Antibiotic Resistance Flashcards
Give an overview of antibiotic resistance?
Definition - the ability of bacteria to survive and grow in the presence of antibiotic concentrations that can be safely achieved in patients at site of infection
= an antibiotic is no longer effective for treatment
With the increase of antibacterial chemotherapy we saw a dramatic increase in antibiotics resistance
Antibiotic resistance is inevitable
What are types of resistance in general?
Acquired and Intrinsic
Acquired resistance is when a previously-antibiotic susceptible bacterial population becomes resistant
Intrinsic resistance results from inherent features of a particular bacterial population
Mycoplasmae are intrinsically resistant to β-lactams and glycopeptides, because they have no peptidoglycan cell wall
E. coli is intrinsically resistant to vancomycin and rifampicin because they can’t get through the outer membrane
Therefore acquired is problematic for us
What are the negative consequences of antibiotic resistance?
Increased mortality
resistant infections often more fatal
Increased morbidity
prolonged illness
increased opportunities for spread of resistant organisms
Increased cost
length of stay in hospital
new, more expensive drugs
Why do we need to prevent antibiotic resistance?
Modern medicine relies on antibiotics
Treatment of bacterial infection
Dentistry, surgery, transplants
Massive benefits to individual health and societies
Survival/ longevity
Quality of life
Productivity
Modern economies underpinned by antibiotics
How has the evolution of antibiotic resistance come about?
Antibiotic resistance is not a new phenomenon - many natural pathogens therefore this has been happening before our discovery of them
In recent history we have now seen resistance in pathogenic bacteria
= evolution by natural selection
A suseptable pathogen can have a mutation or horizontal transfer of resistance genes resulting in a resistant pathogen -> transmission
What is driving antibiotic resistance?
Antibiotic use
Selection pressure - 33K tonnes penicillin manufactured per year
Overuse, misuse and unregulated sales (e.g. Asia)
Antibiotic use in livestock (>10X that for humans)
Large fraction of most antibiotics leave the body in an active form
What are the routes bacteria can take to confer antibiotic resistance?
Endogenous - spontaneous mutation to resistance Point mutation (during replication) often in the gene encoding the drug target
Exogenous - horizontal acquisition of antibiotic resistance
Gene transfer to another bacterial/pathogenic organism
What are the mechanisms of antibiotic resistance?
Altered target site
Decreased uptake
Inactivation/modification
Bypass
Describe altered target site as a mechanism of antibiotic resistance?
Mutation/modification of antibacterial drug target
Simplest mutation = point mutation
Give some examples of altered target site mutations?
Alteration of RNA polymerase mediates rifampicin resistance in S. Aureus
It inhibits RNA polymerase by binding to the catalytic b subunit - blocking the exit of RNA
Substituting just one amino acid the loss in activity of rifampicin is drastic
Methylation of ribosomal RNA by Cfr methylase of a A2503 on 23S rRNA
This is horizontally-acquired resistance to linezolid in S.aureus
Also has resistance to chloramphenicol, lincosamides, streptogramins and pleuromutilins
Vancomycin resistance in enterococci
It normally binds to D-ala-D-ala (peptidoglycan precursor)
Substituting the terminal D-ala with D-lactate
Results in a single hydrogen bond being lost between the drug and the target which rapidly reduces the binding effect to the target
Describe how the mutation effecting vancomycin works?
vanR and vanS are normally functioning in the operon
When vancomycin is detected the resistance machinery is turned on - vanH, vanA, vanX
These produce D-lactate, combine it with the D-ala and prevent any other D-ala binding to D-ala normally
What is another altered target site mechanism?
Target overexpression
It normally interacts with the target in a set ratio of 1:1
When the bacterial cell increases the target you need to try to match the antibiotic level
Therefore the cell can out titrate the antibiotic
This is very energy costly for the cell - less fit
This is often experimented in the lab but not a very widely used method of resistance in the ‘wild’
Give an example of target overexpression to induce antibiotic resistance?
