Dr. Alvin Fox

1
Antibiotics affecting the cell envelope
Dr. Alvin Fox
2
Key Words
Sterilization/disinfection/antisepsis
Vancomycin Antibiotic beta
lactam Selective toxicity
Penicillins Bactericidal

Cephalosporins/cephamycins Bacteriostatic

Monobactam Minimal inhibitory concentration (MIC)
Clavulinic acid Susceptibility testing
Polymyxin B Penicillin
binding proteins
Isoniazid Penicillinase/beta lactamase
Ethioniamide Autolysins

Ethambutol Cycloserine
Resistance Bacitracin
3
STERILIZATION

• All killed
• non-selective

4
Sterilization

• autoclaving
• 121oC (heat/pressure)
• Heat resistant materials
• ethylene oxide
• non heat resistant
• Usually equipment
• being replaced e.g. H2O2/free radicals
• ultra-violet light
• surfaces (e.g. operating rooms)
• not totally effective
• gamma radiation
• mainly food industry

5
Membrane filters
pores
bacteria
6
Disinfection

• Liquids that kill bacteria
• e.g. phenol based
• too toxic for skin surfaces

7
Antiseptics

• Topical (e.g. skin)
• e.g. iodine or 70 alcohol
• reduce bacterial load

8
ANTIBIOTICS

• Selectively toxic for bacteria
• bactericidal (killing)
• bacteriostatic (growth inhibition)
• minimize harm to patient

9
Antibiotics

• destroy structures
• present in bacteria
• not present in host

10

Antibiotics work together with immune system
11
Minimal inhibitory concentration

• lowest level stopping growth
• e. g. zone of inhibition around a disk
  impregnated with antibiotic

12

• Antibiotics that inhibit cell wall
• biosynthesis are bactericidal
• Without cell wall, osmotic pressure
• causes bacteria to burst

13
Cross-linking of peptidoglycan
Old
New
14
Peptidoglycan synthesis
Cell Membrane
Cytoplasm
Cell wall
undecaprenol
sugar
amino acid
15
Cycloserine
alanine (ala) analog - inhibits conversion
L-ala to D-ala - inhibits formation of
D-ala-D-ala
Cytoplasm
X
X
sugar
amino acid
X
X
16
Bacitracin

• Inhibits dephosphorylation

17
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE
Cell membrane
Cell wall
undecaprenol
P
P
18
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE
Cell wall
Cell membrane
undecaprenol
P
19
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE
Cell wall
Cell membrane
undecaprenol
P P
20
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE
Cell wall
Cell membrane
undecaprenol
P P
21
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE
Cell membrane
Cell wall
undecaprenol
P
P
22
MINUS BACITRACIN
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS
MEMBRANE
Cell wall
Cell membrane
undecaprenol
P
23
PLUS Bacitracin
TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS
MEMBRANE
Cell wall
Cell membrane
undecaprenol
P P
24
Vancomycin
Cell wall
Vancomycin

• binds to D-ala-D-ala
• inhibits cross-linking

25
Beta lactam antibiotics

• penicillins
• Cephalosporins/cephamycins
• monobactams

26
ß lactams
Cell wall

• inhibit penicillin binding proteins
• stop cross-linking

ß lactam
Active site of enzyme
Penicillin binding protein
27
STRUCTURE OF PENICILLIN
Site of penicillinase action. Breakage of the ß
lactam ring.
28
Attached to lactam ring

• penicillins
• 5 membered ring
• Cephalosporins/cephamycins
• 6 membered ring
• monobactams
• no second ring

29
Chemical modifications change biological activity

• Early lactam antibiotics
• inactive against Gram negative bacteria
• no penetration of outer membrane

30
Resistance mechansims

• Produce ß lactamase (penicillinase)
• destroys antibiotic
• modified penicillin binding proteins
• dont bind antibiotic
• modified porins
• no internalization of antibiotic

31
Clavulinic acid

• ß lactam
• binds strongly to ß lactamases
• inhibits activity
• limited potency as antibiotic
• used in conjunction with other ß lactams

32
Polymyxin B

• binds
• lipid A
• phospholipids
• disrupts outer membrane, Gram negative bacteria
• toxic to human cells

33
Isoniazid, Ethambutol, Ethionamide  
Anti-tuberculosis drugs
Isoniazid and Ethionamide - chemically related -
blocks mycolic acid synthesis Ethambutol - blocks
arabinogalactan synthesis