MICRO L10 P2Microbiological Testing Techniques

Food Sample Dilution

• Dilution factor calculated by dividing volume of sample by final tube volume
• A series of dilutions are created by adding 1ml of sample to 9ml of diluent.
• The dilutions are used for further analysis.
• Examples of the volumes involved in each step include, 1ml sample and 9ml diluent, resulting in a full 10ml tube as final.

Examination Methods - Microbiological Tests

• Total Viable Count (TVC): Commonly used technique. Also known as Standard Plate Count (SPC) or Aerobic Plate Count (APC)
• Purpose: Determines the number of viable bacteria in a sample.
• Method: Colonies are counted manually using a colony counter.
• Ideal Count Range: 30-300 colonies are ideal for accurate counting.

Examination Methods - Media and Temperature

• Media: Nutrient agar or plate count agar (pH 7.0 to 7.4) are used to culture bacterial growth and aerobic and facultative anaerobes.
• Incubation Temperatures: Specific temperatures are crucial for specific bacteria types.
  • Thermophiles: 55°C
  • Mesophiles: 35-37°C
  • Spoilage bacteria and psychrophiles: 20-30°C

Indicator Organisms - Basic Concepts

• Indicator organisms are used to detect possible pathogens in food.

• Purpose: Presence of Indicator organisms in foods suggests the possibility of pathogens being present. It shows the general microbiological condition of the food.

• Examples: Coliforms, E. coli, Enterococci, Staphylococcus aureus, Listeria monocytogenes, Salmonella....etc.

Coliforms

• Principal Types: Escherichia coli and Enterobacter aerogenes are the principal coliform bacteria.
• Habitat: E. coli resides predominantly in the gastrointestinal tract (a large part of the process).
• Water Indicator: E. coli is a common indicator organism in water samples to detect the possible presence of enteric (fecal) pathogens.
• Food Significance: In food, the presence of E. coli isn't as significant a direct indicator of fecal contamination.
• Enumeration Methods: Coliforms can be counted using
  • Most probable number (MPN) method and Membrane filtration

Enterococci

• Indicator organism that indicates faecal contamination on water but less conclusive evidence in food as they are also found in non-faecal environments.
• Two main species found in the human and animal intestines are E. faecalis and E. faecium.

Staphylococcus aureus

• Produces thermostable deoxyribonuclease (TNase), a helpful measure in enumeration of the bacteria in food.
• Growth ≥ 10^6 and potential toxin production.
• Used as an indicator of improper food handling and processing, as well as a measure for handling ready-to-eat foods and contamination involved with this process.

Salmonella

• Used as an indicator of cross-contamination, environmental contamination, and how dust moves/contaminates food.

Listeria monocytogenes

• Used as an indicator of foods stored in low temperatures (refrigerator or other low-temperature containers).
• Presence indicates insufficient sanitation/sanitary procedures and/or inadequate sanitation practices are in use.

Campylobacter

• Used to indicate recovery of low numbers of this organism from samples of different foods.

Bacillus cereus

• Indicator organism for Bacillus food poisoning. Common in starchy foods, such as rice and cereals.

Escherichia coli

• Indicator organism utilized for products to indicate hygiene in raw and processed plant/animal forms.
• Presence in foods from animals indicates possible contamination/potential pathogens, often in conjunction with animal feces.

Clostridium (botulinum, perfringenes)

• Used to evaluate the impact of heat processing, especially on canned foods.

Yeast and Molds

• Used as indicators of fruit, vegetable, or other food spoilage.

This quiz covers essential techniques in microbiological testing, focusing on food sample dilution methods, total viable counts, and growth media considerations. Understand the processes involved in accurately determining viable bacteria and the importance of incubation conditions.