Vol. XII · Food Safety Plate 2 of 12bacterial growth, doubling time, lag phase, exponential growth, log, Salmonella, temperature, growth curveS

How Bacteria Grow.

One cell dividing every twenty minutes becomes a million in under seven hours and a billion in ten. That is why time matters as much as temperature.

A bacterium grows by taking in nutrients and splitting into two. Moved into a new food, it first spends a lag while it adapts. Then it divides at a steady rate, so its numbers double again and again: 1, 2, 4, 8 … — exponential growth. Because the numbers become so large, microbiologists count in powers of ten, logs: 3 log is a thousand per gram, 6 log a million. Growth stops only when food runs out or waste builds up, near 9 log, a billion per gram, when the food is visibly spoiled.

Temperature sets the doubling time. Each organism has a minimum below which it cannot grow, an optimum where it grows fastest, often near body temperature, and a maximum above which it stops and begins to die (XII.3). Acid and low water activity slow it further (III.9, XIII.7). The workshop grows a bacterium in a food left at the temperature and for the time you choose.

Below: the count of a pathogen in food, in logs per gram, against hours at the temperature you choose (microbe bench: cardinal-temperature growth with a lag), compared with the same food in the fridge and at body heat.

What you will learn
Workshop
Microbe bench: growth rate from the cardinal-temperature model (Rosso et al., 1993) with each organism's published minimum, optimum and maximum temperatures; a lag of about four doubling times; a ceiling of 9 log per gram. Starting from 2 log (100 cells per gram), a light contamination.

Doubling, and doubling again

In this model Salmonella doubles every 21 minutes at 37 °C, every 53 at 21 °C and every 519 at 10 °C. At its fastest a single cell would pass a million in about 6.9 hours, after its lag. A light contamination of 100 cells per gram left at 25 °C for eight hours gains about 2.8 logs: from a hundred to tens of thousands per gram, with no change in smell or look.

Three lessons follow. Growth is invisible until very late: food reaches dangerous numbers long before it spoils visibly. Time is a dial as strong as temperature: the lag buys an hour or two at room temperature, which is the basis of the two-hour rule (XII.3). And cold slows but does not kill: food from the fridge carries its microbes, ready to grow again when it warms. Cooking resets the count only for the organisms heat can kill (XII.5, XII.9).

At the stove

Work it out on paper: if a cell divides every 20 minutes, how many generations is eight hours, and what is two raised to that power? Compare the answer with a million.

The doubling of bacteria was first measured with care by Jacques Monod in the 1940s; his growth curves of E. coli became the foundation of modern microbiology — and of the arithmetic of food safety.

Terms

Lag phase
The time bacteria spend adapting before they divide.
Doubling time
The time for a population to double.
Log
A power of ten: 6 log per gram is a million per gram.
Exponential growth
Growth by repeated doubling.
Bacteria lag, then double at a rate set by temperature; count them in logs. Danger arrives long before spoilage shows.

Sources

  1. Bacterial growth: cardinal-temperature model (L. Rosso et al., J. Theor. Biol. 162, 1993) with cardinal temperatures and optimum rates rounded from ICMSF (1996); teaching values that put each organism in its known range.
  2. ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.