Microbiology Course: Identification of Unknown Microorganism Analysis

This microbiology assignment presents the identification of an unknown bacterium. The student performed various tests, including Gram staining, growth on selective media (MacConkey and EMB agar), and biochemical tests such as indole, methyl red, Voges-Proskauer, citrate, H2S production, lactose fermentation, urea hydrolysis, and gelatin hydrolysis. The results indicated a Gram-negative, rod-shaped bacterium identified as Enterobacter aerogenes. The report includes a detailed discussion comparing the results with other potential bacteria like Escherichia coli, Proteus mirabilis, Citrobacter freundii, and Serratia marcescens, along with a table summarizing the phenotypic and biochemical test results. The appendix provides further test data and comparisons.

Running head: MICROBIOLOGY
MICROBIOLOGY: DETECTION OF UNKNOWN MICROORGANISM
Name of the Student
Name of the University
Author Note

1MICROBIOLOGY
Table of Contents
Results..............................................................................................................................................2
Discussion........................................................................................................................................5
Reference List..................................................................................................................................6
Appendix..........................................................................................................................................7

2MICROBIOLOGY
Unknown
bacteria
Gram positive Gram negative
Gram stain
Indole, methyl
red negative
BacilliCocci
Voges-
Proskauer, citrate
positive
Gelatin
hydrolysis
negative
H2S
negative
Lactose
fermentatio
n positive
Results
The microorganism is Enterobacter aerogenes. Gram-staining revealed that the organism
is Gram-negative and is rod shaped or bacillus. It showed growth on MacConkey and EMB agar.
It showed positive citrate test, glucose and lactose fermentation. It gave negative results for
hydrogen sulphide production as no black precipitate was observed. It showed weak urea
hydrolysis, no gelatin hydrolysis. It showed positive motility as growth was observed away from
slab. The organism showed positive results for ornithine, lysine, sugar aldehyde, sorbitol, o-
nitrophenyl β-D-galactoside, p-nitrophenyl β-D-glucoside, p-nitrophenyl β-D-xyloside, p-
nitrophenyl n-acetyl-β-D-glucosaminide, malonate, γ-Glutamyl β napthylamide and adonitol
tests.

3MICROBIOLOGY
Table 1. Phenotypic and Biochemical tests
Unknown organism: Enterobacter aerogenes
Test Observation/Result of test Interpretation/Meaning
Gram Stain Colour: Pink/Red GRAM -
Cell length (m) 0.1 mm Not Required
Cell shape &
arrangement
Rod shaped Not required
Colonial
characteristics
on TSA
Whole colony shape: Irregular
Edge/margin: undulate
Surface: glistening umbonate
Colour: Translucent white
Not required
Growth on
selective
medium
MacConkey: + growth
EMB: + growth
PEA: - Growth
GRAM -
Oxygen
requirement
Growth in FTM: Growth greatest at
surface
Growth in GasPak: little bit of growth
Facultative aerobe

4MICROBIOLOGY
Citrate Test Positive Green-blue
Indole (SIM) No red -
Voges-Proskauer
Test
Yellow -
Methyl Red Test Yellow/orange Negative results
Glucose
fermentation
(KIA after 24 hours)
Color change, observed growth on
surface
+
Lactose
fermentation
(KIA after 24 hours)
The medium has been lifted due to gas
produced.
+
H2S (KIA or SIM after
48 hours)
No black precipitate. No sulfur
reduction
Negative results
Urea hydrolysis Slightly pink W +, weak positive
Gelatin
hydrolysis
Slightly movement, still gelatin like
consistency
Negative results
Motility Growth observed away from slab +

5MICROBIOLOGY
Discussion
All the microorganisms were Gram-positive rods. However, the microorganisms cannot
be Escherichia coli as it shows positive results for indole and methyl red test. E. coli shows
Voges-Proskauer and citrate test negative (Odonkor & Ampofo, 2013). But, the organism shows
indole and methyl red negative results, while citrate test is positive. Proteus mirabilis and
Citrobacter freundii are known to show positive methyl red tests. The organism shows positive
lactose fermentation. P. mirabilis and Serratia marcescens give negative results for lactose
fermentation, while C. freundii, E. coli and Enterobacter aerogenes show positive lactose
fermentation tests. C. freundii and P. mirabilis shows positive for hydrogen sulphide production,
while the organism shows negative for hydrogen sulphide production (Kumar, Tyagi & Gupta,
2012). Thus, the organism is not P. mirabilis and C. freundii. S. marcescens shows positive
results for gelatin hydrolysis, but the organism gave negative results for gelatin hydrolysis. Thus,
the organism is not S. marcescens. P. mirabilis and C. freundii gives negative for adonitol tests
but the organism showed positive for adonitol tests. On the otherhand, S. marcescens gives
negative for malonate test, but the organism showed positive results for malonate test. Thus, it
can be concluded that the organism is E. aerogenes. However, the errors present is that the
Voges-Proskauer test should have been positive. The urea hydrolysis test should be negative but
here it was weak positive.

6MICROBIOLOGY
Reference List
Kumar, D., Tyagi, N., & Gupta, A. B. (2012). Sensitivity analysis of field test kits for rapid
assessment of bacteriological quality of water. Journal of Water Supply: Research and
Technology-Aqua, 61(5), 283-290.
Odonkor, S. T., & Ampofo, J. K. (2013). Escherichia coli as an indicator of bacteriological
quality of water: an overview. Microbiology research, 4(1).

7MICROBIOLOGY
Appendix
Urea
Arginine
Ornithine
Lysine
Aliphatic thiol
Fatty acid ester
Sugar aldehyde
Sorbitol
-Nitrophenyl-,D-glucuronide
-Nitrophenyl-,D-galactoside
-Nitrophenyl-,D-glucoside
-Nitrophenyl-,D-xyloside
-Nitrophenyl-n-acetyl-,D-glucosaminide
Malonate
Proline--naphthylamide
-Glutamyl--naphthylamide
Pyrrolidonyl--naphthylamide
Adonitol
Tryptophane
Test
Code
UR
E
AD
H
OD
C
LD
C
TET LIP KS
F
SB
L
GU
R
ONP
G

GL
U

XY
L
NA
G
MA
L
PR
O
GG
T
PY
R
ADO
N
IN
D
Escheric
hia coli
DH5
1 15 75 84 2 0 1 90 92 95 1 0 0 1 3 44 1 2 9
8
Enterbac
ter
aerogene
s
4 7 97 97 1 1 96 97 0 98 98 98 97 92 1 93 98 88 0
Citrobac
ter
freundii
11 18 24 0 88 0 94 92 0 92 5 1 0 5 3 88 96 1 1
Proteus
mirabilis 98 13 91 1 98 5 1 0 0 0 0 0 0 4 7 96 0 0 1
Serratia
marcesce
ns
12 74 92 96 62 80 86 88 0 88 95 0 98 3 98 98 92 16 0