Antimicrobial Chemotherapy & Resistance year/Micro/powerpoint/antimicrobial... · Susceptibility...

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Antimicrobial Chemotherapy & Resistance Ken B. Waites, M.D. F(AAM) October, 2008 Learning Objectives To understand how antibiotics work To review how drug resistance develops in bacteria To summarize current status of antimicrobial resistance in selected bacteria To understand how resistant organisms are detected in the clinical laboratory

Transcript of Antimicrobial Chemotherapy & Resistance year/Micro/powerpoint/antimicrobial... · Susceptibility...

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Antimicrobial Chemotherapy &Resistance

Ken B. Waites, M.D. F(AAM)October, 2008

Learning Objectives• To understand how antibiotics work• To review how drug resistance develops in

bacteria• To summarize current status of antimicrobial

resistance in selected bacteria• To understand how resistant organisms are

detected in the clinical laboratory

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Terminology• Antibiotic - An agent that can be naturally occurring,

partially or completely synthetic that selectivelyinhibits the growth of microorganisms at lowconcentrations (penicillin)

• Antiseptic – An agent used to inhibit or eliminatemicrobes on skin or other living tissue (alcohol,iodine, chlorhexidine)

• Disinfectant – An agent used to destroy microbeson inanimate objects (phenols, formaldehyde,chlorine bleach)

SirAlexanderFleming

DiscoveredPenicillin in1928

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Agents Acting on Cell Wall: Beta Lactams

• Penicillins• Penicillinase-

stable• Broad-spectrum• Beta-lactamase

Inhibitorcombinations

Betalactamringresemblessubstrateforpenicillinbindingproteins(enzymes)

Penicillin Mechanism

• Binds active site of transpeptidase enzyme thatcross-links peptidoglycan by joining D-ALA andGLY

• Mimics D-alanyl-D-ALA that would normallybind to this site

• Irreversibly inhibits transpeptidase

• Growth of the bacterial cell wall is inhibited

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Other Beta Lactams

• Cephalosporins 1-4th generations• Monobactams• Carbapenems

Non-Beta Lactams Acting on Cell Wall• Cycloserine

– Inhibits synthesis of D-alanyl-D-ALA within cell callmucopeptide

• Glycopeptides (vancomycin)– binds to D-ALA-D-ALA moiety of precursor subunit

blocking transpeptidation• Bacitracin

– complexes with lipid carrier that transportspeptidoglyan precursors from cytoplasm to cellmembrane

• Isoniazid (INH)– Inhibits fatty acid and lipid components of mycolic acid

synthesis in mycobacteria

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Examples of Drugs Acting onBacterial Ribsome

Tetracyclines preventattachment of tRNA-AAto 30s.

Aminoglycosidesbind 30s, inactivateinitiation complex, misreadmRNA genetic code &prematurely release peptide

Macrolides bind 50s &prevent release ofdeacylated tRNA preventingpeptide elongation

Chloramphenicol &clindamycin bind 50s &prevent peptide bondformation

Oxazolidinones bind50s & preventformation ofinitiation complex

Streptogramins bind 50s,prevent peptide elongationand premature releasefrom ribosome

Agents Interfering with CellMembranes

• Polymyxins• Daptomycin

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Daptomycin Mechanism of Action

Agents Interfering with DNAReplication

• Rifampin – binds DNA-dependent RNA polymerase &prevents mRNA transcription

• Metronidazole – intermediatemetabolites damage DNA inanaerobes

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Antimetabolites• Sulfonamide

– inhibitsdihydropteroatesynthase

– disrupts folic acidsynthesis

• Trimethoprim– inhibits

dihydrofolatereductase

– synergy withsulfonamide

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“Chemotherapy withoutbacteriology is guesswork”

Terminology

• MIC – minimum inhibitor concentration– the lowest concentration of antimicrobial

that inhibits visible bacterial growth• MBC – minimum bactericidal concentration

– the lowest concentration of antimicrobialthat kills 99.9% of an inoculum

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Susceptibility Testing Methods:Susceptibility Testing Methods:Agar Disk DiffusionAgar Disk Diffusion

