Today’s talk

• Dimensions of AB resistance

• History of AB resistance

• Biology of AB resistance– Biology of antibiotic action and measurement– Genetics– Biochemistry– Selection

• Some guiding questions and concepts

Dimensions of Antibiotic Resistance

• Bacterial species– Type of transmission: hospital or community

• Antibiotic• Genetic mechanism of resistance: how does the

bug get the genes to be resistant?• Biochemical mechanism of resistance: what

does the bug do to be resistant?• Mechanisms of selection for resistance

– Individuals– Populations

• Study design to assess these mechanisms

History of Antimicrobial Resistance

Early principles: Paul Ehrlich

• The therapia sterilisans magna consists in this, that by means of one or at most two injections the body is freed from the parasites…. Here, therefore, the old therapeutic remedy is applicable …. frapper fort et frapper vite

• A further advantage of combined therapy is, that under the influence of two different medicines the danger of rendering the parasites immune to arsenic, which naturally would be a very great obstacle in connexion with further treatment, is apparently greatly minimized.

GC, H. flu

ADD DHFR

ADENYLATE

E.coliADD ß-LACTAMASE

METHYLATE RIBOSOME

ACETYLATE

EFFLUX

PHOSPORYLATE

S. pneumoADD PBPS. aureusADD ß- LACTAMASE

All Gram negsADD DHTS

1930 1940 1950 1960 1970 1980 1990 2000

GC Mening

VRE

QM

RSA

Pseud

Kleb

E.coli

ESBLs

AmpCs

? VMRSA

Sulfa

Penicillin

Strept

Tetra

Chlora

Erythro

Oxa/amp/ceph

Vanco

Genta

Trimethoprim

3rd gen cephalosporins

Fluroquinolones

Carbapenems

MRSAADD PBP

MUTATE

Resistance comes fast

Antibiotic Discovery Clinical Use

Resistance identified

Penicillin 1940 1943 1940

Streptomycin 1944 1947 1947,1956

Tetracycline 1948 1952 1956

Erythromycin 1952 1955 1956

Vancomycin 1956 1972 1987

Gentamicin 1963 1967 1970

J. Davies 1997

P. aeruginosa resistant to imipenem

K. pneumoniae non-susceptible to3rd cephalosporins

ICU PatientsNon-ICU Patients

P. aeruginosa resistant to quinolones

Source: NNIS DATA Clinics Chest Med.20:303-315

0

2

4

6

8

10

12

14

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year

Per

cent

Res

ista

nce

0

5

10

15

20

25

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

Year

Per

cent

Res

ista

nce

0

5

10

15

20

25

30

35

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

Year

Per

cent

Res

ista

nce

Resistance increases quickly

Methicillin-resistantS. aureus

Methicillin-resistantCoagulase-negative Staph

Vancomycin-Resistantenterococci

ICU PatientsNon-ICU Patients

Source: NNIS DATA Clinics Chest Med.20:303-315

0

10

20

30

40

50

60

1989 1991 1993 1995 1997 1999 2001

Year

Per

cent

Res

ista

nce

0102030405060708090

100

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

Year

Per

cent

Res

ista

nce

0

5

10

15

20

25

30

35

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

Year

Per

cent

Res

ista

nce

Resistance goes up quickly (2)

Still growing: Resistance in Streptococcus pneumoniae in US

G Doern et al.,

Clin Inf Dis. 2005

Geographic variation in antibiotic resistance

H. Goossens et al. 2005 Lancet

Geographic Variation Resistancein 1999

0

0.1

0.2

0.3

0.4

0.5

CA CT GA MD MN NY OR TN

Pro

port

ion

Pen

icill

in

Res

ista

nt

State

The Tragedy of The Commons

Resistance goes down slowly, if at all

V Enne et al., Lancet 2001

39% R 45% R

Resistance goes down slowly, if at all

Hennessy et al. 2002, CID

~30% decline in prescribing after initial intervention~25% after expanded

But in hospitals, changes can move faster

0%

5%

10%

15%

20%

25%

30%

35%

Jun-79

Jul-79 Aug-79

Sep-79

Oct-79

Nov-79

Dec-79

% in

fect

ed w

ith

gen

tam

icin

-re

sist

ant

MR

SA

Dunkle et al. Amer J Med 1981

Biology of Antimicrobial Resistance

How do antibiotics kill?

