Action of and Resistance to drugs and toxic metals

by

E. Börje Lindström

This learning object has been funded by the European Commissions FP6 BioMinE project

Definitions• Chemoterapi: - Use of chemical substances against parasites in the host

• Antibioticum: Substance that is produced by a micro-organism and that:

- inhibits growth of a micro-organism (-static) or

- kills the micro-organism (-cid)

Producers of antibiotics• Actinomycetes - Streptomyces

• Bacillus - Bacillus

• Saprophytic fungi - Penicillium, Cephalosporium

Targets for some antibiotics

Group Where Target Drug

I

Outside CM

On CM

Cell wall synth- Penicillin

- Bacitracin

II Permeability

(Osmos)

- Nystatin

- Polymyxin

III Inside CM

• DNA repication

• RNA synthesis

• Protein synthesis

• Co-factor synthesis

- Nalidixic acid

- Rifampicin

- Streptomycin

- Sulfa

Penicillins (b-lactams)

CH2CO

CH CO

NH2

Pen G

Amp

R:

Penicillins (b-lactams), contNAM – NAG – NAM – NAG – NAM – NAG

L-ala

D-glu

L-lys

D-ala

D-ala

NAM – NAG – NAM – NAG – NAM – NAG

(D-ala)

D-ala

L-lys

D-glu

L-ala

Penicillins – block the synthesis

• Active only on growing cells

• Lysis of the cell

• active against both G+ and G-

• broad spectrum

• bactericidal

• Action

Penicillins (b-lactams), cont• Side effects on our cells?

• Allergy?

Penicellenic acid

protein

Penicilloyl – protein

antigen

Streptomycin

• active against both G+ and G-

• broad spectrum

• bactericidal

Streptomycin, cont.

• Targets (translation): - initiation complex

- binding to 30S subunit

RpsL-protein

• Results: - misstranslation

- faulty proteins

Streptomycin, cont.• Used clinically? -selldom

- against TBC

• Side effects: -dizziness (balance difficulties)

- lowering the hearing

Note! The 80S ribosome is not effected!

Sulfa drug• Sulfa drugs – not antibiotics – produced chemically

• Growth factor analog

Sulfanilamide PABA

Folic acid

(vitamin)

CoF

Sulfa drug, cont.• Acts as a competetive inhibitor in synthesis of Folic acid

• CoF participates in several biosynthetic reactions – aa, purins etc.

Type of resistance

• no receptors are available

1. Natural, artspecific resistance

• inactivating enzymes present

- Mycoplasma

- penicillinase

2. Acquired resistance - sensitive m.o resistant m.o.

Genetic processes: • mutation

• transformation

• transduction

• conjugation

Type of resistance, cont.Biochemical mechanisms for acquired resistance:

• permeability changes of OM or CM - penG, tetracyclin, actinomycin D

• alternative biosynthesis or

increased production

- sulfa

• changed receptor - streptomycin

• enzym production - penicillinase

Properties of a good antibioticum

• Broad spectrum

• Prevent resistant mutants to arise

• Have no side effects on the human cell

• Leave the flora of our body intact

Effect on a growing cultur

t

log OD/VC + drug

OD

VC

log OD/VC + drug

t

OD

VC

log OD/VC + drug

t

OD

VC

Effect: - static - cid - lytic

Combined usage of antibiotics• Antagonism -drugs acting against each other

- (-cid) + (-static)

- e.g. Penicillin & kloramphenicol/ sulpha

• Synergism - drugs enhancing their effect

- (-cid) + (-cid)

- e.g. penicillin + streptomycin

Mercuric resistance• Action: -Bind to SH- groups

- inhibits synthesis of macro molecules

- most sensitiva are transcription and translation

• Resistance: -usually plasid mediated

- both in G+ and G-; S.aureus, Pseudomonads, At. thioxidans

- enzymatic reduction; Hg2+ Hg0

- Hg0 less toxic

- in organic mercury , C-Hg, Hg is first removed with the enzyme lyas.

Mercuric resistance, cont.

Arsenic resistance• Action: -AsO4

3- ions are transported into the cell via

- phosphate-transport system

- analog to PO43- ions

- inhibits different kinases

• Resistance:

- plasmid mediated

- AsO43- is reduced to AsO2

-

- AsO2- is effluxed (transported to the outside)

Arsenic resistance, cont.• Genetic:

E. coli R773

(plasmid)

arsR arsA arsC arsB arsD

arsR arsB arsC Chromosome

(At. caldus)

• reductase• negative regulator