Antibiotic resistance Flashcards
Why is antibiotic resistance a concern?
Increases mortality
Challenges control of infectious diseases
Threatens return to pre-antibiotic era
Increases cost of health care
Jeopardises health care gains to society
Drug resistant bacteria are NOT MORE pathogenic, we just have fewer antibiotic options for treatment.
History of methicillin resistant Staphylococcus aureus
When penicillin was used more widely, there was an increase in resistance.
Staphylococcus acquired gene that encodes for beta lactamase enzyme, stops antibiotics from working – resistance.
By using antibiotics not intelligently you drive selection of antibiotic resistance in S.aureus, until we got to MRSA infections in 1978.
Superbugs
What happens if you put MRSA organism in the same location as others that are resistant?
Enterococci, gram positive, live in gut, naturally resistant to vancomycin.
Acinetobacter, gram negative, in the gut, multiply resistant. Multiple mechanisms for resistance, can cause wound infections and is hospital acquired infection
MRSA, gram positive, causes gut and wound infections.
If these three organisms co-exist together in the gut, what would happen?
Because bacteria undergo genetic exchange readily, organisms start to exchange bits of DNA and genes by different mechanisms
If MRSA acquires genes that encode for vancomycin resistance
End up with vancomycin resistant MRSA
Occurring in some parts of the world , few options to treat
Mechanisms of antibiotic resistance
Drug inactivation
e.g beta lactamase.
Some bacteria acquire gene that encodes for beta lactamase, it will secrete this enzyme and destroy beta lactase ring in antibiotics and becomes resistant to them
Inactivating drug so organism becomes resistant
Mechanisms of antibiotic resistance
Altered target for antibiotic or acquire a new target
Ribosome
Porin – mutation in porin genes of gram negative bacteria, you can become resistant to multiple drugs, whole range of antibiotics can’t go across transporter porin
PBPs – certain bacteria acquire new enzymes through gene exchange, have new penicillin binding protein that no longer binds penicillin, becoming resistant
DNA gyrase – prevents quinolones from inhibiting DNA gyrase as mutation alters the conformation of enzyme so drug can’t bind
RNA polymerase
Mcr1 and colistin
Mechanisms of antibiotic resistance
Efflux pump
If antibiotic gets in through porin, bacteria can acquire genes that encode for efflux pump, increases their ability to pump out drugs
Although drug is getting into bacteria, they are pumped out faster, antibiotic concentration needed to inhibit bacteria is never achieved
Never achieve MIC
Resistance
Mechanisms of antibiotic resistance
Overproduction of target
Trimethoprim involved in folic acid synthesis
Bacteria overproduces precursors needed, leading to overproduction of targets (enzymes), there isn’t enough drug concentration to inhibit the targets
Overcomes inhibition from antibiotics
Mechanisms of antibiotic resistance
intrinsic permeability
Antibiotics can’t go across cell membrane
No new acquisitions, just intrinsically resistant to antibiotics
Mechanisms of antibiotic resistance
metabolic by-pass
New metabolic pathway that bypasses the part where vancomycin inhibits
No terminal D-ala D-ala, instead is D-ala-Dlac so vancomycin can’t bind
What are the different paths to resistance?
Directed at antibiotic itself
- Degrading the drug
- Modifying the drug
New or altered target
- Antibiotic no longer binds e.g PBPs – PBP2a in MRSA
Altered transport
- Actively pumping drug out – efflux pump
- Porins no longer influx drugs
Metabolic by pass
- Metabolic change D-ala-D-lac and vancomycin
What are the different types of mechaisms of resistance ?
Natural resistance
Genetic mechanisms – acquired
Non-genetic mechanisms (growth phases)
- Tolerance, is reversible. Tolerate certain concentrations of drugs
What are some natural barriers?
Porins, export pumps in organism, so you can’t treat that organism with that particular antibiotic
Natural resistance is part of some species of bacteria but not all
Gram positive peptidoglycan – highly porus – no barrier to diffusion
Gram negative outer membrane – barrier – resistance advantage
A single mutation in porin gene can give you multiple resistance because many drugs can no longer get through porins.
Genetic mechanisms
Chromosome-mediated
Due to spontaneous mutation:
in the target molecule
Or in the drug uptake system
Mutants are selected, NOT induced
Green is sensitive bacteria, add antibiotic, and bacteria become resistant bacteria – THIS IS FALSE, what people used to think.
What is actually happening is that there is a population of bacteria, random mutation occurs.
Selection for these mutations due to antibiotics and kill off sensitive mutations, end up with clonal selection for resistant strains.
Genetic mechanisms
plasmid-mediated gene exchange
Plasmid-mediated gene exchange
Common in Gram-negative bacteria
Transferred via conjugation
Multidrug resistance
How are genes transferred in bacteria?
Mechanism for genetic heterogeneity and evolution
Rapid, cross species
Can transfer virulence genes (toxins), drug resistance, antigens (for immune evasion)
three steps, transformation, transduction, conjugation
What happens during transformation in gene transfer?
Organisms can take up DNA from environment or other organisms that have died around it
Incorporates DNA into its own genome e.g acquires gene for beta lactamase, now in its own genome
What happens during transduction in gene transfer?
Transfer of genetic material through use of phage
Virus that infects on bacteria, incorporates into its phage head and own genome parts of the genome it just infected
When it infects new host, it injects its own genome and genes acquired from previous host by phage
Transduced recipient becomes resistant
Rapid
What happens during conjugation in gene transfer?
Two organisms come together
Conjugational tube encoded for by plasmids in bacteria
Allows genetic exchange from donor bacteria to recipient bacteria
What happens in gram positive and negative bacteria once they acquire resistance to beta lactams?
