Antibacterials

What was the first bacteria observed that was resistant to every antibiotic tried?

case of Klebsiella pneumoniae in 2013

Why has research into new antibiotics dropped off ()?

cost of discovery vs profits

When did Alexander Fleming observed antibiosis against bacteria by Penicillium fungus?

1928

When were the first clinical trials using purified penicillin?

1941

What is an antibacterial?

a substance that kills bacteria or inhibits their growth

What is an antibiotic?

substances produced by living organisms capable of destroying or inhibiting the growth of microorganisms

What is an antimicrobial?

substance that kills microorganisms or inhibits their growth - included antivirals, antifungals, antiparasititcs and antibacterials

Where do antibiotics come from and why (3)?

• the environment (produced by fungi and bacteria)

• secreted to kill / slow competitors for resources

• synthetically or semi-synthetically produced

Where does resistance come from (2)?

• biological phenomenon

• the organism that produces the antibiotic must also be resistant to it (don’t want to commit bacterial suicide)

What do bactericidal antibiotics do (2)?

• kill bacteria

• BUT in low concentrations bacteriostatic

What do bacteriostatic antibiotics do (2)?

• slow bacterial growth or reproduction

• BUT in high concentrations / in combination can be bactericidal

What type of antibiotic is better - bactericidal or bacteriostatic?

trick question - similar outcomes from both

What is the target of beta-lactams class antibiotics?

cell wall (peptidoglycan layer)

What is the ideal antibiotic target?

targets that are not present in humans - reduced toxicity and cross-reactivity

What are some different antibiotic targets (3)?

• cell wall

• cell membrane

• folic acid metabolism

• protein synthesis (can target ribosomes)

Why does antibiotic choice matter when choosing an antibiotic for a gram negative bacteria vs a gram positive (2)?

• gram negative has multiple cell wall layers

• beta lactams target peptidoglycan layer

• larger antibiotics (e.g. vancomycin) cannot penetrate layers well

What are some features of beta-lactams antibiotics (cell wall inhibitors) (5)?

• first antibiotics

• generally bactericidal

• highly selective (no mammalian cell wall)

• disrupt maintenance of osmotic pressure (cell bursts)

• generally more effective against gram positive

What type of bacteria are cell wall inhibitors (e.g. beta lactams) generally most effective against?

gram positive

Which different antibiotics act as cell wall inhibitors (5)?

• beta-lactams

• vancomycin and teicoplanin

• fosfomycin

• cycloserine

• bacitracin

What are the 2 key components of the peptidoglycan layer?

• NAG (N-acetyl glucosamine)

• NAM (N-acetyl muramic acid)

How is the bacterial cell wall peptidoglycan layer formed (2)?

• NAG and NAM cross bridging

• side bridging of amino acid chains

How do beta-lactams antibiotics act (2)?

• inhibit with NAM and NAG crosslinking (prevents peptidoglycan formation)

• mimic D-Ala-D-Ala and bind transpeptidase instead

How does fosfomycin act (cell wall inhibitor) (2)?

• inhibits MurA enzyme

• prevents conversion of NAG to NAM

How does cycloserine act (cell wall inhibitor) (3)?

• cyclic analogue of a component of NAM

• impacts building of peptide chain

• particularly acts on alanine regions

How does bacitracin act (cell wall inhibitor)?

interferes with addition of glycine-5 side chain

How do vancomycin and teicopanin act (cell wall inhibitors) (3)?

• mimic D-Alanine and sit in its place

• prevent side chain being built

• no peptide construct for peptidoglycan layer to be built

What is the difference between beta-lactams antibiotics and beta-lactams type antibiotics (2)?

• beta-lactams = inhibit NAG and NAM interaction

• beta-lactams type = mimic components of NAG and NAM

What is required for NAG and NAM crosslinking to build the peptidoglycan layer?

DD-transpeptidases

Why were synthetic version of penicillin created (3)?

overcome penicillin disadvantages:

• penicillinases susceptibility (i.e. resistance)

• narrow specificity

How were synthetic penicillins created (2)?

• strip side chains and replace with new ones

• stop synthesis of penicillin early and purify the central beta-lactam ring core

What are inhibitors and what is their purpose (2)?

• drug molecules that can interact with microbial enzymes

• ‘distracts’ bacteria from breaking down beta-lactams

What is an example of an RNA synthesis inhibitor?

rifampicin

What does rifampicin have a high affinity for?

prokaryotic RNA polymerase (RNA synthesis inhibitor)

How does rifampicin generally act (3)?

• bacteriostatic

• blocks mRNA synthesis by steric clashes with the growing oligonucleotide

• inhibits elongation of transcript

When is rifampicin most potent?

when DNA is ‘melted’ for replication

What are some antibiotics that act as DNA synthesis inhibitors (2)?

• fluoroquinolones

• metronidazole

What are the features of the fluoroquinolones (3)?

• synthetic, broad spectrum

• low MICs

• rapidly bactericidal

How do the fluoroquinolones act (general)?

target topoisomerase II and IV in DNA replication

How do the fluoroquinolones act (specific) (5)?

• topoisomerase (gyrase)

• gyrase:DNA complex forms = DNA broken but held in place

• fluoroquinolone stabilises complex = DNA cannot be released

• replication blocked, strand broken

• cell death

How does metronidazole act (DNA synthesis inhibitor) (2)?

• reduced by reacting with ferredoxin

• reduced intermediates damage enzymes and form unstable molecules in DNA

What bacteria does the DNA synthesis inhibitor metronidazole act against?

selectively absorbed by anaerobic bacteria

What type of antibiotic do folic acid synthesis inhibitors tend to be?

bacteriostatic

What do bacteria use folic acid enzymes for?

amino acid synthesis = necessary for bacterial protein synthesis

What are 2 examples of folic acid synthesis inhibitors?

• sulphamethoxazole

• trimethoprim

What is the starting molecule used in folic acid synthesis?

PABA (p-amino benzoic acid)

How can folic acid synthesis inhibitors become bactericidal?

when used in a 2 pronged combination targeting different synthesis steps

Why are many antibiotics used in combination ()?

• can act synergistically

• delays the emergence of resistance

How do monobactams act?

similar to beta-lactams

How do polymyxin B antibiotics act?

disrupts the plasma membrane

What is an example of a polymyxin B antibiotic?

colistin

What are 2 antibiotics commonly used against Mycobacteria?

• isoniazid - inhibits mycolic acid synthesis

• ethambutol - inhibits mycolic acid incorporation

Which bacteria are polymyxins most effective against?

gram negative

What other antibiotic class are polymyxins often used in combination with and why?

beta lactams - allow access

How does colistin (polymyxin B) resistance emerge (3)?

• alter LPS biosynthesis

• colistin cannot bind

• also impacts TLR4 signalling