ANTIMICROBIAL RESISTANCE

MECHANISMS

- Dr Mangala Nischal

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

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.

• Two major factors are associated with emergence of antibiotic

resistance:

- Evolution

- Clinical/ Environmental practices.

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.

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.

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

• 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

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.

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)

• Examples :-

• The penicillin-resistant gonococci are less permeable to penicillin G.

• Chloroquine-resistant P. Falciparum accumulates less chloroquine.

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

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

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

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.

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.

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.

• 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

• 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.

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 .

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.

• 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 .

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

• 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

• 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.

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.

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.

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.

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.

• 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.

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.

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.