Threat of Resistance and Their Mechanisms Flashcards
Talk about the growing threat of antibiotic resistance
One of the top global public health and development threats
Estimated that bacterial AMR was directly responsible or 1.27 million global deaths and contributed to 4.95 million deaths in 2019
Thought to contribute to 50 million deaths by 2050
Rapidly increasing prevalence of MDR pathogens compicates treatment options
What is the main driver of resistance?
The misuse and overuse of antimicrobials in humans, animals and plants
What is mant by the dual threat of AMR?
Multi-drug resistance severly limits treatment options
Cross-infection facilitates spread and transmission
How does WHO combat AMR
In 2015 WHO launched the Global Action Plan on AMR in response to the growing AMR crisis
This lead to a need for a One Health approach
Who then launched the Global Antimicrobial Resistance and Use Surveillane System (GLASS)
What are the 5 objectives of the WHO global action plan
Improve awareness and understanding of antimicrobial resistance through effective communication, education and training
Strengthen the knowledge and evidence base through surveillance and research
Reduce the incidence of infection through effective sanitation, hygiene and infection prevenetion measures
Optimise the use of antimicrobial medicines in human and animal health
Develop the economic case for sustainable investment that takes account of the needs of al countries and to increase investment in new medicines, diagnostic tools, vaccines and other interventions
What does GLASS stand for?
Global Antimicrobial Resistance and Use Surveillance System
What has the last GLASS report in 2022 reported?
Highlights alarming resistance rates in 76 countries
42% third gen cephalosporin resistant E. Coli
35% methicillin resistant staph aureus
1 in 5 E. Coli UTIs show reduced susceptibility to ampicillin, co-trimoxazole, fluoroquinolones
K. pneumonia rising reliance on last resort antibiotics
Anticipated x2 surge in resistant to last resort antibiotis by 2035
Rate of resistant in E. Coli
42% third gen cephalosporin resistant E. Coli
Rate of resistance in S. aureus
35% methicillin resistant staph aureus
Rate of resistance in e. Coli UTIS
1 in 5 E. Coli UTIs show reduced susceptibility to ampicillin, co-trimoxazole, fluoroquinolones
How is the UN combatting resistance?
UN Genera Assembly held a high-level meeting on AMR in Septemeber 2024 to address the global threat of AMR
What is the WHO priority list and what is the point of is?
A list containing a catalogue of 15 families of bacteria that pose the greatest threat to human health
Intended to guide and promote research and development of new antibiotics
It classes bacteria as critical group, high group, medium group
What bacteria are part of the critical group according to WHO priority list
(4)
Enterobacterales -> Carbapenem resistant
Enterobacterales -> third gen cephalosporin resistant
Acinetobacter baumannii -> carbapenem resistant
M. tuberculosis -> rifampicin resistant
What bacteria are part of the high group according to WHO priority list
Salmonella typhi - fluoroquinolone resistant
Shigella species - fluoro resistant
E. faecium -> vancomycin resistant
P. aeruginosa -> carbapenem resistant
Non-typhoidal Salmonella -> fluoro resistant
N. gonorrhoeae -> third gen cephalosporin and/or fluoro resistant
S. aureus -> methicillin resistant
What bacteria are part of the medium group according to WHO priority list
Group A streptococci -> macrolide resistant
S. pneumoniae -> macrolide resistant
H. influenza -> ampicillin resistant
Group B streptococci -> penicillin resistant
What are the two classifications of resistance
Intrinsic resistance
Acquired resistance
What is intrinsic resistance?
The natural ability of the microorganism to resist specific antimicrobial agents, stemming from inherent biological characteristics rather than acquired mutations or gene transfer
What is acquired resistance?
This occurs when microorganisms gain the ability to resist agents through genetic mutations or the acquisition of resistance genes via horizontal gene transfer
How does intrinsic resistance affect the genotype of bacteria
There is a no change in genotype
Its a stable genetic property encoded in the chromosome, shared by all members of a genus
What are the two ways intrinsic resistance comes about?
