Experiment 7: Detection of Coliforms & Fecal Coliforms in Food
Objectives of Experiment 7
Determine the Mean Density of Coliforms:
Estimate the average concentration of coliform bacteria in food samples.
Understand MPN Principle:
Apply the Most Probable Number (MPN) method to estimate the number of organisms.
Discuss Biochemical Tests for E. coli Identification:
Review common tests used to identify Escherichia coli specifically.
Materials and Equipment
Juice Drink Samples:
Use both commercially produced and street-vended juice drinks for analysis.
Standard Lab Equipment:
Sterile Petri dishes
Wire Loop
Sample container (300-mL)
Stirring rod
Durham Tubes
Sterile dilution blanks
Test tube
Erlenmeyer flask
Sterile pipettes
Alcohol lamp
Graduated cylinder
Culture Media and Reagents
Media Used in the Experiment:
Brilliant Green Lactose Bile (BGLB) broth
Eosin Methylene Blue (EMB) agar
Lauryl Tryptose Broth
Tryptone broth
Nutrient broth with 0.5% glucose (NBG)
Simmon Citrate Agar
Reagents:
Kovac’s reagent
Methyl red indicator
40% KOH
5% alpha-naphthol
Procedures
A. Sample Collection
Aseptic Techniques:
Samples must be collected under sterile conditions.
Analyze samples within 8 hours of collection.
Maintain cooler than 4°C, do not freeze samples.
B. Sample Preparation
Prepare a dilution of $25 ext{ ml}$ to a $10^{-1}$ dilution.
Conduct serial dilutions up to $10^{-3}$, or further for highly contaminated foods.
C. Presumptive Test
Dispense $10 ext{ ml}$ LST broth in each test tube, add inverted Durham tube, and autoclave.
Inoculate 3 replicate tubes of LST Broth per dilution with $1 ext{ ml}$ from each dilution.
Incubate tubes at $35^{ ext{o}}C$ for $24 ext{±} 2 ext{ hours}$.
Examine for gas formation (indicating a positive test).
Re-incubate negative tubes for another $24 ext{ hours}$ and note growth/gas production.
Record positive results, referencing MPN tables for 3-tube dilution.
Report results as presumptive MPN of coliform bacteria/ml or g of sample.
D. Confirmatory Test for Coliforms
Transfer from gas-positive LST broth to separate sterile tubes of $10 ext{ ml}$ 2% BGLB broth.
Incubate at $35^{ ext{o}}C$ for $48 ext{±} 2 ext{ hours}$.
Check for gas production (indicates coliform presence).
Record positive BGLB tubes and determine MPN.
Report as confirmatory MPN of coliform bacteria/ml or g.
E. Completed Test for Coliforms
Streak a loopful from a positive BGLB tube onto EMB agar plate.
Incubate at $35^{ ext{o}}C$ for 24 hours.
Examine for colonies:
E. coli: Round colonies with dark centers, potential greenish sheen.
Other coliforms (e.g., Enterobacter): Large, mucoidal, spreading colonies without dark centers.
Inoculate sterile LST broth with a typical EMB colony. Incubate for 48 hours at $35^{ ext{o}}C$.
Check for gas formation (positive completed test).
Determine the Mean Density of Coliforms:
Estimate the average concentration of coliform bacteria in food samples.
Understand MPN Principle:
Apply the Most Probable Number (MPN) method to estimate the number of organisms.
Discuss Biochemical Tests for E. coli Identification:
Review common tests used to identify Escherichia coli specifically.
Materials and Equipment:
Juice Drink Samples: Use both commercially produced and street-vended juice drinks for analysis.
Standard Lab Equipment:
Sterile Petri dishes
Wire Loop
Sample container (300-mL)
Stirring rod
Durham Tubes
Sterile dilution blanks
Test tube
Erlenmeyer flask
Sterile pipettes
Alcohol lamp
Graduated cylinder
Culture Media and Reagents:
Media Used in the Experiment:
Brilliant Green Lactose Bile (BGLB) broth
Eosin Methylene Blue (EMB) agar
Lauryl Tryptose Broth
Tryptone broth
Nutrient broth with 0.5% glucose (NBG)
Simmon Citrate Agar
Reagents:
Kovac’s reagent
Methyl red indicator
40% KOH
5% alpha-naphthol
Procedures:
A. Sample Collection:
Aseptic Techniques: Samples must be collected under sterile conditions.
Analyze samples within 8 hours of collection.
Maintain cooler than 4°C, do not freeze samples.
B. Sample Preparation:
Prepare a dilution of $25 \text{ ml}$ to a $10^{-1}$ dilution.
Conduct serial dilutions up to $10^{-3}$, or further for highly contaminated foods.
C. Presumptive Test:
Dispense $10 \text{ ml}$ LST broth in each test tube, add inverted Durham tube, and autoclave.
Inoculate 3 replicate tubes of LST Broth per dilution with $1 \text{ ml}$ from each dilution.
Incubate tubes at $35^{\text{o}}C$ for $24 \text{±} 2 \text{ hours}$.
Examine for gas formation (indicating a positive test).
Re-incubate negative tubes for another $24 \text{ hours}$ and note growth/gas production.
Record positive results, referencing MPN tables for 3-tube dilution.
Report results as presumptive MPN of coliform bacteria/ml or g of sample.
D. Confirmatory Test for Coliforms:
Transfer from gas-positive LST broth to separate sterile tubes of $10 \text{ ml}$ 2% BGLB broth.
Incubate at $35^{\text{o}}C$ for $48 \text{±} 2 \text{ hours}$.
