microbio practical 2 tests
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- If there is a mutation in an amino
- acid biosynthetic pathway gene, the bacteria cannot grow on DMM.
- TSA contains amino acids and can save a
- mutant via supplementation
Plain LB plate
contains no antibiotics
Salmonella-Shigella (SS) Agar
- Selective--- contains bile salts and brilliant green to inhibit Gram positive bacteria and most Gram negative bacilli
- Differential---Lactose/ neutral red -
- if ferment, form red colonies; not, colorless colonies
- Sodium thiosulfate/ Ferric citrate -
- colonies with black center are positive for H2S production
MSA --- Mannitol salt agar
- Selective for halophilic
- bacteria--- staphylococcal spp.
- Differential --- ferment mannitol à
- eg. Staphylococcal aureus
- do not ferment mannitol
- eg. Staphylococcal epidermidis
EMB--- Eosin Methylene Blue agar plates
Selective--- Eosin and methylene blue inhibit the growth of Gram positive bacteria.
- Differential--- lactose
- fermentation (yes, pink; no, colorless
- colonies) ---strong acid production
- produces ‘greenish
- metallic sheen’.
- from other Staphylococcal
- skin flora based upon its ability of mannitol fermentation and
Blood agar medium
used to detect (strongest to weakest) B, a, or y hemolysis
Brain Heart Infusion plate:
- BHI is a rich medium commonly used for the
- isolation of fungi and bacteria
staph catalase and coagulase
- Catalase test---
- Staphylococcal spp.
- Coagulase test ---
- Staphylococcal aureus coagulase
Why is mannitol salt agar used as a selective medium for normal skin flora?
- MSA selects for halophilic bacteria,
- and salt resistant bacteria can be found on the skin’s surface
- Crystal violet---inhibit most G+ bacteria except streptococcus
- Potassium tellurite---inhibit
- G- bacilli
- strep salivarius will produce gum-drops while strep mitis will make small,flat, light blue colonies
- Enterococcus will make dark black blue colonies
Is blood agar selective or differential? Explain
- It is actually an enriched medium,
- as most all bacterial species can grow upon blood agar. It could be considered
- differential based upon hemolysin production, but it is not selective.
What is one test that can be quickly used to differentiate between Staphylococci
- used to isolate gram negative organisms, at the same time differentiate organisms
- according to their lactose fermentation ability.
Bile esculin agar
commonly used to identify members of the genus Enterococcus (E faecalis and E. faecium).
- ---Bile inhibits the growth of other Gram +
- ---Sodium azide inhibits Gram –
- ---Differentiates the ability to hydrolyze esculin
many organisms hydrolyze esculin, but only Enterococcus spp. can do so in the presence of bile.
urea broth inoculations
- differential for urease production
Countable plate numbers
- spread plates are countable if they fall between 25 – 250 isolated colonies.
- for the spot plate dilutions you would look for spots with at least 10 colonies and
- no more than 30 colonies.
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