• Static v. cidal– Bacteriostatic: prevent cell division (e.g. by

preventing protein synthesis) – Bactericidal: kill bacteria directly

• Cidal drugs often kill only dividing bugs– Cell wall synthesis inhibitors– Others

• This makes treatment of latent infection especially difficult (TB)

Antibiotics and the immune system

• Immune responses required to kill alongside bactriostatic drugs

• Also for many bactericidal drugs: phenotypic resistance

Wiuff et al. AAC 2005

“Normal” FloraS. aureus

GNR(E. coli, KlebsiellaEnterobacter)

Enterococcusfaecium

S. pneumoniaeH. influenzaeN. meningitidis

Normal flora: Consequences

• Treatment exerts selection on “innocent bystanders”

• Most of the harm done by use of a drug may be on species OTHER than the target of treatment– Optimal dosing for treatment ≠ optimal to prevent

resistance

• Most of the exposure of a given species to a given drug may be due to treatment of OTHER infections

Measuring resistance: Minimal Inhibitory Concentration (MIC)

• Broth microdilution • Etest

MIC is a simplification

Regoes et al. AAC 2004

Limitations of MIC

• Subpopulations• Depends on in vitro

conditions: pH, etc – not necessarily same as in vivo

• One parameter summary of the curve

• Ignores physiologic variation Regoes et al. AAC 2004

Biological Aspects of Resistance

• Genetics: how is drug resistance coded? – And how can it move from one bug to another?– Think of: floppy disk, memory stick, punch card

• Biochemistry/mechanism: what does a bug do to become resistant?– Think of: iTunes, RealPlayer, Microsoft Media Player

• How resistance is selected: how do we increase the frequency of resistant bugs?– Block that metaphor!

Intrinsic resistance

• All members of a species are resistant, and have been since before clinical use– Tuberculosis and penicillin: naturally encodes

beta-lactamase– Vancomycin-producing species and

vancomycin: alters its cell wall to be insensitive (same as resistance in targets!)

– Don’t activate prodrug• Isoniazid or pyrazinamide and non-mycobacteria:

not chemically altered to become active

Transformation

Plasmidtransfer

Genetic Mechanisms of Resistance Acquisition

Mutation

Implications

• Mutation: easy to get a resistant strain in almost any patient: mutation frequencies ~10-7 – 10-10

– Unless multiple mutations are required to confer resistance!

– Examples: Tuberculosis, HIV• Other mechanisms

– Very complex mechanisms of resistance can evolve, because they can move as a block from one bug to the next

– Can even transfer from one species to another– Emergence of a resistant bug in a single host is

unlikely, unless a mix of resistant bugs and sensitive bugs is present

Courtesy Tom O’Brien, BWH

Epidemic Plasmids

Integrons

• System for combining resistance (and other) genes

• Can take up new genes via integrase and add them to the “package”

• Often on transposons or plasmids

www.mmb.usyd.edu.au/coleman/

• Reduced permeability• Efflux• Degradation• Detoxification• Target alteration: enzyme• Target alteration:

mutation• Target amplification• Inactivate the activator of

the prodrug

Biochemical mechanisms

Mechanisms: their consequences

• Most mechanisms are quite specific to one drug or class of drugs– Enzymes to alter drug or target– Target changes or amplification

• A few mechanisms confer resistance to more than one class of drug– Efflux pump: “MDR transporter”

Mechanisms

• High-level resistance: completely resistant to any achievable concentration

• Partial resistance: small change in MIC

Mechanisms of selection

How Antimicrobial Use Increases Resistance: Mechanisms

Rx

• “Acquired” Resistance: selection within host– Patient infected with a susceptible

organism– Treatment selects a resistant variant

Emerg Inf Dis 2002 8:347

How Antimicrobial Use Increases Resistance: Mechanisms

• “Primary” Resistance: Selection in host population– Patient infected with a resistant organism– Competitive mechanism: Treatment selects

by reducing transmission of susceptible infections Rx

How Antimicrobial Use Increases Resistance: Mechanisms

• Increasing susceptibility to colonization– Patient carries a “normal flora”– Treatment increases susceptibility to

colonization by opening ecologic niche

Rx

Summary of Mechanisms of Selection of Resistance by Abx

Rx

Rx

Rx

Acquired resistance during

Tx

Primary resistance

(Infected with Res strain)

Colonization susceptibility

Examples TB, HIV, Pseudomonas

All? Most Commensals: pneumococcus,

nosocomials

Relation between selection and treatment success - + ? Relation between selection and dose/number of abx - +? +?

Is antibiotic use harmful to

individuals?

Co-selection

• Dental fillings installed and removed from experimental monkeys

Summers 1993 AAC

Concepts and questions

• Antibiotic resistance is interesting: how bad is it?

• How can we measure the costs? Compared to what?

What can we do to/for an individual patient to prevent resistant

infections?

Why have some kinds of drug resistance increased fast, others

slowly or not at all?

What can we do to the population as a whole to reduce the risk of

reistant infections?

In what circumstances does doing what is best for the patient

• Increase the burden of resistance in the community?

• Reduce the burden of resistance in the community?

• Both but on different time scales?

What would happen if we stopped using antibiotics tomorrow?

• To disease?

• To resistance?