BIOL 0510 - Antibiotics and Resistance

What is chemotherapy?

the use of drugs to treat a disease

What do antimicrobial drugs do?

interfere with the growth of microbes within a host

What is an antibiotic?

a substance produced by a microbe that in small amounts, inhibits another microbe

What is a prodrug?

a drug that needs to be metabolised by the liver or microbiome to work

What is a bactericide?

kills microbes directly

What does bacteriostatic mean?

prevents microbes from growing

Where do most antimicrobial drugs come from?

derivatives of natural compounds

Why do most antibiotics come from nature?

microbes exist in naturally competitive environments which requires them to find lots of ways to survive

What did Gerhard Domagk do?

He worked with coal tar and eventually found prontosil. He determined its structure and then saved his daughter’s life with it

Why does prontasil work in animals and humans but not on a plate?

It’s a prodrug that needs to be metabolised by the liver or the microbiome which converts it into sulphonamide

What event catalysed the formation of the FDA?

The Elixir sulfanilamide ethylene glycol Disaster

What do sulpha drugs do?

block bacterial synthesis of folic acid and act as a competitive inhibitor of PABA

Why do bacteria need PABA?

to make their own folate for proteins, RNA and DNA because they can’t get it from food like humans can

Why are sulpha drugs broad spectrum?

most microbes use PABA

Why are sulpha drugs not considered to be antibiotics?

they weren’t isolated from a microbe; they’re synthetic

What is rifampicin?

an antibiotic that inhibits RNA progression by binding to RNA polymerase

Where was rifampicin found?

Isolated from Amycolatopsis rifamycinica in French soil

What is rifampicin used to treat?

tuberculosis and leprosy

How do quinolones act as antibiotics?

Inhibit bacterial DNA replication by binding to gyrase and topoisomerase

How do quinolones affect bacteria?

they prevent the re-ligation step during DNA replication, leading to an accumulation of DNA breaks. So, the replication fork is stalled and the DNA is fragmented

Where were quinolones isolated from?

chloroquine

What were quinolones first used for?

antimalaria medication

Are quinolones bacteriostatic or bactericidal?

bactericidal

What kind of cells are quinolones effective against?

slow growing

When did Fleming discover penicillin?

1928 but it first became publicly available in 1945

What did Ernst Chain and Howard Florey do?

test penicillin further, assemble a team and scale up the production from one dose in 1941 to 2 million in 1942

Penicillin is a antibiotic from what family?

beta lactams

What do beta lactams do?

inhibit the transpeptidases that form peptide cross bridges in peptidoglycan

What is moenomycin?

an inhibitor of glycosyltransferases, which blocks the elongation of NAG and NAM chains in peptidoglycan

How do bacteria become resistant to beta lactams?

gaining the plasmid with the penicillinase gene

What does penicillinase do?

convert penicillin into penicilinoic acid

What’s another way that bacteria become resistant to beta lactams?

change the structure of transpeptidases so that beta lactams can’t bind to it and inhibit them

What is oxacillin?

a narrow spectrum semisynthetic penicillin that’s resistant to penicillinase

What kind of bacteria does oxacillin work on?

gram positive

Describe ampicillin?

it’s like oxacillin but it also works on some gram negative bacteria

How are some semisynthetic penicillins being made now?

as a combination of a penicillin and a penicillinase inhibitor e.g amoxicillin and potassium clavulanate

What is MRSA?

methicillin resistant staphylococcus aureus

What is methicillin?

a semisynthetic penicillin meant to combat bacteria with penicillinase

What makes MRSA resistant to methicillin?

The MecA gene which encodes for penicillin-binding protein 2a which has a low affinity for beta-lactam antibiotics. So, methicillin can’t bind effectively and inhibit bacterial cell wall synthesis

What other virulence factors does MRSA have?

biofilm formation and PVL toxin which kills human leukocytes and inflicts severe tissue damage

What’s an example of a polypeptide antibiotic?

Bacitracin

How does bacitracin work?

it binds to lipid carrier on Gram + bacteria that transports NAG and NAM across the membrane

What is vancomycin?

a glycopeptide antibiotic that is mainly used on Gram + bacteria because it can’t reach the peptidoglycan layer of Gram -

How does vancomycin work?

