Gram staining and biochemical tests

Unknown Identification Guide

SC246: Unit 3 Assignment

Unit 3: Unknown Identification Assignment

� An unknown sample containing two unknown bacteria was submitted to a lab.

� Gram staining and biochemical tests were completed.

� You are provided with the lab results and have been asked to interpret the results and identify the unknown bacteria down to genus and species.

* Visit DocSharing to obtain the template for the unknown identification assignment.

Instructions

1. Review the unknown identification video linked in the assignment section (also found on the MasteringMicrobiology website). This video will walk you through the identification of an unknown bacterium, and will be very helpful in completing this assignment.

2. Access the Unit 3 assignment template (in Docsharing), review the results charts for both unknown samples and interpret the results. Fill in the ‘interpretation’ column after considering the observations from the lab. Consult the section below for assistance with the biochemical tests.

3. Identify the unknown genus and species and record it in the last row of the chart.

4. Answer the questions that follow the results charts.

5. Include all references and citations for any sources consulted (in APA).

6. Upload your assignment to the Unit 3 assignment dropbox in a MS Word document.

* For a full set of instructions, consult the assignment section in unit 3.

Instructions

To complete the charts:

� Fill in the interpretations column and use the identification matrix to identify your unknown.

� Record the genus and species of your unknown in the chart.

Example

� Fill in the interpretations column (highlighted) and use the identification matrix to identify your unknown.

� Record your the genus and species of your unknown in the chart (bottom row).

Test Observation interpretation

Gram stain reaction purple Gram +

morphology cocci cocci

fermentation

glucose Yellow media, gas +

sucrose Yellow media, no gas +

lactose red media, no gas –

indole yellow ring on top of test tube –

Methyl red red media +

urease yellow media –

Unknown organism: Genicus examplai

Biochemical Tests

Gram stain

� Gram negative bacteria stain red following the Gram stain. Record results as Gram negative.

� Gram positive bacteria stain purple following a Gram stain. Record results as Gram positive.

Fermentation (carbohydrate) Test

Negative for fermentation

Positive for fermentation, negative for gas production

+

Positive for fermentation, positive for gas production

+ –

Indole

� The indole test indicates the presence or absence of the enzyme tryptophanase.

Methyl Red

� Used to identify enteric bacteria based on glucose metabolism.

� Checks for mixed acid fermentation.

Urease

� Tests for the presence of urease.

� If the bacteria have urease, they can break urea down into NH3 and CO2.

Positive Negative

Identification Matrix

+ Positive for fermentation + Positive for fermentation and gas

Morphology Glucose Sucrose Lactose Indole Methyl Red Urease

Bacillus sphaericus Gram + rod – – – – – –

Bacillus subtilis Gram + rod + + – – – +

Bacillus megaterium Gram + rod + + – – + +

Bacillus cereus Gram + rod + + – – + –

Staphylococcus epidermis Gram + cocci + + + – + +

Micrococcus luteus Gram + cocci + – – – + +

Micrococcus roseus Gram + cocci – – – – – –

Staphylococcus aureus Gram + cocci + + + – + +

Proteus vulgaris Gram – rod + + – + + +

Enterobacter aerogenes Gram – rod + + + – – –

Serratia marcescens Gram – rod + + – – – +

Escherichia coli Gram – rod + – + + + –

Pseudomonas fluorescens Gram – rod + – – – – +

References and Photo Credits

� Tortora, G., Funke, B., & Case, C. (2007). Microbiology: An Introduction (8th ed.). San Francisco, CA: Pearson.

� Alexander, S., & Strete, D. (2001). Microbiology: A Photographic Atlas for the Laboratory. San Francisco, CA: Benjamin-Cummings.