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Key Words
Sterilization/disinfection/antisepsis
Bacitracin
Antibiotic
Vancomycin
Selective toxicity
beta lactam
Bactericidal
Penicillins
Bacteriostatic
Cephalosporins
Minimal inhibitory concentration (MIC) Monobactam
Susceptibility testing
Clavulinic acid
Penicillin binding proteins
Penicillinase/beta lactamase
Autolysins
Polymyxin B
Cycloserine
Resistance
STERILIZATION
• All killed
• non-selective
Sterilization
• autoclaving
– 121oC (heat/pressure)
* Heat resistant materials
• ethylene oxide
– Non heat resistant
– usually equipment
• ultra-violet light
– surfaces (e.g operating rooms)
– not totally effective
Membrane filters
pores
bacteria
Disinfection
• Liquids that kill bacteria
– e.g. phenol based
– too toxic for skin surfaces
Antiseptics
• Topical (e.g. skin)
– e.g. iodine or 70% alcohol
– “reduce” bacterial load
ANTIBIOTICS
• Selectively toxic for bacteria
– bactericidal (killing)
– bacteriostatic (growth inhibition)
• no harm to patient
Antibiotics
• destroy structures
– present in bacteria
– not present in host
Antibiotics work together
with immune system
Minimal inhibitory
concentration
• lowest level stopping growth
• e. g. zone of inhibition around a disk
impregnated with antibiotic
• Antibiotics that inhibit cell wall
biosynthesis are bactericidal
• Without cell wall, osmotic pressure
causes bacteria to burst
Peptidoglycan synthesis
Cytoplasm
Cell Membrane
undecaprenol
sugar
amino
acid
Cell wall
Cycloserine
1. alanine (ala) analog
2. inhibits conversion of L-ala to D-ala
3. inhibits formation of D-ala-D-ala
Cycloserine
Analog of alanine
Cytoplasm
X X
sugar
amino
acid
X
X
Bacitracin
• Inhibits dephosphorylation
Bacitracin
Cell membrane
undecaprenol
P
P
Vancomycin
• binds to D-ala-D-ala
• inhibits cross-linking
Vancomycin
Cell wall
Beta lactam antibiotics
• penicillins
• cephalosporins
• monobactams
• inhibit penicillin binding proteins
• stop cross-linking
Beta lactams
Cell wall
Penicillin binding protein
Cross-linking of peptidoglycan
STRUCTURE OF PENICILLIN
O
S
C
NH
CH
O
C
CH
N
CH3
C
CH
CH3
COOH
Site of penicillinase action.
Breakage of the  lactam ring.
Attached to lactam ring
• penicillins
– 5 membered ring
• cephalosporins
– 6 membered ring
• monobactams
– no second ring
Chemical modifications change
biological activity
• Early lactam antibiotics
– inactive against Gram negative
bacteria
• no penetration of outer membrane
Resistance mechansims
• Produce beta lactamase (penicillinase)
– destroys antibiotic
• modified penicillin binding proteins
– don’t bind antibiotic
• modified porins
– no internalization of antibiotic
Clavulinic acid
• beta lactam
• binds strongly beta lactamases
– inhibits activity
Polymyxin B
• binds
– lipid A
– phospholipids
• disrupts outer membrane, Gram negative bacteria
• toxic to human cells