Food Microbiology 636

Autumn 2003

Coliform Question Set

Instructor: Erin Horton

Due: Mon. Oct. 27

Cover Page: (2 pts)  Include title of lab, your name, partner’s name, due date and a descriptive title such as:

            The examination of raw, whole chickens for coliform counts using presumptive, confirmatory, and completed tests, with incorporation of the Most Probable Number Technique.

Introduction

1.      What is the significance of coliform presence in food?  (2 pts.)

Coliform presence “may indicate fecal contamination” (1pt)

Used as an indicator microorganism to determine quality and safety of the food (1 pt)

2.      Does a high coliform count in food mean that the food had fecal contamination?  Why or why not?  (2 pts.)

No, a high coliform count in food does not mean that the food had fecal contamination. Coliforms come from both fecal and nonfecal sources (1 pt). Explanation needs to be more than 1 sentence.  Nonfecal sources of coliforms come from somewhere other than intestinal contents of animals.  It is possible that they are in the soil, along with decaying plant matter.  Also, it is helpful to include a species that is in this category.  (1 pt)

3.       Why were raw, whole chickens chosen for testing in this experiment?  What other foods would have been good choices for testing? (4 pts.)

Raw, whole chickens have various sources for contamination: (2pts)

            Growth environment (coop)

            Processing environment

            Coliforms inhabit the intestinal tract of most animals

Other foods to test: sprouts, spinach, ground beef, pork, poultry, raw milk, fresh produce items/organic vegetables (2pts)

Methods

4.      List any information about the food tested that may be relevant to the microbiological quality and/or safety.  (2 pts.)

Chicken was purchased from a “small shop” that sells in bulk on October 6, 2003.  It was stored in the refrigerator (1-4°C) until we tested it on October 8^th.  (1 pt)   Also, what specific area of the chicken (25cm²) was swabbed?  (1 pt)

5.      Explain why both positive and negative controls were used in this experiment.  Be sure to identify what they were.  (2 pts.)

Escherichia coli was the positive control (0.5 pt)

Proteus vulgaris was the negative control (0.5 pt)

Positive and negative controls serve various purposes, but the most important include using them to know what positive and negative reactions on the media look like so the sample can be compared, and to make sure the media is functioning properly. (1pt)

6.      Five selective-differential media were used in this laboratory. Determine the

selective and differential agents in each medium, and comment on their function. Use this information to fill-in the blanks in the following table. (5 pts.)

Results

7.        Reconstruct Table 4.3 using the data that your group obtained from this experiment.  Include a descriptive title and at least three footnotes, including those needed to explain irregularities in results.  Include a sample calculation beneath this table for MPN and either VRB or Petrifilm results.  (7 pts.)

Table 4.3  Group results for presumptive and confirmed coliform counts from 25cm² swabbed area^a of whole, raw chicken (1pt)

                                                (1pt)          (1pt)                              (1pt)

a specific area swabbed (neck, breast, etc)

b Most Probable Number                                                              (1pt)

c Violet Red Bile Agar                                (1 pt)

d Not Available

e volume plated = 1.0 ml

Sample Calculation with correct units is worth 1 point.  

8.      Create Table 4.4 using the average class data of the confirmed coliform counts from the MPN tubes.  Include a title and necessary footnotes.  (2 pts.)

This question asked you to “create a new table”.  You could have used what was online, but you needed to make a separate column for the average.  If this was not present in your table, you lost points because you did not follow the directions of the question.  You could have also averaged different regions that were swabbed, as long as you included all of the averages.

Table 4.4  Average class counts of confirmed coliforms (MPN^a/25cm²) from three raw, whole chickens (swabbed in different locations) could footnote this

a Most Probable number

Discussion

9.      Compare coliform counts (your groups data) from the three different media (LST broth, VRB Agar, E. coli/Coliform Petrifilms).  Describe why using these different media may yield different results.  (4 pts.)

There should be some indication that you are comparing relevant numbers from each type of medium.  You MUST include the numbers, or reference the appropriate table in your discussion.  If you just say that they were similar or different, you did not receive full credit.  Also, there must be some evidence of thought as to why the media might have produced different results.

10.  Does gas production in the LST tubes (from the MPN determination) confirm the presence of coliforms in the sample?  If not, how could this test have been modified to include confirmation? (4 pts.)

The correct answer is NO (2 pts.).  To modify this test (LST broth/presumptive test), addition of a pH indicator would have helped detect acid production, therefore confirming the test. (2 pts).

11.  Compare and contrast the confirmed coliform counts from LST broth that your group obtained with the class average. (4 pts.)

Again, as with Q9, there needs to be some evidence that you thought about why your sample was similar or different to the class average.  By indicating there was a difference, but no explanation (1 pt).  Discussion of the areas swabbed and the different counts achieved (2 pts).  The area your group swabbed in comparison (1 pt).

12.  Based on the results obtained, comment on the microbial quality of the food analyzed.  Remember that there were limited data gathered during this experiment, and it may not be sufficient to judge the suitability of this food for consumption.

(4 pts.)

      Based on the results obtained in the laboratory experiment, the chicken was probably of relatively good quality (1pt).  The coliform counts were between 10¹-10² CFU/25cm².  Remember that chicken spoilage count is ~10⁶-10⁷ CFU/cm², also taking into consideration coliforms, not total microbial counts were determined.  However, some groups detected E. coli, so there is a possibility that there could be a pathogenic strain present, or other, non-coliform pathogens that were not detected, which would need additional testing (2pts).  Most likely, the chicken would be cooked properly before eating it, which would kill most non-sporing microbes (1pt).

 

13.  If the chicken that we analyzed was contaminated with E. coli O157:H7, would we have been able to detect it using the procedures we followed in this experiment?  Why or why not?  (4 pts.) 

NO (2 pts) because the tests we used cannot identify specific strains of E. coli. (i.e. the presumptive/confirmatory tests aren’t enough).  E. coli O157:H7 does not ferment lactose very well, so it might even be missed as a presumptive coliform.  One would need to use the MUG reaction, and additional “detection” methods to know if E. coli O157:H7 was present (2 pts).  There is additionally a specific 3M Petrifilm Test Kit-HEC that will detect this strain.

References: Please include at least 2 outside references in addition to the manual.  Use proper ASM format. (2 pts.)

            PLEASE REVIEW THE PROPER ASM FORMAT.

            1 pt. For ASM format

            0.5 pts. For correctly alphabetizing the references

            0.5 pts. For overall spelling/neatness/consistency