Micro 265 - Antibiotics (7)

antibiotic

naturally occurring compound produced by microorgs to inhibit growth of other microorgs.

In soil and aquatic habitats

what produces antibiotics?

bacteria, fungi

why produce antibiotics?

eliminate competition for limited resources

do antibiotics affect viruses?

no

antibiotic targets

antibiotics + world wars

cell wall synthesis inhibitors

INHIBIT cross-linking of peptidoglycan cell wall

cell wall synthesis inhibitors - examples

1.) beta-lactam antibiotics

• penicillin, ampicillin, methicillin

2.) glycopeptide antibiotics

• vancomycin

** KILL ONLY GROWING BACTERIA, not persistent ones, doesn’t break existing cross-links

do cell wall synth inhibitors kill growing bacteria?

yes, only growing ones. Doesn’t break existing crosslinks and doesn’t kill persistent bacteria

beta-lactam antibiotics

cell wall synthesis inhibitors

Ex: penicillin, ampicillin, methicillin

how do beta-lactams work?

structural similarity of peptides and ß-lactam ring of penicillin

(transpeptidase is blocked by penicillin binding)

penicillin

cell wall synthesis inhibitor

ampicillin

cell wall synthesis inhibitor

methicillin

cell wall synthesis inhibitor

glycopeptide antibiotics

effective against Gram-positive bacteria, particularly methicillin-resistant Staphylococcus aureus (MRSA)

Ex: vancomycin

vancomycin limit

cell wall synthesis inhibitor - doesn’t penetrate membrane of Gram neg

protein synth inhibitors

bind to 30S or 50S ribosomal subunit, interfere with translation

Ex: aminoglycosides — streptomycin, gentamicin, erythromycin, tetracycline

protein synth inhibitors - examples

aminoglycosides:

streptomycin, gentamicin, erythromycin, tetracycline

erythromycin limit

doesn’t penetrate membrane of Gram neg

streptomycin

protein synth inhibitor

gentamicin

protein synth inhibitor

erythromycin

protein synth inhibitor

tetracycline

protein synth inhibitor

inhibitors of cell membrane fxn

disrupt plasma mem of cells, causing ion and macromolecule leakage

Ex: polymyxins like colistin

colistin

a polymyxin that inhibits cell membrane function

colistin limit

doesn’t penetrate cell walls of Gram positive

nucleic acid synthesis inhibitors

RNA synth inhibitors target RNA pol —> rifampicin

DNA synth inhibitors target DNA gyrase —> ciprofloxacin

rifampicin

RNA synth inhibitor that inhibts RNA pol

ciprofloxacin

DNA synth inhibitor that inhibts DNA gyrase

folic acid synthesis inhibitors

inhibit diff enz in the folic acid synthesis pathway. Typically used together

Sulfanilamide, trimethoprim

folic acid synthesis inhibitors - examples

Sulfanilamide, trimethoprim

sulfanilamide

folic acid synthesis inhibitor

trimethoprim

folic acid synthesis inhibitor

antibiotics limits

• not all effective against each bacterium

• toxic or have harsh side effects

• 1% of all known antibiotics usable in humans

• allergies

• persistent bacteria start growing again if not taken long enough

• broad vs narrow spectrum

vancomycin limit

Gram neg outer membrane prevents vancomycin from entering cell

ciprofloxacin limit

nerve and tendon damage

tetracycline limit

hepatitis

antibiotic resistance

to inactivate antibiotics or prevent their damage —> naturally, to keep fighting for limited resources

antibiotic resistance mechanisms

overuse of antibiotics

can cause antibiotic resistance

horiz gene transfer

many antibiotic resistance genes are encoded on mobile genetic elements (plasmids, transposons), move easily btw bacteria via horizontal gene transfer

antibiotic resistant strains

MRSA

VRSA

VRE

CRE

MRSA

methicillin-resistant Staphylococcus aureus

• common

• resist B-lactams

VRSA

vancomycin-resistant S. aureus

• rare

• resist vancomycin

VRE

vancomycin-resistant enterococci

• getting common

• resist vancomycin

CRE

carbapenem-resistant enterics

• rare

• resist B-lactams

preventing antibiotic resistance

1.) decide with professional if antibiotic treatment is actually useful

2.) prevent infections via hygiene - reduce the need for antibiotics

Kirby-Bauer test

to test susceptibility/resistance of bacteria against antibiotics.

• spread dilute bacteria culture over Mueller-Hinton plate = lawn

• small paper discs w known [antibiotic]

• plate culture is incubated

• if bacteria are affected, zone of growth inhibition around disc. Resistant bact grow closer to the disc

how to inoculate with swab

why gram stain for KB test?

minimal inhibitory concentration (MIC)

best effective, yet lowest dose conc. of an antibiotic

avoids creating resistance opportunities in patients

how to determine MIC?

inoculate same bact culture into medium with different defined concs of an antibiotic

McFarland standard

to measure turbidity of diluted bacteria in saline. Hold against index card with black lines

MIC lab - serial dilution?

1:10

make serial dilution of antibiotic containing medium (TSB + 100 ug/mL gentamicin) into regular TSB.

Will later incolulate all 5 tubes with same amount of bacteria.

what do you need to do mix serial dilutions of gentamicin?

vortex

swirl all tubes well

remember this when comparing against McFarland standard

bacterial growth makes the liquid medium ________

turbic

biomass description

colony description

colony description words

colony shape

colony elevation

colony margins

colony optical quality