AMPC and ESBLs - Resistance Mechanisms Flashcards
What are our ESKAPEE organisms?
E. faecium
S. aureus
Klebsiella
Acinetobacter
Pseudomonas
Enterobacter
E. Coli
In general what resistance are we concerned with in gram negatives
B-lactamase mediated resistance emerging at an alarming rate
Increased resistance to aminoglycosides and quinolones
MDR GNBs becoming an epidemic public health threat
Clonal spread of resistant phenotypes e.g. epidemic strains of E. coli and klebsiella
Talk about B-lactamase mediated resistance in GNBs
ESBLs -> transferable resistance seen in Klebsiella pneumonia and E. coli
Chromosomal AmpC -> E.cloacae, C. fredii, S. marcescens, P. aeruginosa and M. morganii
Plasmid mediated AmpC -> transfers resistance
CPE - ‘Big 5’ -> P. aeruginosa, Enterobacterales
Give the common different resistance mechhanisms in GNBs
Porin reduction to decrease permeability
Broad-specificity efflux pumps
Beta-lactamase enzymes
Target site modification - Beta-lactam-insensitive cell-wall transpeptidases
What are the different classifications of B-lactamases?
Bush system - Functional classification
Ambler system - Structural classification
Talk about the Bush system of classifying b-lactamases
Divided into 4 primary class (1-4) based on functionality
Not really used anymore as organisms could gain mutations which would result in them changing classes
Talk abut the Ambler system
Categorised into 4 molecular groups A to D
Offers stability against mutations that may influence enzyme activity
The majority of clinically significant B-lactanases re found in Ambler A and C
This system is used over the Bush system nowadays as these are more stable categories
What are the Ambler class A?
Serine beta-lactamases
Works on penicillins, early cephalosporins with ESBLs working on 3rd generation cephalosporins
Inhibited by clavulanate
Examples include ESBLs (CTX-M) and carbapenemases such as KPC
Talk about Ambler class B
Metallo-beta-lactamases
Broad spectrum including carbapenem resistance but not monobactam resistance
NDM, VIM and IMP
Require zinc for their activity
Talk about Ambler class C
AmpC beta-lactamases
Resistance to penicilliins, cephalosporins (1st and 3rd gen), monobactams not but not carbapenems
Example = AmpC
Can be Chr or plasmid mediated - inducible in some bacteria e.g. enterobacter
Talk about Ambler class D
Oxacillinases (OXA)
Penicillins, cloxacillin, carbapenems
OXA-48 = example
Common in Acinetobacter and CR-enterobacterales
What is the mechanism in AmpC B-lactamases
A type of Class C, Group 1
They are typically chromosomally encoded but can also be plasmid-mediated -> therefore less likely to see transmission
AmpC enzymes hydrolyse penicillins and cephalopsorins including many third gens
What is the resistance spectrum of AmpC B-Lactamases
Active against penicillins, cephalosporins (including cephamycins like cefoxitin) and monobactams
Not inhibited by clavulanic acid but may be inhibited by substances like boronic acids or cloxacillin
Resistant to third-generation cephalosporins like ceftriazone and cefotaxime but are less susceptible to B-lactamase inhibitors
What are the most common AmpC organisms
Enterobacterales like Enterobacter spp, Citrobacte freundii, Serratia marcescens and Pseudomonas aeruginosa
Also seen in plasmid-mediated forms in E. coli and Klebsiella species
Talk about the clinical relevance of AmpC B-lactamases, how do we detect it
Resistance can emerge via the induction or depression of chromosomal AmpC genes
Use of cefoxitin or cefepime in testing can help identify AmpC producers
Talk about Extended-Spectrum B-lactamases
ESBLs are an Ambler clas A B-lactamases
ESBLs are a major public health threat associated with significant morbifity, mortality and increased healthcare cosrs
They hydrolyse most penicillins and cephalosporins including cefuroxime and 3rd and 4th generation cephalosporins
Have no activity on cephamycins or carbapenems
What is the resistance profile of ESBLs
Resistant to most penicillins and cephalosporins e.g. cefuroxime, cefotaxime, ceftrazidime and ceftriaxone also aztreonam
No activity against carbapenems or cephamycins such as efoxitin and cefotetan
Inhibited by clavulanic acid, sulboactam and tazobactam
Level of expression nad presence of other mechanisms such as AMP C leads to a variety of resistance phenotypes
What do ESBLs have no activity against?
No activity against carbapenems or cephamycins such as efoxitin and cefotetan
What is ESBL inhibited by
Inhibited by clavulanic acid, sulboactam and tazobactam
What are some of the different mutants of ESBLs
TEM and SHV
CTX-Ms such as CTX-M-15
OXA
VEB
PER
GES
Talk about TEM and SHV mutants, how did the originate
Original ESBLs
Mainly drive resistance in healtchare
Arose form mutations in the parent beta-lactamases found in Escherichia coli (TEM) and Klebsiella pneumonia (SHV)
What is the mechnism of TEM and SHV ESBLs
Mutations in the TEM and SHV genes typicaly result in the substitution of specific amino acids in the beta-lactamase active site, allowing these enzymes to break down a broader range of beta-lactam antibiotics
Ability to hydrolyse exended-spectrum penicillins and 3r and 4th generation cephalosporins
Talk about CTX-Ms
Dominant ESBL enzymes globally
Resistance in both hospitals and CA infection
Widespread in common pathogens such as:
- E. Coli - most frequent carrier in community infections
- Klebsiella pneumoniae - especially in healthcare-associated infections
Resistance is spread due to plasmid-mediated mobility facilitating horizontal gene transfer
Spectrum of CTX-Ms
They hhydrolyse third generation cephalosporins e.g. cefotaxime and ceftriaxone
How do we detect CTX-Ms
Resistant to cefotaxime (marker)
Will also be resistant to ceftriaxone
Talk about the mechanism of resistance in CTX-Ms
CTX-M genes encoded on plasmids and often co-locate with resistance genes for other antibiotic classes such as aminoglycosides or fluoroquinolones
Mobility of genes allows rapid dissemination across species and geographical boundaries - hence widespread
Mobility also allows co-resistance - MDR through linkage with other resistance determinants
What is the most prevalent CTX-M ESBL vaiant?
