mecanism of resistance

1. Mechanism of Resistance to
Antimicrobial Agents
Dr.Tshema.V
Pharmacology PG
Coimbatore Medical College

2. Definition – Drug Resistance
• It refers to unresponsiveness of a
microorganism to an antimicrobial
agent(AMA)
• when aspecies-existence threat
- chemical,
-evolvesmechanismsto survive
-development of resistance.

3. •Twomajor factors areassociated with
emergenceofantibiotic resistance:
- Evolution
- Clinical/ Environmental practices.

4. NATURALRESISTANCE:-
•Somemicrobeslackthe metabolic processor the
target sitefor particular drug.
•e.g:- Gram-negativebacilli arenormally unaffected
bypenicillinG
- M. tuberculosis isinsensitive totetracyclines.
- aerobic organisms – not affectedby
metronidazole
- anaerobic bacteria– not inhibited by
aminoglycosides
•Thisresistancedoesnot poseasignificantclinical
problem.

5. ACQUIRED RESISTANCE:-
•It isthe development of resistance byan
organism(whichwas sensitive before) due
to the prolonged useof anAMA.
• Somebacteria arenotorious for rapid
acquisition ofresistance
e.g.staphylococci,coliforms, tubercle
bacilli.

6. RESISTANCEMECHANISMS
MECHANISMS
•Antimicrobial resistancecandevelopat anyoneor
more of stepsin the process
• Reducedentry of antibiotic intopathogen
• Enhancedexport of antibiotic byeffluxpumps
• Releaseof microbial enzymesthat destroy the
antibiotic
• Alteration of targetproteins
• Developmentof alternative pathwaysto those
inhibited bythe antibiotic

8.  ReducedEntryof Druginto Pathogen:-
•Smallpolar molecules& antibiotics, enter the cell
throughprotein channelscalledPorins.
•Absence
• mutation
• lossof afavouredporin channel-slow the rate of
drug entry into acell or prevent entry altogether -
reducingdrug concentration at the targetsite.
•If targetisintracellular - mutation or phenotypic
changethat slows or abolishes this transport
mechanism- resistance.

9. • Examples:
• Sleeping sickness
• Trypanosoma brucei – early stage – treated
with Suramin and pentamidine
• CNS disease (sleeping sickness) – melarsoprol
and eflornithine
• P2 – active transport of Melarsoprol
• Lack or mutation P2 – resistant to melarsoprol

10.  ResistanceDueto DrugEfflux:-
•Microorganisms canoverexpress efflux pumpsand
then expel antibiotics to which theirsusceptible.
•Fivemajor systemsof effluxpumps
- Themultidrug andtoxic compoundextruder
(MATE)
- Themajor facilitator superfamily(MFS)
transporters
- Thesmallmultidrug resistance(SMR)system
- Theresistancenodulation division (RND)
exporters
- A
TPbinding cassette (ABC)transporters

12. Example – Drug resistance
P.falciparum :
• Antimalarial drugs – chloroquine, quinine, mefloquine,
halofantrine, lumefantrine
• ABC transporter – encoded by Pfmdr 1
gene(P.falciparum multidrug resistance gene 1)
• Point mutation of gene – drug resistance and failure of
chemotherapy
Chromosomal resistance – S.pnuemoniae and
M.tuberculosis
• Induction of efflux pumps early stage – increases MIC –
further microbial replication – Chromosomal mutation

13.  ResistanceDueto ReducedAffinityof Drugto Altered
TargetStructure:-
• A reducedaffinity of drugfor its target – single or
multiple point mutations – amino acid composition
change – change in target protein.
• Mutation of the naturaltarget (e.g.,fluoroquinolone
resistance)
• Targetmodification(e.g.,ribosomal protection
typeof resistanceto macrolidesandtetracyclines)
• Acquisition of aresistant form of the native, susceptible
target (e.g., staphylococcalmethicillin resistancecausedby
production ofa low-affinity penicillin-bindingprotein)
• HIV resistance – reduced affinity for drugs
• Benzimidazoles – mutation in Beta tubulin gene

