Antimicrobial Stewardship: An Important Consideration for all ...Most common plasmid-mediated -lactamases in Gram-negative bacteria Drugs stable in presence of these Extended-spectrum

ANTIMICROBIAL RESISTANCEMECHANISMS AND IMPLICATIONS THAT AFFECT PRACTICE

David J. Feola, Pharm.D., Ph.D., BCPS

Assistant Professor

University of Kentucky College of Pharmacy

Disclosures

Research Funding

Pfizer

Honoraria

GlaxoSmithKline

bioMerieux

Needs Statement

Antimicrobial resistance mechanisms affect practice in a variety

of ways. Preventing their emergence and altering therapies

for the treatment of resistant organisms are important concepts

for all practitioners. This activity will meet this need by

reviewing common and important mechanisms of antimicrobial

resistance, and then applying how these traits affect the

practice of the prescribing of antimicrobial agents.

Objectives

Discuss general mechanisms of antimicrobial

resistance that are clinically important

Apply knowledge of resistance mechanisms to

infectious diseases pharmacotherapeutics

Establish rules of management dictated by the

emergence of antimicrobial resistance

CP stands for “clinical pearl” which will designate

important clinical applications of the discussed

resistance mechanisms

Dellit TH et al. CID 2007;44:159-77.

The Critical Balance

Importance of appropriate

empiric therapy

Effect of broad-spectrum

therapy on resistance

Mortality increases

when initial therapy

is inappropriate

Resistance increases

when broad-spectrum

agents are needed;

Resistance has a

negative impact on

outcomes

“Collateral damage”

Antimicrobial Use and Resistance

Changes in use parallel changes in resistance

Patients with resistant infections more likely to have

received prior antimicrobials

Hospital areas of highest resistance associated with

highest antimicrobial use

Increased duration of therapy increase likeliness of

colonization with resistant organisms

Dellit TH et al. CID 2007;44:159-77.

Example: Oximinocephalosporins

Cefotaxime, ceftazidime, ceftriaxone cause

Extended-spectrum beta-lactamase production

Selection of stably de-repressed isolates in SPACE bacteria

Selection of VRE

Contribution to MRSA emergence

Increased cases of Clostridium difficile associated diarrhea/colitis

Dancer SJ. J Antimicrobial Chemother 2001; 48: 463-478

New Resistant Bacteria

Emergence of Resistance

Susceptible Bacteria

Resistant Bacteria

Resistance Gene Transfer

Resistant StrainsRare

Resistant Strains Dominant

Antimicrobial Exposure

Selection for Resistant Strains

Mechanism Classes

Mechanism Affected Agents

Drug modification/degradation -lactams, FQ, AGL, TCN, macrolides,

linezolid, clindamycin

Decreased bacterial

permeability

Sulfa, AGL, TCN, daptomycin,

carbapenems

Alteration of target site -lactams, FQ, TCN, vancomycin, linezolid,

clindamycin, macrolides

Efflux pumps FQ, AGL, TCN, macrolides, carbapenems

Enzyme Modification/Degradation

-lactamase production

Penicillins, cephalosporins, carbapenems, aztreonam

Acetylation

Fluoroquinolones

Hydroxylation

Aminoglycosides

Nucleotransferases

Clindamycin

Decreased Bacterial Permeability

Cell wall changes

Sulfa, daptomycin, vancomycin

Porin channel loss

TCN, carbapenems, macrolides

Alteration of Binding Site

Altered proteins

Penicillin binding protein (all -lactams)

D-ala-D-ala (vancomycin)

DNA gyrase, topoisomerases (FQ)

Methylation of ribosomal target site

Linezolid

Macrolides and clindamycin (erm)

Proteins protect and shield ribosome

FQ, TCN

Efflux Pumps

FQ (norA, MexAB-OprM)

Aminoglycosides

TCNs (tet, MexAB-OprM)

Macrolides (mefA, msrA)

-Lactamases

More than 340 different types have been described

More than 120 ESBLs

New ESBLs identified monthly

Classified by:

Plasmid vs. chromosomally mediated

Genes located on plasmids can spread

Constitutive vs. inducible production

Expression relates to -lactam exposure

Bush K. Clin Infect Dis. 2001;32:1085-1089.

Livermore DM. Clin Microbiol Inf. 2008;14:S3-10.

-Lactam Hydrolysis

Sites of -lactamase Hydrolysis

Dever LA et al. Arch Intern Med. 1991;151:886-895.

TEM and SHV -Lactamases

TEM-1, TEM-2, SHV-1

Most common plasmid-mediated -lactamases

in Gram-negative bacteria

Drugs stable in presence of these

Extended-spectrum cephalosporins resist hydrolysis

-lactamase inhibitors protect parent -lactam compound

Carbapenems


Rice LB. Pharmacotherapy. 1999;19:120S-128S.


