Clinical Uses of Antibiotics & Resistance Flashcards
Ceftriaxone…
Pseudomonas?
No.
other 3rd gen cephs do, but ceftriaxone does not.
Why are we reluctant to use Aminoglycosides?
Nephrotoxicity
Ototoxicity
Do aminoglycosides work on pseudomonas?
yes.
What are the 4 big considerations for empirical therapy?
- Infectious syndrome (site)
- Pathogens that cause this syndrome?
- Antibiotic resistance of the possible bugs?
- Most appropriate AntiB to use…?
Why are E. Coli more likely to undergo R-factor plasmid transfer via conjugation than Pseudomonas?
Both are Gram ? C or R?
E. Coli are enteric so they hang out and conjugate with one another. Pseudomonas are non-enteric, less likely to conjugate.
Both are GNR
What are the Bactericidal Drugs (by class) drugs and the exceptions.
- Cell Wall Synth. Inhibitors
PCN, Ceph, Vanc. - Aminoglycoside (protein synth. inhib, exception)
- DNA gyrase fn. inhibitors
What are the Bacteriostatic (by class) durgs and what are the exceptions?
Protein Synthesis Inhibitors are bacteriostatic
…with the exception of aminoglycosides which are bacteriocidal.
Aminoglycosides don’t work well at _______ pH.
low
Breakpoints for whether an antimicrobial will be effective against a given bug…
- Susceptible - likely to get a response
- Intermediate - intermediate or uncertain response. Higher dose may work.
- Resistant - probably no response.
The labratory-defined in vitro susceptibility testing breakpoints do not take into account …
- infection site (penetrate?)
- # of bugs (abscess needs to be drained first)
- host conditions (pH at infection site)
- patients host defenses (immuno-compromised?)
The minimum requirement for considering a bug to be resistant to an anti-bug…
Cp > MIC
maximal serum concentration should be greater than the minimum inhibitory concentration
Why is E. Coli resistant to penicillin?
What sort of resistance is this?
What drug (in the same class) is effective on E. coli and why?
PCN can’t get thru the PORINS in the outer membrane of E. coli (gram neg).
Intrinsic Resistance
Ampicillin is able (more hydrophobic side chain) to enter thru the porin to reach the PBP and inhibit c.w. synth.
Why is Mycoplasma resistant to beta lactams?
No cell wall –> no effect from the c.w. synth inhibitors.
Intrinsic Resistance.
Why is pseudomonas resistant to multiple agents?
…
3 types of resistance
Intrinsic
Tolerance
Acquired
Give 4 tolerance resistance mechanisms
Biofilms
Metabolic bypass (sulfonamides example)
Anaerobic growth
Stationary phase
3 acquired resistance mechanisms
- Inactivate or modify the drug
- Alter the antibacterial target
- Prevent the drug & target from meeting (efflux)
Pseudomonas resistance mechanism(s) to Fluoroquinolones
- Efflux
2. Altered bacterial target site
Pseudomonas resistance mechanism(s) to beta lactams?
Decreased entry
MRSA resistance mechanism(s) to beta lactams?
- Altered bacterial site (alter PBP)
2. Enzymatic degradation (beta lactamase)
Streptococci resistance mechanism to sulfonamides?
Bypass pathway (metabolic)
Porin channels exist on which bugs?
gram negative only
Role of porin channels in gram negative bacteria?
Selective uptake of nutrients etc.
Conduit for hydrophillic antiB’s to get inside cells
How can porins contribute to antiB resistance
Can be intrinsic: prevent antiB entry
Can be acquired…alter structure or # or porins
Efflux pumps exist on which bugs?
Gram (+) and Gram (-)
What is the normal function of efflux pumps?
get rid of substrates from bacterial cytoplasm
efflux pumps are plasmid or chromosomally encoded?
both
how do efflux pumps contribute to antiB resistance?
can pump antiB out of cell preventing drug from reaching target
are efflux pumps drug specific or multi-drug?
can be both.
pseudomonas - multi drug resistance via efflux pumps
How do bacteria become resistant to beta lactams?
- MODIFY THE DRUG: BETA LACTAMASES
- MODIFY THE TARGET: CHANGE PBP
(also porin channels and efflux pumps)
Where are the worst beta lactmases?
GNR
Enteric bacteria
What does a beta lactamase do?
How are they encoded?
enzymatically splits the beta lactam ring
plasmid or chromosomal
Where are beta lactamases found?
gram(+) and gram(-)
but worst are GNR enteric bact.
