Unit 2 - Practical Microbiology and Infectious Diseases
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Unit 2 - Practical Microbiology
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Unit 2 - Practical Microbiology and Infectious Diseases
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PEARSON (PEARSON)
This is the second part to the Unit, detailing Practical Microbiology. It has criteria for pass, merit, and distinction which are all required for a distinction. It does involve data and photos used during our own experiments conducted with specific microbials and antimicrobials, so these areas wil...
Unit 2 - Practical Microbiology and Infectious Diseases
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2c:
Compare techniques to cultivate microorganisms
Spread plate- where bacteria is pipetted onto the agar plate before being spread with a sterilised glass
spreader.
Streak plate- Where bacteria is applied to the agar using a sterilised inoculating loop in a cross-hatch
method, ensuring bacteria doesn’t grow over each other.
Pour plate- Where bacteria is grown by pipetting the bacteria into an empty agar plate. Then cool, liquid
agar is poured on top of the bacteria before being closed and slightly swirled to mix the bacteria and
agar.
Advantages Disadvantages
Spread plate Bacteria shapes can be Fungal colonies may grow1
observed1
Streak plate Creates well isolated colonies2 Not suitable for isolation of
different microorganisms3
Pour plate Sensitive, good for growing Time consuming, prone to
small sample numbers4 error4
Reflect on quality of results
I think the best type of culturing technique is the spread plate. This is because using the spread plate, we
can obtain an even spread of bacteria on the agar, which then allows us to be able to complete further
investigations, such as observing how antimicrobials affect the growth of that bacteria. This is done by
measuring zones of inhibition around the antimicrobials. Streak and pour plates are good in their ways,
depending on what you are trying to sample. If you are trying to sample individual colonies, pour and
streak plates may be better.
Compare staining
There are three main types of stains used. They are: Methylene blue, Gram stain, and India ink stain.
Methylene blue stains the nucleus of the cell to analyse the genetic info, Gram stain stains the whole cell
to distinguish between gram positive and gram-negative bacteria, and India Ink staining stains cells to
observe the morphology of cells.
Stains Advantages Disadvantages
Methylene Blue Accurate stain Not very concentrated
Gram stain Differentiates bacteria, helps Inaccurate perception of what
in finding suitable antibiotic is gram positive or negative-
colour
, India ink stain Cell viewed clearly Internal structures cant be
viewed
Colony charactarisation as method of identification
Colony characterisation is done by observing the characteristics of a bacterial colony and describing
them based on their particular features. There are different categories, like: shape, margin, elevation,
size, texture and appearance. For example, bacteria might be classified as being circular, raised and with
an enticed margin.
However there can be an issue with using colony characterization. This is because a particular bacterial
colony might look like it fits into a particular colony when they don't. This is problematic as it can lead to
making false conclusions and identifying bacteria wrong. It can also lead to potential development of an
antimicrobial that isn’t effective. It is a method that can be prone to error due to the inaccuracy of
observation. But there are some things that can be done to increase the accuracy of characterisation.
This includes ensuring the bacteria has been properly spread on the agar plate15, and just generally
observing the colonies in a lot more detail. This can be done by comparing bacterial colonies to others to
increase the likelihood of its accuracy.
Benefits of using differential media
What is…
Liquid media- Things like broth which contain nutrients for the microbes to grow
Solid media- Agar. Bacteria is usually grown on agar. Provides nutrients for it to grow
Semi-solid media- Made with lower agar concentrations to give media a ‘custard’ consistency⁵
Benefits of…
Liquid media- Easy to replace, easy to handle, provides high level of nutrients⁶
Solid media- Individual bacterial colonies can be observed and characterises/counted⁷
Semi-solid media- Results are observable without microscope and are easily used⁸
Reason for health and safety and aseptic techniques
Health and safety regulations, like keeping long hair back, rolling sleeves, wearing safety goggles, to
ensure that we could be kept safe and to prevent the spread of bacteria and contamination. Health and
safety is important to ensure that people are kept safe. We used aseptic techniques to ensure that our
data is valid and reliable. This way, if the experiments were replicated, we could be quite sure that
results would be similar.
Evaluate methods for quantitative analysis of culture
One method of quantitative analysis of culture we did was use of a haemocytometer, We used a
Haemocytometer to measure the number of yeast cells in a dilute yeast solution. We were able to count
the individual number of yeast cells in the amount of solution we pipetted.
, Strengths Limitations
Inexpensive equipment Reduced accuracy, Human
error in counting
Easy to do More time consuming
Another method of quantitative analysis of culture methods is with the use of a colorimeter. It measures
the stock dilution of different solutions, which is also quantitative. We used the colorimeter to measure
dilutions of solutions containing yeast.
Strengths Limitations
Simple to use16 Not always accurate16
Quick results16 Calibration is
necessary16
Evaluate methods of Microbe plating
Advantages Disadvantages
Spread plate Bacteria shapes can be Fungal colonies may grow1
observed1
Streak plate Creates well isolated colonies2 Not suitable for isolation of
different microorganisms3
Pour plate Sensitive, good for growing Time consuming, prone to
small sample numbers4 error4
Evaluate selective culture methods
Selective culture methods are certain ways in which different types of bacteria can be grown in specific
growth medium. Selective culture methods include penicillin being tested on agar⁹. Another is
Cephalosporins, which is grown in a broth. We use selective culture methods to isolate a specific kind of
culture. Once the specific type of bacteria is grown in the selective media any other types of bacteria
that aren’t needed, are disposed of⁹. This way, the specific type of bacteria can be isolated⁹.
Selective culture is made by using an antibiotic, which inhibits the growth of some bacteria, but the
growth of the other bacteria isn't affected, causing the growth to be selective. One culture technique can
include growing bacteria on things like agar (maybe using different inoculating techniques like pour
plate, stab culture, etc). These methods can be selective to particular types of bacteria, for example
some agar may include eosin methylene blue which will only allow gram negative bacteria to grow, (it
inhibits growth of gram positive)17. Another method of selective culturing of bacteria is using liquid
culturing. The liquid media can be selective to a particular culture, for example, Terrific broth is used to
selectively grow E. coli species that have combined DNA.
Advantages Disadvantages
Selective culture makes it easier to observe and Limited knowledge of how bacteria will grow in
analyse a specific bacterial strain¹¹ presence of other microorganisms¹²
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