FSC-342 Quiz #1: Lecture 4 - Intrinsic Factors that Impact Microbial Growth

What is binary cell division in bacteria?

Bacteria grow by binary cell division, meaning each cell divides into two, doubling the population.

What are the four phases of the bacterial growth curve?

Lag phase, Exponential phase, Stationary phase, Death phase.

What happens during the Lag phase?

No increase in cell number; bacteria adapt to their environment.

What happens during the Exponential phase?

Rapid cell division and population increase.

What happens during the Stationary phase?

Growth stops; number of cells remains constant.

What happens during the Death phase?

Bacterial cells begin to die off, reducing population size.

How do food microbiologists control food safety using the growth curve?

They extend the Lag phase or increase the Death phase to prevent exponential growth.

What are intrinsic factors in microbial growth?

Properties inherent to the food itself, such as pH and water activity.

What are extrinsic factors in microbial growth?

Environmental factors external to the food, such as temperature.

Why is understanding intrinsic and extrinsic factors important?

It allows prediction and control of microbial growth, survival, and death in food.

What are the three key controllable factors in food safety?

pH, water activity (aₜ), and temperature.

What is pH?

A logarithmic measure of acidity, defined as pH = -log[H⁺].

What does each pH unit represent?

A 10-fold change in hydrogen ion concentration.

What is the optimal pH range for most bacteria?

Between 6.0 and 7.5.

At what pH do most foodborne pathogens stop growing?

Below pH 4.4 (rare exceptions exist).

Which microbes can grow in acidic foods?

Lactic acid bacteria (LAB) and fungi.

What is the homeostatic response to acid exposure?

A constant response that uses proton pumps to keep internal pH stable when external pH > 6.0.

What is the Acid Tolerance Response (ATR)?

A stress response triggered near pH 5.5–6.0 that requires protein synthesis and helps bacteria survive acid and other stresses.

What are some stresses ATR provides cross-protection against?

Heat, salt, nisin, and alcohol in pathogens like E. coli O157:H7, Salmonella, and Listeria.

What are acid-shock proteins?

Proteins produced at pH 3.0–5.0 that differ from ATR proteins.

What are possible outcomes of bacterial acid exposure?

Cell death, injury, or adaptation.

What does water activity (aₜ) measure?

The amount of free (available) water in food for microbial growth.

How is water activity (aₜ) defined?

aₜ = P / P₀, where P is the vapor pressure of food and P₀ is that of pure water at the same temperature.

What is the typical aₜ value of high-moisture foods?

≥ 0.98 (e.g., fruits, vegetables, meat, fish).

How does decreasing water activity affect bacterial growth?

It increases the lag phase, decreases growth rate, and lowers the stationary population.

Which group of bacteria require the highest water activity?

Gram-negative bacteria.

What aₜ does Pseudomonas require?

Greater than 0.96–0.97.

What aₜ do Enterobacteriaceae require?

Greater than 0.93.

How do Gram-positive non-spore-forming bacteria compare in aₜ tolerance?

They are less sensitive to low aₜ than Gram-negative bacteria.

At what aₜ can Staphylococcus aureus grow?

As low as 0.86, but it does not produce toxin below 0.93.

What aₜ prevents Bacillus cereus outgrowth?

Approximately 0.93–0.97.

What is the minimum aₜ for Clostridium perfringens growth?

Between 0.95–0.97.

Do molds and yeasts grow at lower or higher aₜ than bacteria?

Lower — many can tolerate drier conditions.

What are osmotolerant yeasts?

Yeasts such as Zygosaccharomyces that grow at low aₜ in high-sugar or salty foods.

How do preservation methods like dehydration and freeze-drying ensure food safety?

They lower aₜ, limiting microbial growth.

What common additives lower aₜ in food?

NaCl, KCl, glucose, and sucrose.