Inaccessible Target
Target Not Susceptible to Antibiotic
Give an example of intrinsic resistance because of an inaccessible target
Anaerobic organisms that are inherently resistant to aminoglycosides because they lack an oxygen-dependent transport system to move the drgus across the cytoplasmic membrane
Give an example of intrinsic resistance because a targets not sussceptible to the antibiotic
Enterococci are inherently resistant to B-lactam antibiotics due to their low affinity of their PBP to them
How does acquired resistance affect the genome?
Arises by Alteration in genotype
What are the two methods of acquired resistance?
By Mutation through vertical gene transfer
Acquisition of exogenous genetic material through horizontal gene transfer
How is resistance acquired through mutation via vertical gene transfer?
This occurs within the endogenous makeup of a bacterial cell
This involves changes in the host chromosome, passed down to a daughter cell
If resistant mutats survive they usually show only reduced susceptibility i.e. low level resistance
Full resistance in a single step is rare
How is resistance acquired through acquisition of exogenous genetic material via horizontal gene transfer
Mechanisms include conjugation, transformation, or transduction
New resistance genes are incorporated into the bacterial chromosome
OR resistance genes are contained on a plasmid
MGEs such as transposons and integrons enhance the transfer of resistance genes through various mechanisms
MGEs provide bacteria numerous ways to spread and propagate resistance efficiently
What level of resistance is seen with vertical gene transfer, why is this?
Low level resistance
This is because its rare to see full resistance mutations evolveing in just a single step
-> way more likely to occur over multiple generations etc
What are the three mechanisms of horizontal gene transfer that can bring about resistance?
Conjugation
Transformation
Transduction
Where are resistance genes found in acquired resistance through horiontal gene transfer
Either incorporated into the bacterial chromosome or contained on a plasmid
What mobile genetic elements are responsible for acquired resistance through horizontal gene transfer?
Transposons and integrons
What is meant be selection of resistance?
Survival and proliferation of microorganisms with resistance traits under selective pressure, such as exposure to antimicrobial agents
How does selective resistance occur?
Antibiotics kill or inhibit susceptible bacteria allowing resistant strains to survive and multiply
Even low concentrations of antibiotics can create selective environments that favour resistant bacteria
What are three sources of selective pressure?
Inappropriate antibiotic use e.g. overprescription or misuse of antibiotics in healthcare settings
Agricultural Use e.g. antibiotics used in livestock for growth promotion or disease prevention
Environmental Contamination e.g. antibiotic residues in water, soil and wastewater
What are the four main mechanisms of resistance?
Enzymatic inactivation
Modification of target sites
Efflux pumps
Reduced permeability
What two mechanisms of resistance often go hand in hand?
Efflux pumps and reduced permeability
What is the mechanism behind enzymatic inactivation as a form of resistance
Bacterial production of specific enzymes that inactivate antibiotics
These enzymes hydrolyse, acetylate or otherwise modify antibiotics, preventing them from interacting effectively with bacterial targets
What are the three most common methods on enzymatic inactivation
Hydrolysis
Group transfer/acetylation
Redox process
Give two examples of hydrolysis as a methood of antibiotic inactivation
B-lactamase enzymes on B-lactams
Esterase on Macrolides
Give an example of group transfer as a methood of antibiotic inactivation
transferase-mediated resistance against aminoycosides
Most common in enterobacterales and enterococci
Give an example of a redox reaction as a method of enzymatic inactivation of antibiotics
Tetracycline modified into 11a-hydroxy-tetracycline
-> addition of O2
Talk about B-lactamase enzymes
These can degrade the B-lactam ring of penicillins, cephalosporins and carbapenems
This renders these drugs ineffective
In general where are B lactamase enzymes most commonly found
Enterobacteriaceae
Talk abut penicillinases
B-lactamases against penicillins
Mainly in gram positives
Talk about cephalosporin b-lactamase enzymes
ESBLs
Mostly in GNBs
Talk about carbapenemases
B-lactamases against carbapenems
Mostly in enterobacteriacae and pseudomonas
Talk about the prevalence of B-lactamases
Prevalence of ESBLs and CPEs is concerning especially in hospital settings where these pathogens contribute to healthcare-associated infections
Talk about acetylate/group transfer mediated resistance
Aminoglycoside resistance
Transferase mediated resistance
Commonly associated with enterobacterales and enterococci
What are the three therapeutic implications of enzyme mediated resistance
Compelled to rely on B-lactamase inhibitors in combination with B-lactam antibiotics to counteract resistance
Evolution of carbapenemase-producing organisms has diminished the efficacy of even these combinations, this pushes clinicians towards alternatives like colistin or polymyxins which are toxic
The clinical burden of treatening infections with these last-resort antibiotics has heightened, demanding careful patient monitoring and increasing healthcare costs
What are our two main B-lactamase inhibitors?