• Rapidly growingbacteria

• Qualitative• Inverse relationship

between zone size &MIC

• CLSI sets criteriaused by labs tointerpret

Agar GradientAgar GradientDiffusion (Etest)Diffusion (Etest)

• Quantitative (MIC)• Use for many types

of organisms• MIC read where

ellipse of inhibitionintersects plasticstrip containinggradient of drug

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Automated Bacterial Identification andSusceptibility Testing Systems: MicroScan

WalkAway 96

Microbroth DilutionSusceptibility Testing

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Agar Dilution• Reference method• Too time consuming and

expensive for routine useas primary method

• Excellent for screeningsingle drugs– MRSA (oxacillin screen

agar)– Vancomycin-resistant

enterococcus agar

Lab Report Terminology

• Susceptible - appropriate Rx with recommendeddosage

• Intermediate - MIC approaches blood/tissue levelfor which response may be less than for susceptibleisolates. May Rx for infection in sites where drug isconcentrated or if high dose is used

• Resistant - organism not inhibited by achievablesystemic concentration with normal dosage and/orralls in range where clinical efficacy unreliable

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Desirable Properties of Antibiotics

• Selective toxicity• Water soluble• Bactericidal• High serum levels achieved for several hours• Broad spectrum• Minimal effect on normal flora• Low potential for inducing resistance• Minimal side effects/toxicity

Clinical Pharmacology ofAntibiotics

• Serum/tissue concentrations• Route of elimination• Half life• Oral vs. parenteral• Duration of treatment• Bateriostatic vs. bactericidal• Kinetics of killing

• Concentration vs. time dependent

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Indications for Cidal Drugs

• Life threatening conditions• Endocarditis• Meningitis

• Immunosuppressed host

Combination Therapy

• Broadspectrum

• Polymicrobialinfections

• Preventresistance

• Synergy

• Risk of toxicity• Superinfection• Cost• Antagonism

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Antimicrobial Prophylaxis

• Prevent endocarditis following dentalwork

• Contacts of meningococcal meningitis• Opportunistic infections in AIDS• Recurrent UTIs• Prevent post-surgical infections

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“ Antibiotic resistance has been a fact of life since the dawn ofthe antibiotic era in the mid 20th century. Clinical deploymentof every new antimicrobial agent has been greeted with thedevelopment of resistance in one or more bacterial species.”

Selective Pressure“Environmental conditions enhance ability ofbacteria to develop resistance and proliferate.Ability to survive may be the result ofspontaneous mutation or acquisition of new DNA.Organisms with new mutations or genes probablywould not survive if it were not for environmentalconditions that encouraged their emergence.”

FC TenoverHosp Pract, Feb, 1999

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Tertiary Care Hospitals

Potential Routes for Spread ofDrug-Resistant Bacteria

Antibiotics in Animal Food

Day Care Centers

Community Outpatients

CommunityHospitals

Nursing Homes

3rd World Countries

Other Factors That May IncreaseAntimicrobial Resistance in Hospitals

• Greater severity of illness• Immunocompromised patients• New devices and procedures• Ineffective infection control practices• Increased use of antimicrobial prophylaxis• Empiric polymicrobial antimicrobial therapy

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Organism CharacteristicsFavoring Resistance

• Intrinsic resistance to some drugs• Ability to exchange genetic

information• Ability to survive adverse

environmental conditions• Easily colonize, infect, and transmit• Reservoirs in body

Impact of AntimicrobialResistance

• Prolonged illness/hospitalization• Increased mortality• Inappropriate therapy• More expensive/toxic therapy• More lab tests• Spread of infectious organisms that may

have no effective treatment and that may beimpossible to eradicate from hospital

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How Does Antibiotic ResistanceAffect the Biology of the Bacteria?

