This is the main page for the second laboratory exercise of an 8-week introductory-level college microbiology class. The following two links take you to the documents that you need to print off to prepare for lab. Below the links is basic information on microscopy.
Files Associated with Media & Culture Lab #2
1. Lab Exercise and Report #2 (Word .doc)
This is the word document that HCR120 students must print off to prepare for lab #2 and the associated assignment (lab report).
2. The PowerPoint presentation associated with Lab #2 can be accessed at Microbiology Lab PowerPoints.
Media & Cultures
In this lab we learn about different types of media that are used to grow colonies of bacteria. Some types of media will grow just about any type of bacteria whereas others are more selective and only grow specific types of bacteria.
Normal Flora
Our bodies are inhabited by a quadrillion bacterial cells, known as our normal flora. Normal flora is found on any exposed surface of the body, like skin and mucus membranes, but internal organs are sterile---no bacteria live there.
Normal flora take up space, (preventing pathogens from moving in and setting up housekeeping), consume scant nutrients available (again, competing with potential pathogens) and put out waste products that keep out pathogenic microbes.
Culturing Bacteria
Medical microbiologists routinely "culture" a particular site to determine whether bacteria that cause microbial disease are present. In this lab exercise you will be culturing various environments and body surfaces in order to examine that normal flora from the surface of our bodies and bacteria in the environment.
We will do this by taking a sample and then plating it out on nutrient media. When a single cell comes in contact with the media it reproduces and its progeny reproduces, until a mound of cells, call a colony, becomes visible. Visible colonies are composed of a million or more cells.
Requirements for Bacterial Growth
Bacteria and other microbes have particular requirements for growth. When they reside in and on our bodies or in the environment, they harvest their food from us or from the environment.
When we grow bacteria in lab, we are essentially creating a captive environment for bacteria – like a bacteria zoo. So we must provide the bacteria we grow in lab with all of the materials that they need to grow and thrive.
Growth media (singular: medium) are used to cultivate bacteria. Media are mixtures of nutrients that the microbes need to live, and the necessary moisture and pH to support microbial growth.
Types of Media
Many normal flora and clinically important microbes can be grown either in liquid medium (sometimes called broth), or on Petri dishes (also called Petri plates, or just “plates”). Agar is a seaweed extract that becomes at room temperature.
When we want to create a solid surface to grow bacterial cultures on, we add agar to the liquid medium so that, when cool, the medium has the consistency of very stiff Jello.
TSY (Tryptic Soy Agar)
The medium that we use most often in lab is called "TSY" (Tryptic Soy Agar), a complex nutrient medium which supports the growth of a wide variety of microbes.
Selective and Differential Media
There are many other types of solid media that are more specific in what they grow, or that give us information about the type of microbes growing there. MacConkey’s, Blood agar and Mannitol Salt are three examples or more specific types of media.
If a medium is selective, that means that it grows only certain types of microbes and inhibits the growth of other types of microbes.
A growth medium is considered differential if, when certain microbes grow on that medium, it exhibits a color change that gives us information about the type of microbe growing there.
Types of Differential and Selective Media
Here are links to pages covering three types of selective and/or differential media:
- MacConkey’s Agar (MAC) to identify Gram- lactose fermenters
- Mannitol Salt Agar(MSA) to ID Staph epi and Staph aureus
- Blood Agar(BAP) to identify Streptococcus pyogenes
Sources:
Microbiology class laboratory material appearing on this website is adapted from the Applied Microbiology laboratory manual by Cynthia Schauer.
Images:
To be updated.