Clavulanic acid or tazobactam
NB: new resistance to these
How does modification of target sites work as a mechanism of resistance
Bacteria can evade antibiotics by mutating or modifying the drug’s target site within the bacterial cell
This reduces the binding affinity of the antibiotic, this allowing the pathogen to survive
Altered target sites often arise from point mutations or horizontal gene transfer of resistance genes
Through what two ways can altered target sites arise?
Point mutations or horizontal gene transfer
What are the four ways that a bacteria can modify its target site?
Alteration of target enzyme e.g. peniciliin binding proteins
altered cell wall target
alteration of ribosomal target sites
production of auxotrophs bypass inhibited steps
Give an example of alteration of target enzymes inducing resistance against penicillins
PBPs are enzymes located on the outer surface of the bacterial cytoplasm
These catalyse the cross-linking of peptidoglycan in the bacterial cell wall
These also serve as the target sites for B-lactam antibiotics
Thus mutations in these PBPs lead to PBPs losing afinity for B-lactam antibiotics
Reduced number of PBPs can also further limit the bindin of these B-lactam drugs (usually see both together)
What are PBPs actually?
Enzymes such as transglycosylases and transpeptidases
What is peptidoglycan also known as?
Murein
Give five specific examples of alteration of a target enzyme
Methicillin resistance in S. aureus
Penicillin resistance in PNSP
Reduced cephalosporin susceptibility in N. gonorrhoeae
Quinolone resistance
Rifampicin resistance
Talk about MRSA as an example of an altered target enzyme mechanism of resistance
Acquisition of the mecA gene which encodes a modified PBP2a which has reduced affinity for B-lactam antibiotics
This modification renders all B-lactam antibiotics ineffective against MRSA
Talk about PNSP as an example of an altered target enzyme mechanism of resistance
Altered PBPs cause reduced susceptiblity to penicillin in certain S. pneumoniae strains
Talk about N. gonorrhoeae as an example of an altered target enzyme mechanism of resistance
Altered PBP2 associated with reduced susceptibility to ceftriaxone
Talk about quinolone resistance as an example of an altered target enzyme mechanism of resistance
Quinolones inhibit DNA synthesis, target bacterial DNA gyrase
Mutations in the gyrA gene (encoding NA gyrase) lead to broad-spectrum resistance against all fluoroquinolones
Talk about Rifampicin resistance as an example of an altered target enzyme mechanism of resistance
Rifampiin inhibits RNA synthesis by binding to the B-subunit of RNA polymerase, thereby blocking mRNA synthesis
Mutations that alter the B-subuit of RNA polymerase reduce the drugs binding affinity, resulting in resistance to rifampicin
How does alteration of cell wall precursor targets induce resisitance
Resistance arises through acquisition of genes encoding ligases which synthesis abnormal pentapeptide precursors with altered termini, reducing the binding affinity of antibiotics such as glycopeptides
Give an example of where alteration of cell wall precursor targets induces resistance
Glycopeptide resistance in enterococci, such as VREs
Explain glycopeptide resistance as an exaple of alteration of cell wall precursor targets as a mechanism of resistance
Vancomycin and teicoplanin bind to the D-ala-D-ala termin of pentapeptide chains in peptidoglycan subunits, inhibiting the cross-linking of the bacterial cell wall
Give two examples of alteration of ribosomal target sites as a mecahnism of acquired resistance
Amionglycoside resistance e.g. streptomycin resistance in mycobacterium tuberculosis
Macrolide-Lincosamide-Streptogramin MLS Resistance
Talk about aminoglycoside resistancfe as an example of alteration of ribomsal target sites
Aminoglycosides bind to multiple sites on the ribosom to block protein synthesis
Resistance occurs through the production of altered ribosomal binding proteins
e.g. streptomycin resistance in MTB
Talk about macrolide-lincosamide-streptogramin resistance as an example of alteration of ribomsal target sites
Drugs like erythromycin and clindamycin bind to the 50S ribosomal sunit interfering with protein synthesis
Production of ribosomal enzymes with reduced macrolide binding affinity incur resistance
What does MLS resistance stand for?
