Listeria Growth Curve — Effect of Temperature, Water Activity, and MAP: 04-BS-13, December 2019, Q8

Question 8: Listeria Growth Curve — Effect of Temperature, Water Activity, and MAP (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. Listeria monocytogenes is a psychrotolerant (cold-tolerant), facultatively anaerobic, moderately halotolerant pathogen. Each condition below is reasoned from its known growth-rate response to temperature, water activity, and modified atmosphere relative to the baseline curve (15°C, Aw 0.99, pH 6.8).

Fig. 3: baseline Listeria growth curve (15°C) compared with the three modified conditions — (a) 37°C (shorter lag, faster exponential phase, slightly lower/shorter-lived stationary plateau due to faster substrate depletion and quicker onset of decline); (b) 37°C + Aw 0.42 (essentially flat — no measurable growth); (c) 15°C + MAP 20% CO₂/80% N₂ (longer lag and slower exponential phase than the 15°C baseline, reaching a similar final plateau more slowly).

(a) 37°C, Aw 0.99, pH 6.8. 37°C is close to Listeria's optimum growth temperature (its accepted range spans roughly 0–45°C, with an optimum near 30–37°C), so both the lag phase and the generation time shorten substantially relative to the 15°C baseline — the curve rises earlier and much more steeply, reaching stationary phase sooner. Since none of the other growth-limiting factors (Aw, pH) have changed, the final population density in stationary phase is similar to (or slightly below) the baseline's, but reached in a fraction of the time.
(b) 37°C, Aw 0.42. Although 37°C alone would favor rapid growth, Listeria's minimum water activity for growth is approximately 0.90–0.92; Aw 0.42 is far below this limit (a value more typical of a low-moisture dry product, e.g. crackers or dried herbs). The curve is therefore essentially flat — no measurable increase in cfu/ml over time — because the cells are metabolically inactive/dormant (though not necessarily dead) at such severe osmotic/water stress, regardless of how favorable the temperature is.
(c) 15°C, Aw 0.99, pH 6.8, MAP (20% CO₂, 80% N₂). Elevated CO₂ is bacteriostatic against many spoilage and pathogenic organisms (it dissolves into the aqueous phase, forms carbonic acid, lowers intracellular/local pH, and inhibits several enzyme systems), and Listeria, while more CO₂-tolerant than many competing spoilage bacteria, still shows a measurably extended lag phase and reduced growth rate under high-CO₂ MAP relative to an air-stored control at the same temperature. The resulting curve keeps the same shape as the 15°C baseline but is stretched out — longer lag, shallower exponential slope, reaching a similar final density only after a longer total time (this partial-inhibition, not-fully-static behavior is precisely why MAP is used as a shelf-life extension tool rather than a guarantee of no Listeria growth, and refrigeration is always required in addition).

Condition	Qualitative growth-curve behaviour vs. 15°C baseline
(a) 37°C, Aw 0.99, pH 6.8	Much shorter lag and exponential phase; stationary phase reached far sooner
(b) 37°C, Aw 0.42	Flat curve — no growth (below the water-activity minimum for growth)
(c) 15°C, MAP 20% CO₂/80% N₂	Longer lag, slower exponential rise; growth still occurs (partial inhibition only)

Topic: Effect of temperature, water activity, and modified-atmosphere packaging on the microbial growth curve.

Key relations: growth rate roughly follows an Arrhenius-type temperature dependence up to the organism's optimum; growth requires Aw above an organism-specific minimum threshold, below which growth ceases regardless of temperature; elevated CO₂ extends lag phase and reduces growth rate (bacteriostatic) without necessarily preventing growth outright.

Why this works: each environmental factor acts through a distinct physiological mechanism (enzyme kinetics/membrane fluidity for temperature, osmotic water availability for Aw, intracellular pH/enzyme inhibition for CO₂), so the three panels change the growth curve in qualitatively different ways — a rate change (a), a hard threshold/switch-off (b), and a lag/rate change without a threshold (c).

Common pitfall: assuming any single hurdle (low temperature or MAP alone) fully prevents Listeria growth — it is specifically psychrotolerant and CO₂-tolerant relative to many competitors, which is exactly why it is a persistent concern in refrigerated, MAP-packaged ready-to-eat foods; only a sufficiently low Aw (as in panel (b)) produces a true growth-preventing hurdle here.

Source: Madigan et al., Brock Biology of Microorganisms, Ch. 5–6 (microbial growth, environmental factors); Toledo, Fundamentals of Food Process Engineering, Ch. 9 (modified atmosphere packaging and shelf-life control).

Question 8 of 9: Listeria Growth Curve — Effect of Temperature, Water Activity, and MAP

Question 8: Listeria Growth Curve — Effect of Temperature, Water Activity, and MAP (20 marks)