Antibiotic Resistance Flashcards
What is selection pressure?
Factors that contribute to selection which variations will provide the individual with an increase chance of surviving over others –> organisms with certain phenotypes have an advantage
e.g. predation, disease and competition
How can selection pressure lead to antibiotic resistance?
Bacteria are constantly subject to spontaneous gene mutations.
Mutations that confer a survival advantage favour the growth and propagation of the mutant strain.
If the organism is growing in the presence of a sub-inhibitory concentration of antibiotics the development of a resistance gene will confer a survival advantage.
The resistant mutant will have a survival advantage and out-compete other strains.
A resistant strain is born.
Define a sub-inhibitory conc of antibiotics?
Not completely producing inhibition (as of bacterial growth)
How can the transmission of resistant organisms lead to antibiotic resistance?
Resistant bacteria gain access to the human microbiome directly or via environmental sources (fomites).
Resistant genes can be transferred between bacteria of different species (this commonly happens in the large bowel).
The result is a human microbiome that contains a mixture of sensitive and resistant bacteria.
What are fomites?
Objects which are likely to carry infection, such as clothes or utensils
If a microbiome that contains a mixture of sensitive and resistant bacteria is exposed to antibiotics, what happens?
Sensitive bacterial strains will die out and strains carrying resistance genes will become the dominant strains.
Subsequent endogenous infection is more likely to be caused by antibiotic-resistant organisms.
What is an endogenous infection?
A disease arising from an infectious agent already present in the body but previously asymptomatic.
Gut and antibiotic resistance explained:
The gut contains very large numbers of bacteria. Changes in the composition of the gut flora, due in particular to antibiotics, can lead to the selection of highly resistant bacteria and Candida species.
These resistant organisms may remain for months in the gut of the carrier without causing any symptoms or migrate to cause infections (e.g. UTIs). These infections then are antibiotic-resistant.
Especially: E. coli
What are the 2 distinct ways in which antibiotic resistance can arise in a patient?
- Exposure to antibiotics (and spontaneous mutation)
2. Transmission of resistant organisms
What methods are used to prevent antibiotic resistance?
- Reducing antibiotic exposure to the minimum safe level - antibiotic stewardship
- Minimising transmission through infection prevention & control (IPC)
If antibiotic exposure is ceased, the resistant organisms will often be out-competed and replaced with sensitive organisms.
Why is this?
What is the benefit of this?
Resistance mutations usually affect vital bacterial cell functions e.g. cell wall construction, DNA synthesis etc
For this reason antibiotic resistant organisms are often less fit than strains that do not carry resistance genes
If antibiotic exposure is ceased, the bacteria will eventually be susceptible to antibiotics again.
What are the main 2 ways in which antibiotic resistance can be identified?
- Antimicrobial sensitivity testing
2. Detection of antimicrobial resistance genes
What are antimicrobial resistance genes?
Genes that are known to encode resistance mechanisms
How does antimicrobial sensitivity testing work?
- Try to grow the organism in the presence of an antibiotic
- If it grows in the presence of a high concentration (high MIC) it is “resistant”
- If it is killed at a low concentration (low MIC) it is “sensitive”
Therefore, the lower the MIC the more sensitive is the organism to antibiotics (killed more easily).
What is MIC?
The minimum inhibitory concentration (MIC) –> the lowest concentration of a drug which prevents visible growth of bacteria
The MIC value must then be compared with a breakpoint value.
What is a breakpoint value?
The breakpoint is the highest plasma concentration of the drug that can safely be achieved in the patient and defines whether an organism is susceptible or resistant to the drug.
If the organism MIC is LOWER than the breakpoint MIC, what does this mean?
Then the organism is said to be sensitive
If the organism MIC is higher than the breakpoint MIC, what does this mean?
Then the organism is said to be resistant
How is disk sensitivity testing (solid media) carried out?
- Add organism to agar plate
- Add different antibiotics
- Incubate
- Growth around some antibiotics, no growth around others (zone of inhibition)
What is the zone of inhibition?
