Lab 11: Bacteria of the Skin
Learning Objectives:
 Identify the growth of skin bacteria on different types of selective and differential media.
 Identify pathogenic from non-pathogenic bacteria
Introduction:
Our skin forms the first line of defense. It has several characteristics that makes it an inhospitable environment for the growth of most microbes. Some of these
characteristics of skin include: high percentage of salt, low pH from 4-5.5, sebum, antimicrobial chemicals; an outer layer of dead skin cells that acts as a barrier to
pathogens; mechanical defenses such as continuous sloughing off of skin, and dendritic macrophages. Skin also harbors a range of resident microbiota including
Staphylococcus and Diptheriae species that act as normal antagonist microbiota. Only the organisms that can tolerate high levels of salt, approximately 7.5% salt
concentration are able to grow on skin.
Mannitol salt agar (MSA) plates mimic our skin asthey contain a high salt concentration. Other components of MSA plates are sugar mannitol and the pH indicator
phenol red (yellow at pH <6.8 and red at pH higher than that). High salt concentration makes this growth medium selective for bacteria that can tolerate 7.5% salt.
Furthermore, pathogenic bacteria use Mannitol most commonly as a source of carbon. When sugar breaks down by fermentation it produces acids; this lowers the
pH of media and turns the indicator yellow. Hence this plate also acts as differential media.
Note: Use personal protective equipment (PPE) at all times. Follow instructions at all times. Do not open the plates containing skin swab sample.
Materials:
MSA plates, nutrient agar plates, sterile water, sterile swabs, marker, ruler, pure cultures of B. subtilis, E. coli, S. marcescens and S. epidermidis, inoculating loops,
incinerators
Method:
1. For this experiment, you will work in a team of 4 students.
2. Obtain two MSA plates and two nutrient agar plates per team.