Antimicrobial mechanism Flashcards
Which microbes are beneficial
Microbiome, 95% of bacteria are in the GI tract
What features are on the outside of a bacteria cell
Cell wall (peptidoglycan) cell membrane
What features are on the inside of a bacteria cell
- circular DNA
- ribosomes (70s not 80s)
- unique aspects of metabolic pathways
How does bacterial DNA fit inside the cell
Supercoiling
Negative supercoiling is usually in which direction
Left-handed
Features of negative supercoiling
Compact and loosen the two strands of DNA
Thermophilic bacteria have what kind of supercoiling
Positive supercoiling
Why do thermophilic bacteria have positive supercoiling?
protects DNA from denaturation in extreme conditions
What enzyme is responsible for supercoiling in gram-negative bacteria?
DNA Gyrase
What enzyme is responsible for supercoiling in gram-positive bacteria?
Topoisomerase IV
How does DNA gyrase and topoisomerase IV achieve supercoiling?
- cutting one region of DNA double-helix
- threading a second DNA region through the break
- resealing the break
Name a first-generation quinolone
Nalidixic acid
What was Nalidixic acid used to treat?
Active against mainly gram -ve bacteria, treatment for UTIs, bacteriocidal
Second generation quinolones are also known as
Fluoroquinolones
What did fluoroquinolones have an increased activity on
Gram +ve bacteria
Name two examples of fluoroquinolones
norfloxacin
ciprofloxacin
Name an example of a third-generation quinolone
Levofloxacin
What benefit did third generation quinolones have over previous generations?
Active against streptococci
sufficiently broad spectrum to be used before bacteria identified
indicated in pneumonia and urinary tract infections
What environment do nitroimidazoles require?
Anaerobic environment
What is the nitro group in nitroimidazoles reduced by?
Bacterial electron-transport system
Name the two radicals formed in nitroimadazoles?
nitro radical anion
imidazole radical
What does the imidazole radical extract from DNA
A H atom
Name a nitroimidazole
Metronidazole
What is the mechanism of action of metronidazole?
nitro radical anion formed and then imidazole radical which extracts a H from DNA, breaking the strand apart.
What are nitroimidazoles used to treat?
anaerobic bacterial and protozoal infections
e.g. bacterial vaginosis, acute oral infections, and C.difficile
What are the side effects of nitroimidazoles?
alcohol intolerance, swollen, red or hairy tongue
What organelle makes proteins?
Ribosome
What is there a higher chance of in bacteriostatic antibiotics?
antibiotic resistance
Oxazolidinones block….
50s/30s binding
Chloramphenicol/ macrolides block…
peptide chain transfer inhibits protein synthesis, bacteriostatic
How do tetracyclines inhibit protein synthesis?
interfere with tRNA binding which means that the corresponding amino acids cannot join together so protein synthesis is disrupted and inhibits bacteria cell growth.
Oxazolidinones
binds to 50s subunit
active against multiple-resistance bacteria including MRSA
used against gram +ve hospital-acquired infections e.g. Staphylococcus aureus, S. epidermis and steptococcus pneumonia, where other treatments are unsuccessful
Tetracyclines
interfere with tRNA binding binds to 30s and stops aminoacyl tRNA from binding
very widely used antibiotic, with broad spectrum
What are tetracyclines used to treat?
Lyme disease, malaria and acne
Macrolides
binds to 50s subunit blocks peptide chain transfere
active against most gram +ve bacteria
e.g. ear infections, streptococcal pharyngitis (bacterial tonsillitis)
used as a first-line treatment for penicillin-allergic patients
Erythromycin
binds near peptidyl transferase centre
blocks exit tunnel for the growing peptide
inhibits peptide bond formation and translocation
Chloramphenicol belongs to which class
amphenicols
Chloramphenicol works by
binds to 50s subunit and also prevents peptidyl transferase reaction
What two antibiotics cannot be used in combination
Chloramphenicol and macrolides as they occupy the same region
Synthesis of DNA uses which molecule
Folate
What does DHFR (dihydrofolate reductase) do?
Dihydrofolate reductase (DHFR) catalyzes the reduction of dihydrofolate to tetrahydrofolate (THF). THF is needed for the action of folate-dependent enzymes and is thus essential for DNA synthesis and methylation.
Metabolic pathway of folate
folate > DHFR > dihydrofolate > DHFR > tetrahydrofolate
DNA synthesis inhibitors are sometimes known as
Antimetabolites
Name two DHFR inhibitors
Trimethoprim
Methotrexate
Trimethoprim mimics which ring of folate
pteridine ring
Trimethoprim is used in combination with other antibiotics to treat which condition?
UTI
Methotrexate mimics which ring
Pteridine ring of folate (closer mimic than trimethoprim)
What does methotrexate target?
human DHFR so mainly used against cancer and rheumatoid arthritis
Sulfa Drugs
inhibits DHPS by mimicking PABA substrate of DHPS
Sulfamethoxazole
used with trimethoprim in co-trimoxazole
What is sulfamethoxazole used to treat
UTIs and pneumocystis carinii pneumonia in AIDs patients.
Which enzyme catalyses dihydrofolate in bacterial cells?
dihydropteroate synthase
How do sulfa drugs work?
Inhibit DHPS mimicking PABA substrate of DHPS
Why is sulfamethoxazole used in combination with trimethorpim
Folic acid is made from PABA through a step-by-step process involving two enzymes. The sulfa drug inhibits the first enzyme and trimethoprim inhibits the second enzyme. This double inhibition is called the sequential blockade and produces the death of a bacterial cell.
What can resistance bacteria increase the production of to compete with sulfa drug
PABA
Explain the process of sequential blocking
action of two or more inhibitors each of which acts on the same metabolic pathway but upon different enzymes.