14.  ResistanceDue to ReducedAffinity of Drug to Altered
Target Structure :-
• Areduced affinity of drug for its target or the enzyme
that converts the prodrug to active drug. Such
alterations may be dueto
 Mutation of the natural target (e.g., fluoroquinolone

15. Incorporation of drug
• Uncommon – organism becomes resistant –
but also requires drug for growth
• Eg – Enterococcus – vancomycin resistance on
prolonged exposure – developes vancomycin
requiring strains
• Streptomycin for Tuberculosis – mutant goes
into dormancy in absence of drug

16.  ResistanceDueto Destruction of Antibiotic:-
•Druginactivation isacommonmechanismof drug
resistance.
•Bacterialresistance to aminoglycosides -
aminoglycoside-modifying enzyme
• β-lactamantibiotics - β-lactamase

18.  Hetero-resistanceandViralQuasiSpecies:-
•It issaidto bepresentwhen only asubset of the
total microbial population isresistant.
•Bacteria – vancomycin in S.aureus, E.faecium
•Colistin in acinetobacter
•Rifampin, isoniazid in Tb
•Fungi – fluconazole in C.neoformans and candida
•Increasedtherapeutic failures andmortality isseen.
• Viral evolution dueto drug andimmune pressure-
Quasispecies.
•Quasispeciesareresistant to antiretroviral agents
- failureof antiretroviraltherapy.

19.  Resistancedueto EnhancedExcisionof incorporated
drug:-
•Thesedrugsareincorporated into the viral DNA
chainandcause chaintermination.
•E.g. Nucleoside reverse transcriptase inhibitors such as
zidovudine are 2′-deoxyribonucleoside analogues - 5′-
triphosphate andcompetewith natural nucleotides.
•Resistance – mutation – reverse transcriptase gene –
phosphorolytic excision of incorporated chain –
terminating nucleoside analogue

20.  MUTATION–
•Mutation and antibiotic selection of the resistant
mutant are the molecular basis for development
of resistancein manybacteria, viruses,andfungi.
•Mutations arenot causedbydrug exposure,They are
random,spontaneous eventswhichconfersurvival
advantage,when drug ispresent.

21. • Mutations mayoccurin the geneencoding
(1) Thetarget protein, altering its structure sothat it
no longer binds the drug
(2) Aprotein involvedin drugtransport
(3) Aprotein important for drug activation or
inactivation
(4) In aregulatory geneor promoter geneaffecting
expressionofthe target, atransport protein, or an
inactivatingenzyme

22. • Suboptimaldosingstrategies - selectivekill of themore
susceptible population, which leavesthe resistantisolatesto
flourish.
• Asingle-stepmutation - highdegreeofresistance.
• M.tuberculosis katG,Ser315mutation–resistancetoisoniazid
• M814Vmutation–reversetranscriptasegeneofHIV-1–
resistance tolamivudine
• TheMulti-step mutation - clinically significantresistance.
• E.g: Combination of pyrimethamine andsulfadoxineinhibits
Plasmodiumfalciparum’s folate biosynthetic pathwayvia
inhibitionof dihydrofolate reductase(DHFR)by
pyrimethamine and dihydropteroate synthetase(DHPS)by
sulfadoxine.

