Using an “Unknown Characterization” Activity as the Midterm Lab Practical in a Large Enrollment, Multi-Lab Section Microbiology Course

Using an “Unknown Characterization” Activity as the Midterm Lab Practical in a Large Enrollment, Multi-Lab Section Microbiology Course

Ned T. Barden
Massachusetts College of Pharmacy and Health Sciences, Boston, MA 02115.

INTRODUCTION

The characterization and identification of “unknowns” is an often-used laboratory activity in microbiology courses. As course enrollments become large, the logistics of providing unknown culture mixes, tubed and plated culture media, plus incubator and refrigerator space become quite daunting. A few years ago we discontinued the three-week Unknowns lab exercise when enrollment reached 240 or so students. In its place we have developed an hour long, midterm lab practical that incorporates smear preparation and the Gram stain, biochemical media, rapid tests, and a streak plate. This easy-to-grade laboratory assessment tool has taken the place of the more traditional station-to-station lab practical, and has worked well for the instructors responsible for administering practicals to 480 students in 13 lab sections over a two-day period.

PROCEDURE

A. Lab practical setup

After five to six weeks of microscopy, culturing, and performing a select set of biochemical characterizations, students are given a lab session to practice the staining and rapid tests on stock cultures, and negative and positive results for the biochemical tests are made available for review. At their individual stations, the students are required to interpret a set of prepared biochemical tests, make a smear and perform the Gram stain, perform the catalase and oxidase tests from a pure culture plate, and prepare a streak plate from a mixed broth culture – all from memory. No laboratory notes or written procedures are allowed.

Each station has a rack of inoculated and incubated tubes of various biochemical tests with uninoculated controls (all labeled), a TSA culture plate containing a streak of a pure culture, a mixed broth culture containing two species of bacteria, and one uninoculated TSA plate. Each student also gets a scoring sheet with instructions on one side and a response/grading page on the other. The biochemical tests are actually tubes that the students themselves have inoculated, incubated, and interpreted in previous labs. These tubes are collected and saved in the cold room as the students complete specific exercises. With over 400 students it is easy to accumulate 40–50 sets of good, typical reactions during the biochemical test exercises in the five to six weeks before the practical.

In our very first attempt at this exercise a few years ago, the instructors meticulously matched up the biochemical tests at each of the 40 stations with the bacterial species on the culture plate to be Gram stained. A master chart with the Gram reaction and biochemical test results and the Genus species names for the various species used was provided at each station. Not surprisingly, some enterprising (or desperate) students were able to identify the species using just the pattern of biochemical test results, no Gram staining needed. To avoid this shortcut we now require, and give points for, the correct results of each of the tests. Since we no longer require the name of the species, no chart is necessary and the biochemical tests provided do not necessarily have to match the species that is on the pure culture plate. However, we feel the “name your species” variation could work for smaller classes.

The streak plate must be performed using proper aseptic technique, and the plate properly labeled and placed inverted in a wire basket for incubation. The plates are subjectively scored for proper technique and isolated colonies after incubation. Points are deducted for poor technique, nonseparation, and/or improper (right side up) incubation.

After completing the practical, the students must clean and put away their microscope, swab their work station, put their streak plate in an incubator basket – properly inverted – and turn in the answer sheet.

B. Grading sheet (side one) microbiology mid-term lab practical

Name______________________________________

Lab station # __________ Lab day/time ___________

Read the following directions thoroughly before starting the practical!

This side of paper must face up at all times when answers are not being recorded.

Record your station number in the blank provided above

Use the bacterial culture on the TSA plate at your station to perform Gram stain, catalase test, and oxidase test.

Use the rack of inoculated tubes to analyze biochemical tests.

Necessary reagents have already been added.

DO NOT shake or invert tubes! You may pick them up to observe reactions

Record all results on the next page. Complete the answer table at the bottom of the page.

Streak Plate

Make a TSA streak plate from the TSB tube labeled “mix” that is in the rack in front of you. Use proper aseptic technique and label the plate with your full name PRINTED clearly along edge of the bottom of the plate. Place the plate in your lab section’s basket after you complete the practical.

You will have 1 hour during your usual lab section to complete the practical. Be sure to manage your time – we recommend that you begin by making your bacterial smear for the Gram stain so it can air-dry while you work on other components of the practical.

Microscope lens should be cleaned before and after use. Points will be deducted for oily lenses.



 

Record final results below using letter result for #1–11 (A, B or C) and abbreviations for #12 &13.



 

CONCLUSION

Overall, this lab practical gives the students an incentive for learning the various lab tests and procedures in the five to six weeks leading up to the practical. The students are reminded during their first five weeks in lab that for the upcoming practical they will have to be able to make a smear and perform a Gram stain on their own, will have to know how to focus a microscope without assistance, and will have to determine a positive and negative reaction for all the biochemical tests that we do in the lab. The lab practical becomes a goal and focus of the lab exercises as each student, even in a group of four, knows that he or she will have to know every reaction and will have to be proficient in using the microscope.

This method of giving a lab practical also provides the lab instructors a way to give a meaningful lab assessment to hundreds of students in multiple lab sections that meet over a two-day period. It is also easy to grade, and the instructors can give the students timely feedback. Grades are usually posted in Blackboard the day after the streak plates are scored. Although we haven’t done so yet, the grading sheet could be used in conjunction with an electronic grading system such as Parscore.

ACKNOWLEDGMENTS

The author wishes to acknowledge Joe Holloway and Melanie Grandy, our Biology Laboratory Faculty Associates, for refining this lab assessment tool over the past several semesters and developing the scoring sheet used for grading.

*Corresponding author. Mailing address: School of Arts and Sciences, Massachusetts College of Pharmacy and Health Sciences, 179 Longwood Avenue, Boston, MA 02115. Phone: 617-732-2944. Fax: 617-732-2959. E-mail: ned.barden@mcphs.edu .

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DOI: 10.1128/jmbe.v11i2.221
Journal of Microbiology & Biology Education , December 2010
Copyright © 2010 American Society for Microbiology . All Rights Reserved



JMBE
ISSN: 1935-7885