Practicals Flashcards
what happens in the catalase test
catalase enzyme catalyses the dissociation of hydrogen peroxide to oxygen and water
what bacterias are catalase positive
staphylococci
corynebacteria
bacillus
what bacterias are catalase negative
streptococci
lactobacilli
clostridium
what is the procedure of the catalase test
pick a colony of chosen microbe, smear over clean glass slide, place a drop of hydrogen peroxide onto this smear
what shows a positive catalase test
organisms will fizz
what is the problems with catalase test
hydrogen peroxide is corrosive and oxidising
what happens in the oxidase test
detects enzyme cytochrome C oxidase, more discriminatory for Gram-negative than Gram-positive bacteria
what is an example of enterobacteriaceae
E. coli
what is the oxidase test good for distinguishing species
particularly useful for distinguishing species of Pseudomonas and Vibrio from members of Enterobacteriaceae
what is the procedure for the oxidase test
use plastic loop, get colony from plate, smear lightly on strip of filter paper impregnated with oxidase reagent, denoted by development of deep purple colour, will occur in 15 seconds
what happens if colour develops after 15 seconds
after a long period should be ignored
what happens in the coagulase test
Staphylococcus aureus has the useful differential characteristic of coagulating plasma whereas the related Staphylococcus epidermis lacks this property
what the procedure for the coagulase test
loop to place 2 separate drops of saline on microscope slide
remove 2 or 3 colonies of test organism with loop and gently rub into each drop of saline, form creamy suspension
add loopful of plasma to one of the bacteria suspensions
new loop, loopful of saline to the other
mix gently by rocking the slide, if coagulase positive, bacteria will quickly clump to give speckled granular appearance
what is the second suspension for in the coagulase test
serves as a saline control to check for auto-agglutination
what happens in the DNAse agar test
Staphylococcus aureus produces a deoxyribose whereas coagulase-negative staphylococci do not
distinguishing characteristic can therefore be tested
what is the procedure of the DNAse test
loop to get colony of test organism and stab inoculate Aa DNAse plate (puncture agar)
inoculate plate with control Staphylococcus epidermis, space between the two inoculations
incubate 37degrees C overnight
flood plate with concentrated hydrochloric acid and examine clearing zones of inoculation sites
what is the problem with DNAse test
hydrochloric acid is corrosive
how is streptococci identified
method uses Prolex Streptococcal grouping latex kit, provides most rapid serological identification of groups A, B, C, D, F and G Lancefield groups of streptococci grown on agar plates
what is the procedure to identify streptococci
add 1 drop of reagent to round bottom tube
pick complete isolated beta-haemolytic colony from streptococcal-selective plate gently mix with reagent 1, ensure turbid solution
add 1 drop of reagent 2 to tube, gently mix
add 5 drops of reagent 3 to tube, gently mix
label cards A, B, C, D, F, G dispense drop of each specific latex suspension, mix with loops
group Streptococci
what is a problem with Streptococci grouping
take care when preparing suspensions of bacteria to avoid generating aerosols
what happens in the API test
differentiation between gram-negative enteric bacteria based on variable abilities to metabolise selection of biochemical substrates
substrates are provided in test strip, after addition of test bacterium, overnight incubation, results are scored, used to generate a numerical identifier
what is the procedure of the API test
inoculate tube of distilled water with loop of organism
gently homogenise solution with sterile Pasteur pipette
inoculate bacterial suspension into each well with sterile pipette used to homogenise above
incubate strip at 37degrees C for 18-24hrs
add appropriate reagents
interpret results of test
what is a problem of API test
when preparing suspension of bacteria to avoid generating aerosols
which bacteria are bacilli
E.coli P.aeruginosa C.perfringens B.cereus S.typhimurium
what bacteria are cocci
S.aureus
S.pyogenes
what is a strict anaerobe
will only grow in complete absence of oxygen
what is a strict aerobe
can only grow when oxygen present
what is a facultative anaerobe
prefer aerobic conditions but can also grow aerobically
what is indicated if bacteria grow on MacConkeys agar
bile tolerant as agar contains bile
also contains lactose, bacteria can use this lactose to grow and form bright red colonies = lactose fermentation
if not red then no lactose fermentation
what is MacConkeys agar used for
indicator medium
what does oxolinic acid do
chemotherapeutic agent will inhibit the growth of most gram-positive bacteria, much less inhibitory to streptococci than other species
what is COBA
colistin oxolinic acid blood agar
what does COBA agar growth show
used to selectively isolate streptococci bacteria
Colistin inhibits the growth of gram negative bacterias
Oxolinic acid inhibits the growth of most Gram positive bacteria other than Streptococci
how is enterobacteriaceae be identified
API strips, biochemical activities
antibiotic sensitivity tests examples
- diffusion tests
- broth dilution
- agar dilution tests