Livermore DM et al. J Antimicrob Chemother. 2001;48(suppl):59-64.

TEM and SHV -Lactamases

Extended spectrum beta-lactamases (ESBL)

Mutants of classical enzymes

Hydrolyze most extended-spectrum cephalosporins and

aztreonam

Carbapenems are spared

Inhibited by clavulanic acid

Organisms that produce ESBLs

Klebsiella, E. coli, other Enterobacteriaceae and non-

fermenting Gram-negative bacteria

Amino acid position

Ceftazidime

Enzyme MIC (μg/mL) 104 162 237

TEM-1 256 Lys Ser Glu

Modified from Rice LB. Pharmacotherapy. 1999;19:120S-128S.

Molecular Basis of ESBLs

Why Are ESBL Producers Important?

Plasmid-mediated resistance facilitates spread

Significant laboratory detection issue

Therapeutic implications

Formulary implications

Widespread unawareness of clinicians due to

underreporting by microbiology laboratories

CP: Laboratory Detection Problem

No simple marker for presence of ESBL

ESBLs give variable MICs to the extended-spectrum

cephalosporins

May not reach currently defined breakpoint for resistance

( 32 µg/mL)

Present susceptibility break points for ceftazidime

Susceptible < 8 mcg/ml

Intermediate 16 mcg/ml

Resistant > 32 mcg/ml


CP: Recommended ESBL Detection

ESBL screening

If MIC 2 µg/mL to ceftazidime, cefotaxime, or

ceftriaxone, then must do an:

ESBL confirmatory test

3 two-fold concentration decrease in an MIC for an

antimicrobial agent tested in combination with clavulanic

acid or > 5 mm increase in ceftazidime/clavulanic acid

zone diameter

If an ESBL producing isolate identified, must report

RESISTANT to all cephalosporins and penicillins

Laboratory Detection of ESBLs

Etest® ESBL Prescribing Information; AB Biodisk.

CP: Treatment of ESBL Producers

Carbapenems: current drugs of choice

Cefepime: more stability but reports of treatment

failures

Little reported experience with trimethoprim/

sulfamethoxazole, aminoglycosides and fluoroquinolones

Tigecycline may be an option

Paterson DL, et al. CID 2004; 39: 31 – 37

AmpC β-Lactamases

Different from ESBLs

Not inhibited by β-lactamase inhibitors

Differing susceptibilities

Usually chromosomally encoded

Generally confers resistance to

First-, second-, and third-generation cephalosporins and

aztreonam

Broad-spectrum penicillins associated with β-lactamaseinhibitors

Pfaller MA, Segreti J. Clin Infect Dis. 2006;42(suppl 4):S153-S163.

Jones RN. Diagn Microbiol Infect Dis. 1998;31:461-466.

AmpC β-Lactamases

Most AmpCs are chromosomal and inducible

SPACE bacteria

Some however are plasmid-mediated

Klebsiella spp, Salmonella spp, Proteus mirabilis

Drugs that induce AmpC, highest potential to lowest

Carbapenems, cephamycins, aminopenicillins, carbenicillin,

ticarcillin, piperacillin, cephalosporins, clavulanic acid,

cefepime, aztreonam

Pfaller MA, Segreti J. Clin Infect Dis. 2006;42(suppl 4):S153-S163.

Jones RN. Diagn Microbiol Infect Dis. 1998;31:461-466.

CP: Chromosomal, Inducible AmpCs

Produced by the SPACE Bacteria Serratia marcescens

Pseudomonas aeruginosa

Acinetobacter species

Citrobacter species

Enterobacter species

β-lactamase under the control of the ampC gene (turn on) and repressor gene (turn off)

Mutation is loss of the repressor gene – terminology is the

isolate becomes “stably de-repressed”

Drugs of choice: carbapenems, cefepime

Bush, K. Clin Infect Dis 2001; 32: 1085 - 1089

CP: Stable Derepression

Selection of stable derepressed mutants: susceptible when

tested, then resistance 3 days later

Resistant Strain

Rare

Selection During

Therapy

Resistant Strain

Dominant

Livermore DM, Woodford N. Trends Microbiol. 2006;14:413-420.

Bonomo RA, Szabo D. Clin Infect Dis. 2006;43(suppl 2):S49-S56.