Name 3 GNR enteric bacteria
- E. coli
- Enterobacter
- Klebsiella
Name 3 GP bacteria
Staph. aureus
Strep. pneumoniae
Enterococcus (faecalis, faecium)
Name 1 GNR non-enteric bacteria
Pseudomonas
Pseudomonas is usually hospital or community acquired?
hospital
very resistant to antiB’s
How do beta lactams work?
irreversibly bind to and inactivate PBP’s
Two beta-lactamase inhibitors
Tazobactam
Clavulanate
S. aureus resistance profile
Plasmid
gene: bla
PCN, amp, amox
beta-lactamase
E. coli resistance profile
Plasmid
gene: TEM-1
PCN, amp, amox
beta-lactamase
Klebsiella pneumonia resistance profile
CHROMOSOME
gene: SHV-1
PCN, amp, amox
beta-lactamase
How common is E. coli resistance via beta lactamase?
rare
How common is Klebsiella resistance via beta lactamase?
All.
Gene is chromosomal, so all klebsiella will carry the resistance gene
How common is Staph. aureus resistance via beta lactamase?
All.
assume all…its possible but highly unlikely to have a non-beta-lactamase-resistant S. aureus
How common is Streptococcus resistance via beta lactamase?
Never.
Strep will never be resistant to abx via a beta-lactamase.
Will be resistant via alteration of PBPs
What is an ESBL and how did it appear in bugs?
Extended spectrum beta lactamase
mutation of the orig. beta-lactamases: TEM-1, 2 and SHV-1
What does ESBL add to the resistance profile of bugs that carry it
adds Cephalosporin resistance to the PCN, ampi and amox resistance.
Are ESBL’s susceptible to beta-lactamase inhibitors?
yes (most)
e.g. clavulanate
Which bugs are likely to carry an ESBL plasmid?
E. coli
Klebsiella pneumoniae
Which bugs (do we need to know) carry the ampC gene?
Enteric GNR: Enterobacter
Non-Enteric GNR; Pseudomonas
Where is the ampC gene located?
Chromosome
How common is Enterobacter resistance via the ampC gene?
ALL.
the gene is chromosomal
How common is Pseudomonas resistance via the ampC gene?
ALL
the gene is chromosomal
How is the ampC gene regulated?
Can be inducible or constitutive.
The inducible gene is turned on by ampicillin & cefazolin (hence the gene name)
Gene can become mutated (during therapy) to be constitutivley on
Resistance profile of a bug carrying an inducible ampC gene
e.g. enterobacter or pseudomonas
Resistant to: Amp & Cefazolin
Susceptible to: Ceftriaxone, Pip-Taz,
Ertapenem (carbapenem)
Resistance profile of a bug carrying a constituitively on ampC gene
e.g. enterobacter, pseudomonas
Resistant to: Amp, Cefazolin, Ceftriaxone, Pip-Tazo
Susceptible: Entrapenem (carbapenem)
What is the mechanism of action of the ampC resistance gene?
hydrolyzes beta lactams
The resistance spectrum of ampC is similar to ESBLs. So, why is it worse?
is not inhibited by the beta-lactamase inhibitors that used to work on ESBLs
What are some important bacteria that harbor the ampC gene in their chromosome?
Enterobacter
Pseudomonas
(Serratia)
Since a bug with an inducible ampC gene is not turned on by Ceftriaxone (3rd gen Ceph) is this a good tx choice?
No.
Prolonged tx with a 3rd gen Ceph can select for a de-repressed ampC gene…making it now constitutively on –> drug no longer works.
What is the tx of choice in a bug with an inducible (or constitutive) ampC gene?
Carbapenems
Entrapenem
What genes are now emerging as Carbapenemases? (confer resitance to Carbap’s)
KPC & NDM-1
What bugs (that we have to know) are most commonly found as carrying carbapenemase genes?
our old friends…
Klebsiella & E. coli
(+ some other enterobacteriaceae)
Are the KPC and NDM-1 genes plasmids or chromosomal?
Plasmids…so far.
Of the two bugs that we know to carry the KPC gene, which is more common?
Klebsiella pneumoniae (rare)
E. coli (even rarer)
What does KPC stand for?
Klebsiella pneumoniae carbapenemase
yay, an appropriately named gene/protein
What does NDM-1 stand for?
New Dehli metalo-betalactamase
after the place where it was first identified
What beta-lactam does NDM-1 not hydrolyze?
aztreonam (monobactam)
It has been observed that NDM-1 has transfered ______ from ______ to ________ by conjugation.
via plasmid from K. pneumoniae to E. coli
Which bugs (that we need to know) are associated with altered PBPs as a resistance mechanism?
Staphylococcus spp
Streptococcus pneumoniae
Enterococcus faecium
What is the PBP alteration exhibited by Staphylococcus spp?
mecA –> PBP2a
inherited new gene, encodes PBP2a low affinity PBP
What is the PBP alteration exhibited by Streptococcus pneumoniae?
mosaic PBP from pieces of DNA
–> low affinity PBP
What is the PBP alteration exhibited by Enterococcus faecium?
