Mechanisms of antimicrobial action and

resistance; Beyond the cell wall

Mechanisms of antimicrobial action and

resistance; Beyond the cell wall

Cary Engleberg, M.D.Division of Infectious Diseases,University of Michigan Medical School

Mechanisms of antimicrobial resistanceMechanisms of antimicrobial resistance

Drug-modifying enzymes(e.g., - lactamases)

Altered drug targets (e.g., PBPs ribosomes, DNA gyrase)

Altered uptake oraccumulation of drug(e.g., altered porins, efflux)

Drug-modifying enzymes

• Aminoglycosides (enzyme add inactivating group the drug)–acetyltransferases

–phosphoryltransferases

–nucleotidyltransferases

• Fluoroquinolones–Aminoglycoside acetyltransferase has been modified

to acetylate ciprofloxacinShown on the next slide . . .

Modification and inactivation of ciprofloxacin

Ciprofloxacin (active) Acetyl-ciprofloxacin (inactive)

Altered drug targets

• Ribosomes

–macrolides and clindamycin

–streptomycin

–linezolid

• DNA modifying enzymes

–Fluoroquinolones• DNA gyrase (most important in Gram-negatives)

• Topoisomerase II (most important in Gram-positives)

Two mechanisms of macrolide resistance

• Enhanced efflux of the antibiotic from the cell by an active membrane pump

• Acquisition of an enzyme that methylates the 23S rRNA and blocks drug binding–depicted in the next slide . . .

E

23S rRNA

ribosome

mRNA

tRNAgrowing polypeptide

Erythromycin binds to the 23S rRNA subunit and blocks protein synthesis

In resistant strains, methylation of 23S rRNA binding site by an acquired enzyme occurs

Erythromycin cannot bind; protein synthesis proceeds

E

Inhibition of protein synthesis

Initiation Factors

30S mRNA50S

70SInitiation Complex

30S &mRNA

fMet-tRNALinezolid

Elongation FactorsElongation

Cycle

Termination

Peptide Product

ClindamycinMacrolidesTetracyclinesAminoglycosides

Resistance to linezolid

• Originally thought to be unlikely given chemical origin and mode of action of the drug

• 2 of 169 patients treated on a compassionate use protocol developed resistant strains–rRNA mutation (G2576U)

–Both involved prolonged indwelling lines

• Induction of resistance in the laboratory by prolonged passage:– in staphylococci, frequency is 10-9 - 10-11

–among enterococci, E. faecalis is more likely to develop resistance (G2576U) than E. faecium

DNA damage byfluoroquinolone

FQ

Normal DNA gyrase function

Fluoroquinolone

Fluoroquinolones use DNA gyrase as a target

in Gram-negative bacteria

Efflux pumps

Resistance by reducing the cytoplasmic concentration of antibiotics

• Efflux pump systems exist in the cell membrane and transport small molecules out of the cytoplasm

• The systems can be acquired genetically or undergo mutation to more efficiently eliminate a particular antibiotic

• Commonly affected antibiotics–Tetracyclines (except tigecycline)

–Macrolides (except telithromycin)

–Fluoroquinolones