How to use a Dichotomous Key 8/22/22

UNKNOWN TESTING - Applying your RESULTS!

(The "Word" questions on the Midterm) 

As you complete your Unknown Testing, remember to record all of your observations, changes in procedure, errors, repeats, and results in your Laboratory Record Book(LRB).  It is best to make a "chart of your results" in your LRB.... perhaps in "landscape" form in the LRB.  Remember to record the color changes for each test as you may misinterpret a result and later need to review each data point for accuracy. All data tests should be listed in your paper alphabetically.

Deciding on the GENUS:

Upon completion of your Unknown Testing and recording of your results, follow the Dichotomous keys (provided on the this website ONLY) according to your Gram Stain and morphology observations...  This allows you to determine your GENUS. CONFIRM your result with your instructor. 

Next, select the BEST reference text to be your "standard."  Usually this is a Bergey's Manual of Determinative Bacteriology (Editions 6,7, 8, or 9).  You may also use Jean MacFaddin's 3rd Ed. of the text GUIDE to the IDENTIFICATION of MEDICAL BACTERIA (it is very hard to use though).  Remember that the 6th and 9th Ed. of Bergey's is not good for Gram positive microbes.  The 8th Ed. of Bergey's is considered one of the best for both Gram negative and Gram positive microbes.  Be sure and review and chart the GENUS results for your microbe as well as charting the pertinent species results to use as a guide in determining your speceis (see below).  *To determine your species (specific epithet), practice charting using the "LIBRARY Assignment Bonus" below...  

Determining your species:

To determine your species... first confirm your Genus with the instructor.  Then, you must select a text for your main reference and chart the Genus results - remember that certain editions are better for gram negative and/or gram positive.  The best all around most recent text to use is Bergey's Manual of Systematic Bacteriology... Then, to determine your exact species, compare a chart of all your results with the charts of the species that are possible.  Remember, you must read and chart each species in your reference text (OMIT the species that are pathogenic to healthy humans as this is NOT your unknown)... 

NOTE: If you are a Gram positive endospore positive rod, ask the instructor to "narrow" the number of species choices for you...  

Compare your chart of results to each species and select the top two most alike.  Remember to "weigh" some tests that you trust more than others when deciding... 

Confirm your species with your instructor.  Write the Formal Unknown Report and submit it for "PRE-GRADING" so that you may correct your paper and make the highest score when it is submitted in your FINAL EXAM PACKET on Final Exam day.

You might search for other keys to your unknown data online at: http://www.google.com/search?q=%22dichotomous+key%22&num=10

CONFLICTING TEST RESULTS!

If tests conflict trust them in this order 1 best to 10 worst...

CARBOHYDRATE FERMENTATION: PR Broths, KIA, Plates, blank, blank, Litmus Milk Broth

MOTILITY: Stab gel deep, hanging drop, plate observation for "steps" along growth line

CASEASE: Plate, most anything in the universe, Litmus Milk Broth

HYDROGEN SULFIDE PRODUCTION: KIA, Peptone Iron Deep, blank, blank, Hektoen Agar Plate

CITRATE: Simmon's, Koser's

Determining the Gram Stain: Gram Stain, Acid Fast+ are Grm+ rods, Endospore+ are Grm + rods, Gram positive if grows on SALT agar, Gram negative if grows on EMB/Hektoen/MacConkey's Agar

Grows slowly = anaerobe or lipid covered

Candle jar, anaerobic jar, CO2 incubator, mineral oil covered slant = Obligate Anaerobe, Aertolerant or fac anaerobe

Vibrates =  no motility atrichous
Tumbles = motile



Click on the BLUE button below to follow that link...

CLICK HERE FOR special Dichotomous Key for Cocci

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Dichotomous Key