clinical microbio lab methods Flashcards by Keri Courtnage

aerobic likes what atmosphere conditions

oxygen rich

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anaerobic likes what atmosphere conditions

oxygen deplete

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microaerophilic likes what atmosphere conditions

low oxygen

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describe what an obligate/strict aerobe does

bacteria gathers at the top of the test tube to absorb maximal amount of oxygen

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describe what a microaerophile does

gathers at the upper part of the test tube, not at the top (requires oxygen, but at a low concentration)

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describe what a facultative anaerobe does

gathers mostly at the top, since aerobic respiration is most beneficial, but as lack of oxygen does not hurt them, they can be found all along the test tube

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describe what an aerotolerant does

bacteria are not affected by oxygen and they are evenly spread along the test tube

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describe what an obligate (strict) anaerobe does

bacteria gather at the bottom to avoid oxygen

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what are the steps of the gram stain

apply crystal violet (purple dye)
apply iodine (mordant)
alcohol wash (decolorization)
apply safranin (counterstain)

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a pink stain means gram ___

negative

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a purple stain means gram ____

positive

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no/minimal stain means ____

atypical

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cocci means ___

sphere

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bacilli means ___

rod

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why is gram stain important

typically first result provided by lab

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what are common configurations that gram stains reveal

pairs, chains, clusters

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what are common morphologies that gram stains reveal

cocci, bacilli

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cocci in clusters are ____

staphylococcus

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cocci in chains are ____

streptococcus

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coagulase positive staph is ____

staph aureus

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coagulase negative staph is _____

staph epidermidis, staph. saprophyticus, staph. lugdunensis

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group A strep is ____

pyogenes

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group B strep is ____

agalactiae

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strep pneumoniae may appear in ____

pairs (diplococci)

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what pathogens are rods (bacilli)

bacillus cereus, corynebacterium, listeria monocytogenes, clostridium

cocci in pairs and chains are ____

enterococcus (E. faecalis, E. faecium)

growth characteristics on visual inspection of culture plates: staph. aureus?

golden colonies grow on the agar

______ are extra-cellular bacterial proteins that degrade red blood cell lipid membranes

hemolysins

what pathogens have alpha hemolysis (green agar, partial hemolysis)

strep pneumoniae, viridans strep.

what pathogens have beta hemolysis (clear zone of hemolyzed RBCs around bacteria)

strep pyogenes, strep agalactiae, staph aureus

what pathogen has gamma hemolysis (no hemolysis)

enterococcus

what does the catalase test differentiate

streptococci and staphylococci

streptococci is catalase _____

negative

staphylococci is catalase ____

POSITIVE

what does the coagulase test differentiate

staphylococcus aureus from other staphylococci

staphylococcus aureus is coagulase ____

positive

staph. epidermidis is coagulase ____

negative

what are the methods for streptococci differentiation for presumptive ID?

1. catalase negative 2. hemolysis patterns on blood agar 3. agglutination 4. resistance: optochin disk test, bile solubility

what determines the lancefield groups (a, b, c, d, f, g)

agglutination: the beta-hemolytic group is further differentiated by antibody-antigen agglutination reactions

what are some biochemical differentiation tests for gram negatives?

lactose fermentation, indole test, enzyme expression

describe lactose fermentation test?

acid production and color change of the growth medium can indicate which sugars are used by the organism

describe indole test?

product of tryptophan metabolism

describe enzyme expression tests?

urease, oxidase

what are biological differentiation tests for gram negatives?

motility

what are the commonly encountered gram (-) bacilli lactose fermenters in the HS?

citrobacter freundii, escherichia coli, klebsiella aerogenes, enterobacter cloacae, klebsiella pneumoniae, klebsiella oxytoca, serratia marcescens

what are the commonly encountered gram (-) bacilli non-lactose fermenters in the HS?

pseudomonas aeruginosa, stenotrophomonas maltophilia, proteus mirabilis and vulgaris, providencia stuartii

proteus is urease __

pseudomonas is oxidase _

what are the commonly encountered gram negative cocci in the HS?

moraxella catarrhalis, neisseria meningitidis, neisseria gonorrhea, acinetobacter baumanii, haemophilus influenzae

what are the common anaerobes in the HS

actinomyces, bacteroides, clostridial

what are the common atypicals in the HS

legionella pneumophilia, chlamydophila pneumoniae, mycoplasma pneumoniae

what bacteria are acid-fast (do not take up gram stain)

mycobacterium tuberculosis, non-tubercular mycobacteria

what are rapid tests??

non-culture based: no culture growth is required

advantages of rapid tests

genus and species identification in hours versus days, may identify presence of resistance gene determinants, reduce time to effective therapy

what are limitations of rapid tests

typically no susceptibility data, and no culture prepared to conduct susceptibility testing. only some resistance gene determinants available for detection. not uniformly adopted across health systems due to cost

bacterial resistance genes can be ____ or _____

inherent to the organism (chromosome mediated) or acquired (transferred on plasmids)

what resistance genes are detectable by GenMark Eplex

mecA, mecC for MRSA vanA, vanC for VRE

what are 3 testing methods for antibacterial susceptibility

disk diffusion (kirby bauer), broth dilution, e-test

kirby bauer (disk diffusion) determines _____

zone diameter of bacterial growth inhibiton

broth-dilution and E-test determine ___

MIC

MIC and zone of inhibition are referenced to a set of standardized interpretations known as ___

breakpoints

what is the MIC

the lowest concentration of an antibiotic that inhibits visible growth of a microorganism

the ____ the MIC on an E-test, the more potent the antibiotic

smaller

the ____ the zone of inhibiton on a kirby-bauer test, the more potent the antibiotic

larger

who provides the standards for interpreting susceptibility testing results in the united states

CLSI- clinical laboratory standards institute

what are some rapid susceptibility tests for detection of resistance genes

molecular: DNA FISH (fluorescence in situ hybridization), PCR (mecA/mecC, VanA/VanB, CTXM) rapid phenotype others: PBP2a, lateral flow, agglutination

what kind of bacteria are difficult to grow and test susceptibility?

anaerobes due to environmental growth requirements

what is a full susceptibility panel

all results are released

what is a scaffolded susceptibility panel

results are released for broad/novel spectrum agents only if the organism is resistant to multiple drug classes

what is a selective susceptibility panel

results are released for only agents not expected to develop resistance during treatment

types of antibiograms

by region, by institution, by site of infection, for a specified period of time

who requires antibiograms

hospitals and LTCFs

what is the minimum number of isolates required for antibiogram reliability