Antibacterial susceptibility testing

Drug classes

Methods for testing

Laboratory strategies

Basic principles of antimicrobial action

1. Agent is in active form- pharmacodynamics: structure & route

2. Achieve sufficient levels at site of infection- pharmacokinetics

Serum CSF Urine

Ampicillin + + +Ceftriaxone + + +Vancomycin + ± +Ciprofloxacin + ± +Gentamicin + - +Clindamycin + - -Norfloxacin - - +Nitrofurantoin - - +

Anatomic distribution

Basic principles of antimicrobial action

3. Adsorption of drug by organism

4. Intracellular uptake

5. Target binding

6. Growth inhibition (bacteriostatic) or death (bactericidal)

- Resistance can develop at any point

Mechanisms of action

Beta-lactams

Penicillins, cephalosporins, carbapenemsInhibit cell wall synthesis by binding PBPsActive against many Gram + and Gram – (varies)

Aminoglycosides

Gentamicin, tobramycin, amikacin, streptomycinInhibit protein synthesis (30S ribosomal subunit)Gram + and Gram – but not anaerobes

http://www.life.umd.edu/classroom/bsci424/Definitions.htm

Beta-lactams

http://gsbs.utmb.edu/microbook/ch011.htm

Aminoglycosides

Mechanisms of action

Fluoroquinolones

Ciprofloxacin, levofloxacinInhibit DNA synthesis by binding to gyrasesActive against many Gram + and Gram – (varies)

Glycopeptides

VancomycinInhibit cell wall synthesis by binding precursorsGram + only

http://gsbs.utmb.edu/microbook/ch011.htm

Quinolones

Glycopeptide

Mechanisms of action

Macrolides-lincosamides

Erythromycin, azithromycin, clindamycinInhibit protein synthesis (50S ribosomal subunit)Most Gram + and some Gram –

Tetracyclines

Tetracycline, doxycyclineInhibit protein synthesis (30S ribosomal subunit)Gram + and Gram – and intracellular orgs.

http://gsbs.utmb.edu/microbook/ch011.htm

Macrolides

Tetracycline

Mechanisms of action

Oxazolidinones

LinezolidInhibit protein synthesis (50S ribosomal subunit)Gram + and Gram – including multi-resistant

Streptogramins

Quinupristin/dalfopristin (Synercid)Inhibit protein sythesis (50S ribosomal subunit)Primarily Gram + organisms

Linezolid

http://www.kcom.edu/faculty/chamberlain/Website/Lects/Metabo.htm

Streptogramins

Mechanisms of action

Trimethoprim

Sulfonamides

Usually combined (Trimeth/sulfa)Inhibit different parts of folic acid pathway

affects DNA synthesisGram + and many Gram –

http://gsbs.utmb.edu/microbook/ch011.htm

Mechanisms of resistance

Biologic

- physiologic changes resulting in a decreasein susceptibility

Clinical

- physiologic changes have progressed to a pointwhere drug is no longer clinically useful

Mechanisms of resistance

Environmentally-mediated

Physical or chemical characteristics that alter theagent or the organism’s physiologic response tothe drug

pHanaerobiasiscationsmetabolites

Mechanisms of resistance

Microorganism-mediated

Intrinsic predictable

Gram neg vs. vancomycin (uptake)

Klebsiella vs. ampicillin (AmpC)

Aerobes vs. metronidazole (anaerobic activation)

Mechanisms of resistance

Microorganism-mediated

Acquired unpredictable- this is why we test- mutations, gene transfer, or combination

Mechanisms of resistance

These factors are taken into account to attemptto standardize in vitro testing methods.

In vitro methods are not designed to recreatein vivo physiology.

In vivo physiology affects clinical response suchthat in vitro testing cannot be used to predictclinical outcome.

Mechanisms of resistance

Common pathways

1. Enzymatic degradation or modification of agent

2. Decreased uptake or accumulation of agent

3. Altered target

4. Circumvention of consequences of agent

5. Uncoupling of agent-target interactions

6. Any combination of above

Emergence of resistance

Mixing of bacterialgene pool

Selective pressure fromexcessive antimicrobial

use and abuse

Survival of the fittest

Emergence of resistance

1. Emergence of new genes- MRSA, VRE, GISA

2. Spread of old genes to new hosts- Pen resistant GC , GRSA

3. Mutations of old genes resulting in more potent resistance- ESBLs

4. Emergence of intrinsically resistant opportunistic bacteria- Stenatrophomonas