Vancomycin-intermediate S. aureus
Numerous genetic/ biochemical changes described in VISA strain (Mu50)
Common denominator: significant thickening of cell-wall
Affinity-trapping of vancomycin
Reduced cross-linking increases the presence of D-ala-D-ala that can bind to vancomycin
They act as decoy sites to bind vancomycin before it can act on the deeper areas of the bacteria
Describe decreased antibiotic uptake as a mechanism of antibiotic resistance?
Either:
Reduced permeability - can’t penetrate the cell membrane
Seen in gram negative - they can delete specific porins (but not common)
OR
Efflux - transport back out the cell
Describe active efflux?
Toxic compounds pumped from the cell by a transporter protein or complex (‘efflux pump’)
There are several different families of efflux pumps
Commonly require proton motive force or ATP to function
They can recognise multiple antibiotic classes, dyes, heavy metals, solvents, detergents
Resistance may arise from up-regulation of an endogenous pump, or from horizontal acquisition of a new pump
Give an example of decreased uptake of antibotics?
Multiple Drug Efflux Systems, such as AcrAB/TolC System in E. Coli
This is very important due to a broad substrate profile = large recognition of substrates
Therefore can often undergo upregulation
Efflux transporter connected to adaptor protein linked to OM channel protein
Drug crosses OM and/or CM
Transporter pumps drugs out, bypassing OM
Describe enzymatic inactivation or modification as a mechanism of antibiotic resistance?
This is catalysing a chemical change within the antibiotics - which reduces the activity of these compounds
Destruction/modification
Give an example of a enzyme that mediates resistance to an antibiotic?
B-lactamase
This mediates resistance to β-lactam antibiotics
>1500 described
Catalyse hydrolysis of the cyclic amide bonds of β-lactam ring
The open ring forms cannot bind to target sites
Serious problem in gram negative bacteria as B-lactams are fundamental in curing this type of bacterial infection
Describe b-lactamases?
They are categorised into 4 classes
These categories are established due to their protein sequence
Class A, C and D have serine in the active site
They share an evolutionary history - similar proportions of alpha/beta sheets
Class A were most under concern - as they catalysed 1st, 2nd and 3rd cephalosporins
This drove towards the use of carbapenems (heavy weight antibiotics)
Class B have a metallo enzyme - contains 2 metal ions usually zinc
Responsible for catalysing the hydrolysis of b-lactams
Give another example of an enzymatic modification of aminoglycosidases?
Inhibitors of protein synthesis
The groups added interfere between the interaction of the drug and the target
Aminoglycoside adenyltransferase (ANT) - catalyses ATP-dependent adenylation of hydroxyl group Aminoglycoside acetyltransferases (AAC) - catalyses acetyl CoA-dependent acetylation of an amino group Aminoglycoside phosphotransferases (APH) - catalyses ATP-dependent phosphorylation of a hydroxyl group
Describe target bypass as a mechanism of antibiotic resistance?
Acquisition of alternative target of an enzyme that can substitute for the original drug target - but it is not antibiotic sensitive
This is acquired via horizontal gene transfer from another bacterial/pathogenic organism
When the antibiotic binds, it will inactive the drug target - but the bacteria can still function
Give an example of target bypass for antibiotic resistance?
Methicillin resistance in S.aureus
MRSA carries mecA which encodes PBP2a - mainly B-lactam insensitive
PBP2a catalyses sufficient cross-linked peptidoglycan to allow the organism to survive
What is significant about these antibiotic resistant mechanisms?
They can work together to achieve the resistance
What can we do in the future to improve antibiotic resistance?
Better training for prescribers
Co-ordination of surveillance of resistance in human and animal sectors
Better guidelines for therapy
Cycling of antibiotics to reduce selective pressure
Restriction on use of antibiotics as growth promoters
Improved infection control practices
Development of novel antibacterials to circumvent existing resistance mechanisms - but difficult, lengthy, costly