Coliforms Everywhere! Using Microbiology to Teach the Scientific Method

doi:10.1128/jmbe.v11i2.163

Coliforms Everywhere! Using Microbiology to Teach the Scientific Method †

Coliforms Everywhere! Using Microbiology to Teach the Scientific Method ^†

Cindy R. Cisar ^*, John S. de Banzie
Department of Natural Sciences, Northeastern State University, Tahlequah, OK 74464.

INTRODUCTION

The scientific method is a fundamental concept in science. In this exercise the scientific method is taught as a hands-on investigative laboratory experience. Students generate a hypothesis concerning the environmental distribution of coliforms, design and execute an experimental test of that hypothesis, and analyze the resulting data. The exercise is safe and straightforward. It is appropriate for use in undergraduate laboratory courses for science majors and secondary school students and undergraduate non-majors with the appropriate mathematical backgrounds. Students learn both the process by which science progresses, as well as more advanced concepts in microbiology and statistics.

PROCEDURE

This laboratory exercise uses the Path-Chek Hygiene Pathogen Detection System for Coliforms (Microgen Bio-products Ltd., Camberley, Surrey, UK) to examine the environmental distribution of coliforms. Background information on this system, the scientific method and coliform bacteria is given in a handout provided before class and in an introduction at the beginning of class. The use of coliform bacteria as indicators of fecal contamination and the potential presence of human pathogens is emphasized.

Students are given an example of an observation concerning coliform distribution that leads to a null hypothesis. For instance, “Based on observations of people washing their hands, one might hypothesize that men are more likely to have coliforms present on their hands than women. The appropriate null hypothesis would be: men and women are equally likely to have coliform bacteria present on their hands.” Students are assigned to small groups (2–4 students) and asked to develop their own null hypothesis involving the distribution of coliforms in the environment. Certain limitations are imposed for practical, safety and ethical reasons. For example, “experiments must be performed during the lab period today” and “experiments cannot include direct detection of coliforms on humans”. The instructor visits with each group during this process to ensure that students are developing appropriate hypotheses and that the null hypotheses are phrased correctly.

Each group’s hypothesis is written on the blackboard and the class is asked to select one hypothesis for the class to test. The importance of sample size for statistical analysis is introduced to explain why this step is necessary. Student groups are encouraged to “sell” their null hypothesis to the class. The result is often an animated discussion of the pros and cons of each hypothesis.

Once the class has selected a null hypothesis, the instructor guides the students in development of an experimental protocol. This is a highly valuable part of the exercise, as students are often naïve concerning experimental design. Continuing with the men’s hands vs. women’s hands example described above, students might propose to swab hands. If so, students should be asked questions such as, “Should you swab the right or the left hand?”, “Might it make a difference?”, and “What if some individuals are right-handed and some are left-handed?” By asking such questions the instructor shows how to formulate a detailed experimental protocol that eliminates variables that might affect analysis of the data. The students then collect their Path-Chek swabs and perform the selected experimental protocol. After overnight incubation of the samples, the students compare their results with negative and positive controls provided by the instructor and record whether coliforms were present on the surfaces they swabbed.

The following laboratory period, a class dataset is assembled on the blackboard from individual results. A two-row, two-column table (category 1/category 2, positive for coliforms/negative for coliforms) is used to facilitate analysis. The statistical test to be used in accepting or rejecting the null hypothesis (Fisher’s exact test of independence) is described in the laboratory handout and by the instructor. Concepts such as the chi square statistic, degrees of freedom, and probability are reviewed. Students then work in small groups on the statistical analysis of their data with the instructor interacting to answer questions.

In our class, students are required to write a lab report. In grading these reports, clear statements of the null hypothesis tested, the methods used, the results obtained, the result of the statistical analysis and conclusions drawn regarding the null hypothesis, as well as a discussion of confounding factors, are considered.

We have used the Path-Chek Hygiene Pathogen Detection System for Coliforms with good results in our classes. The kit is qualitative and results are recorded as either coliforms present or coliforms absent.

CONCLUSION

This exercise has been used very successfully over several semesters in the introductory microbiology class at Northeastern State University. Since the instructor closely monitors the lab exercise, it is possible to observe and guide students as they learn concepts such as how to develop a null hypothesis, how to design a good experimental protocol, and how to use statistics. The students are enthusiastic about the exercise and are very interested in their results because they can test objects with which they have personal contact on a daily basis such as cell phones, keys, shoe soles, restroom door handles, etc. In addition, we have observed instances where students have applied principles of experimental design taught in this exercise to the analysis of data generated in later laboratory exercises. For example, students who observed differences between their individual results in a later lab exercise began, without prompting by the instructor, to discuss whether variations in individual experimental protocols might explain the observed differences.

The handouts provided to students performing the laboratory exercise are available at http://arapaho.nsuok.edu/~debanzie/ColiformsEverywhere.html.

In addition, the authors would be pleased to discuss the exercise with interested instructors. This laboratory exercise was the subject of a Microbrew presentation at the 2010 ASM Conference for Undergraduate Educators.

SUPPLEMENTAL MATERIALS

Lab Exercise

*Corresponding author. Mailing address: Department of Natural Sciences, Northeastern State University, 600 N Grand Avenue, Tahlequah, OK 74464. Phone: (918) 444-3841. Fax: (918) 458-9693. E-mail: cisar@nsuok.edu .

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^†Supplemental material available at http://jmbe.asm.org ( Return to Text )

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ISSN: 1935-7885