Antibiotic susceptibility testing

chemical substance which has the capacity to inhibit the growth of bacteria
often derived from a fungi

Antibiotic

peptidoglycan, nucleic acids, folic acid pathway, protein synthesis

Targets for antibiotic

target Penicillin-binding proteins. PBP facilitate the cross-linking of peptidoglycan. Beta lactams bind to PGPs and results in failure to cross-link peptidoglycan

Beta lactams

because they have a thicker layer of peptidoglycan

Why are beta lactams most effective against gram pos bacteria

these have the mecA gene which encodes on altered PBP protein called PBP2a. Beta-lactams can't bind with this PBP

Methicillin Resistant Staph aureus (MRSA)

PCR or antigen testing

How do we detect a MRSA organism

forms hydrogen bonds with the amino acid chains that connect the sheets of peptidoglycan. Prevents the polymers to form cross-links. Useful for MRSA

Vancomycin

interfere with the folic acid pathway

Sulfonamides and Trimethoprim

sulfonamides interrupt in the beginning of the pathway, then trimethoprim interrupts later on. Synergistic

How do sulfonamides and trimethoprims work

sulfamethoxazole/trimethoprims

strep pyogenies is resistant to what drug

binds to DNA topoisomerase and gyrase. Inhibits the action of these enzymes resulting in decreased functionality in DNA replication

Quinolones

this drug blocks the activity of RNA polymerase, preventing chain elongation

Rifampin

This drug prevents the binding of tRNA to the smaller 30s ribosome A site. Causes mis-translation and the production of abnormal proteins (bactericidal)

Aminoglycosides

this drug binds to the 30s ribosomal subunit preventing the proper docking of tRNA. Causes premature release of the tRNA and termination of protein synthesis (Bacteriostatic)

Tetracyclines

bind to the 50s ribosome. Terminates translation by slipping into the cavity between arriving tRNA

Macrolides

antibiotic is serially diluted, inoculated, incubated, observed for growth, minimal inhibitory concentration (MIC) is the tube with the least amount of antimicrobial showing no growth of the organism
MIC=8 for picture

In Vitro susceptibility testing-tube dilution tests
In the picture what is the MIC

diluted, lyophilized drugs placed in 96 well trays, dilution of organism added, susceptible intermediate or resistant determined by which dilution is showing inhibition

Broth microdilution tests

MIC-minimum inhibitory concentration

bacteriostatic test

MBC-minimum bactericidal concentration

bactericidal test

Kirby-Bauer assay: standardized suspension of the organism is compared to a .5 McFarland standard. Spread over the surfact of a Mueller-Hinton agar plate, incubate, zones of inhibition are measured

Disc Agar diffusion method

innate characteristics that are transmitted vertically (mother cell to daughter cell)

intrinsic drug resistance

caused by changes in the usual genetic makeup of a microorganism, leading to altered physiology and structure (horizontal transfer)

Acquired drug resistance

SXT

Strep A is intrinsically resistance to this drug

organism makes beta lactamases-usually intrinsic, bind to beta lactam antibiotic and inactivates them

Intrinsic resistance to beta lactam antibiotics

detects the enzyme that splits the beta lactam ring, if the disk turns pink the organism has beta lactamase enzyme

beta lactamase testing

Methicillin resistant staph aureus

MRSA

staph aureus acquired mecA gene that codes for an altered PBP results in reduced affinity for penicillin drugs

What is MRSA

Inhibit cell wall synthesis

beta lactams, glycopeptides

beta lactams

carbapenems, cephalosporins, monobactams, penicillins

depolarize cell membrane. inhibit ion transfer.

lipopepties

inhibit protein synthesis by binding to 30s ribosomal subunit

aminoglycosides, tetracyclines

inhibit protein synthesis by binding to 50s ribosomal subunit

chloramphenicol, lincosamides, macrolides, oxazolidinones, streptogramines

inhibit nucleic acid synthesis

quinolones/fluoroquinolones

inhibit metabolic pathways

sulfonamides, trimethoprim

what are bacteroides (anaerobes) intrinsically resistant to

aminoglycosides, many beta lactams, quinolones

all gram positives are intrinsically resistant to

aztreonam

enterococci are intrinsically resistant to

aminoglycosides, cephalosporins, lincasamides

listeria monocytogenes is intrinsically resistant to

cephalosporins

all gram negatives are intrinsically resistant to

glycopeptides, lipopeptides

e. coli is intrinsically resistant to

macrolides

klebsiella spp. are resistant to

ampicillin

serratia marcescens is resistant to

macrolides

pseudomonas aeruginosa is resistant to

sulfonamides, apmicillin, cephalosporins, chloramphenicol, tetracycline

stenotrophomans maltophilia is resistant to

aminoglycosides, beta lactams, carbapenems, quinolones

acinetobacter spp. are resistant to

ampicillin, glycopeptides