Macrolide-Lincosamide-Streptogramin resistance
Give two examples of resistance whereby production of auxotrophs bypass inhibited steps
Sulphonamide and trimethoprim
-> both drugs work by competitively inhibiting the folic acid metabolic pathway which is essential for bacterial survival
Talk about sulphonamide resistance as an example of auxotroph production
Acquired resistance arises from the production of an altered dihydropteroate synthase enzyme with reduced affinity for sulphonamides
Talk about trimethoprim resistance as an example of auxotroph production
Resistance develops due to the production of a new dihydrofolate reductase enzyme that has reduced susceptibility to trimethoprim
Talk about efflux pumps as a mechanism of resistance
Active efflux is a reconised common mechanism of resistance
Normal levels of efflux contribute to low level intrinsic resistance but bacteria can increase expression for high level
Resistance occurs due to active extrusion of drugs from the cell
This extrusion is mediated by multi-drug efflux pumps
Efflux pumps are categorised into five distinct protein superfamilies
What is the function of efflux systems
They facilitate the removal of various secondary metabolites including antibiotics
They may be specific to a single substrate or broad spectrum expessling diverse compoints including antibiotics from multiple different classes (MDR)
Talk about the expression of efflux pumps
Overexpression of efflux pumps or amino acid substitutions enhances pump efficiency, potentially leading to high-level multidrug resistance
Increased efflux pump activity may be an initial step in a bacterium’s progression to full resistance
Multi subunit efflux pumps are often encoded on the bacterial chromosome
What is the effect of elevated efflux activity
Reduces intracellular antimicrobial levels enabling bacterial survival and facilitating the accumulation of additional resistance mutations
Where are multi subunit efflux pumps encoded?
Encoded on the bacterial chromosome
What are the five categories of efflux pumps?
Major Facilitator Superfamily (MFS)
Resistance-Nodulation-Division (RND)
Small Multidrug Resistance (SMR)
ATP-Binding Casette (ABC)
Multidrug and toxic compound extrusion (MATE)
Talk about major facilitator superfamily of efflux pumps
MFSs found in E. Coli and S. aureus
Efflux antibiotics such as tetracyclines as well as dyes and sugars
Talk about the resistance nodulation division of efflux pumps
RNDs found in P. aeruginosa and E. Coli
Efflux antibiotics such as beta lactams and fluoroquinolones as well as detergents
Talk about the small multidrug resistance family of efflux pumps
SMRs found in E. Coli and M. tuberculosis
Efflux disinfectants and antiseptics
Talk about the ATP-binding cassette family of efflux pumps
ABCs found in S. aureus and E. Coli
Efflux antibiotics such as macrolides and peptides
Talk about the multidrug and toxic compound extrusion family of efflux pumps
MATEs found in Vibrio cholerae and P. aeruginosa
Efflux antibiotics such as fluoroquinolones and organic cations
Talk about efflux pumps in GNBS
Efflux pumps in GNBs often span both the inner and outer membranes
This enables efficient extrusion of antibiotics directly into the external environment
RND pumps are major contributors to MDR in HAI caused by GNBS
Many of these pumps handle a variety of antibiotics, biocides and toxic compounds making infections very difficult to treat
Why are we most concerned with RND family efflux pumps in GNBs
These pumps often work in conjunction with other resistance mechanisms such as enzyme degadation or permeability changes
Talk about spread of resistance and efflux pumps in GNBs
Efflux pumps in pathogens like K. pneumoniae and A. baumannii are associated with global dissemination of MDR strains
Talk about the role of efflux pumps in lastresort drug resistance
Pumps such as KpnEF in K. pneunoniae are linked to resistance against last-resort antibiotics like tigecycline
Talk about efflux pumps in gram positives
Resistance mediated by efflux transporters located in the cytoplasmic membrane
The Major Facilitator Superfamily is the key efflux pump family in gram positives
Efflux pumps can bring about intrinsic or acquired resistance in gram positives
Talk about resisistance mediated by efflux pumps in gram positives
Seen in S. aureus and S. pneumonia
intrinsic and transferable resistance
Resistance to a wide range of antimicrobials and biocides
What are the two types of efflux pumps in gram positives, what is the importance of these
Plasmid encoded efflux pumps which facilitate horizontal spread of resistance
Chromosomally encoded pumps provide intrinsic resistance to key antibiotics
Give two exampls of plasmid encoded efflux pumps in gram positives
QacA/B
MefA
Give three examples of chromosomally encoded efflux pumps in gram positives
NorA
NorB
NorC
How does multidrug resistance come about, in relation to porins?