• It does not typically make the organismmore virulent in its ability to cause diseaseor produce pathologic lesions in the host

• Slight resistance may still be overcome withhigher doses of antibiotic, especially if thedrug is concentrated in the site of infection

• Highly resistant strains will not respond andwill require alternative treatments

Types of AntimicrobialResistance

• Innate or Primary• Acquired

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How Bacteria Pick Up Resistance GenesHow Bacteria Pick Up Resistance Genes

Clonal spread of S. pneumoniae Spain23F

USA

Mexico

ColombiaBrazil

ArgentinaUruguayChile

SouthAfrica

SingaporeMalaysia

ThailandPhilippines

Hong KongTaiwan

South KoreaSpain

FranceBM42001978 ?

Finland

Several well-characterized PRSP clones have spread worldwide, driven by selectiveantibiotic pressureUp to 85% of PRSP in U.S. belong to one of 9-10 clonal groups, often MDR

Corso, Microb Drug Res, 1998

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Bacterial Strain Typing PFGE

100%

95%

Figure 2. Pulse field gel electrophoresis (PFGE) patterns and dendrogram of a single clone of VRE isolated from 14 UAB patients at UAB Hospital. Five isolates were from 1997; 8 isolates were from 1998; and 1 isolate was from 1999. Patterns were gene rated using PFGE on Sma1 digested genomic DNA and the dendrogram produced using Pearson correlation in GelComparII (Applied Maths, Kortrijk, Belgium).

PFGE patterns and dendrogram of a single clone of vancomycin-resistant E. faecium isolated from14 patients at UAB. 5 isolates were from 1997; 8 isolates were from 1998; and 1 isolate was from1999 indicating persistence of infecting organisms in the nosocomial environment for many months.

Mechanisms of Antimicrobial ResistanceMechanisms of Antimicrobial ResistanceActive Efflux ErythromycinErythromycin

Bacterial Defense

Mechanism

EnzymeInactivation

PenicillinPenicillin

Altered TargetFluoroquinolonesFluoroquinolones

Antimicrobial Antimicrobial agentagent

BypassTrimethTrimeth/sulfa/sulfa

Decreased permeability

Aminoglycoside

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Susceptible Resistant

Penicillin-binding proteinsβ-Lactamases

β-Lactams: Mechanisms of Resistance

β

β β β

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Susceptible Resistant

Ribosome

Drug efflux

M M

M

Esterases

M

M

M

MBacterium

M

Macrolides: Mechanisms of Resistance

Sulfonamide Resistance:Metabolic Bypass

•Sulfonam

ides inhibitfolic acid synthesisby binding todihydropteroatesynthase, a criticalstep in D

NA

synthesis

•R

esistance occursw

hen bacterialstrains developalternative enzym

esystem

s tosynthesize folic acid

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Methicillin resistant Methicillin resistant Staphylococcus aureus Staphylococcus aureus (MRSA)(MRSA)

•• Staphylococcus aureusStaphylococcus aureus–– skin and wound infectionsskin and wound infections

–– bacteremia bacteremia•• > 50% of > 50% of S. aureus S. aureus are MRSA in many hospitals)are MRSA in many hospitals)•• Carrier spread within and between institutions byCarrier spread within and between institutions by

healthcare workershealthcare workers•• Most MRSA are resistant to almost all drug classesMost MRSA are resistant to almost all drug classes

except vancomycinexcept vancomycin

•• Community-acquired MRSA becoming more commonCommunity-acquired MRSA becoming more common

Problematic resistance in nosocomial infections: Problematic resistance in nosocomial infections:

MRSA: An Escalating ProblemUAB Hospital (1996-2006)

0

10

20

30

40

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1996

1997

1998

1999

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2001

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2004

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% R

esis

tan

t Is

ola

tes

MRSA attack rate: 5/1000 admissions

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ESBLs: Plasmid-Mediated Resistance

Transposons:(mobile genetic elements)

Transferable Plasmid carries genetic information,

including codes for various resistance

factors

Controlling Drug Resistance

• Education• Healthcare providers and consumers• Limit unnecessary antibiotics• Proper dose and duration of therapy• Limit prophylaxis• Target pathogens• CDC guidelines