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What are gram negative rods that can ferment glucose?
enterobactericeae
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What are coliforms?
Enterobacteriaceae that can also ferment lactose (lactose positive)
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What are non-coliforms?
Enterobacteriaceae that can not ferment lactose (lactose negative)
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What do lactose fermentation tests do?
help to distinguish coliform from non-coliform Enterobacteriaceae
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Which Enterobacteriaceae are coliform (lactose positive)?
- Escherichia coli
- Klebsiella pneumoniae
- Enterobacter aerogenes
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What are the three gram negative media and testing?
- eosin methylene blue agar (EMB)
- MacConkey agar (MAC)
- triple sugar iron (TSI)
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What is the eosin methylene blue agar (EMB)?
- selective media for gram negatives
- the dye in the agar is bacteriostatic to gram positives
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What does EMB contain?
lactose and pH indicators
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What does the uninoculated color of EMB look like?
clear red
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What does non-lactose-fermenters look like on EMB?
- colonies appear light colored, transparent
- lactose negative
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What does lactose fermenters look like on EMB?
- take up some of the dye from the media
- colonies appear purple red
- lactose positive
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What will Escherichia coli look like on EMB?
will have a metalic/green sheen
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What is MacConkey agar (MAC)?
- selective media for growing gram negatives
- growth or no growth indicator of gram negative bacteria
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What is MAC a preliminary test for?
enteric pathogens
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What does uninoculated MAC look like?
clear light red
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What nutrients are in the MAC?
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What is the bile salt for in MAC?
selective for Enterobacteriaceae and salt tolerant gram negatives - suppresses gram positives
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What is crystal violet for in MAC?
suppresses growth of gram positives
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What are the indicators in MAC?
- lactose: differentiates between lactose and non-lactose fermenters
- neutral red indicator: colorless at pH above 6.8, red at pH less than 6.8
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What does lactose fermentation produce in MAC?
acid
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What do lactose fermenters look like in MAC?
- grow as pinkish, purple-red, brick-red colonies
- turn MAC agar red
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What do non-lactose fermenters do in MAC?
attack the peptone in the agar
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What kind of reaction does non-lactose fermenters have in MAC?
alkaline reaction
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What do non-lactose fermenters look like in MAC?
light pink, white, transparent colonies
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What are clinical specimens usually cultured on?
both blood agar and MacConkey agar
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What does no growth on MAC and a good growth on blood agar mean?
strongly suggests that the pathogen is gram positive
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What is the TSI agar for?
initial differentiation of enteric bacteria
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What nutrients are in the TSI agar?
- glucose, lactose, sucrose
- iron
- phenol red pH indicator
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What does the TSI agar demonstrate the ability for the organisms to do?
- to ferment sugars
- to produce gas by fermentation of sugars
- to produce hydrogen sulfide
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What does an uninoculated TSI agar look like?
clear red
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What does sugar fermentation in the TSI agar do?
- causes production of acid products
- acid: yellow
- alkaline: red/pink
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What may be produced in the agar with fermentation of sugar?
gas
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TSI agar detects the ability of the organism to reduce _____ to _____.
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What reacts with the hydrogen sulfide to produce iron sulfide and what color does it produce in the agar?
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What do we do an inoculation slant in a TSI agar?
- stab inoculation with needle
- use only one colony
- when withdrawing the wire, streak the surface of the agar
- leave cap slightly loose
- incubate for 16 hours and read
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How do we read the TSI agar?
- check slant and butt for...
- acid or alkaline reaction
- hydrogen sulfide production
- gas production
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What happens early in incubation with the TSI agar and why is this important for us to know?
- early in incubation the butt and slant will turn yellow due to glucose metabolism
- important for use to know so that we don't read it to early and think it is an acidic reaction
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What happens if we leave a TSI agar in an anaerobic condition?
- the slant will stay yellow
- always loosen the cap of the slant
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