Microbiology 102 – Experiment 7:
Simplified Key to the Twelve Known Cultures

Updated October, 2013
See very bottom of this page for links to more detailed keys with photos


organism gram
rx.
cellular
shape &
arrange-
ment
glucose
fermen-
tation*
lactose
fermen-
tation*
nitrate
reduc-
tion to nitrite
nitrate
reduc-
tion to N2
motility indole
produc-
tion
starch
hydro-
lysis
catalase
rx.
some notes on
colonial characteristics
Escherichia
coli
short rods FG FG + + + + Note that only two (SM and ML) are brightly pigmented (dark red and canary yellow, respectively). Also, PF has a yellowish-green pigment that diffuses into the medium. Your unknown would not be any of those organisms if you did not see that pigment. The other species on the list are generally whitish to gray.

Of the three gram-positive cocci, SE has small, white, opaque colonies; EF has small, gray, translucent colonies; ML has small, canary-yellow, opaque colonies.
Klebsiella
pneumoniae
short rods FG FG + +
Serratia
marcescens
short rods FG or F (F) or + + +
Pseudomonas
fluorescens
rods + + or +
Alcaligenes
faecalis
rods +
Micrococcus
luteus
+ cocci in clusters +
Staphylococcus
epidermidis
+ cocci in clusters F F + +
Enterococcus
faecalis
+ cocci in chains F F (+) or
Lactobacillus
plantarum
+ rods F F (+) or
Bacillus
sphaericus
+ large rods + or +
Bacillus
cereus
+ large rods F + + or + +
Bacillus
polymyxa
+ rods FG FG + + or + +

*  For Glucose and Lactose Fermentation Broths:
      F = fermentation without gas in Durham tube.
      FG = fermentation with gas in Durham tube (corresponding
          to the circled F on page 34 in the Manual).
       = negative.


A Dichotomous Key which shows one of many ways these organisms can be differentiated from each other is shown here.

Links to Keys with Photos:  Hopefully you were able to see positive and negative reactions for the various differential media in the lab for Exp. 7 (Period 2). If you did not get to see each of the known cultures that were set up by others in your general area, the results are shown with corresponding photos in the on-line manual here except for two new species (Bacillus sphaericus in place of B. subtilis and Alcaligenes faecalis in place of Chromobacterium violaceum). Dr. Tim Paustian is responsible for this great set of photos and descriptions.

  • For motility reactions (which are generally hard to interpret), use the simplified key above instead of the on-line photos.

  • You will find a special photo page of tubes and colonies of Alcaligenes faecalis here.  Both A. faecalis and Bacillus sphaericus are non-reactive (i.e., negative) in nearly all tests.