Agar plates Flashcards
You are asked to review a 32-year-old man who has been admitted with profuse watery diarrhoea 2 days after returning from Bangladesh.
On examination he is lethargic and profoundly dehydrated and you arrange for immediate fluid resuscitation. He is apyrexial and has a soft non-tender abdomen, although he opens his bowels twice during this time. The stool is of high volume and very watery with small amounts of mucus and no blood. He also reports occasional vomiting.
You visit the lab and review his stool culture plates which are 24 hours old. What would you expect to see?
clear colonies on sorbitol MacConkey agar
green colonies on TCBS agar
pink colonies on sorbitol MacConkey agar
red colonies on XLD agar
yellow colonies on TCBS agar
- Vibrio cholerae - diffuse watery diarrhoea
- yellow colonies on
TCBS (Thiosuplhate citrate bile salts sucrose) agar - TCBS inhibits other enteric bacteria, but allow Vibrio to grow
- V. cholerae forms large yellow colonies
- Other Vibrio species are green
clear colonies on sorbitol MacConkey agar - non-lactose fermenter
green colonies on TCBS agar - other vibrio species
pink colonies on sorbitol MacConkey agar - lactose fermenter
red colonies on XLD agar - shigella
run out of X and V discs so they are resorting to other methods to identify Haemophilus influenzae while they wait for new stock to arrive.
What other test could be used instead?
motility nitrocefin disc reverse CAMP test staph streak string test
Staph streak - culture Haemophilus, then streak Staph through it. Will demonstrate presence of a V factor requirement.
nitrocefein disc - turns red if beta lactamase production present
reverse CAMP - used to test for Clostridium Perfringens
string test - identify gram negative bacteria by adding KOH and observing stickiness of mixture
he sample is from a 55-year-old man who is on high dose steroids for treatment of an autoimmune condition.
On the blood and chocolate agar, there are dry chalky cream-coloured colonies. Gram stain shows long, branching gram-positive rods.
On Kinyoun’s staining they are not acid-alcohol fast. On repeat ZN with 1% sulphuric acid decoloriser they are weakly acid fast.
What is the most likely identify of the organism?
Actinomyces israelii Mycobacterium avium intracellulare Mycobacterium kansasii Nocardia asteroides Streptomyces spp
Nocardia asteroides
Nocardia is extensively branching, chalky appearance, and is acid fast with modified Kinyoun’s staining
Actinomyces is branching, but are not acid fast (even with Kinyoun’s)
Streptomyces is branching, but are not acid fast (even with Kinyoun’s)
Mycobacteria are not branching, and would be acid fast
What is importance of adding colistin or meropenem disc with identifying bacteria?
Stenotrophomonas
Serratia
Resistance or sensitivity to an antimicrobial disc can provide clue towards identification and early resistant pattern of organism
Meropenem - Stenotrophomonas is intrinsically resistant
Colistin - Serratia is intrinsically resistant
What colour are these species on XLD agar?
Shigella Salmonella E. coli Proteus Klebsiella Pseudomonas
Shigella - red (clear) colonies
Salmonella - red colonies with black centres
E.coli - yellow colonies
Proteus - yellow colonies
Klebsiella - yellow colonies
Pseudomonas - pink colonies
XLD Agar is a selective differential medium for the isolation of Gram-negative enteric pathogens from faecal specimens and other clinical material.
Which is a selective agar for Bordetella pertussis?
charcoal blood agar with cefalexin New York City (NYC) agar Hoyle’s tellurite agar cefixime tellurite sorbitol MacConkey (SMAC) agar cefsulodin irgasan novobiocin (CIN) agar
Bordetella pertussis
charcoal blood agar with cefalexin
Which is a selective agar for Corynebacterium diptheriae?
charcoal blood agar with cefalexin New York City (NYC) agar Hoyle’s tellurite agar cefixime tellurite sorbitol MacConkey (SMAC) agar cefsulodin irgasan novobiocin (CIN) agar
Corynebacterium diptheriae
Hoyle’s tellurite agar
It is enriched with horse blood and potassium tellurite inhibits growth of other organism
Which is a selective agar for Neisseria?
charcoal blood agar with cefalexin New York City (NYC) agar Hoyle’s tellurite agar cefixime tellurite sorbitol MacConkey (SMAC) agar cefsulodin irgasan novobiocin (CIN) agar
Neisseria
New York City (NYC) agar
Which is a selective agar for Yersinia entercolitica?
charcoal blood agar with cefalexin New York City (NYC) agar Hoyle’s tellurite agar cefixime tellurite sorbitol MacConkey (SMAC) agar cefsulodin irgasan novobiocin (CIN) agar
Yersinia entercolitica
cefsulodin irgasan novobiocin (CIN) agar
Which is a selective agar for E. coli O157?
charcoal blood agar with cefalexin New York City (NYC) agar Hoyle’s tellurite agar cefixime tellurite sorbitol MacConkey (SMAC) agar cefsulodin irgasan novobiocin (CIN) agar
cefixime tellurite sorbitol MacConkey (SMAC) agar
It is a modified MacConkey agar containing 1% D-sorbitol instead of lactose. The medium is made selective by the addition of cefixime and potassium tellurite which suppress other non-sorbitol fermenters but do not affect E. coli O157.
HACEK organisms - suspicion should be raised if gram neg rods fail to grow on MacConkey.
What unique feature can help identify the organism?
Haemophilus spp
factor X and V dependency spot indole positive pitting of the agar beta-haemolysis star-shaped colonies
factor X and V dependency
HACEK organisms - suspicion should be raised if gram neg rods fail to grow on MacConkey.
What unique feature can help identify the organism?
Aggregatibacter actinomycetemcomitans
factor X and V dependency spot indole positive pitting of the agar beta-haemolysis star-shaped colonies
star-shaped colonies
HACEK organisms - suspicion should be raised if gram neg rods fail to grow on MacConkey.
What unique feature can help identify the organism?
Cardiobacterium hominis
factor X and V dependency spot indole positive pitting of the agar beta-haemolysis star-shaped colonies
spot indole positive
HACEK organisms - suspicion should be raised if gram neg rods fail to grow on MacConkey.
What unique feature can help identify the organism?
Eikenella corrodens
factor X and V dependency spot indole positive pitting of the agar beta-haemolysis star-shaped colonies
pitting of the agar
HACEK organisms - suspicion should be raised if gram neg rods fail to grow on MacConkey.
What unique feature can help identify the organism?
Kingella kingae
factor X and V dependency spot indole positive pitting of the agar beta-haemolysis star-shaped colonies
beta-haemolysis