Check for gas production (indicates coliform presence).
Record positive BGLB tubes and determine MPN.
Report as confirmatory MPN of coliform bacteria/ml or g.
E. Completed Test for Coliforms:
Streak a loopful from a positive BGLB tube onto EMB agar plate.
Incubate at $35^{\text{o}}C$ for 24 hours.
Examine for colonies:
E. coli: Round colonies with dark centers, potential greenish sheen.
Other coliforms (e.g., Enterobacter): Large, mucoidal, spreading colonies without dark centers.
Inoculate sterile LST broth with a typical EMB colony. Incubate for 48 hours at $35^{\text{o}}C$.
Check for gas formation (positive completed test).
The rationale of this experiment is to assess the safety and quality of juice products by determining the concentration of coliform bacteria, which are indicators of fecal contamination and potential pathogens. This experiment applies the Most Probable Number (MPN) method, which is a statistical approach used to estimate the number of organisms in a sample based on dilution and growth conditions. Understanding the presence of coliforms, and specifically Escherichia coli, helps ensure food safety and adherence to health standards, thereby protecting public health.
The quality and safety of juice products remain critical concerns for public health, particularly with regard to microbial contamination. Coliform bacteria, particularly Escherichia coli, serve as important indicators of fecal contamination and the potential presence of pathogenic microorganisms. The rationale for this experiment centers on assessing the concentration of coliform bacteria in juice drink samples, thereby providing insight into the microbiological safety of these products. By employing the Most Probable Number (MPN) method, we can effectively estimate the viable counts of coliforms in various juice beverages, which includes both commercially produced and street-vended options. This statistical approach analyzes dilutions and growth patterns to establish a reliable estimate of bacterial presence, aiding in ensuring compliance with health standards that protect consumers from foodborne illnesses.
The problem addressed in this experiment is the inadequate monitoring of coliform bacteria in juice products, which can lead to health risks for consumers. Despite regulatory frameworks designed to uphold food safety, lapses in adherence to best practices during production and distribution may result in contaminated beverages entering the market. The potential for outbreaks of foodborne diseases, linked to the consumption of contaminated juices, underscores the necessity of this research. Through systematic testing and analysis, this study aims to illuminate the levels of microbial contamination that may be present, supporting efforts towards improving food safety protocols and ensuring the protection of public health from the dangers posed by unsafe food products.
The presence and estimated concentration of coliform and fecal coliform bacteria in juice drinks, both commercial and street-vended, can be determined through a series of microbiological testing procedures utilizing the Most Probable Number (MPN) method. This approach involves several key steps:
Sample Collection: Juice drink samples should be collected under aseptic conditions to prevent contamination. It is crucial to analyze these samples within 8 hours of collection while keeping them maintained at temperatures below 4°C.
Sample Preparation: This step includes preparing a dilution of the juice sample to a concentration of $10^{-1}$ and conducting serial dilutions up to $10^{-3}$ or beyond for highly contaminated products. These dilutions allow for a manageable estimation of the viable coliform population.
Presumptive Test: In this phase, $10\text{ ml}$ of Lauryl Sulfate Tryptose (LST) broth is dispensed into test tubes, followed by the addition of inverted Durham tubes. The samples are inoculated with $1\text{ ml}$ from each dilution and incubated at $35^{\text{o}}C$ for 24 \text{±} 2 hours. Detection of gas formation in the Durham tube indicates a presumptive positive result for coliform presence. Negative tubes are re-incubated for an additional 24 hours to reassess gas production.
Confirmatory Test for Coliforms: For tubes showing gas production, a transfer to separate sterile tubes containing $10\text{ ml}$ of 2% Brilliant Green Lactose Bile (BGLB) broth is performed, followed by another incubation at $35^{\text{o}}C$ for 48 \text{±} 2 hours. Gas production again confirms coliform presence.
Completed Test for Coliforms: Finally, a loopful from a positive BGLB tube is streaked onto Eosin Methylene Blue (EMB) agar, incubated for 24 hours at $35^{\text{o}}C$, and colonies are examined. Identification of colonies can determine whether E. coli or other coliforms are present, distinguished by morphological characteristics observed on the agar media.
Quantification: Results from the presumptive and confirmatory tests allow for the calculation of the Most Probable Number (MPN) of coliforms per milliliter or per gram of the juice sample, providing a comprehensive assessment of microbial safety in the analyzed juice products.
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The presence and estimated concentration of coliform and fecal coliform bacteria in juice drinks, both commercial and street-vended, can be determined through a series of microbiological testing procedures utilizing the Most Probable Number (MPN) method. This approach involves several key steps: firstly, sample collection should occur under aseptic conditions to prevent contamination, with analysis required within 8 hours while maintaining temperatures below 4°C. Next, sample preparation includes diluting the juice sample to a concentration of $10^{-1}$ and conducting serial dilutions up to $10^{-3}$ or beyond for highly contaminated products, which allows for a manageable estimation of the viable coliform population. The presumptive test entails inoculating Lauryl Sulfate Tryptose (LST) broth with the dilutions and incubating, with gas formation indicating presumptive coliform presence. Confirmatory tests involve transferring samples from gas-positive LST broth to Brilliant Green Lactose Bile (BGLB) broth, followed by incubation to confirm coliform presence through gas production. Finally, the completed test includes streaking from a positive BGLB tube onto Eosin Methylene Blue (EMB) agar, where morphological examination of colonies can identify E. coli and other coliforms. The results from these tests allow for the calculation of the MPN of coliforms per milliliter or per gram of juice sample, thus providing a comprehensive assessment of microbial safety in the analyzed juice products.