23. Hypermutable Phenotypes
• Genetic continuity – replicative and repair
activities of DNA polymerases , post
replicative repair systems
• Mutator phenotype: defect in repair
mechanism – high degree of mutations in
many genes
• Emergence of multidrug resistant strains of
M.tuberculosis

24. GENETRANSFER:-
•Drugresistance maybeacquiredbypassageof the trait
verticallyto daughtercells,but more commonly it is
acquired byhorizontal transfer of resistanceby,
- Transduction
- Transormation
- Conjugatifon

25. • Horizontaltransfer of resistancegenesisgreatly
facilitatedby Mobile geneticelements
Plasmids Integrons
T
ransducing
phages
T
ransposable
elements
Gene
cassettes
Insertionsequences Transposons Transposablephages

26. •Insertion sequencesdo not encoderesistance,but
they functionas sitesfor integration of other
resistance-encodingelements.
•Transposonsareinsertion sequencesthat also
codefor drug resistance& otherfunction.
•Transposonmovebetween chromosome andplasmid
thus “hitchhike” the resistant geneout of the host
andinto arecipient.

27. Transduction - Isacquisition of bacterial DNAfrom a
phagethathas incorporated DNAfrom aprevious
resistant host bacterium.
e.g.strainsof S.aureus.

28. Transformation - Isthe uptakeandincorporation
into the host genomebyfree DNAreleasedinto the
environmentby other bacterial cells.E.g.Penicillin
resistancein PneumococciandNeisseria.

29. Conjugation- Isgenetransfer bydirect cell-to-cell contact
througha sexpilusor bridge.
• Multiple resistancegenescanbetransferredin asingleevent.
• Genetictransfer byconjugationiscommonamonggram-
negative bacilli,andEnterococci.

30. CROSSRESISTANCE
•Acquisition of resistanceto oneAMAconferring
resistanceto another AMAto which the
organismhasnot beenexposed
e.g.- resistanceto onesulfonamide means
resistance to allothers,
-resistanceto onetetracycline meansinsensitivity
to allothers
• Partialcrossresistanceissometimes seenin unrelated
drugs
e.g. - between tetracyclinesandchloramphenicol
- between erythromycin andlincomycin.

31. •Crossresistance may be
Two-way,e.g.between erythromycin andclindamycin
andviceversa
One-way,e.g.development of neomycinresistanceby
enterobacteriaceaemakesthem insensitive to
streptomycin butmany streptomycin-resistant
organismsremain susceptible to neomycin.

32. Preventionofdrug resistance:-
Noindiscriminate andinadequateor unduly prolongeduse
ofAMAs shouldbemade.
Preferrapidly actingandselective(narrow spectrum)AMAs.
Usecombination of AMAsfor prolongedtherapye.g.
tuberculosis, SABE.
Intensivetreatment for notoriousorganisms.

33. Over view of Drug resistant TB
Isoniacid(H)
• MOA: inhibition of synthesis of mycolic acids
• fatty acid component of mycobacterial cell wall
• Gene – InhA & Kas A
• Catalase peroxidase enzyme – active metabolite
Resistance to Inh (H)
• 1 in 10^6 bacilli – inherently resistant to INH
• Most common- Catalase peroxidase (KatG) gene – INH
stopped
• Mutation in InhA – overcome by High dose INH
• Efflux of INH from bacterial cell wall

34. Rifampicin (R)
• MOA: interrupts RNA synthesis
• Binding to DNA- dependent RNA
polymerase(encoded by rpoB gene)
Resistance – rpoB gene mutation – reduces
affinity for drug

35. • Drug sensitive TB – bacilli susceptible for all first
line anti TB drugs
• Multidrug resistant TB (MDR-TB)– Both R and H
resistance with or without resistance to other 1ST
line drugs
• Rifampicin resistant TB (RR-TB) – resistant to R
not H, with or without resistance to other ATD
• Mono-resistant TB – resistant to one 1ST line ATD,
but not R resistant
• Poly drug resistant TB(PDR – TB) – resistant to
more than one 1ST line ATD, but not R and H
resistant
• Extensive Drug resistant TB (XDR- TB) – additional
resistance to FQ and one 2nd line injectable ATD

36. REFERENCES:-
•Essentialsof Medical Pharmacology- 8thEdition - KD
TRIPA
THI
•Goodman& Gilman’sThePharmacologicalBasisof
therapeutics