Porin alterations o ften coexist with other resistance mechanisms such as eflux pumps or enzymatic degradation, making infections much more difficult to treat
In what four ways can porins induce antibiotic resistance?
Reduced permeability leads to resistance
Mutations or loss of porins result in resistance
Altered porins slow drug penetration
Limited porin availability reduces efficacy
Give an example of where reduced porin permeability leads to resistance
Normally beta lactams diffuse through general porins e.g. OmpF
But reduced permeabiblity leads to resistance
Give an example of where mutations or loss of porins result in resistance
Carbapenems utilise specific porins such as OprD in P. aeruginosa but mmumtations in these or a loss of these porins results in resistance
Give an example of where altered porins slow drug penetration
Fluroquinolones normally diffuse through porins into the cytoplasm
altering these porins slows down the penetration of the drug
Give an example of where limited porin availability reduces efficacy
Aminoglycosides enter via porins and interact with ribosomes
Limiting porin availability prevents entry
Talk about carbapenem resistance induced by porins
Loss or modification of specific porins like OmpK36 in K. pneumonia or OprD in P. aeruginosa is a hallmark of carbapenem resistance
Talk about porin induced resistance and their relevance in HCAI
Pathogens like Acinetobacter baumannii and K. pneumoniae with altered porins are common causes of HCAI leading to treatment failures
How do different mechanisms of resistance work together
- individual class of drugs inactivated by different mechanisms
- individual organism employs different mechanisms to generate resistance to single drug/different classes of drugs -> these mechanisms interact todetermine actual level of resistance
What is synergistic resistance?
Whereby most clinically signigicant bacteria employ at least two or more resistant mechanisms for the same antibiotic class
A combination of mechanisms that leads to higher levels of resistance, often termed extensivelt drug-resistant or multidrug resistant
What is cross-class resistance
Whereby one mechanisms has affects across more than one class of antibiotics
e.g. efflux pumps and permeability changes often provide cross-class resisistance impacting multile antibiotic classes simultaneously
Talk about synergistic resistance in K. pneumoniae
Resistance to B-lactams by multiple mechanisms
Beta lactamase production e.g. ESBLs, KPC
Porin loss to reduce permeability -> porin Ompk35/OmpK36 are altered
Efflux pumps to actively remove beta-lactams from the cell
Talk about synergistic resistance in P. aeruginosa
Resistance against fluoroquinolones through multiple mechanisms:
- efflux pumps encoded by MexAB-OprM system removes fluoros
- target modification encoded by mutations in DNA gyrase (gyrA) or topoisomeras IV (parC)
- outer membrane changes to limit drug penetration
Talk about synergistic resistance in S. aureus
Resistance to macrolides through multiple mechanisms:
-target modification - ribosomal methylation via erm genes (ermA or ermC) reduces macrolide binding to 50S ribosomal subunit
-efflux pumps: MsrA pump actively expels macrolides
Conclusions on resistance mechanisms
The molecular mechanismms underlying bacterial resistance highlight the formidable challenge facing modern medicine
Each resistance mechanism - enzymatic degradation, target modification, efflux pumping, or reduced permeability - constrains available treatment options and necessitates new approached in both clinical management and drug development
Multifaceted approach combining clinical vigilance, therapeutic innovation and policy support is essential to manage and mitigate the impact of resistance effectively
