Modern Biology students are expected to grasp the terminology and technology of six common methods of identifying and classifying bacteria.
- Gram staining: This is a quick and simple technique all your students need to know. It involves staining the bacteria with crystal violet and iodine and observing them under a microscope. Bacteria are classified as gram-positive or gram-negative based on the color they retain after staining.
- Biochemical tests: These tests are used to identify specific enzymes and metabolic processes that are characteristic of certain types of bacteria. For example, catalase, which breaks down hydrogen peroxide, can help identify staphylococci bacteria.
- Serological tests: These tests use specific antibodies to detect and identify bacterial antigens. For example, the enzyme-linked immunosorbent assay (ELISA) can be used to identify specific strains of Escherichia coli.
- DNA sequencing: This is a molecular technique that involves sequencing the DNA of a bacterial sample to identify it. It can involve sequencing a specific gene or the entire genome.
- MALDI-TOF mass spectrometry: This is a rapid and accurate method for identifying bacteria by measuring the masses of molecules in the sample, which can help identify the bacterial species.
- Whole-cell matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF): This method involves analyzing the mass spectrum of a bacterial sample to identify its protein profile, which can help identify the species.
But if you asked your students to take samples and identify some of the bacteria in their everyday world, what would they do?
Low-Tech, Low-Cost Ways to Introduce Students to Microbial Identification
Our experience with thousands of biology classes across the United States suggests most students would go about their assignment something like this:
Students collect bacteria from what they believe to be “germy” places
Your students will take swabs of places they think are the dirtiest in their everyday lives. Most will sample toilet handles, toilet seats, or bathroom mirrors. Some knowledgeable students will take a swab of their kitchen sink.
Or your students might take swabs of the floors in their biology classrooms! They will transfer bacteria from their swabs to agar plates, place them into the incubator, and return the next class period to see what happened.
Some swabs don’t seem to have bacteria, even though they probably do
Not every swab will result in bacterial growth. This allows your students to generate some hypotheses about the reasons why.
Were there no bacteria on the biology classroom floor, or did they need a different medium in their Petri dish to grow? Here’s an opportunity to use the scientific method. But how can they run their tests?
Students learn to use Gram stain
When your students grow bacteria, they will need to figure out what kind of bacteria they have. Since 1884, the most basic method of bacterial identification has been Gram staining, developed by Hans Christian Gram to look for tuberculosis bacteria in sections of the lungs.
Your students can learn Gram staining in a single lab period. Then they can start to think through the implications of Gram-positive and Gram-negative status not just for the initial identification of bacteria but also for identifying chemical agents and antibiotics that may control them.
Students use selective and differential growth media
Your students can also test their bacteria with selective media and differential media. For example, mannitol salt sugar agar is selective for Staphylococcus bacteria, the only bacteria that measurably grow on it. Or your students could use differential media, which support multiple kinds of bacteria, but encourage growth at different rates.
Blood agar is a differential medium for different Streptococcus species, which break down the red blood cells in the medium at different rates. No change in the color of the media means you have S. viridans. Partial cell hemolysis leads to brown or green staining on the media, which means you have S. haemolyticus. A complete breakdown of red blood cells, so the medium fades, means you have S. pyogenes.
Your students use simple biochemical testing
Biochemical testing doesn’t have to be expensive. Your students can test for the presence of the catalase enzyme in a bacteria sample, for instance, with just a drop of hydrogen peroxide placed next to the bacterial colony in the Petri dish. For instance, they might use this kind of test to distinguish catalase-positive Staphylococci from catalase-negative Streptococcus, which are both Gram-positive.
Next, your students can use their pipettes to fill the wells of an assay plate to test which nutrients encourage the greatest growth. This test will give them dozens of ways to narrow down the identity of their sample, all before ever looking at the bacteria through the microscope and without DNA sequencing.
With Modern Biology, you ground your students in the basics of good lab technique and reward them with thought-provoking experiments that burnish their credentials as the scientists of the future. You receive all the supplies you need for each exercise (other than the basics you use every day), teacher preparation notes, and the assurance that thousands of biology teachers just like you have used these experiments to create successful science students who become science majors and enter science-based professions.
Modern Biology has helped over 8,000 biology teachers reach over 500,000 students. Modern Biology is a leader in providing biology teachers with the tools to train students. We are ready to answer all of your questions about our products. We are here to help your students excel. Call us at (765) 446-4220 or email us today!