Gram positive bacteria
- Acquires B-lactamase enzymes (Pencillinase), breaks down penicillin
- Have alterations of transpeptidase enzyme (PBP)
Gram negative
- Acquire B-lactamase
- Alterations of porins, stops drugs from entering
B-lactamase destroys the active part of the penicillin molecule, the beta lactam ring.
What is the drug augmentin/ co-amoxiclav a combo of?
What does it inactivate?
A combo of clavulanic acid and amoxicillin.
Binds to and inactivates beta-lactamases
No anti-bacterial activity of its own (clavulanic acid), it binds to beta-lactamase enzyme of bacteria to block it
allows amoxicillin to inhibit protein synthesis and gives broad spectrum activity
Beta lactam resistance in gram negative bacteria
How does beta-lactam normally work?
Beta lactam goes across porin, binds to PBP, stops cross linking of peptidoglycan and bacteria can’t survive
What happens to beta lactam in gram negative bacteria when there is porin mutations?
If porin mutations or a new porin type is acquired through gene exchange means beta lactam can’t cross through, organism get multi drug resistances
What happens to beta lactam when PBP mutates in gram -ve bacteria?
PBP mutates or bacteria acquires a new PBP
(How MRSA became MRSA, by acquiring new PBP)
Drug can get through porin but can’t bind or inactivate enzymes involved in peptidoglycan synthesis
What happens to beta lactam in gram-ve bacteria when bacteria acquires a beta-lactamase enzyme?
Bacteria acquires a beta-lactamase enzyme
Drug can get into periplasmic space, but beta lactamase binds to and degrades beta-lactase antibiotics
bacteria becomes resistance to antibiotics like penicillin
Mechanisms by which bacteria become resistant to pencillin
Produce penicillinases / beta lactamases that cleave the beta lactam ring – penicillin is inactivated
Acquire alternative forms of / or mutations in penicillin binding proteins (PBPs) - penicillin can’t bind
Acquire alternative forms of/mutations in porins – penicillin cannot get into cell
Acquire alternative forms of/mutations in efflux pump – penicillin’s are pumped out faster
Treatment of MRSA
What drug is used?
How does it work?
Only effective treatment is vancomycin, a 1.5kDA glycopeptide
Binds to terminal D-ala-D-ala residues, stops enzymes from binding and bacteria can’t make peptidoglycan
Blocks availability of active site of enzyme
Bacteria instead of synthesising terminal D-ala-Dala synthesises terminal D-ala lac, causing vancomycin resistance
How can a bacteria become vancomycin resistant?
Acquisition of van operon by transposition - causes altered target, drug can no longer bind
Makes D-ala-D-lactate terminal instead– prevents vancomycin binding
Some hydrogen bonds are missing in D-ala-D-lac, so drug doesn’t bind and doesn’t inhibit
Bacteria becomes resistant
Non-genetic mechanism of resistance
Inaccessibility to drugs
e.g abscess, TB lesion, it is hard to get drugs to the site of infection
So it doesn’t seem like person is responding to drugs because drugs aren’t getting to where they should be in high enough concentrations.
Non-genetic mechanisms of resistance
Stationary phase/vegetations and biofilms
When bacteria grow in body they reach stationary phase e.g like biofilm growth you might get on heart valves, not growing and dividing anymore but cause signs and symptoms
Organisms create complex biofilms of organisms
Not susceptible to inhibitors like antibiotics inhibiting cell wall synthesis because they’re not turning over their cell walls
Have to use antibiotics that target other targets in bacteria that are still active
How can you prevent or overcome antibiotic resistance?
Control use
- Not in animal feeds
- Complete course (DOTS for TB, need 6 months, if you stop after 2-3 months you’re selecting for pre-existing mutations for resistance)
- Appropriate prescribing
New or modified drugs
- Few in past 25 years – difficult to do
Combination therapy
- Different targets, limits selection for pre-existing mutants
- Overcome mutation rates
Infection control
- Individual – ward – society
Re-establish susceptible flora?
What is the issue of treating Neisseria gonorrhoea?
How is it treated now?
In 60’s/70s to treat you gave single I/M of PenG, high dose
Gradually increase in pencillin resistance to gonorrhoea 15%, can’t give PenG now because it could be resistant and would cause it to transmit to others
Switched to ciprofloxacin, inhibits DNA gyrase, high dose
Risk emergence of ciprofloxacin, now 30% resistant to ciprofloxacin
Now treatment is short therapy:
- Single oral dose of cefixime
- Then I/m ceftriaxone + 1g Azithromycin
- 125 – 250 – 500mg increasing MIC
- 2019 now 1g injection of ceftriaxone and no azithro due to resistance
Carbapenems treat gram negative bacteria like E.coli and Klebsiella
E.coli and Klebsiella main cause for UTIs, now there is CR E.coli and Klebsiellas, making it difficult to treat.
New strains destroy antibiotics – resistance
- Acquired new gene, nmd1
- An extended spectrum beta lactamase – ESBLs
- E.coli and Klebsiella’s have ESBLs which mean they’re very resistant to most beta lactam antibiotics, and to carbapenems
- Final drug to use to treat these are colistin, targets bacterial cell membranes, very toxic as eukaryotic membranes are similar to prokaryotic membranes
What is antibiotic stewardship?
Anyone involved in prescribing or using antibiotics needs to now be aware of antibiotic stewardship
about knowing how antibiotics work, drug resistance and antibiotic therapy combinations
Restrict use of antibiotics and how we use them