It inhibits peptidoglycan polymerisation by binding to the D-Ala-D end if NAM so transpeptidase can’t work

Why is vancomycin considered bacteriostatic?

it can only inhibit bacterial growth

What are antimycobacterial antibiotics?

antibiotics against acid fast bacteria

How does isoniazid work?

inhibits mycolic acid synthesis

How does ethambutol work?

inhibits incorporation of mycolic acid

What kind of bacteria do polymyxins inhibit?

gram negative

How do polymyxins work?

Bind to lipopolysaccharide (LPS) and disrupt the structure of the bacterial cell membrane by interacting with its phospholipids

Provide examples of antibiotics that inhibit protein synthesis

tetracyclines, streptomycin and chloramphenicol

How does streptomycin inhibit protein synthesis?

it changes the shape of 30S portion of the ribosome causing code on mRNA to be read incorrectly

What family of antibiotics does streptomycin belong to?

Aminoglycosides

Where was streptomycin derived from?

Streptomyces

Aside from streptomycin, what are examples of aminoglycosides?

gentamicin and neomycin

What did Selman Waksman do?

hypothesised that the soil is a battleground of microorganisms therefore it must contain substances that microbes use to kill or inhibit it

What do broad spectrum antibiotics do?

act against a wide group of bacteria

When should broad spectrum bacteria be used?

when you’re unsure of what bacteria is causing an infection

What do narrow spectrum antibiotics do?

act against a limited number of bacteria

Why is narrow spectrum used?

to avoid promoting antibiotic resistance and killing helpful bacteria

What’s one type of superinfection?

being reinfected after being ill with an infectious agent

What’s a second type of superinfection?

a new infection as a result of antimicrobial therapy

What’s a 3rd type of superinfection?

a secondary infection that develops during or immediately after an infection

Why do superinfections occur?

previous infections can deplete the level of immune cells, increase oxidative stress and alter the cellular environment in a way that makes bacterial infection easier

Describe the setting of a broth dilution test

a 96 well plate, each row has a different antibiotic/antimicrobial agent in it. Concentration of it decreases as you go down the row

What is the broth dilution test?

observing the growth of a bacteria at different concentrations of different antibiotics

What’s a disadvantage of the broth dilution test?

only one antibiotic can be tested at a time

What is the epsilometer (E) test?

A rectangular paper strip is embedded with a gradient of antibiotic concentrations and added to a plate with bacteria spread on it

What does MIC stand for?

minimal inhibition concentration

What do antibiograms show?

the percentage of bacteria from an institution that are susceptible to the antibiotics normally used to treat them

Why are antibiograms helpful?

helps identify the most effective treatment and helps compare between different institutions and areas

What is antibiotic resistance?

ability of microbes to grow in the presence of of a chemical that would normally kill it or limit its growth

What’s one method of antibiotic resistance?

preventing penetration or entry of the drug

What’s a second method of antibiotic resistance?

enzymatic destruction of the drug e.g use of penicillinase

What’s a 3rd mechanism of antibiotic resistance?

alteration of the drug’s target site e.g MecA gene which creates a transpeptidase with no penicillin binding site

What’s a 4th method of antibiotic resistance?

rapid ejection of the drug using efflux pumps

What are examples of beta-lactam resistance?

MecA gene to alter the drug binding site, penicillinase and efflux pumps to eject penicillin from the cell

Why are biotech companies making less antibiotics?

they aren’t needed all the time like medicine for chronic diseases and bacteria could become resistant to it in a couple years so it stops being used

Ice core primary literature tells us that…

Antibiotic resistance comes naturally to microorganisms as a way to compete with each other

What is heteroresistance?

a single strain of bacteria harbours both resistant and susceptible cells

Why do susceptible cells grow faster than resistant cells?

they spend less metabolic energy transcribing genes needed for antibiotic resistance

What is the fitness cost for resistant cells?

When there’s no drug present, they grow slower due to the energy spent transcribing genes for resistance

How can antibiotic resistance be reduced?

By using antibiotics thoughtfully because they run the risk of creating an environment of natural selection that promotes the expansion of persister cells

Misuse of antibiotics selects for…

resistant mutants

What are ways that antibiotics are misused?

use on livestock, use of outdated antibiotics, not completing prescribed regimen, using antibiotics for a viruses e.g a cold

What are potential future chemotherapeutic agents?

phage therapy, siRNA, blocking quorum sensing, HINS light and exploring other animals like cockroaches as they are highly resistant to bacterial infections

Why might a doctor prescribe two different antibiotics for a single infection?

To allow the use of lower, less toxic doses while achieving a greater killing effect than either drug alone