Mutation or over-expression of PBP5 - a low affinity PBP
Recall, all Staph spp (aures, coag-neg) harbor the gene ______ that confers resistance to _______.
This led to development of what drugs?
bla gene, PCN, ampicillin
developed: Penicillinase-resistant drugs - methicillin, nafcillin, oxacillin, dicloxacillin
After the development of methicillin, nafcillin, oxacillin, dicloxacillin (Penicililnase-resistant drugs) what new resistance mechanism appeared in some bugs?
mecA gene –> MRSA
SCCmec
What is SCCmec?
large gene (cassette) called Staphylococcal Chromosome Cassette that contains the mecA gene conferring resistance to Staph. aureus via alterations in the PBP
(PBP2a low affinity PBP)
What are the only exceptions (drugs) that are effective against bugs carrying the mecA (PBP2a) gene?
5th generation Cephalosporins
Ceftaroline & Ceftobiprole
Why not use a beta-lactamase inhibitor against bugs that harbor mecA (PBP2a)?
PBP2a is not a beta-lactamase!!!
its just a low affinity PBP
Of the Staph isolates at the Denver VA, ________% are MRSA. Of the MSSA ________% carry a beta lactamase.
40%
90%
What % of Streptococcus carry a beta lactamase?
0%
Strep do NOT carry beta lactamases.
How do Steptococcus pneumonia resist beta lactams?
Alter their PBPs
How does Strep acquire its beta lactam resistance?
picks up DNA and inserts it into its PBP gene(s) to create a “mosaic” gene. So you get all sorts of wacky altered PBP’s
They primarily get these DNA fragments from neighboring resistant staph.
“transformation”
Distinguish between the resistance to beta lactams exhibited by Enterococcus faecium and Enterococcus faecalis
E. faecium: mutaion or overexpression of PBP5 (low affinity PBP)
E. faecalis: Beta-lactamase on plasmid (rare)
The combo Ampicillin-Sublactam is effective against E. faecium or E. faecalis?
Why?
E. faecalis: its resistance mechanism is a beta lactamsase
This drug combo is not effective against E. faecium b/c E. faecium’s resistance mechanism is an altered/overexpressed PBP5 NOT a beta lactamase
How does Vancomycin work?
Targets the D-alanine-D-alanine terminal of the 5-peptide chain precursor peptidoglycan molecule for c.w. synthesis. (stage 2)
Elaborate on Vancomycin’s mechanism of action…
Vanc binds the D-ala-D-ala terminal of the peptide. Vanc is so large that it blocks both transpeptidase and transglycosylase enzymes for c.w. synth.
Recall: PCN binds/blocks transpeptidase only
Is Vancomycin bacteriostatic or bacteriocidal?
bactericidal
blocks c.w. synth…kills the cell
Does vancomycin work on gram negative bacteria?
No. Vanc is too large to pass through the porins to reach the c.w. and inhibit synthesis.
What are the two mechanisms of Vanc resistance?
- Alter target (D-ala-D-ala —> D-ala-D-lactate)
2. thicker cell wall with free-hanging D-ala-D-ala; binds vanc & prevents it from getting to precursors.
Which bugs are resistant to Vancomycin via D-ala-D-lactate alteration?
Almost exclusively Enterococcus spp (fecalis & faecium)
Very rarely S. aureus
Which bugs are resistant to Vancomycin via a thickened cell wall with free-hanging D-ala-D-ala that bind & trap Vanc?
Staphylococcus aureus
What gene encodes for the D-ala-D-lactate mutation that confers Vancomycin resistance?
VanA
If a bug harbors the VanA gene, does it have a high or low level of resistance?
What about the thickened cell wall with free-hanging D-ala-D-ala precursors?
VanA high!
low-level resistance is the super rare enhanced cell wall thickness resistance mechanism
What is VRE?
Vancomycin Resistant Enterococcus
via the VanA gene
Is VanA plasmid or chromosomal?
Plasmid
Which VRE is more common?
E. faecium or E. faecalis?
E. faecium
VanA is encoded on a plasmid in Enterococci. Why is this worrisome?
plasmid transfer to S. aureus…
—> vanc. resistant S. aureus
VanA-carrying S. aureus has happened but only about 8-10 times in the past decade.
How are VISA isolates resistant to vancomycin?
Vancomycin Intermediate Staph aureus
thicker cell walls, free-hanging D-ala-D-ala chains, not joined, not going to be joined, just bind vanc and prevent it from reaching target.
VISA strains are rare. Where do they usually pop up?
in prolonged vancomycin therapy
