Antimicrobials 1 Flashcards by Jasleen Sambhi (2025)

What type of bacteria are macrolides effective against?

Gram pos

What are macrolides particularly useful for?

Mild staph or strep infections in patients who are allergic to penicillin

Recall 2 macrolide antibiotics

AzithromycinClarithromycin

What type of bacteria are tetracylines effective against?

They are broad spectrum

Recall a class of antibiotic you should never give to children or pregnant women

Tetracyclines

Recall one side effect of tetracycline antibiotics

Photosensitivity --> Light-sensitive rash (believed to be because tetracycline can absorb UV light and release that energy to skin cells in form of reactive oxygen species)

Recall the broad mechanism of action of macrolides

Bind to the 50s subunit of ribosomes

Recall the broad mechanism of action of tetracyclines

Bind to 30s subunit of ribosomes\*Bacteriostatic - still useful in certain situations, especially with MRSA\*

What type of bacteria is chloramphenicol effective against?

Many - it is v broad spectrum

Why is chloramphenicol rarely used?

Risk of aplastic anaemia and grey baby syndrome in neonates due to inability to metabolise drug\*Still useful for meningitis when patients have penicillin anaphylaxis\*

Recall 2 specific bacteria that macrolides are effective against

Campylobacter spLegionella pneumophila(macrolides = erythromycin/ azithromycin/ clarithromycin)

Recall the broad mechanism of action of chloramphenicol

Binds to 50s subunit of ribosomes - inhibits formation of peptide bonds during translation

Recall the broad mechanism of action of oxazolidinones

Binds to the 23s portion of the 50s ribosome subunit to prevent 70s subunit formation

Recall two types of bacteria that oxazolidinones are particularly active against

Highly active against gram positive organisms - especially MRSA and VRE

Recall an example of oxazolidinones

Linezolid

Recall one potential side effect of oxazolidinones

Thrombocytopaenia and neurological side effects if used longer than 4 weeks

Which antibiotics inhibit protein synthesis by binding to the:30s subunit50s subunit

30s = aminoglycosides, tetracyclines50s = macrolides, chloramphenicol, oxazolidinones (binds to the 23s portion)

Recall the broad mechanism of action of fluoroquinolones

Act on alpha subunit of DNA gyrase

Recall the uses of fluoroquinolonesWhich bacteria types are susceptible

Complicated UTIMeningococcal chemoprophylaxisPneumoniaAtypical pneumoniaBacterial gastroenteritis\*BROAD SPECTRUM antibacterial activity against gram negatives\*

Recall examples of fluoroquinolone antibiotics

LevofloxacinMoxifloxacinCiprofloxacin

How do nitroimidazoles work?

Under anaerobic conditions, an active intermediate is produced which causes DNA strand breakage

Give examples of nitromidazole antibiotics

MetronidazoleTinidazole

Recall types of organisms that metronidazole is effective against

AnaerobesProtozoa

When should metronidazole be taken?

Right after visiting the toilet as it sits in bladder

Recall the broad mechanism of action of rifampicin

Binds to DNA-dependent RNA polymerase to inhibit RNA synthesis

Recall the main use of rifampicin

TB treatment

Recall one side effect of rifampicin

Turns secretions orange

Recall one condition of rifampicin prescription

Should never be prescribed alone as resistance develops very quickly\*Resistance is caused by a single amino acid change\*

Colistin is very toxic. Why is it coming back into use?

It is active against certain multi-drug resistant bacteria

What is daptomycin licensed for the treatment of?

MRSAVRE

Recall the 2 classes of antibiotic that inhibit folate synthesis

SulphonamidesDiaminopyrimidines (e.g. trimethoprim)

Give an example of a sulphonamide

Sulfamethazole\*Sulfonamides aren't used on their own - should be in combination with trimethoprim (co-trimoxazole)

What is the main use of trimethoprim

Uncomplicated UTI

Which antibiotic is best for treating pneumocystis jirovecii?

Co-trimoxazole

Give 2 examples of cell membrane toxins

Daptomycin (lipopeptide with limited activity to gram positives - potential alternative to linezolid and synercid for MRSA and VRE infections)Colistin (old antibiotic which is very nephrotoxic but it is active against gram negative organisms like pseudomonas)

What are the 4 main mechanisms of resistance

***inactivation, target, accumulation, bypass ***1. Inactivation of the antibiotic (eg beta lactamases)2. Altered target - so antibiotic no longer binds* E.g. **penicillin resistant pneumococci or MRSA** where bacteria change the penicillin-binding protein* E.g. protein-synthesis inhibitors where the binding of the ribosome subunit is prevented3. Reduced accumulation (most important in **gram negs** - either due to enhanced efflux or to reduced uptake)4. Bacteria bypasses antibiotic sensitive step (particularly important for **folate inhibitors** - bacteria can change the enzyme they use)

How is MRSA resistant to all beta lactams?

**MRSA survives beta-lactams by using PBP2a (from mecA) – a version of the target that the antibiotics can’t bind to**.mecA gene encodes novel penicillin binding protein (2A) / novel PBP 2aLow affinity for binding beta lactamsSubstitutes for essential functions of high affinity PBPs at otherwise lethal concentrations of antibiotics

How does Strep pneumoniae develop beta lactam resistance?

Penicillin resistance is the result of acquisition of stepwise mutations in PBP genes\*Lower level resistance can be overcome by increasing dose of penicillin used\*

How do bacteria become resistant to macrolides?

**erm gene → methylates 23S rRNA → macrolide can’t bind → resistance**Adenine-N6 methyltransferase modifies 23S rRNA -\> reduces binding of MLS antibiotics and results in resistanceEncoded by **erm gene** (erythromycin ribosome methylation)\*If bacteria is resistant to erythromycin in this manner but still sensitive to clindamycin, only use clindamycin with caution - sometimes the in-vitro tests aren't reliable\*

Which bacteria typically forms "gram pos cocci in clusters"?

Staphylococcus

Which bacteria typically forms "gram pos cocci in chains"?

StreptococcusStrep sounds like 'stripe' = chain

What gram stain status are enterococci?

Positive("Enter-o-coccus" = like letting someone in, positive thing to do)

Is streptococci gram pos or neg?

Gram pos

Is pseudomonas gram pos or gram neg?

Gram neg(Pseudo"moan"as - 'moan' = negative)

Is neisseria meningitis gram pos or gram neg?

Gram neg(Neisseria starts with N = negative)

Is haemophilus gram pos or neg?

Gram negHa"emo"philus - emo = negative

Is listeria gram pos or neg?

PositiveLister = good man = positive