This is my IB Biology SL Internal Assessment. I hope this exemplar can help you better structure & format your IA & guide your analysis. Best of luck :)
I scored a 7 in IB Biology SL & 44/45 overall in my Diploma
A study on the effect of benzoyl peroxide concentration on the growth
of Staphylococcus epidermidis
Introduction
Background Information
Acne vulgaris or ‘acne’ is a skin condition characterised by the clogging of individual lesions in hair
follicles, leading to a build-up of dead skin cells and excess sebum. Acne occurs commonly among
adolescence in areas where sebaceous glands are most populated, such as the face and upper back. In
bacteria’s presence, hair follicles connected to sebaceous glands become inflamed and infected,
resulting in more severe acne (Mayo Clinic, 2020).
Benzac AC ® is a typical, current-day over-the-counter treatment for acne; it comes in the form of
face washes, creams, and lotions, and the active ingredient is benzoyl peroxide. Benzoyl peroxide
(C₁₄H₁₀O₄) is a chemical compound known for its antibacterial and anti-inflammatory effects (Skin
Therapy Letter, 2021). The compound was initially discovered in 1965 by Dr William E. Pace as an
effective treatment of acne vulgaris when combined with sulphur cream (PACE, 1965).
When applied to the skin, benzoyl peroxide is converted to benzoic acid. The remaining benzoic acid
that is not excreted by the renal system is metabolised by the amino acid cysteine, resulting in the
oxidation of proteins in bacterial cell membranes. Benzoyl peroxide penetrates deeply into
pilosebaceous follicles, treating and preventing acne by shedding layers of dead skin cells and excess
sebum. Benzoyl peroxide is considered a bleaching agent when converted into benzoic acid; therefore,
at the higher concentrations between 2.5% - 10% of over-the-counter treatments, it can cause skin
sensitivity reactions, such as skin peeling, and visible skin whitening in long-term use (Cotterill,
1980).
Benzoyl peroxide exhibits antibacterial effects against Propionibacterium acnes (P. acnes) that cause
inflamed, bright red, puss-filled acne (Healthline, 2019). Benzoyl peroxide prevents the selection of
erythromycin-resistant strains of Staphylococcus epidermidis (S. epidermidis) (Skin Therapy Letter,
2021), a bacterium found on human skin often isolated from human epithelia for the analysis of
infections, often as a lawn culture on agar (Claudel et al., 2019). The nutrient medium of agar allows
for the cultivation of bacteria and supports the growth of non-fastidious organisms (Aryal, 2018).
Research Question
What is the effect of varying concentrations of benzoyl peroxide (%m/v) as found in Benzac AC ® on
the bacterial growth of S.epidermidis as indicated by the radius of the zone of inhibition (mm)?
Personal Engagement
As acne has been my biggest insecurity since middle school, I chose to research acne treatment.
Despite acne’s commonality during pubertal development to indicate mental distress and hormonal
changes, its appearance as inflamed, red, puss-filled spots on my face had severely decreased my self-
esteem. With determination to reduce my acne, I researched common active ingredients in skincare
and their effects on the skin. This lead to my discovery of benzoyl peroxide.
As benzoyl peroxide is a hazardous, oxidising agent, I could not source the active ingredient on its
own for a high school laboratory experiment. Instead, I decided to research and experiment with
salicylic acid, another common active ingredient used in acne treatment that is safe for student use in
a high school laboratory. I conducted a trial experiment by diluting salicylic acid powder to create
variations of concentrations. Despite this, the salicylic acid powder did not dissolve into water to form
a sufficient consistency that filter paper disks [± 0.5mm] could absorb. I ultimately chose Benzac AC
®, the acne treatment with benzoyl peroxide as its only active ingredient. This allowed me to
experiment on an existing brand in the skincare industry widely used and purchased by adolescents. I
understand that the skincare industry is ever-changing. With this comes a surge of product marketing
and viral trends. I have noticed how much marketing has made customers like myself overlook the
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, long list of ingredients and inspect the effects of each chemical to benefit the skin. Evaluating the
efficacy of an authentic product in the market, Benzac AC ®, would give me insightful knowledge of
the product’s efficacy on the skin and the effects of having benzoyl peroxide as the active ingredient.
Hypothesis
It can be hypothesised that as the concentration of Benzac AC ®, and therefore, benzoyl peroxide
increases, the efficacy of benzoyl peroxide on oxidising proteins in S. epidermidis cell membranes
would increase. Consequently, the radius of the zone of inhibition will increase. This is because there
will be a greater amount of molecules that interact with the bacteria’s cell membranes, proposing a
greater inhibitory effect.
Variables
Independent Variable
Concentration of Benzac AC ® as measured by %m/v. The concentrations are: 0% (negative control
of deionized water), 1%, 2.5%, 5%, 7.5%, 10%
Concentration increments used in the experiment are within the range of over-the-counter benzoyl
peroxide acne products. Specifically, 2.5%, 5%, and 10% are actual concentrations seen in Benzac
AC ® products. Employing 1% and 7.5% allow for closer observation in trends across concentrations.
Dependent Variable
The amount of bacterial growth of Staphylococcus epidermidis (S. epidermidis) is measured by the
smallest radius of the zone of inhibition (mm) based on visual approximation. The technique to
measure the radius is visualised in Figure 1 below.
Figure 1 – Measuring the smallest radius of the zone of inhibition based on visual approximation
(Hudzicki, 2009)
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