The Zone of inhibition is a circular area around the spot of the antibiotic in which the bacteria colonies do not grow.
What is the zone of inhibition then compared with?
Zone diameter breakpoint values
If the zone of inhibition is larger than the zone diameter breakpoint, what does this mean?
Organism is sensitive
If the zone of inhibition is smaller than the zone diameter breakpoint, what does this mean?
Organism is resistant
How is the detection of antibiotic resistance genes carried out?
Nucleic acid amplification tests –> Mainly the polymerase chain reaction (PCR)
How is MRSA gene testing done?
Nose/skin swabs.
Detect a gene that produces a pencillin binding protein (PBP2a). If this gene is present, then the mutation is present. Then the organism will be MRSA.
How is the presence of Carbapenemase producing Enterobacterales (CPE) done?
Rectal/faecal swabs
What are Enterobacterales?
Enteric gram-negative bacilli that live naturally and harmlessly in people’s guts,
What are Carbapenemase producing Enterobacterales (CPE)?
Bacteria that are likely to be resistant to most antibiotics.
Enterobacterales: Enteric gram-negative bacilli that live naturally and harmlessly in people’s guts,
Carbapenems: a class of broad spectrum β-lactam antibiotics which are highly effective against most Gram-negative infections and are important in the treatment of infections resistant to most other antibiotics.
Carbapenemase: an enzyme that some bacteria produce, which make them resistant to important antibiotics, including carbapenems.
What are the advantages and disadvantages of antibiotic resistance genes testing?
Advs: Sensitive and fast
Disadvs: Presence of a gene does not always correlate with resistance
What are the 2 broad categories of resistance mechanisms?
- Innate
2. Acquired
What is innate antibiotic resistance?
A fundamental property of the bacterium/antibiotic combination that usually relates to permeability/entry of the antibiotic into the cell.
E.g. Glycopeptides and daptomycin don’t get into Gram-negatives
What are glycopeptides and daptomycin unable to treat?
Gram-negative bacteria negatives due to their inability to cross the outer membrane
What are glycopeptides and daptomycin used to treat?
Gram-positives
What is Aztreonam not effective against?
Gram-positive bacteria or anaerobes
What is Aztreonam effective against?
Gram-negative aerobic bacteria (e.g. Neisseria, Pseudomonas, Haemophilus influenzae)
What are aminoglycosides not effective against? Why?
- Not effective against anaerobes as their uptake across bacterial cell membranes depends on energy derived from aerobic metabolism
- Not effective against streptococci
What is acquired antibiotic resistance?
A gene that encodes an antibiotic resistance mechanism has been acquired by an organism:
- New mutation
- Horizontal transfer
What are the most commonly acquired resistance mechanisms?
Usually an antibiotic-modifying enzyme or an alteration of the antibiotic target site (e.g. cell wall)
What are the 5 main methods of antibiotic resistance?
- Absent target
- Decreased permeability
- Target modification
- Enzymatic degradation
- Drug efflux
Explain when there is an ‘absent target’ during antibiotic resistance
Target of drug is not present:
- Antibacterial agents against fungi
- Antiviral agents against bacteria
What is ‘decreased permeability’ regarding antibiotic resistance?
This is an example of innate resistance.
Example: Vancomycin/Gram-negative bacilli
- Gram-negatives have an outer membrane that is impermeable to vancomycin.
Example: Gentamicin/anaerobic organisms:
- Uptake of aminoglycosides requires an O2 dependent active transport mechanism
What is Vancomycin not effective against?
Gram-negatives
What is ‘target modification’ regarding antibiotic resistance?
Alteration in the target sites of antibiotics.
What has MRSA conferred resistance to? How?
Resistance to all beta-lactam antibiotics, including methicillin, flucloxacillin, cephalosporins, and carbapenems.
By target modification: Altered penicillin-binding protein does not bind β-lactams
What is VRE?
Vancomycin-resistant enterococci
(Enterococci are bacteria that live in the GI tract of most people without causing illness.).
Vancomycin is an antibiotic used to treat infections caused by enterococci.
When enterococci become resistant to vancomycin, they are called VRE.
