A simple method is presented to show kids the size of a microbe – a fungus hypha – compared to a human hair. Common household items are used to make sterile medium on a stove or hotplate, which is dispensed in the cells of a weekly plastic pill box. Mold fungi can be easily and safely grown on the medium from the classroom environment. A microscope capable of 200–400× is necessary. Students can use a hair from their own head to view a fungus and a hair side-by-side on the same slide. They will see that a microscopic fungus hypha is 20–50× smaller in diameter than a hair. Older students will also learn that microbes are measured in micrometers, that fungi are ubiquitous, and that decay is an inevitable part of Earth’s processes.
One of the greatest things a teacher can do is to show kids something that will stay with them forever. The microbial world is as vast as it is small. A verbal explanation is usually not enough for kids to understand just how small microbes (or microorganisms) really are. Microbes can be difficult to visualize for all of us, simply because we are so enormous in comparison. Bacteria and fungi are two ubiquitous microbes that share the world with us. Bacteria are smaller and can be difficult to see microscopically, but fungi can be viewed without staining with a simple light microscope at 200 to 400 power.
Kids especially have a difficult time understanding just how vast and how small the microbial world is, because it is something they have never seen. Seeing is believing, as they say, and a teacher can easily show kids how small a microbe is with a few simple household items and a microscope. It is easy to grow common mold fungi on media made in the classroom or kitchen without the need of an autoclave or other special equipment. If a few simple safety precautions are used, the molds grown will present no harm to students. By growing mold fungi from room dust or other debris, students will also learn that mold spores are ubiquitous.
Molds are a group of fungi easily found in any outdoor environment, home, or classroom (Christensen, 1975; Moore-Landecker, 1982; Cooke & Rayner, 1984; Manion, 1991). Mold spores in small amounts, comparable to a small amount of dust in the air, are not hazardous to most people (Christensen, 1975; Money, 2004). However, in larger amounts, airborne mold spores can produce allergic responses (Christensen, 1975; Carlile et al., 2001; Money, 2004), so care must always be taken to avoid acute exposure. This exercise will not expose students to dangerous amounts of mold or mold spores; however, teachers can provide a face mask to students who may have a history of asthma or allergies. Students should be advised to wash their hands following the exercise.
The growing cells of fungi are called hyphae (singular hypha). A mass of hyphae is called a mycelium. The cultured fungus hyphae can be examined with a microscope and compared side-by-side to a hair from a student’s own head on the same slide. This provides a striking and memorable image of the size of each, and shows that a microbe is beyond that which can be seen with the unaided human eye. In addition, students are always keen to see something from their own body under the microscope.
Materials (see Figure 1)
Packet of unflavored gelatin
Plastic, daily pill container with seven cells (like the ones free at the pharmacy)
Table sugar (sucrose)
Student microscope (200–400×)
Glass microscope slides and plastic cover slips
Boil a cup of water (or slightly less to make thicker gelatin). Add a packet of powdered gelatin and a teaspoon of sugar and stir to dissolve. Pour the liquid gelatin into the cells of the pill box, about half to three-quarters full. This amount will fill three or four pill boxes. Use a measuring cup to decant the liquid gelatin. Prepare pill boxes on a small tray because they are easier to handle. Close the lids on each cell and refrigerate to solidify the gelatin.
Once the gelatin has cooled and solidified, fungi can be introduced to the pill-box cells. If gelatin is to be kept for a week or more, the pill boxes can be sealed in a plastic bag in the refrigerator to prevent evaporation. Introduce fungi to the gelatin (i.e., inoculate) by having kids rub their fingers (or cotton swab) on a dusty bench or shelf top, or let them be creative, like touching their shoe, etc. Kids can be very creative at this stage, imagining where fungus spores are lurking. Then have kids gently touch the surface of the gelatin. A student can be assigned one or more cells to touch, identified by the day of the week of the cell, the color of the pill box, etc. Alternatively, cell lids can be labeled with students’ initials.
Close the lids on the pill-box cells, place the inoculated gelatin on a shelf at room temperature, and plan to examine them in about a week. If the pill boxes are not going to be examined for a couple weeks, store them in a sealed plastic bag on the shelf to conserve moisture.
Examining Fungus Hyphae Compared to a Hair
After a week or so, blue, gray, yellow, pink, or green growth will appear on the surface of the gelatin. These are species of mold fungi in the form of hyphae and spores (Figure 2). The spores will not become airborne unless they are blown across or the boxes are opened in a vigorous manner. Inform students to open the pill box lids carefully and to not touch the molds.
Remove a small amount of the fungus-colonized gelatin with the wide end of a flat toothpick and place it in a drop of water on a microscope slide. Then have students take a hair from their head and place it in the middle of the drop of water with the fungus specimen. Small scissors can be used to cut the hair as long as the slide so that it does not accidentally catch on something and get pulled out of the slide during manipulation. Then place a cover slip over the drop containing the fungus and hair and squash it down flat. Place the slide on the microscope stage and examine at 200–400×. It is easiest if the hair is perpendicular on the slide rather than length-wise, because the stage can be shifted back and forth until the hair comes into view.
