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TEST BANK FOR PRESCOTT'S MICROBIOLOGY, 12TH EDITION BY JOANNE WILLEY ALL CHAPTERS COVERED
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Test Bank for Prescott's Microbiology, 12th Edition by Joanne Willey Chapter 1 – 42
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MIB 3704 – PORTFOLIOBY
SHAYLENE MOONILAL
STUDENT NUMBER – 55202063
,Shaylene Moonilal – 55202063
MIB 3704 Portfolio
Week 1: Section A – Bacteriology and Environmental Microbiology
Experiment 1: Identification of the skin microbiota/ Bacteria on selected areas of the human skin.
ABSTRACT:
The skin is the body’s largest organ home to trillions of bacteria, the skin bacteria can be collectively
referred to as skin microbiota. The aim of this experiment was to isolate and identify bacteria on
three selected areas of the skin; the back part of the neck, outer edge of an ear (retroauricular
crease) and outside forehead (glabella). To isolate and identify, we used general purpose, differential
and selective media. The results showed that the selected three areas were dominated by gram
positive bacteria.
INTRODUCTION:
Skin is at the interface between the complex physiology of the body and the external, often hostile,
environment, and the semipermeable epidermal barrier prevents both escape of moisture and entry
of infectious or toxic substances. (Segre, 2006).
Microbes found on the skin are usually regarded as pathogens, potential pathogens or innocuous
symbiotic organisms. (Cogen et al, 2008). Where innocuous means that it is not harmful to the host
or in this case the skin.
We need to understand this vast array of microbes living on the skin, as these microorganisms play a
huge role in human health. Most of the microbes found on the skin are mutualistic. That is, both
species benefit and depend on each other for survival (Cogen et al, 2008). The other skin microbes
are commensal, that is, they offer no benefit to the host, but are also not harmful.
However, according to Cogan et al (2008), many microbes found on the skin can be regarded as
pathogens as mentioned above. It is therefore important to identify the different types of
microorganisms inhabiting the human skin. So that we can determine which bacteria pose a risk to
us and which bacteria are harmless or beneficial.
The aim of this experiment was to isolate and identify bacteria found on the back of the neck, outer
edge of the ear (retroauricular) and outside forehead (glabella). To achieve this isolation and
identification, general purpose, selective and differential media would be used in the form of agar
plates.
,Shaylene Moonilal – 55202063
MIB 3704 Portfolio
MATERIALS AND METHODS:
Bacterial cultures were prepared on the following agar plates; Eosine Methylene Blue (EMB) agar,
Mannitol Salt Agar (MSA), and Nutrient Agar (NA). The agar plates had been previously prepared (on
Day 2 of the practical session). NA is a general purpose medium, while EMB and MSA are both
selective and differential media.
Using a moistened sterile swab, the outer edge of the ear of one of the group members was
swabbed. The swab was spread over the surface of the NA plate. The process was repeated for EMB
and MSA, using a fresh swab and a different person of our Group in each case.
The above process was repeated for the other selected areas of the skin; the neck, and outside
forehead. This yielded a total of nine plates (3 EMB, 3 NA, and 3 MSA). The plates were incubated at
37°C for approximately 24hours.
After incubation and observation, the agar plate/plates which have most growth will be gram-
stained to identify under the microscope.
RESULTS:
Back of Neck Outer edge of Ear Outside Forehead
EMB No growth Less growth No growth
( 2 colonies)
MAN No growth No growth No growth
NA Less growth (3 A lot of growth A lot of growth
colonies) (300> colonies) (<300 colonies)
Table 1: shows a summary of the observations after incubation. (Where EMB = Eosine Methylene
Blue, NA= Nutrient Agar and MSA = Mannitol Salt Agar).
Figure: A Figure: B
Figure 1: A – Shows Cocci shaped cells in clusters from NA plates from outer edge of ear and B –
shows Rod- shaped cells in clusters from NA plates from outside forehead.
, Shaylene Moonilal – 55202063
MIB 3704 Portfolio
DISCUSSION:
Eosin Methylene blue agar – Also known as EMB, is a selective and differential medium used to
isolate fecal coliforms. They also serve to inhibit the growth of most gram positive organisms.
(Microbugz, online).
According to Willey et al. (2008) EMB differentiates between lactose fermenters and non-lactose
fermenters. EMB contains Lactose salts and two dyes- eosin Y and methylene blue.
In the experiment, there was no growth on 2 of 3 EMB plates and there was 1 plate with very little
growth, about 2 colonies only on the outer edge of the ear. This results show that the other two
areas swabbed contained no gram positive bacteria. This agrees with what was presumed. According
to Chiller et al. (2001), these areas of the skin (Forehead and Neck) are predominated by
Corynebacterium and Propionibacterium, which are both gram positive. This results also suggest
that the outer edge of the ear has some gram negative bacteria, hence growth on the agar.
Mannitol Salt agar – Also known as MSA, is a selective and differential media for gram positive
bacteria. That is, it allows the growth of gram positive bacteria while inhibiting growth of gram
negative bacteria (Willey et al, 2008).
The high salt concentration (7.5%) selects for members of genus Staphylococcus, since they can
tolerate high saline levels. MSA also contains the sugar mannitol and the pH indicator phenol red. If
an organism can ferment mannitol, an acidic by-product is formed that will cause the phenol red in
the agar to turn yellow. (Microbugz, online).
From the above results, we had no growth on any of the mannitol salt agar plates. This could have
been because the group member who swabbed for this agar plate used skin products with strong
antiseptic properties.
Nutrient agar – Also known as NA, it is a general purpose nutrient medium used for the cultivation
of microbes supporting growth of a wide range of non-fastidious organisms. (Microbiology info,
online) Nutrient agar is popular because it can grow a variety of types of bacteria and fungi, and
contains many nutrients needed for bacterial growth. This means that most microorganisms,
whether gram positive or gram negative will be able to grow on this medium.
This explains why there was growth for all the skin areas selected, from the above results.
CONCLUSION:
We have achieved our aim of this experiment. We have managed to successfully isolate and identify
bacteria that are found on the back of the Neck, outer edge of the Ear (retroauricular crease) and
outside forehead (glabella).
From the results, it is clear that these areas are dominated by gram positive bacteria which include
the absence of growth on 2/3 EMB agar plates that seems to agree with theory that these skin areas
are dominated by gram positive bacteria and only some gram negative bacteria.
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