Bacterial Growth Curve

A bacterial growth curve is a model showing how a bacterial population changes over time under a defined set of conditions. In microbiology, a typical growth curve for bacteria growing in a closed system is divided into four phases: lag, log (exponential), stationary and death or decline. The length and shape of each phase depend on the organism, food or growth medium, temperature, pH, water activity and other conditions.

The four phases of bacterial growth

• Lag phase: cells adapt to their environment and prepare for growth. The population may show little increase while cells adjust.
• Log phase: cells divide rapidly under favourable conditions, and the population increases exponentially.
• Stationary phase: the rate of new cell formation is approximately balanced by the rate of cell death, often because nutrients become limited or waste products accumulate.
• Death phase: viable cell numbers decline as conditions become less favourable.

Why the growth curve matters in food safety

Time and temperature strongly influence how quickly many foodborne bacteria multiply. Refrigeration slows the growth of many organisms, but it does not sterilize food and does not stop every pathogen; some bacteria can grow at refrigeration temperatures. Warm holding, slow cooling and extended storage can allow certain organisms to multiply or produce toxins.

What changes bacterial growth?

Important factors include temperature, acidity (pH), available moisture, oxygen, nutrients, competing microorganisms and the food’s structure. Different bacteria have different growth limits, so one set of conditions does not affect every organism in the same way.

Example: A cooked food left warm for an extended period may provide favourable conditions for bacterial multiplication. Rapid cooling and refrigerated storage are used to limit growth, following the time and temperature limits in the operation’s food-safety plan.

Chef’s tip: Do not use the growth curve to calculate a safe holding time without validated data. Control time and temperature, prevent contamination and follow approved cooling, storage and reheating procedures.