Beta-Lactam Resistance in Gram Negative Bacilli Flashcards
Beta-Lactam Antibiotics: Mechanism of Action
• Penicillin binding proteins (PBPs) are responsible for assembly, maintenance, and regulation of peptidoglycan (cell wall) metabolism • Beta-lactams bind and inhibit PBPs • Disruption of peptidoglycan synthesis • Bactericidal
Beta-lactam antibiotics
• Penicillins – Benzylpenicillin (natural) – Ampicillin (extended spectrum) – Piperacillin (Ureidopenicillin) – Ticarcillin (Carboxypenicillin) • Beta-lactam/beta-lactamase inhibitors – Ampicillin/sulbactam (Unasyn) – Amoxicillin/clavulanate (Augmentin) – Ticarcillin/clavulanate (Timentin) – Piperacillin/Tazobactam (Zosyn)
B-lactam antibiotics : Cephalosporins
• First Generation cephalosporins – Cefazolin – Cephalothin – Cefadroxil • Second Generation oral antibiotics – Cefuroxime (many others) • Second Generation cephamycins – Cefoxitin – Cefotetan
Beta-lactam antibiotics : Cephalosporins continued
• Third generation cephalosporins – Cefotaxime – Ceftriaxone – Ceftazidime – Cefixime • Fourth generation cephalosporins – Cefepime • Monobactams – Aztreonam
Beta-lactam antibiotics : Carbapenems
• Carbapenems – Imipenem – Meropenem – Ertapenem – Doripenem
Resistance to Beta-Lactams
• Six “P’s” that can explain resistance to β-lactams:
– Penetration: poor access to intracellular bacteria
– Porins: prevent entry of β-lactams into bacterial cell
– Pumps: prevent accumulation of β-lactams in bacterial cell (efflux pumps)
– Penicillin-binding proteins: production of PBPs that do not bind β-lactams (or only bind certain types of beta-lactams)
– Peptidoglycan: some bacteria do not have a cell wall
– Penicillinases (β-lactamases): production of enzymes that hydrolyze β-lactams
Location of β -lactamases
• Most β -lactamases of gram-negative species are periplasmic in location.
Classification of β-lactamases
• Four ways based on: – Spectrum and preference of subsrates – Susceptibility to inhibitors – Chromosomal vs plasmid mediated – Molecular Sequence • Classification schemes include: – Jack and Richmond (1970) – Richmond and Sykes (1973) – Ambler (1980 based on molecular phylogeny) – Bush (1989, 1995) based on substrate preference
Molecular classification of β-lactamases
• Four molecular groupings: A,B,C, and D
– A,C, and D are all serine β-lactamases
– B is reserved for metallo β –lactamases (eg zinc requiring)
• There is good correlation between the Bush scheme and molecular taxonomy
Chromosomal β -lactamases:
Miscellaneous Gram-negative
• Pseudomonas aeruginosa
– Inducible, chromososmal Amp C
– Totally derepressed mutants occur at low frequency but can be selected by cephalosporin or ureido-penicillin therapy
• Stenotrophomonas maltophilia
– Produces 2 inducible, chromosomal enzymes both
regulated by the same induction system.
• One is a Class B metalloenzyme
• One is a Class A
– Between the two, the hit almost all β -lactams
Chromosomal β –lactamases–Klebsiella
• Primary Class A chromosomal enzymes
– K. pneumoniae = SHV-1
– K. oxytoca = KOXY
• All are constitutive but at levels to produce
resistance to ampicillin, amoxicillin, ticarcillin, etc
- Regardless of MIC, no unprotected penicillin should be used against Klebsiella
- Higher and broader levels of resistance in Klebsiella are usually caused by plasmid-mediated or hyperproduced B-lactamases
ESBL - Extended Spectrum Beta-Lactamase
• Class A, plasmid encoded enzymes • Derivatives (mutants) of original TEM-1 and SHV-1 -lactamases • Spreading throughout the Enterobacteriaceae • More than 200 described to date • Labile – 1st and 3rd generation cephalosporins – Ureidopenicillins • Stable – Cephamycins – Carbapenems – Cefepime** • Inhibited by Beta-lactamase inhibitors – Tazobactam – Sulbactam – Clavulanate
Extended Spectrum β-Lactamases
● Extended Spectrum β-Lactamases (ESBLs) are Ambler class A enzymes.
● Encoded on either the chromosome or mobile genetic elements.
● Inhibited by β-lactamase inhibitors e.g., clavulanic acid.
AmpC β-lactamases
• Can be encoded on chromosome (SPACE
organisms)
– Serratia, Providencia/Proteus
vulgaris/Pseudomonas, Acinetobacter,
Citrobacter freundii complex, Enterobacter
• Constitutive expression
• Penicillins, First- and Second- generation cephalosporins are hydrolyzed (and usually 3rd
-gen but R to 3rd gen not always detected in vitro)
• In Enterobacteriaceae, expression can be “upregulated” by exposure to beta-lactam antibiotics (esp carbapenems) and beta-lactamse inhibitors
• Cefepime and carbapenems are not hydrolyzed
AmpC β-lactamases
Continued
• Plasmid mediated AmpC
– Kleb, E. coli
– Clue=cefoxtin resistance in these organisms
• ESBL “skips” a generation—cephamycins are not
hydrolyzed, first and third generation cephalosporins
are usually resistant
AmpCs
● AmpCs are Ambler class C enzymes.
● Encoded on the chromosome (SPACE organisms) or mobile genetic elements.
● Expression of chromosomal-encoded enzymes can be induced.
● Inhibited by cloxacillin
ESBL vs. AmpC
• Plasmid (ESBL) vs. Chromosomal (AmpC)
• Inhibited by β-lactamase inhibitors (ESBL) vs. not
inhibited (AmpC)
• Cephamycin S (ESBL) vs. Cephamycin R (AmpC)
• Sometimes confers Cefepime R (ESBL) vs. does not confer Cefepime R (AmpC)
