Treatment of Infection

Define antimicrobial

Any chemical which kill or inhibit microbial growth in or on a body surface

Define antibacterial

Any chemical which specifically targets bacteria and kills or inhibits its growth

Define antibiotic

A drug used to treat bacterial infections

What are the sources of antibacterials?

Microorganisms

Synthesis

Semi-synthesis

How are most antibiotics produced?

Via semi-synthesis - microorganisms produce a compound and then we modify it in the lab

Define selective toxicity

selectively kills or inhibits the target organism, whilst causing no or minimal harm to the host

antimicrobials need this feature

Why is it harder to find selective toxicity in antifungals/ antivirals?

because they don’t have cell structures and enter the host cell, this makes it hard to find a therapeutic agent that won’t effect us

both of the cells are also eukaryotic, so alot of the structures and processes are the same as in our cells

What are the two types of antibiotic?

Bacteriostatic

Bactericidal

Define bacteriostatic

Slows down or stalls bacterial growth, once the antibiotic is taken away, it’ll start growing again

This allows time for our immune system to kill it

Define bactericidal

Kill bacteria

Why is it hard to treat dormant infections?

most antibiotics only kill actively growing bacteria because most antibiotics target enzymes and active processes

What are the ideal antimicrobial properties of antibacterials?

Specific - interact with a defined target

Selective - selectively kills/ inhibits the target organism whilst causing no/ minimal harm to the host (therefore less side effects)

Bactericidal - kill bacteria

Minimal emergence of resistance to the drug

What are the ideal pharmacological properties of antibacterials?

Non-toxic to the host

Long plasma half life

Good tissue distribution - most can’t cross the blood-brain barrier so it’s hard to treat brain infections

Low plasma protein binding - more that’ll cross into the tissue

Oral and parenteral availability

No interference with other drugs

Why do we have few drugs which target the bacteria cell membrane?

Because our cell membrane and a bacterial cell membrane are quite similar, so we don’t want to target ourselves or there will be increased risk of side effects

What are the 4 main sites of action of antibacterials?

Cell wall

Cell membrane

Protein synthesis

Nucleic acid

What is the biggest target of antibacterials?

Cell wall

What are the bactericidal sites of action of antibacterials?

Cell wall

Cell membrane

Nucleic acid synthesis

What are the bacteriostatic sites of action?

Protein synthesis because it just stops growth/ replication

Which antibacterials inhibit cell wall synthesis?

beta-lactams

glycopeptides

fosfomycin

cycloserine

bacitracin

What drug classes are classified as beta-lactams?

penicillin’s

cephalosporins

carbapenems

monobactams

Which drug classes are classified as glycopeptides?

Vancomyocin

Teicoplanin

Are gram negative or gram positive bacteria harder to treat?

Gram negative because they have an extra outer membrane which sandwiches the peptidoglycan layer

What is the peptidoglycan cell wall formed of?

A series of alternating NAM and NAG sugars linked by glycosidic bonds

NAM always have a peptide side chain attached, which cross links to another glycosidic chain

What are the two key enzymes for the formation of peptidoglycan cell walls and their roles?

Transpeptidases - cross link the peptidoglycan chains via the amino acid side chains

Glycosylases - forms glycosidic bonds between monomers, adds more monomers on

What is the direct MoA of beta-lactams?

1. The penicillin binding protein (transpeptidases) catalyse the cross-linking of the peptide side chains

2. Beta-lactams bind and inhibit the action of penicillin binding proteins

3. This blocks cross-linking of the peptidoglycan chains

4. Leading to an unstable peptidoglycan cell wall

5. And cell lysis due to osmotic pressure

What is the less common MoA of beta-lactams?

The beta-lactams get incorporated into the peptide side chain which prevents cross-linking, therefore preventing stable formation of peptidoglycan and leading to cell lysis

What is the indirect MoA of beta-lactams?

Beta-lactams stimulate the bacteria to produce enzymes called autolysins, these then break down the peptidoglycan cell wall leading to cell lysis due to osmotic pressure

How many modes of action do beta-lactams have?

3

Which types of bacteria do beta-lactams work on?

Gram positive

Work on most gram negative bacteria however not all of them can get through the porin channel on the outer membrane

What is the MoA of glycopeptides?

1. It binds to the terminal amino acids (D-ala - D-ala)

2. This binding prevents glycosidic bonds (as glycosylase enzyme can’t function) and transpeptide bonds from forming

3. The bacteria is unable to form a stable peptidoglycan cell wall

4. Leads to cell lysis due to osmotic pressure

What type of bacteria do glycopeptides work on?

Gram positive only - because they are large hydrophilic molecules so can’t fit through the cells porin channels on gram negative bacteria

Why are glycopeptides usually administered by IV?

they have poor absorption in the gut because they are large hydrophilic molecules

Which antibiotic is used to treat Clostridioides difficile infections?

Oral vancomyocin

Which drug class targets the bacterial cell membrane?

Polymyxins

Which type of bacteria do polymyxins target?

Gram negative only because it targets the LPS which gram positive bacteria don’t have

What is the MoA of polymyxins?

1. They bind to lipid A of LPS

2. This distorts the membrane

3. Causing penetration of the cell wall

4. This then disrupts the membrane integrity and allows leakage of cytoplasmic contents

What do polymyxins treat?

Serious gram-negative infections

What do antibacterials acting on nucleic acids do/ effect?

Metabolic inhibitors of nucleic acid synthesis: trimethoprim

Affects DNA replication: fluoroquinolones

Affects RNA polymerase: rifampicin

Affects DNA: metronidazole

What do fluoroquinolones target and what’s their MoA?

