Microbiology - Antibiotic Resistance

Question 1

What is the chemistry and drug class of flucloxacillin?

Answer 1

Is a small molecule
Is penicillin
Is resistant to breakdown from beta-lactamases. (penicillinase resistant)
Others examples include: temocillin

Question 2

When is flucloxacillin mainly used?

Answer 2

Mainly targets bacteria caused respiratory tract of skin/soft tissue infections mainly staphylococcus auerus, but also other gram -ve and +ve infections
Infections due to beta lactamases producing staphylococci including otitis external
Adjunct in pneumonia
Impetigo
Cellulitis
Secondary bacterial infection of eczema
Diabetic foot infection

Question 3

What is the mechanism of action of flucloxacillin?

Answer 3

Contains a beta lactam ring that mimics the d-ala-d-ala sequence found in peptidoglycan so are able to bind to the transpeptidase enzyme (a penicillin-binding protein) preventing the cross-linking of peptidoglycan, this inhibits bacterial cell wall synthesis and causes cell wall weakening.
Causes cell lysis mediated by autolysins - bacteriocidal
Possible that flucloxacillin interferes with an autolysin inhibitor as well.

Question 4

What is the method of bacterial resistance against flucloxacillin and how common is it?

Answer 4

MRSA is resistant - produced an altered PBP2a - this reduces the affinity for flucloxacillin binding - reduce effectiveness of drug.

Around 1% of SA is resistant to flucloxacillin.
MRSA is carried by 2% of the population.
MRSA resistant breakouts are decreasing over time. Around 6% of total infections in 2021.

Question 5

What is the chemistry and class of ceftriaxone?

Answer 5

Is a cephalosporing beta lactam antibiotic (cef)
Contain a beta lactam ring
Other examples include - cefmeonxime, cefditoren

Question 6

What is the mechanism of action of ceftriaxone?

Answer 6

Is a cephalosporin
Contains a beta lactam ring that mimics the d-ala-d-ala sequence found in peptidoglycan so are able to bind to the transpeptidase enzyme (a penicillin-binding protein) preventing the cross-linking of peptidoglycan, this inhibits bacterial cell wall synthesis and causes cell wall weakening.
Causes cell lysis mediated by autolysins - bacteriocidal

Question 7

For what type of infections might ceftriaxone be used?

Answer 7

bacterial infections in lower respiratory tract ( CA and HA pneumonia), intra-abdominal, complicated UTIs, skin and meningitis
Acute axacerbations of COPD
Mainly targets: streptococcus pneuomina, methicillin susceptible staphy, H, influenza, Moraxella catarrhalis and Neisseria spp.
Usually gram positive

Question 8

What is the mechanism of resistance against ceftriaxone and how common is it?

Answer 8

Hydrolysis by beta lactamses - such as in E.coli producing ESBL
Alteration of penicillin binding proteins - changes affinity
Deceased permeability - by altering membrane structure
Second two methods are done by Neisseria Gonorrhae, increasing with resistance reported in 10 cases in 2022.

Question 9

What is the class and chemistry of vancomycin?

Answer 9

Is a glycopeptide antibiotic
Other examples include dalbavancin or telavancin.

Question 10

What type of infections des vancomycin normally treat?

Answer 10

Used to treat gram-positive infections
Typically staphylococcus aureus, enterococci and streptococci.
Important use in MRSA where resistance against vancomycin is rare.

Question 11

What is the mechanism of action of vancomycin?

Answer 11

Targets the cell wall of activly divind susceptible gram-positive bacterium.
Inhibits transglycosylases enzyme
binds forming hydrogen bonds to the acyl-D-ala-D-ala portion.
Prevents cell wall from forming the cross-linking necessary to increase strength
NAM and NAG subunits are not incorporated in cell wall
Cell begin to produce more proteoglycan as a compensation - actives feedback loop where degradative enzyme to break down peptidoglycan are activates - contibte to cell wall destruction
Causes bacterial lysis by osmotic burst - bacteriocidal

Question 12

What is the mechanism of resistance against vancomycin and how common is it?

Answer 12

Most common - vancomycin-resistant enterococci - a gene that changes the acyl-D-ala-D-ala amino acid chain to acyl-D-ala-D-lactate - prevents vancomycin binding as reduced affinity.
Myobacterium TB has also developed efflux pumps to expect vancomycin causing resistance.
Resistance if fasirly stable - E, faecium 22% in 2022

Question 13

What is the class and chemistry or ciprofloxacin?

Answer 13

Small molecule
Bicyclic core
Fluoride and carboxylic acid
Is a fluoroquinolone
Other examples include: levofloxacin, ofloxacin and moxifloxacin.

Question 14

What is the mechanism of action of ciprofloxacin?

Answer 14

Reversibly binds to DNA gyrase and tropoisomerase IV enzymes, complex stabilising the form of the complex that promotes double-stranded DNA cleavage.
Inhibits supercoiling of bacterial DNA and DNA synthesis.
Inhibits DNA replications
Inhibits cell cycle at interphase preventing bacterial colony growth (bacteriostatic)
Resulting DAN fragmentation cause cell death (bacteriocidal)

Question 15

What type of infections is ciprofloxacin normally used in?

