SCI 250 Week 1 Identifying Bacteria

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SCI 250 Week 1 Identifying Bacteria

Review Figure
6.25 and Table 9.5 of Microbiology.

Complete Appendix
B to classify bacteria and identify a strain of bacteria responsible for a
simulated food poisoning outbreak.

Appendix B – Identifying Bacteria

 

Bacteria
identification is accomplished in a number of ways. Two common tools
microbiologists use to identify unknown bacteria include dichotomous key and
biochemical tests. The dichotomous key is useful when a microbiologist only
needs to know which group an unknown microbe belongs to on a general level.
When a microbiologist needs to identify a specific bacterium, biochemical tests
are used.



PART ONE: GENERAL
BACTERIA IDENTIFICATION



Review the dichotomous key in Figure A, the bacterial shapes in Figure
B, and the Gram stain information below. You will use all three to determine to
which major group unknown bacteria belong.

 




Figure A: A dichotomous key for classifying major groups of
bacteria.


[Figure
9.4 in Microbiology text]















 




Figure B: The most common bacterial shapes.
[Figure
4.1 in Microbiology text]










 
Shape Types:
 
Comma, or
Club-shaped, rods = Vibrio
Rods =
Coccobacillus, Bacillus
Spherical =
Coccus
Spiral or
Helical = Spirillium, Spirochete
 




Gram Stain Results:

 

Purple = Gram positive

Red = Gram negative

Neither purple nor red = No cell wall (neither Gram positive nor Gram
negative)

 

 

 

Review Figure C on the following page. Use Figure C to identify the type
of arrangement displayed by the unknown bacteria. Read through the Sample
Identification on the following page for an example of how to identify
bacterial groups and arrangements using Figures A, B, C, and the Gram stain
results.




Figure C: Arrangement of Bacteria




Rods
and Clubs





 
Single
 
 
Strepto-




Cording
 


Snapping
 
 
Palisades
 
 
 
Chinese
Letters


 





Cocci




 
 
Single
 
Diplo-



Tetrad





Sarcinae









Staphylo-






Strepto-
 


 





Sample
Identification


 

After performing a Gram stain, you observe the following under a
microscope:

 



(Cells are stained red.)

 

·        
Begin with item 1 on the dichotomous key (Figure
A). Because the organisms are red, they are not Gram-positive. According to the
key, continue to item 3.

·        
Because the organism is red (not clear or another
color), it is indeed Gram-negative according to item 3. The key directs you to
item 5.

·        
Consider the shape of individual cells for item 5.
According to Figure B, the shape type is closest to spherical. (Note: spherical
is circular.)

·        
The bacteria belong to the Gram-negative cocci group.

·        
Now use Figure C to describe arrangement of bacteria.
(The bacteria are not rod- or club-shaped, so focus on the cocci arrangements.)
Identify the arrangement that is most prominent.

·        
The bacteria are arranged in a diplo- fashion.

·        
Enter the group and arrangement in the table.

 

Application

 

Use Figures A, B, C, and Gram stain results to identify group and
arrangement of bacteria. Continue to Part Two after completing the table.

 

 

 

 







View from Microscope


Group of Bacteria


Arrangement




 
(stained red)


Gram-negative cocci


Diplo-





(stained purple)


 


 





(stained purple)


 


 





(stained red)


 


 





(stained red)


 


 





(stained purple)


 


 





(stained purple)


 


 







 



PART TWO: CAse study
scenario: identify specific bacteria through biochemical testing





To gain an understanding of the processes involved with identifying bacteria
through biochemical tests, access Chapter 6 of the text in WileyPlus located on
the Week One course page. Once in WileyPlus (Chapter 6), select the “Bacterial
Identification by API” link located under the heading, Take Another Look. Once selected, review the information and watch
the Flash-animated movie (animation) located within this link. Then, review the
following scenario and answer the questions that follow.

Case Study

A recent outbreak of food poisoning has occurred
in a community. One possible source of contamination may be the produce that is
grown and distributed locally at a farmer’s market. A test sample of some of
the produce revealed evidence of bacterial contamination. The bacteria sample was
tested in a microbiology lab and showed the results that follow. The laboratory
also performed a Gram stain of the isolated bacteria and ran a number of
biochemical tests to aid identification. The biochemical tests were assayed
using the Analytical Profile Index (API) 20E system for identification of Enterobacteriaceae
and other gram-negative bacteria.

Figure 1.
Microbiology laboratory results – Unknown bacteria present on produce (API
Results)




Figure 2.
API Results of Unknown Bacteria in Text Format







Gram
stain:



Gram-negative


ONPG


ADH


LDC


ODC


CIT


H2S


URE


TDA


IND


VP


GEL


GLU




Bacteria: Unknown


+


-


+


+


-


-


-


-


+


-


-


+






 






MAN


INO


SOR


RHA


SAC


MEL


AMY


ARA




+


-


+


+


-


+


-


+






Based
on Gram stain results and the knowledge that this bacterium caused food
poisoning, the laboratory is able to narrow down the possibilities to three
bacterial strains. Figure 3 shows the API results:

Figure 3.
API Results of Known Bacterial Strains in Text Format







Gram
stain:

Gram-negative


ONPG


ADH


LDC


ODC


CIT


H2S


URE


TDA


IND


VP


GEL


GLU




Bacteria: Salmonella


-


-


+


-


-


-


-


-


-


-


-


+






 






MAN


INO


SOR


RHA


SAC


MEL


AMY


ARA




+


-


+


-


-


+


-


-






 

 

 






Gram
stain:

Gram-negative


ONPG


ADH


LDC


ODC


CIT


H2S


URE


TDA


IND


VP


GEL


GLU




Bacteria: Shigella


-


-


-


-


-


-


-


-


-


-


-


+






 






MAN


INO


SOR


RHA


SAC


MEL


AMY


ARA




-


-


-


-


-


-


-


-






 






Gram
stain:

Gram-negative


ONPG


ADH


LDC


ODC


CIT


H2S


URE


TDA


IND


VP


GEL


GLU




Bacteria: E. coli


+


-


+


+


-


-


-


-


+


-


-


+






 






MAN


INO


SOR


RHA


SAC


MEL


AMY


ARA




+


-


+


+


-


+


-


+






 

Application

Compare
the API biochemical test results in Figure 2 (unknown bacteria from produce)
with API test results in Figure 3 (known bacterial strains identified in the
gray boxes on the left). The bacterial strain in Figure 3 that matches Figure 2
will identify the bacterial strain causing illness. Based upon your
observations, which of the bacteria in Figure 3 is the most likely cause of the
food poisoning outbreak? 
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