unit 5 microb

AST step 1

patient presentation- symptoms of infection, collect blood wound or urine sample

AST step 2

empiric therapy- ab educated guess given immediately, based on history, local antibiograms, and gram stain

AST step 3

lab ID and AST- days1-3, isolation of pathogen, ID, and resistance tests

AST step 4

targeted therapy- ast results determine clinical action, de escalate to narrow spectrum drugs or optimize dose

AST

definitive diagnostic tool that enables targeted therapy, minimizes adverse events and maximizes drug efficacy

goal of antimicrobial agents

determine the extent of an organism’s acquired resistance via IN VITRO methods to guide IN VIVO therapy

vitro vs vivo

vitro= reaction in tube, vivo= what should be mimicked in patient

MIC minimum inhib concentration

lowest concentration of ab that inhib visible growth of an organism (ex broth dilution series)

MBC minimum bactericidal concentration

lowest concentration of ab that kills 99.9% of organism (ex subculture to drug free agar)

bacteriostatic

prohib org from growing and reproduction without killing

bactericidal

actively kill bacteria by attacking cell walls, disrupting proteins, or interfering with dna, irrivesible

inoculum standards

4-5 pure colonies, 16-24 hrs old (log phase), 0.5 mcfarland suspension, too few orgs= false S, too many= false R

ATCC

creates known quality control orgs to test accuracy of our tests

media

mueller-hinton M-H agar or broth, use with blood for specific fastidious orgs or CHOC

envirionment

35 C, ambient air, 16-24 hrs, rapid reads causing error with delayed resistance

CLSI AST M100

the flagship- annually updated, contains the MIC, zone diameter breakpoints, drug groupings, and quality control parameters

CLSI AST M02

the methods, materials, and procedures for the disk diffusion sus test

CLSI AST M07

standardized procedures for agar disk diffusion and broth dilution sus testing for aerobic bacteria

CLSI AST M11

dilution methods for testing antimicrobial sus of anaerobic bacteria

AST interpretive categories

S> sus-dose-dependent SSD> intermediate I> R> nonsus NS

susceptibility

high likelyhood of therapeudic sucess at standard dosing regimens

susceptible-dose dependent

success likely, must alter dose higher or more frequent to reach efficacy

intermediate

gray area, may work if the drug naturally concentrates at the infection site (urine) or maximum safe dosage is given

resistant

high likelihood of therapeutic failure, org not inhib by achievable safe serum levels

nonsusceotible

used when no resistant criteria exists for an org, prompts mandatory re ID and confirmation

diffusion disk or kirby bauer

agar diffusion of single concentration, qualitative S I R, cheap, flexible drug choice, labor intensive, lacks exact MIC

gradient diffusion or e-test

agar diffusion of exponential gradients, quantitative MIC, precise MIC, fastidious isolates, very expensive

broth microdilution

broth doubling dilutions, quantitative MIC, gold standard, automated, LIS-interfaced, expensive, fixed commerical panels limit drug flexibility

kirby bauer

disk diffusion- swab 0.5 mcfarland suspension in lawn, place discs within 15 min, incubate within 15 min, measure diameter of inhib zone, read back of plate with reflected light against black background, transmitted light for oxacillin vs staph or vancomycin vs enterococci, if BAP read from top of plate

e-test gradient diffusion

plastic strip with exponential concentration of gradient of a single ab and numerical scale, read mic when elliptical growth intersects strip (if in between read higher), used in s pneum, anaerobes, or to determine precise mic for a drug

foundation (vitek, microscan) broth microdilution

manual, ab are lyophilized in two-fold doubling dilutions across a tray, mic is the lowest concentration well with no visible turbidity, time consuming but customizable

modern lab microdilution

automated, sealed disposable cassette, advanced optical arrays to detect subtle bacterial growth or color change before human eye, reduces turnaround time from 24+ to 4-18, directly to LIS eliminating transcription errors, tests many orgs for class

automated databases

enter raw mic values and org ID= AMR report, therapeutic corrections (fix misleading in vitro results), flag atypical phenotype / hold for testing (bio impossibilities)

selective reporting strategies

only write need to know results, in vitro results guide therapy but do not garantee in vivo response, only report cascade down the line until first sus result, narrow-spectrum then R broad-spectrum

predictive patterns of sus

certian abx act as representatives for the whole group, if a abx is S other drugs in that class are not tested bc they follow the same pattern, R can change pattern of sus

mechanisms of R

enzymatic degradation (B-LAC attack abx), porin / membrane changes, enzymatic alteration (of drug molecules), efflux pumps (removes abx), target site modification (prevent binding of drug)

GP R- staphylococci and MRSA / ORSA

encoded by mecA gene (modifies penicillin binding proteins PBP2a), R in vivo (patient) to all B-LAC ab except teflaro, screen with cefoxitin or oxacillin, D zone clindamycin R = erythromycin R clindamycin S

GP R- enterococci

R cephalosporins, vancomycin R VRE (agar dilution on brain heart infusion BHI agar supplemented with 6 ug/ml vancomycin)

GP R- enterococci synergy efficacy

screening for high level gentamicin / streptomycin combined with a cell wall agent

GP R- strep pneumoniae

screen for relative vs frank penicillin R using MHA supplemented with 5% sheep blood and an oxacillin disk, less than 19mm= R confirm by MIC, >20= penicillin S

GR R- AmpC B-LAC

hydrolyzes all b-lac except cefepime (4th gen), no inhib by clavulanate AVOID cephalosporin treatment

GR R- CRE carbapenem- Resistant- enterobacteriaceae CRE

hydrolyzes carbapenems found via ertapenem R, no inhib by clavulanate and R to subclass III cephalosporins, confirm with MHT modified hodge test or molecular methods, stronger

GN R- ESBLs general

stronger than CRE, enzymes mediate R to extended spectrum cephalosporins (cef…), monobactams, and extended penicillins like B-lac, found in e.coli, k. pnem, and p. mirabilis

clavulanate rule

ESBLs are blocked by B-LAC inhib (clavulanic acid), but have no effect on cephamycins (cefoxitin) or carbapenems

if cefotaxin or ceftazidime tests R

add clavulanic acid fliping result to S increasing the inhib zone= ESBL producer

antibiogram

backbone of emperic therapy decisions and outbreak investigations before individual patient cultures are finalized, compiles MIC for individuals to determine drug use, bridge gap between bench testing and population health

diagnostic stewardship

the lab- run right test on right sample, provide timely and accurate MICs, compile cumulative AST data to build local antibiograms

antimicrobial stewardship

the clinic- de escalate therapy with lab targeted AST report, move from broad to narrow spec drugs

diagnostic and antimicrobial stewardship goal

preserve the efficacy of existing abx and slow the emergence of pan-resistant superbugs

Chromogenic Cephalosporinase Test (Cefinase) is used to detect the production of

B-lact