The Evolution of Antibiotic Resistance AND Beta-Lactams and Resistance To Them Flashcards
What are the 4 main methods of antibiotic resistance
1) Target site modification –> not a common mechanism of resistance as mutations often compromise bacterial fiteness
2) Decreased permeability, increased efflux –> can convey resistance to many classes of Abx, mainly a method used in gram negatives
3) Enzymatic degradation –> the most important and most common method of antibiotic resistance
4) Replacement of antibiotic target by a non-susceptible target
What is the most common cause of antibiotic resistance
Enzymatic degradation –> the most important and most common mechanism of resistance
What are the 3 methods used to determine the MIC of an antimicrobial
1) Agar dilution test
2) Disc susceptibility - Zone of Inhibition
Describe the agar dilution test
Put different concentration of antibiotic in the agar and see if the bacteria can grow
Why is breakpoint setting political
if you set a higer breakpoint than it looks like you have less resistance
Name 2 bacteria that are naturally competent
H. Influenzae and Strep Pneumoniae
What does it mean that bacteria are ‘naturally competent’
The bacteria can take up naked linear Dna from the environment and then combine it with the host genome via non-reciprocal homologus recombination
This trasnforms the phenotype of the recipient cell
Describe the process of non-reciprocal homologous recombination
The naked DNA is taken up from other dead bacteria
DNA enters as a linear strang
RecA proteins takes one strand of DNA and you get association of homolous segments
Strand seperation adn apiring –> endonucleases nick the donor and host strands –> the gaps in the strand are filled and ligated.
Can allow many mutations to be passed on
Give an example of target site mutation conveying resistance in S. Pneumoniae
can accumulate 83 mutations in the PBP2 alpha protein nthat can retain the function of the PBP protein but prevents penicillin binding. Mainly gets this by creating ‘mosaic’ genes as non-reciprocal homologous recombination allows the individual mutations to accumulate
UNDERDOSING OF ABX FRIVES THIS –> however bacteria that aren’t naturally competent hence won’t really evolve resistance this
describe how penicillin resistance arose
BlaZ penicillinase enzyme was produced
Penicillin was first used to treat gram +ves in 1941, just 3 years later 50% of S. aureus were resistant
rWhat mobile element is BlaZ carried on
Tn552
What is a transposon
A chromosomal segment that can undergo transposition –> they can move its location around the genome
How do transposons actually work
Inverted repeat sequences ovvur at each end of the transposon
The transposon always encode a tnp enzyme that is responsible for moving the transposon!
Some transposons just cut themselves out and move, whereas other transposons cut themeselves out, duplicate themselves and then move
Give an example of acquired resistance in S. Aureus
MecA gene in S. Aureus
This arose when methicillin was used to combat penicillin resistance in 1961.
However just 2 years later this arose –> MecA encoded an altered PBP2’
This is resistance through acquisition of an altered target
What transposons is MecA being carried on
Generally carried on complex transposons. The transposons often carry resistance to other Abx.
Tn554 –> encodes erythromycin.spectinomycin
pUB110 –> encodes karamycin/neomycin
Describe how the TEM: Beta-lactamase arose
Ampicillin was first used to treat E. Coli in 1961 but resistance was observed just 4 years later
This was due to TEM Beta-lactamase on a plasmid !!
TEM is responsible for 95% of ampicillin-resistance in E. Coli in 1995
What antibitoic is E. Coli fundamentally resistant to
Pencillins as penicillin can not enter.
What transposon is TEM generally carried on
Tn1
Name an alternative beta-lactamase discovered other than TEM
SHV –> was first observed in K. Pneumoniae but wasn’t mobile or at high enoguh levels to convey resistance
SHV then seen in E. Coli, had been mobilised and seems to have activated it to incrased its expression
Describe the transposons involved in SHV
SHV is part of a composite transposon
The Taiwanese transposon is common –> in this transposon the insertion sequence is right next to the SHV gene and drives expression of the gene making resistance more likely
When were cephalosporins introduced into the UK
In the 1980s –> and killed E. Coli and K. Pneumoniae even TEM and SHV
What did the use of cephalosporins fuel the rise of
Caused the explsion of previously rare pathogens that had intrinsic, inducible ESBLs and AmpC e.g. Beta lactamases
After just a few years EmpC hyperproductive was seen in E. COli
Describe the mutations that arose in TEM and SHV to convey
Mutations at 238+240 i.e. Gly238–>Ser in SHV-2 and TEM –> this mutation gives the beta-lactamase a bigger mouth so that it can break down larger cephalosporins and are known as ESBLs
What are the the 2 steps in PBP crosslinking
1) Transglycosylase reaction –> forms glycosylic bond between GlcNac and MurNAc and adjacent monomers to form the peptidoglycan chain
2) Transpeptidation reaction –> catalyses the crosslink between the amino group and the D-Ala residue on adjacent stem peptides
