Module 8 Flashcards
What is the basis for carbohydrate fermentation?
Acid formation is detected by pH indicators and indicates carbs have been fermented
Aerogenic vs anaerogenic
What pH indicators are used for carb fermentation?
Phenol red- red (k) and yellow (a)
Bromcresol purple- purple (k) and yellow (a)
Bromthymol blue- blue (k) and yellow (a)
What can cause erroneous results in carb fermentation?
Over incubation- reversion/false neg
Peptone utilization in oxygen- accumulation of alkaline products/false neg
Sterilization via heat
What is the basis for TSI testing?
Agar contains lactose, sucrose, glucose
Slanted media gives deep butt (oxygen free environment)
Loose caps for oxygen exposure on slant
How are TSI results interpreted?
K/A- ferments glucose, peptone metabolism in oxygen
A/A- lactose and/or sucrose
K/K, NC/NC- no fermentation
Aerogenic fermentation- split medium/bubbles
H2S- thiosulfate metabolism
What is KIA agar?
Same as TSA but without sucrose
What can cause false negatives in TSI?
Shallow butt
Tightly capped tube
Extended incubation (reversion)
What is the basis for ONPG testing?
Ability to ferment glucose
Substrate moves into cell without permease and is hydrolyzed to a yellow product (orthonitrophenol) and galactose if galactosidase is present
What enzymes are required for lactose fermentation?
Permease- moves lactose into cell
Galactosidase- hydrolyzes lactose
What are the categories of lactose fermentation and their ONPG reaction?
LF- both enzymes, pos
NLF- neither enzyme, neg
LLF- permease only induced by lactose, delayed, pos
What do LF look like on MAC?
Pink colonies
What is the basis for indole testing?
Determines ability to decompose tryptophan to indole using tryptophanase
Reagent reacts with indole to give red coloured product
Requires high tryptophan concentration
Most Enterobacteriaceae are positive
What are the types of indole testing?
Tube- Kovac’s reagent added to broth, pos = red, neg = yellow
Spot- rapid, growth add to filter paper with reagent, pos = blue-green or pink-lavender, neg = yellow-brown
What is the basis for Voges Proskauer (VP) testing?
Tests for the production of butylene glycol from carb metabolism
Detects intermediate product (acetoin)
Oxygen required
Neg- colourless/brown
What types of VP tests are there?
Methyl red- no longer used
Rapid- inoculum where fermentation has occurred is added to creatine solution followed by KOH and alpha naphthol, red = pos
Incubated- broth incubation, reagents added
What is the basis for citrate testing?
Determines whether citrate can be the sole use of carbon
Alkaline pH results in deep blue colour
Simmon’s citrate- bromthymol blue (neutral = k, blue = a)
What causes false results in citrate?
Liquid medium- C and N carryover/false pos
Oil overlay- no oxygen/false neg
Not completely filled- false neg
What is the basis of urease testing?
Determines ability to decompose urea to CO2 and NH3 using urease
Phenol red is pH indicator
Pos- alkaline pH, red
What types of urease testing are there?
Broth- highly buffered, PO4 resist pH changes, less sensitive, oil overlay
Christensen’s agar- less PO4, more sensitive, glucose fermented by negative organisms (yellow)
What is the basis for phenylalanine deaminase (PDA) testing?
Determines whether deaminase present to remove amino acid and give keto acid and NH3
Pos- acid detected by ferric chloride, dark green
Neg- ferric chloride gives yellow
Correlates with TDA
Commercial systems- deamination gives black colour
What is the basis for decarboxylase testing?
Determines ability to remove carboxyl group from an amino acid using decarboxylase
Use lysine and ornithine
Miller medium- bromcresol purple and creosol red are pH indicators
Incubated free of oxygen (oil overlay)
Requires control without amino acid
What are the results for decarboxylase testing?
Pos- acid from glucose fermentation promotes decarboxylation, alkaline amine neutralizes and accumulates, deep purple
Neg- ferments glucose, remains yellow, no neutralization
What can cause false results in decarboxylase testing?
Over incubation- reversion/false pos
Oxygen- false pos
What are conventional Enterobacteriaceae ID tests used for?
Screening isolates for enteric pathogens
Supplemental tests for commercial systems
Presumptive ID
