Teaching Bacterial Arrangements and Morphologies with Candy

INTRODUCTION

Determining bacterial morphology and arrangement is a first lesson in undergraduate microbiology or in an introductory lecture to the Prokaryotes in general biology. By identifying cell arrangement and morphology, students can successfully communicate descriptives in microscopy-based labs, can take the first steps in identifying unknown organisms, and are able to understand genus and/or species names of standard laboratory bacteria. This activity is intended to help students visualize and practice naming basic bacterial morphology and arrangements, and is appropriate in either the laboratory or lecture setting of a general microbiology or introductory biology course. Students with a weak background and non-science majors will benefit most from this activity.

PROCEDURE

Materials

Each student should be supplied with candy that represents the three basic bacterial morphologies. For ease of distribution during the activity, candy should be placed in a small plastic bag: one to two Twizlers representing spirillum/helical morphology (and the spirochetes), eight Skittles representing coccus morphology, and three small Tootsie Rolls representing bacillus morphology. Other candy may be used as long as they represent the basic morphologies of bacterial cells.

If this activity will be completed in a laboratory setting, it is recommended that Styrofoam packing peanuts (the S shaped peanuts) be used for spirillum/helical morphology, glass beads (such as those used for spread plating of cultures) or marbles be used for coccus morphology, and cornstarch packing peanuts (biodegradable bullet shaped peanuts) be used for bacillus morphology. In activities where colored glass beads are used in place of colored candy, glass beads may be stained using bacteriological stains (e.g., crystal violet, nigrosin, methylene blue, or safranin). Glass beads should be exposed to stain for 48 hours at room temperature, then placed on paper toweling to air dry. Washing beads with water will remove most of the stain; darker stains (e.g., crystal violet) work better than lighter stains (e.g., safranin).

Activity

The instructor reviews the basic bacterial morphologies (spirillum/helical, coccus, and bacillus) as part of the pre-lab or classroom lecture. Students should group candy into the appropriate classification as the instructor defines the basic morphologies. The instructor should check each student’s grouping to ensure that candy is grouped appropriately. Next, the instructor reviews the basic bacterial arrangements (strepto-/ chains, staphylo-/clusters, single, diplo, tetrad, etc.) while students arrange their candy into appropriate groupings. The instructor should check to ensure each student has grouped the candy correctly. Now that basic arrangements and morphologies have been defined, the instructor should call out different groupings so that students may arrange their candy appropriately. Examples of suggested groups are: Staphylococcus, streptobacillus, or Micrococcus (tetrad arrangement, coccus morphology).

Other potential activities

Candy usage in class can help students to understand the concepts of mixed culture versus pure culture, serial dilution, and disease transmission. Listed below are a few suggestions for the use of candy as a teaching tool in general microbiology. Alternatives to candy are provided in the Materials section above.

1. Students may be given multiple colors of Skittles to teach the concept of a mixed culture. Students are asked to separate like colors into groups. The instructor would describe that separating microbes in the laboratory is done by a dilution process called the isolation streak method. The instructor should draw an isolation streak on the chalkboard or use this to transition into a laboratory exercise.

2. Working in groups of four, students are given small round candy of the same color in a plastic cup and several empty plastic cups. Students could then be asked to demonstrate serial dilution technique by moving the candy into the remaining plastic cups. Serial dilutions may be taught as a decrease by ten-fold (or other dilution), with the candy representing the number of cells transferred at each dilution. Candy may be counted and Colony Forming Units (CFU)/ml calculated.

3. Disease transmission may be taught by giving students plastic cups containing candy of the same color. One student within the class is given a cup of candy of a different color. Students are instructed to move around the room, exchanging candy with another student by transferring the contents of each cup back and forth a few times, then dividing the contents between the two participants. After a few rounds of exchange with different partners, students count their candy by color. Class data is generated after each transfer for disease transmission (rate of incidence, prevalence, morbidity, and mortality). The mortality and morbidity rates are set by the instructor (e.g., ten red pieces of candy for mortality; one to nine pieces for morbidity). Mortality is the number of deaths per infected individuals. Morbidity is the number of new cases of infection within a period of time (e.g., each transfer) per total population. Incidence rate is determined by counting the number of students who have an infectious candy. Prevalence rate would be calculated as the total number of students who have an infectious candy at some point within the activity but may have lost the infectious candy at a later transfer (1).

CONCLUSION

A pre-activity quiz and post-activity quiz using the same PowerPoint images was used to determine if the activity helped in student learning. Students were asked to identify the bacterial morphology and arrangement of four images. Of the four laboratory sections surveyed, 42 students participated in the candy exercise (19 students in fall 2009, 23 students in spring 2010) while 46 students did not participate in the activity (23 student in fall 2009, 23 students in spring 2010). The average quiz score for the students who participated in the exercise was 2.8 ± 0.2 out of 4.0. Students who did not participate in the exercise had an average score of 2.0 ± 0.2 out of 4.0. Data is reported with standard error.

Students have commented that they enjoyed the activity as it is different from the usual lecture format — and they get to eat the candy after the activity. This activity is especially fun when students compete with one another.

As students participate in individual and group exercises, they increase the likelihood of remembering basic concepts for lecture, and may find laboratory concepts easier to understand and link to lecture topics while interacting with their classmates. This opens opportunities for forming study and project groups.

This activity may be applied to grades 7–12, and would serve as an outreach activity when working with teen students at local schools or after-school clubs. The activity could also be used to engage students in campus preview days. Fun activities tend to attract both young and old and may be instrumental in planting the seed of scientific inquiry in a young mind.