[Microbiology] Re: issues with suspected E. coli

JEDilworth via microbio%40net.bio.net (by bactitech from nospamhortonsbay.com)
Fri Apr 11 21:24:34 EST 2008


In order to accurately assess colony morphology, you HAVE to streak for 
isolation. A blob won't tell you much.

Touch your loop to just a little of your growth. You do NOT need a huge 
blob of growth. Take it to a new plate and streak for isolation. It's 
best to do in four quadrants, flaming after the initial inoculation 
spread and the first turn. Check your plate after overnight growth if 
you can.

What are your results? EC should be SIM Motile/Indole pos/H2S negative, 
Citrate negative (very important - if you are getting a positive citrate 
you do NOT have EC), VP negative, TSI A/A and maybe gas (can't 
remember), EMB should have metallic sheen. Streak for isolation on 
non-selective medium to make SURE your isolate is pure. Read all tests 
after overnight incubation. Don't be lazy and let these sit for a few 
days before reading. The clinical standard is 18-24 hours.

We used traditional media on my very first job in the 70's. Traditional 
media is still the gold standard. Most clinical labs use automated ID 
methods now, but the best way to learn is on media where you can see 
results. Our standard setup for lactose fermenters at my first job was 
SIM, Citrate, MRVP, Ornithine decarboxylase, Lysine decarboxylase, and 
control. We had more media for nonlactose fermenters. We used TSI and 
LIA in combination to screen nonlactose fermenting stool isolates. We 
also added urea and SIM to this setup also.

If you are taking an introductory micro class, chances are you will not 
be given anything too weird. There are nonlactose fermenting EC's out 
there clinically. Not sure if they would give a sheen on EMB. Clinical 
labs usually use MacConkey medium instead of EMB.

Micro is not an easy course. I tell our beginning techs fresh out of MT 
school that it will take them a year to feel comfortable in micro and 
that's working full time. I've been doing it since 1974 and still see 
new things all the time.

Good luck.

Judy Dilworth, M.T. (ASCP)
Microbiology


"Nathan Tisdell" <amurion from gmail.com> wrote in message 
news:mailman.105.1207838929.19248.microbio from net.bio.net...
>I am currently taking an introduction to microbiology class and am 
>having
> issues with a strain of bacteria. 



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