34 Attacking the enemy: antibiotic resistance Flashcards
What are general ways in which mechanisms of resistance develop?
Drug target modified so can no longer bind
Reduced cell wall/ membrane permeability
Efflux drug
Enzyme destruction of drug
Alternate metabolic pathway
Resistance mechanism -
Drug target modified so can no longer bind
Some mutations can involve single nucleotide changes, so can arise rapidly. But may not spread as quickly as they are chromosomal in nature (not plasmid)
What drug targets can be modified?
Cell surface protein
Ribosome
Topoisomerase enzymes
How does MRSA change its cell surface protein to become resistant?
gene from mec family encodes for different penicillin-binding-protein 2a
Hows does glycopeptide resistance arise?
Mutation in peptidoglycan precursor structure causes subsitution with d-alanyl-d-alanine termination
So glyocpeptides cannot bind
VanA gene confers high level resistance vanc/ teicoplanin
Van B gene confers low level resistance to vancomycin only
Ribosomal mutation can prevent drug binding.
What are common mutations?
Target modification by methylation of 23S RNA in the 50S subunit
erm gene codes for this
Encodes resistance to MLSb - macrolides, lincosamides, streptogrammin B and sometimes linezolid
What mutation confers resistance in topoisomerase?
GyrA gene mutation renders fluoroquinolones ineffective
Reduction in cell wall/ membrane permeability is relatively uncommon cause of resistance.
What is an example of this?
Pseudomonas - porin loss can cause beta-lactam resistance
What are examples of genes and species which increase drug efflux?
mefA gene streptococcus - efflux macrolides
tetk gene streptococcus - efflux tetracyclines
Mex gene pseudomonas - efflux beta-lactams, fluoroquinolones, aminoglycosides
Acr gene E. coli - effluoc beta-lactams, fluoroquinolones, trimethoprim
Metabolic pathways can be altered to provide resistance
How do trimethoprim/ sulphonamide drugs become inactviated?
New folate synthesis pathway enables bypass
Metabolic pathways can be altered to provide resistance
How does metronidazole resistance occur?
Inactive until chemically reduced
Reduced form cytotoxic, damaging DNA
nim family of genes, particularly in bacteroides species, encodes alternative enzyme which can process metronidazole into less cytotoxic form
Metabolic pathways can be altered to provide resistance
How does nitrofurantoin resistance occur?
Nitrofuranotin is inactive until chemically reduced
Reduced form cytotoxic, damaging DNA
E. coli mutates so has alternative metabolic pathway, producing inert drug
Patient has colectomy, and post-op infection. Enterococcus grown, resistant, and has erm gene.
which antibacterial will retain its activity?
clarithromycin clindamicin erythrmycin pristinamycin vancomycin
vancomycin
erm genes codes for MLSb resistance
What is the Ambler classification used for?
Molecular method used to classify beta-lactamases
Classes
A-D
What are classes of Ambler Classification
A-D
A -
Narrow spectrum beta-lactamases - penicillin
Extended spectrum beta-lactamases - ESBL
Serine carbapenemases - hydrolyse carbapenems
B -
mettalo-beta-lactamases - hydrolyse carbapenemas
C-
cephalosporinases - hydrolyse cephamycins (AmpC)
D -
OXA-type enzymes
What is function of AmpC enzymes?
What antibiotics are inhibited?
Ambler class C
Hydrolyse cephalosporins and penicillins.
Not inhibited by beta-lactamase inhibitor clavulanic acid.
Resistant - co-amoxiclav tazocin carbapenems ceftriaxone/ ceftaz/ cefotazime
Patient initially improves on antibiotics, then deteriorates. AmpC gene detected
How is this explained?
If patient given co-amoxiclav/ tazocin for these infections, may initially improve.
Then AmpC genes switch on, and resistant strain begins to multiply. Patient then deteriorates
If cefoxitin resistant in vitro, that is indication that has AmpC gene. Even if sample sensitive to co-amoxiclav/ tazocin, do not use these are resistant strain will multiply
what is function of ESBL enzyme?
Which antibiotics are inhibited?
Ambler class A
hydrolyse broader range of beta-lactams, including cephalosporins, monobactams
Tazocin
Cefotaxime - 4th generation
Aztreonam (memorise as group A Ambler class)
Often sensitive to co-amoxiclav, but better to use carbapenem as always sensitive
Genetically speaking, where are ESBL and AmpC genes carried?
ESBL - plasmid - so can spread quickly
AmpC - chromosomal. Occasionally plasmid
Following urine culture, and antibiotic susceptibility testing, when are isolates tested for -
ESBL
AmpC resistance
CPE/ CPO
ESBL - Any evidence of Cefpodoxime (3rd generation) resistance triggers further testing
AmpC - cefotaxime/ ceftazidime and Cefpodoxime resistance
CPE/ CPO -
- Eneterobacteriaceae - any evidence of ertapenem/ meropenem resistance
- Pseudomonas - meropenem/ ceftaz/ tazocin resistance
Urine culture is resistant to cefpodoxime. You decide to check for ESBL production.
How is this performed?
Combination disk test - based on premise that most ESBLs are inhibited by clavulanic acid (AmpC are not inhibted by clavulanic acid)
Add disc with ceftazidime/ cefotaxime/ cefepime
Add disc with ceftazidime/ cefotaxime/ cefepime + clavulanic acid
Clavulanic acid inhibits beta-lactamases. If inhibited growth around this disc >5mm, and growth around other disc, suggests ESBL (as resistant to the ceftazidime/ cefotaxime)
Which organisms have -
- plasmid encoded AmpC
- intrinsic chromosomal AmpC
- plasmid encoded AmpC E. coli Klebsiella Proteus Salmonella
- intrinsic chromosomal AmpC Enterobacter Citrobacter Morganella Providencia Serratia
Urine culture is resistant to cefpodoxime, and ceftazidime. You decide to check for AmpC production.
How is this performed?
Add cloxacillin disc
If inhibited by cloxacillin, suggests AmpC production
If not inhibited by cloxacillin, then resistance seen is via other mechanism
cannot use clavulanic acid test, as AmpC not inhibited by this
