromycin and clindamycin and vice versa
 One-way, e.g. development of neomycin resistance by
enterobacteriaceae makes them insensitive to streptomycin but many
streptomycin-resistant organisms remain susceptible to neomycin.

35. Prevention of drug resistance :-
 No indiscriminate and inadequate or unduly prolonged use of AMAs
should be made.
 Prefer rapidly acting and selective (narrow spectrum) AMAs.
 Use combination of AMAs for prolonged therapy e.g. tuberculosis,
SABE.
 Intensive treatment for notorious organisms.

36. REFERENCES :-
• Essentials of Medical Pharmacology Sixth Edition - KD TRIPATHI MD
• Goodman & Gilman’s The Pharmacological Basis of THERAPEUTICS
• LaSarre B, Federle MJ. Exploiting Quorum Sensing To Confuse
Bacterial Pathogens. Microbiology and Molecular Biology Reviews :
MMBR. 2013;77(1):73-111. doi:10.1128/MMBR.00046-12.