How has VRE conferred resistance?
Altered peptide sequence in Gram-positive peptidoglycan reduces binding of vancomycin 1000-fold
What is ‘enzymatic degradation’ regarding antibiotic resistance?
Many bacteria code for enzymes that can modify/degrade antibiotics:
Example: Beta-lactamases inactivate certain penicillins and cephalosporins
What are beta-lactamases?
Enzymes produced by bacteria that provide multi- resistance to β-lactam antibiotics such as penicillins, cephalosporins, cephamycins, and carbapenems
What is ‘drug efflux’ regarding antibiotic resistance?
Active transport mechanism of bacteria that results in antibmicrobial agent being removed from the target cell.
Drug efflux is a key mechanism of resistance in what bacteria?
Gram-negative bacteria
What are many resistance mechanisms encoded by?
Single genes e.g. antibiotic-modifying enzymes, altered antibiotic targets
Resistance genes can be encoded in plasmids.
What is a plasmid?
Circular DNA sequences transmitted within and between species, mainly by conjugation.
Spread of resistance genes can either be by horizontal or vertical transfer.
What is horizontal transfer?
- Enabled by transposons and integrons: DNA sequences designed to be transferred from plasmid to plasmid and/or from plasmid to chromosome
- Often contain cassettes with multiple resistance genes
i.e. movement of resistance genes from one species to another
What is vertical transfer?
Chromosomal or plasmid-borne resistance genes transferred to daughter cells on bacterial cell-division
i.e. vertical transfer of all genetic material to progeny
Chromosomal vs plasmid mediated resistance?
Plasmid-mediated permits intraspecies and interspecies transfer to occur
Chromosomal resistance can only be passed on to progeny.
What is a gene cassette?
A gene cassette is a type of mobile genetic element that contains a gene and a recombination site
Examples of antibiotic resistant bacteriaL
Meticillin-resistant Staphylococcus aureus (MRSA)
Vancomycin/glycopeptide-resistant enterococci (VRE/GRE)
Extended-spectrum β-lactamase-producing Enterobacterales (ESBL)
Carbapenemase-producing Enterobacterales (CPE)
Multi-drug resistant tuburculosis (MDR-TB)
Extremely-drug resistant tuberculosis (XDR-TB)
What is empiric therapy?
Best guess therapy used at the early stages of an infection
What is the risk of antibiotic resistance in empiric therapy?
Risk of under-treatment if ‘traditional’ antibiotic is used
Use of excessively broad-spectrum treatment if risk of resistance is taken into account
What are the implications of antibiotic resistance in targeted therapy?
Requires the use of alternatives which may be:
1) expensive
2) toxic
3) ‘last line’
What are last lie antibiotics?
The last treatment options for patients infected with bacteria resistant to other available antibiotics
e.g. carbapenems for multi-resistant Enterobacterales
Antibiotic resistance can be monitored via ‘local surveillance’. What is this?
Local (hospital) surveillance for individual cases eg MRSA, CPE etc to trigger immediate IPC interventions.
Detects trends trends in resistance eg gradual increases in prevalence of resistant organisms.
Where does resistance tend to be highest in Europe?
In the South
The Indian subcontinent is a worldwide resistance ‘hotspot’ for which antibiotic?
Carbapenems
A septic patient has a bacteraemic urinary tract infection caused by E. coli. The minimum inhibitory concentration (MIC) of antibiotic A for the E. coli is 0.0125 mg/L and the MIC of antibiotic B is 1.0 mg/L. Both the MICs are below their respective breakpoint MICs for E. coli.
Which antibiotic should the patient be treated with?
There is insufficient information available to guide an antibiotic choice.
The relationship between MIC and clinical response depends on the concentration of antibiotic that is available within the patient, so a lower MIC does not necessarily indicate a higher likelihood of clinical response. The E. coli is sensitive to both antibiotics so the infection might respond to either.
However, the ultimate choice of antibiotic depends on a combination of antibiotic sensitivity results and patient factors such as the allergies, interacting drugs and potential contraindications.