Bind to and inhibit topoisomerases II and IV when complexed with bacterial DNA

This means that bacterial DNA transcription and packaging can occur

This causes cell lysis

What do inhibitors of protein synthesis exploit?

They all exploit the differences between the mammalian and bacterial ribosomes

Bacterial ribosomes have a 50S and 30S subunit whereas mammalian have a 60S and 40S subunit

How do tetracyclines work?

Binds reversibly to A-site on 16S rRNA in 30S subunit
Inhibits binding of tRNA to A-site, thus inhibiting protein synthesis

Selectivity through better binding to bacterial ribosome than human ribosome higher accumulation in bacterial cells - it does bind to mammalian ribosomes which can lead to side effects

What is a key side effect of tetracyclines?

Sensitivity to the sun

Which class of drugs inhibit proteins synthesis?

Tetracyclines

Aminoglycosides

What is the MoA of aminoglycosides?

They bind irreversibly to the A-site on 16S rRNA in 30S subunit

Have 4 modes of action:

1. Prevent formation of initiation complex

2. Inhibit binding of tRNA to A-site

3. Cause misreading of the codons (dysfunctional proteins)

4. Increases bacterial membrane permeability

Give an example of an aminoglycoside

Gentamicin

What are the side effects of aminoglycosides?

Accumulates in the ear canals and kidneys - this can cause irreversible ototoxicity and nephrotoxicity

Requires therapeutic dose monitoring

Reserved for serious infections - delivered via IV

What is antimicrobial resistance a consequence of?

Natural selection

How does natural selection lead to antimicrobial resistance?

As within a population, some of the organisms will have mutations which arise when DNA is copied

These mutations may give more/ less desirable effects for the bacteria - this can lead to antibiotic resistance

What is the governments plan to reduce antimicrobial resistance?

• Reducing need for and unintentional exposure to antimicrobials - prevent infection, education and unintentional exposure

• Optimising use of antimicrobials - antibiotic stewardship

• Innovation, supply and access - reducing market barriers, vaccines, looking at health disparities

• Being a good global partner - how antibiotics are used in other countries affects how we use them in the UK, standards help everyone work together

What factors promote antimicrobial resistance?

• Bacterial conjugation - plasmids are passed between bacteria

• Misuse of antibiotics - overprescribing, not completing course of treatment, misprescribing

• Aging population - more people with more complex infections stay longer in hospitals, so are more likely to get a hospital acquired infection

• Poor infection control in healthcare settings

• Industrialisation of use and production of antibiotics - e.g in farming

• Lack of development of new classes of drugs

What are some examples of organisms of particular concern?

Acinetobacter baumannii

Klebsiella pneumoniae

Pseudomonas aeruginosa

Escherichia coli

MRSA

Neisseria gonorrhoeae

MDR tuberculosis

Candida auris

What are the two types of antimicrobial resistance?

Intrinsic / innate - just is resistant to the antimicrobial naturally

Acquired

What are the two types of antimicrobial resistance?

Genetic - irreversible

Phenotypic - reversible

Explain what phenotypic antimicrobial resistance mean?

Refers to the lifestyle stage - when they go into a biofilm, extracellular matrix prevents the antibiotics from penetrating, but when they leave they are susceptable again

Also occurs in the dormant stage as bacteria tend to target active processes which aren’t happening

What are the types of genetic antimicrobial resistances?

Mutations - single base mutation confers a low level of resistance but accumulating mutations results in a high resistance to high concentration antibiotics

New genetic material - mostly acquired on plasmids through conjugation

How does bacterial conjugation occur?

Bacteria have pili which they use to transfer a plasmid into the cytoplasm of a new bacteria - this can occur between different species

You can get multi-drug resistance which can lead to

What are the three mechanisms of antibiotic resistance?

Inactivate / modify drug

Alter the drug site

Altered transport

What are beta-lactamases ?

enzymes produced by bacteria which hydrolyse beta-lactam rings - this inactiviates the beta-lactam antibiotics

How does resistance to beta-lactams arise?

Chromosomal (intrinsic resistance)

Plasmid-mediated - most common

What is the most problemtic beta-lactamase?

plamid mediated beta-lactamases produced by gram negative bacteria

What is an example of a beta-lactamase inhibitor?

Clavulanic acid - inactivate the enzyme

Do beta-lactamase inhibitors have any antimicrobial activity?

No

How does altering a target site arise to beta-lactam resistance?

Mutations in the PBPs prevent the beta -lactams from binding

This is where the beta-lactam rings target

How does altered uptake lead to beta-lactam resistance?

The bacteria reduce the number oor size of porins

This decreases the permeability and prevents uptake

Can all bacteria alter uptake to resist beta-lactams?

Only gram negative

How does resistance to glycopeptides, such as vancomyocin arise?

D-ala-D-ala is changed to D-ala-D-lactate which prevents the glycopeptides from being able to bind

How does resistance to aminoglycosides occur?

Through drug inactivation

Aminoglycoside modifying enzymes add side chains onto the drug which makes it inactive

How does resistance to fluoroquinolones occur?

Chromosomal mutations in DNA gyrase or topoisomerase IV inhibit binding of the antibiotic to its target

How does resistance to tetracyclines occur?

Efflux pumps - actively pump substrates out of the bacterial cells

Present in gram positive and gram negative bacteria

Are inducible - can be switched on/ off in the presence of tetracycline

Not just limited to tetracycline, e.g pseudomanas has a large number of efflux pumps resistant to a lot of unrelated drugs

This mutation can be upregulated

What is the purpose of antimicrobial stewardship?

prescribing the right antibiotic for the right patient at the right time with the right dose and the right route, causing the least harm to the patient and future patients

Define antimicrobial stewardship

An organisational or healthcare system-wide approach to promoting and monitoring judicious use of antimicrobials to preserve their future effectiveness