Carbapenemases

Characteristics

Two categories: serine -lactamases and

metallo- -lactamases (MBL)

Can be chromosomally encoded or plasmid encoded

Can lead to resistance to carbapenems and

antipseudomonal cephalosporins/penicillins

Carbapenemases

Transferable MBLs

Important bacteria found to carry these genes

on integrons Pseudomonas aeruginosa Serratia marcescens

Acinetobacter baumannii Klebsiella pneumoniae

Klebsiella oxytoca Citrobacter freundii

Escherichia coli Proteus mirabolis

Enterobacter cloacae

KPC –Carbapenemase Outbreak

96 isolates from 10 Brooklyn Hospital

Carbapenem MICs > 32 mcg/ml

Potential antimicrobial therapy

Polymyxin B – 91% susceptible in-vitro

Tigecycline – 100% susceptible in-vitro

In-vitro synergy testing

Polymyxin B + rifampin (15/16 isolates)

Polymyxin B + imipenem (10/16 isolates)

Bratu S, et al. J Antimicrobial Chemother 2005;56:128-132

CP: PCN Binding Proteins

MRSA – loss of the target site – PBP2 which is replaced by PBP2a

mecA gene, associated with other resistance genes in the Staphylococcal chromosome cassette (SCCmec)

PBP2a confers resistance to all beta-lactams

Streptococcus pneumoniae

PCN resistance when mutations in 4 PBPs

If PCN resistant, increased macrolide resistance, FQ resistance still low

S. pneumoniae Susceptibility

CP: CA-MRSA vs. HA-MRSA

Characteristic CA-MRSA HA-MRSA

Susceptibility

Chloramphenicol Usually susceptible Frequently resistant

Clindamycin Usually susceptible Frequently resistant

Erythromycin Usually resistant Usually resistant

Fluoroquinolone Geographic variability Usually resistant

TMP/SMZ Usually susceptible Usually susceptible

SCC mec type IV II

Lineage USA 300, USA 400 USA 100, USA 200

Toxins More Fewer

PVL Common Rare

Weber JT. CID 2005;41

Efflux Pumps

Some drug specific (tet), some non-specific (confers

MDR)

Tetracyclines

Genetically-mobile tet genes

tet A-E: pumps drugs out of cell

tet M, tet O: protects ribosomes

CP: Tigecycline D-ring side chain gives steric hindrance

as a substrate of these pumps

Piddock LJV. Clin Microb Rev 2006;19(2):382-402

Efflux Pumps

Some confer baseline resistance

Example MexXY-OprM in Pseudomonas aeruginosa

Tigecycline is a substrate, causes inactivity

Overexpression confers resistance

Drugs of many classes are substrates

Confers multidrug resistance

Key point: do not always confer resistance to

substrates

FQ mutation in topoisomerase gene plus efflux pump

Carbapenems efflux pump in combination with porin

channel lossPiddock LJV. Clin Microb Rev 2006;19(2):382-402

CP: Combination of Mechanisms

Meropenem and P. aeruginosa, Acinetobacter

Up-regulation of efflux pump gene

Loss of porin channel protein

Both mutations needed for resistance development

MIC 0.12–0.5 µg/ml (before mutation)

MIC 2-4 µg/ml (with one mutation)

MIC >8 µg/ml (with both mutations)

Livermore DM. JAC 2001; 47: 247-250

Efflux Pump Examples

P. aeruginosa

MexAB-OprM, MexXY-OprM (constitutive)

Tigecycline

MexCD-OprJ, MexEF-OprN (inducible)

FQ, TCN, chloramphenicol, some β-lactams

E. coli

Concern: ESBL producers, now being treated with second-

line agents, often efflux substrates, increasing selection

pressure


Efflux Pump Examples

S. aureus

NorA: structurally similar to tet proteins

Present on MRSA and MSSA

MDR to FQ, chloramphenicol, some disinfectants

NorB: FQ, tetracylines, disinfectants


CP: Clindamycin Inducible Resistance

Caution in macrolide resistant strains

Inducible clindamycin resistance

Disc diffusion test: D Test

Presence of erm genes

Erythromycin induces expression,

decreasing activity of clindamycin

With a positive D Test, do not

use clindamycin

Vancomycin

Glycopeptide antimicrobial that inhibits

transpeptidation reaction

D-ala–D–ala

Mechanisms of resistance: change in bacterial target

D-ala–D–lac (VanA, B, D)

D-ala–D–ser (VanC, E, L)

Enterococcus sp.

E. faecium incidence of resistance higher that E. faecalis

VanA gene is on a plasmid

Am J Health Syst Pharm 2000;57:S4-9

Clin Pharmacokinet 2004;43:925-942

Clin Infect Dis 2006; 2006; 42:S35–9

Vancomycin

CP: MRSA increasing MICs to vancomycin (1-2 mcg/ml)

Still susceptible, but increase in treatment failures

VISA mechanism: increased cell wall structural material

VRSA mechanism: MRSA acquires the VanA gene

Rare in the US

Am J Health Syst Pharm 2000;57:S4-9

Clin Pharmacokinet 2004;43:925-942

Clin Infect Dis 2006; 2006; 42:S35–9

Documents

Antimicrobial Stewardship: An Important Consideration for all ...Most common plasmid-mediated -lactamases in Gram-negative bacteria Drugs stable in presence of these Extended-spectrum