Food Microbiology

Why study microbiology?

Because microbes are abundant and influential — e.g., ~10¹³ microbes in a baby vs. 10¹¹ stars in our galaxy. Microbiology helps us understand health, food safety, and the environment.

What are plasmids and where are they found?

Circular extrachromosomal DNA in prokaryotes; not essential for growth but often carry genes for unique properties like toxin production or antimicrobial resistance.

What is the structure of the Gram-positive bacterial cell wall?

Thick peptidoglycan layer with teichoic acids; provides rigidity and protection.

What is the structure of the Gram-negative bacterial outer membrane?

Contains lipopolysaccharide (LPS), made of: Lipid A (endotoxin), Core polysaccharide, and O-specific polysaccharide.

What are bacterial endospores, and which genera produce them?

Highly resistant survival structures produced inside cells; protect against heat, desiccation, chemicals, and radiation. Produced mainly by Bacillus spp. and Clostridium spp.

What is the function of bacterial flagella?

Enable locomotion by rotating like a propeller, allowing bacteria to move toward nutrients or away from harmful substances.

What are fimbriae and pili?

Fimbriae are hair-like structures used for attachment to surfaces and host tissues. Pili are longer structures used for DNA transfer during conjugation.

What are bacterial capsules, and why are they important?

Polysaccharide layers outside the cell wall; protect against phagocytosis, aid in attachment, and contribute to biofilm formation.

Describe the stages of biofilm formation.

1. Planktonic cells

2. Initial attachment

3. Irreversible attachment 4.

4. Biofilm formation & maturation

5. Dispersion.

What are the three mechanisms of genetic exchange in prokaryotes?

Transformation: uptake of free DNA; Transduction: DNA transfer via bacteriophages; Conjugation: direct transfer between cells via pilus (plasmid-mediated).

What are biosafety levels (BSL) in microbiology labs?

Containment levels BSL-1 to BSL-4. BSL-1: low-risk microbes. BSL-4: highest containment, dangerous pathogens (e.g., CDC facilities).

What is the purpose of classical food microbiology methods?

To ensure food safety, assess spoilage risk, and monitor hygiene and processing standards.

What are the five main classical methods in food microbiology?

1.Standard Plate Count (SPC) 2. Most Probable Number (MPN) 3. Membrane Filter Technique 4. Microscopic Counts 5. Dye Reduction Tests.

How does the Standard Plate Count (SPC) method work?

Sample → Homogenisation → Serial dilutions → Plating → Incubation → Colony counting. Results reported as cfu. Widely used in milk, meat, and RTE foods.

What is the Most Probable Number (MPN) method used for?

A statistical method (growth no growth in tubes) used for low numbers or unevenly distributed microbes (e.g., coliform testing in water).

What is the membrane filter technique in microbiology?

Large sample volumes are filtered through a membrane, which is then placed on growth medium. Faster than MPN, useful for water and beverage testing.

What is the Direct Microscopic Count (DMC), and where is it used?

Stain and count microbes directly under a microscope. Fast, but counts both live and dead cells. Common in the dairy industry (Breed count).

What is the Direct Epifluorescent Filter Technique (DEFT)?

Fluorescence microscopy method distinguishing viable vs. non-viable cells using dyes. Faster than DMC, but requires specialised equipment.

What types of culture media are used in microbial identification?

General purpose (e.g., Nutrient agar), Differential (e.g., MacConkey agar), Selective (e.g., XLD for Salmonella, VRBA for coliforms), and Selective-differential (combined).

What are biochemical test panels (e.g., API 20E system)?

Standardised strips with mini-tests for enzymes, fermentation, etc. Generate metabolic profiles to identify microbes. Widely used in food and clinical labs.

What are dye reduction tests, and how do they work?

Use dyes like methylene blue or resazurin. Colour change time is inversely proportional to microbial load. Rapid, cheap, and simple.

What are VBNC (Viable But Non-Culturable) cells?

Bacteria that do not proportional to microbial load. Rapid, cheap, and simple. remain alive. They may regain activity in favourable conditions.

What is the key takeaway from classical food microbiology methods?

They form the foundation of testing. Each method has strengths and limitations; understanding microbial biology is essential to choose the right method.