What Kids Will See
Through the microscope, the hair will appear as a large, straight, brown, rectangular line that will take up most of the frame, while the fungus hyphae will appear as much smaller, clear or dark lines curving around and across the hair (Figure 3). If fungus spores are present, they will appear as small dots scattered around the hair. Most of the mold fungi cultured will be common species of Penicillium, Aspergillus, and Cladosporium, which are blue to gray and have round or oval spores (Christensen, 1975; Wang & Zabel, 1990; Barnett & Hunter, 1998).
Depending on the age of the students, the teacher can explain the actual dimensions of the fungus and the hair. A millimeter is about as thick as a thumbnail. Microscopic things are measured in micrometers (µm). There are 1000 µm in a millimeter. The thickness of a human hair can range from 11 µm to >160 µm (Piersol, 1930), but the size range for most people is commonly 75–100 µm. Individual hairs can be seen with the naked eye, but things smaller than this require magnification.
Although variable, the average diameter of a hypha or spore of common mold fungi is 2–4 µm (Wang & Zabel, 1990; Carlile et al., 2001). Therefore, a fungus hypha or spore is about 20 to 50 times smaller than a human hair! We can see a hair with the naked eye, but we can’t see a microbe. Students can be asked, upon viewing the difference in size between a fungus and a hair, how many fungus hyphae can be placed side-by-side to equal the diameter of the hair.
Additional Learning about Fungi
Fungi are heterotrophic organisms just like a bug or a dog or a human. They make up their own kingdom of organisms (Carlile et al., 2001; Stephenson, 2010). Like us, fungi get their food from outside sources, in contrast to plants that make their own food from sunlight, carbon dioxide, and water (autotrophs). Another way to explain this is that plants are the “great producers” on earth and fungi are the “great recyclers.” Fungi take apart the carbohydrates and other fixed carbon products made by plants and turn those substances back into carbon dioxide and water.
Common mold fungi that grow on the gelatin are opportunists, waiting for an available food in order to grow and cause decay (Cooke & Rayner, 1984). Spores, like seeds, are alive but dormant. All they need is water and a source of energy to revive. The energy is provided in the form of sugar – a carbohydrate. Other nutrients are provided by the gelatin – mainly protein. All the minerals and micronutrients that fungi need in very small amounts are sufficient in the water and gelatin.
Fungi grow as tubular filaments (hyphae) containing organelles and protoplasm that penetrate the gelatin, absorbing food as they extend. Some fungi may liquefy the gelatin by breaking down the structure of the protein. When their food source is used up, the fungi will produce spores that can travel through the air, ready to settle on a new available food source. Mold spores are usually colored (Barnett & Hunter, 1998). Students can use a toothpick to carefully touch the spores and transfer them to fresh gelatin to demonstrate the start of a new fungus colony.
Mold fungi cause decay, but decay is simply a form of recycling, and decay is a natural part of the Earth’s processes (Christensen, 1975; Cooke & Rayner, 1984; Stephenson, 2010). When we say that a food or other natural product is “rotten,” we mean that it has been colonized by fungi or bacteria. Fungal colonization and decay can be demonstrated by carefully transferring mold spores to a food item, such as a slice of orange or apple, or a piece of moist bread. Use a toothpick to carefully transfer the mold spores as described above. The food item can be inoculated with spores in several places. Seal the food item in a clear plastic bag for observation and place it on a shelf. Within a week or two, colonies of fungi and decayed areas will develop wherever the food item was touched with the mold spores. The food item should be very moist to foster mold growth. The bag will not need to be opened for observation; thus, all molds that develop will be contained and will not present a hazard to students.
For older students, teachers can structure the exercise to give them practice with the scientific method. Questions such as these can be asked: “Where do molds come from?” “Do dust and debris from the air contain living organisms?” “What habitats are likely to contain mold spores?” After a hypothesis is constructed and an experiment designed, samples can be obtained from different sources with clean cotton swabs, and gelatin can be inoculated as described above. After a week or more of incubation, inoculated cells can be examined for mold growth and compared among sample sources. Uninoculated gelatin can serve as a control. Depending on the age level of the students, the concepts of adequate samples, controls, and replicates in experiments can be discussed. Students can be asked to define the terms microbe, micrometer, mold, fungus, hypha, inoculate, and ubiquitous.
Fungi are microbes present in all human and natural environments. We see the effects of fungi in the form of decay and rot, but a microscope is needed to see the working cells of a fungus. With a few precautions, fungi can be safely grown from dust in the classroom on media that are easily prepared without specialized ingredients or equipment. By observing fungus hyphae through the microscope in the same frame as a human hair, students will gain an appreciation of the size of a microbe better than what can be achieved with words alone. For older students, teachers can extend the exercise to demonstrate the infectivity of spores and the initiation of decay. Students will learn that fungi are ubiquitous and begin to appreciate that decomposition is a natural part of Earth’s processes.