Laboratory Exercise 7: Microbiology of Bacteria

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Questions and Answers

What is the primary reason why staining procedures are not sufficient to identify bacteria at the species level?

Because different bacterial species have different metabolic activities

Which of the following biochemical tests is used to identify bacterial species based on their ability to break down casein?

Casein hydrolysis

What is the purpose of using differential and selective media in bacterial identification?

To distinguish between different bacterial species based on their metabolic properties

Which of the following is an example of a differential media?

EMB agar Signup and view all the answers

What is the term used to describe the ability of bacteria to break down carbohydrates and produce energy?

Fermentation Signup and view all the answers

What is the term used to describe the ability of bacteria to break down proteins and produce energy?

Hydrolysis Signup and view all the answers

What is the primary reason why different bacterial species produce different types of enzymes?

Due to differences in their genetic makeup Signup and view all the answers

What is the purpose of performing biochemical tests in bacterial identification?

To relate the results of the tests to the different types of metabolic activities Signup and view all the answers

What is the primary factor that affects the strength of a reaction in differential media?

Number of bacteria in the initial inoculum Signup and view all the answers

Which of the following is NOT a characteristic of fermentation?

Production of ATP in the presence of oxygen Signup and view all the answers

What is the purpose of pH indicators in differential media?

To measure the pH change in the medium Signup and view all the answers

Which pH indicator changes color at a pH of 7.4?

Phenol red Signup and view all the answers

Why is it important to choose the correct differential media when identifying unknown bacteria?

Because not all bacterial species will give definitive positive or negative results on all differential media Signup and view all the answers

What is the ultimate substrate molecule for fermentation?

Glucose Signup and view all the answers

What is the significance of the end products of fermentation in differentiating between bacterial species?

They are characteristic of individual bacterial species Signup and view all the answers

What is the purpose of glycolysis in fermentation?

To break down glucose into pyruvate Signup and view all the answers

What is the purpose of cooling the gelatin deeps to below 25oC in a bacterial growth experiment?

To slow down the liquefaction of gelatin Signup and view all the answers

What is the function of amylase in bacterial metabolism?

To break down starch into glucose and maltose Signup and view all the answers

What is the color of the agar where starch has been broken down by amylase?

Light yellow or gold Signup and view all the answers

What is the purpose of using milk agar in a bacterial growth experiment?

To detect the presence of bacterial caseinases Signup and view all the answers

Why is starch not directly transported into bacterial cells?

Because it is too large Signup and view all the answers

What is the function of proteinases in bacterial metabolism?

To break down proteins into amino acids Signup and view all the answers

What is the result of a positive gelatinase test?

The liquefaction of gelatin Signup and view all the answers

Why is a negative control important in a bacterial growth experiment?

To provide a baseline for comparison Signup and view all the answers

Study Notes

Laboratory Exercise 7: Metabolic Activities of Bacteria

Objectives

Learn how to use differential and selective media to identify bacterial species
Perform biochemical tests: carbohydrate fermentation, gelatin hydrolysis, starch hydrolysis, casein hydrolysis, urea hydrolysis, citrate utilization, catalase activity, tryptophan hydrolysis (indole test)
Relate test results to metabolic activities of individual bacterial species

Introduction to Identification of Bacterial Species

Staining procedures can discriminate between bacteria based on morphology, arrangement, cell wall structure, and presence of structures like flagella, capsules, and spores
Additional methods are required to identify bacteria at the species level
Each bacterial species has a characteristic metabolism that can be used to distinguish it from others
Metabolic properties can be used to identify unknown bacteria based on their ability to carry out biochemical reactions

Differential Media

Used to identify bacteria based on their metabolic properties
Examples: EMB agar, MS agar
Reactions in differential media can vary based on incubation time, temperature, number of bacteria, and inoculation method
Not all differential media provide meaningful results for all bacterial species
pH indicators are used to measure pH changes in the medium: phenol red, bromocresol purple, bromothymol blue

Fermentation

A metabolic process that breaks down glucose when O2 is not available
Includes glycolysis and additional reactions that produce end products (acids, alcohols, gases)
End products are characteristic of individual bacterial species
Bacteria can be differentiated based on their ability to ferment various carbohydrates and the end products that result

Gelatin Hydrolysis

Used to detect the presence of gelatinase enzyme
A positive result is indicated by the gelatin becoming liquefied due to enzyme activity
Important to note that the nutrient gelatin deeps must be cooled to below 25°C before recording a result

Starch Hydrolysis

Used to detect the presence of amylase enzyme
Starch agar is used to determine if bacteria can produce amylase
Bacteria that secrete amylase will break down starch in the medium surrounding the colony
Iodine is poured on the plate to detect starch breakdown

Casein Hydrolysis

Used to detect the presence of caseinase enzyme
Milk agar is used to detect the presence of bacterial caseinases
Caseinases break down casein outside of the bacterial cell so smaller products can be transported inside

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