CTX-M-15
global variant - known for its efficient dissemination
Talk about the OXA variant of ESBL
A type of Class D organism
Primarily oxacillin-hydrolysing enzymes, associated with carbapenem resistance
Talk about the VEB, PER variants of ESBLs
ESBLs found occasionally in Pseudomonas or Acinetobacter
Talk about the GES variant of ESBLs
A rare group that can confer resistance to carbapenems in addition to cephalosporins
Talk about rates of ESBL E. Coli in Europe
Decrease of 3.6% compared to 2019
Ireland consisent at 10% seen over last 5 years
Bulgaria at 40%
Talk about 3rd generation cephalosporin resistant KLPN in europe
34.8% on average
Bulgaria at 80%
How can you distinguish a TEM and SHV ESBL from a CTX-M ESBL
TEM and SHV are resistant to ceftazidine but variable to cefotaxime
CTX-M are resistant to cefotaxme but variable to ceftazidine
What should you do with a positive ESBL
Must distinguish an ESBL from:
- AmpC B-lactamase
- Carbapenem hydolysing B-lactamase
How would you screen or an ESBL
Put up against cefotaxime and ceftazidine -> if resistant to one or both = ESBL
If AmpC ESBL confirm with cefepime +/- clavulanic acid
If ESBL variant confirm with ceftazidime and cefotaxime +/- clavulanic acid -> if intermediate query AmpC and test accordingly
What combination of antibiotics are used fr ESBLs
Cefotaxime or ceftriaxone AND ceftrazidime or cefpodoxime
Talk about agar for ESBLs
ESBL chromagars are available for use with rectal swabs
- Chromagar 98% sens, 72% spec
- ChromID 98% sens, 73% spec
- Brilliance 99% sens, 58% spec
No medium is fully ESBL selective though - derepressed AmpC B-lactamase and P. aeruginosa
Complementary tests needed to confirm
What is the marker of all ESBLS
Cefpodoxime resistance seen in all ESBLs but low level resistance is common ni the absence of ESBLs
Cefpodoxime most sensitive individual indicator but lacks specificity -> combination of cefotaxime and ceftrazidime allows for better specificty
What is the marker for TEM and SHV
Ceftazidime resistance
What is the marker for CTX-M resistance
Cefotaxime resistance
What is the molecular detection method for ESBLs
Check-Direc ESBL
Talk about Check Direct for ESBLs
Can identify ESBLs directly from rectal swabs in 2 hours
Covers clinically prevalent ESBLs: CTX-M-1, CTX-M-2, CTX-M-9 and SHV
Early studies show good performanc and greated sensitivity then culture
Detects 80 CTX-M, 160 TEM, 110 SHV variants
What are some of the common confirmatory tests for ESBLS
Combinaion disk test
Double-disk synergy test
Gradient strips
Talk about the combination disk test for ESBLs
Ceftazidime + ceftazidime + clavlanic acid
Cefotoxime + cefotozime + clavulanic acid
Cefepime + clavulanic acid or AmpC inhibitor to detect AmpC activity
> 5mm increase in zone indicates an ESBL
Talk about double-disk synergy test
Expansion of indicator cephalosporin inhibition zon towards amox/clav (augmentin) disk -> augmentin and cefodoxime
Disk spacing is critical -> may be reduced or expaned for strains with high or low level of resistance
Either use of cefepime disc or add clox to agar for AmpC producers
Talk about gradient strips for ESBLs
Double ended strip - cephalosporin + cephalosporin + clav
> 8 fold reduction in MIC = ESBL
Phantom zone in middle = ESBL
Deformed ellipse = ESBL
Non determinable could be due to AmpC activity - should test cefepime/clav
What are the pros and cons of each ESBL confirmatory method?
Double disc test: cheap but best disc spacing varies with strain
Combination disc test: cheap, does not require critical spacing, sensitive and specific but batch variation and negative controls critical
Gradiant strip: sensitive, accurate, internally controlled, mor epensivr
What bacteria do we not screen for ESBLs for
Theres no test developed for Acinetobacter spo and P. aeruginosa
Stenotrophomas maltophilia also causes false positives
Why can we not ESBL screen for acinetobacter
Acinetobacter spp is often susceptible to clavulanate alone
Why do we not have an ESBL screen for P. aeruginosa
ESBLs are not common in these and should not be sought routinely
ESBL should only be considered in ceftolozane/tazobactam resistant isolates
Why do we not have an ESBL screen for Stenotrophomonas?
S. matlophilia can cause positive results due to inhibition of L-2 chromosomal B-lactamase ubiquious in the species