Answer 15

Is a broad-spectrum antibiotic
Uncomplicated UTIs, chest infections (pneumonia), skin and bone infections
Affects both gram-negative (best) and positive strains.
For example, E.coli, N. gonorrhoea, N. meningitides.

Question 16

What is the main mechanism of resistance against ciprofloxacin and how common is it?

Answer 16

Gram-negative bacteria frequentl mutate gyrA subunit in DNA gyrase - reduce drug affinity
Gram-positive bacteria mutate ParC subunit of topoisomerase 4 - reduce drug affinity
Plasmids contain genes to produce Qnr proteins, bind to DNA qyrase and topoisomerase to protect from quinolones - spread by horizontal gene transfer via plased mediated conjugation transfer.
Resistance is common around 50% of gram negative bacteria in England in wales, higher in GBMSM.

Question 17

What is the chemistry and class of clarithromycin?

Answer 17

Is a small molecule
Is a macrolide antibiotic
Other examples of this class include: azithromycin and erythromycin

Question 18

What is the mechanism of action of clarithromycin?

Answer 18

Is a semisynthetic macrolide, reversibly binds to the 23rRNA in the 50 S subnunit of bacterial ribosome, inhibits the translocation of aminoacyl tRNA, blocks the polypeptide exit tunnel,
preventing peptide chain elongation.
Leading to bacterial cell death.

Question 19

What type of infections does clarithromycin normally treat?

Answer 19

Skin, respiratory and soft tissue infection
Inlcudes gram positive such as staphylococcus auerus, P,penuomnia and S.pyrogenes)
Includes gram negative aerobic bacteria such as H. influenza.
is broad spectrum.

Question 20

What is the mechanism of resistance against clarithromycin and how common is it?

Answer 20

Point mutations in the 23sRNA genes - mainly in position 2142 and 2143
Transition of adenosine to guanine

H.pylori shows the highest levels of resistance.

USA and UK prevalence of clarithyrmycin resistance in H.pylori strains in 22.2%, worldwide average is 18%.

Question 21

What is class and chemistry of trimethoprim?

Answer 21

Is a small molecule
is a diaminopyrimidines - antifolate
Other examples of this antibiotic class include - sulfamethoxazole and sulfalazine.

Question 22

What is the mechanism of action of trimethoprim?

Answer 22

is a reversible competitive inhibitor of dihydrofolate reductase
Inhibits conversion of dihydrofolic acid to tetrahydrofolic acid
THF is essential for bacterial nucleic acid synthesis, therefore inhibition prevents amino acid and in turn protein formation
Binds to bacterial DHFR with higher affinity than mammalian DHFR due to difference in tertiary protein structure.

Question 23

What infections does trimethoprim usually treat?

Answer 23

Effective against gram-positive cocci and some gram-negative bacilli.
Often used in combination with sulfamethoxazole.
Commonly respiratory tract infections, prophylaxis of r,UTI, acne.
Main pathogens include: E.coli, proteus mirabilis, Klebsiella pneumonia, enterobacter species and coagulase negative staphylococcus.

Question 24

What is the main mechanism of resistance against trimethoprim?
How common is it?

Answer 24

Mutation in dihydrofolate reductase enzyme - lower affinity for the drug.
Efflux pump - SmeDEF to reduce cell permeability to drug.
Only 34% of UTI samples are resistant to trimethoprim
Aureus strains and E.coli tend to have higher rates of resistance.

Question 25

What is the class and chemistry of tobramycin?

Answer 25

Class - aminoglycoside antibiotic
Other examples include: gentamycin, streptomycin
Are small molecules, core amino sugar structure with glycosidic bonds, is positively charged to allow it to attach to outer membrane/

Question 26

What is the mechanism of action of tobramycin?

Answer 26

Is an aminoglycoside
Targets 16s ribosomal RNA of the bacterial 30s ribosomal unit - binds forming hydrogen bonds
Acts as an irreversible inhibitor
Causes inhibition of protein syntheses, specifically translation, by preventing elongation or by directly inhibiting initiation leading to a bactericidal effect
Can also cause mRNA misreading resulting in incorrect insertion of amino acids forming non-functional or toxic peptides which damage the cell membrane.
Bacterocidal

Question 27

What type of infections if tobramycin normally used against?

Answer 27

Broad spectrum - gram-positive (narrow and hardly used clinically) and gram-negative organisms (broad - aerobic)
Can be given via nebuliser to treat Pseudomonas aeruginose lung infection in CF patients.
Local eye infections, meningitis, septicaemia billitary tract infections, pneumonia, UTI, prostatitis.

Question 28

What is the mechanism of resistance against tobramycin?
How common is it?

Answer 28

Most common mechanism is the production of aminoglycoside modifying enzymes (AMEs) - including N-acetyltransferases, O-nucelotidyltransferases and O-phosphotransferases.
Modified drug is unable to inhibit 30S ribosomal subunit
Also - alteration at ribosomal binding sites
27.5% of P.aueroginose is resitant to tobramycin.