Microbial Growth Phases

Why This Matters

When you culture bacteria in a lab or study how pathogens multiply during an infection, you're watching a predictable pattern unfold. The microbial growth curve isn't just a graph to memorize—it's a framework for understanding population dynamics, metabolic regulation, and environmental adaptation. These concepts appear throughout microbiology, from calculating antibiotic efficacy to optimizing industrial fermentation.

You're being tested on your ability to explain why cells behave differently at each phase, not just what happens. Exam questions often ask you to predict how changing conditions (nutrients, temperature, antibiotics) would shift the curve, or to identify which phase is most relevant for a given application. Don't just memorize the four phases—know what drives the transition between them and why each phase matters for real-world microbiology.

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Phases of Active Preparation and Growth

Before cells can divide, they must sense their environment and gear up their metabolic machinery. These early phases reflect the cell's ability to adapt and then exploit available resources.

Lag Phase

• No increase in cell numbers—cells are metabolically active but not dividing, as they synthesize enzymes, replicate DNA, and adjust to new conditions
• Duration varies with prior history—cells transferred from rich to minimal media experience longer lag phases because they must produce new biosynthetic enzymes
• Environmental sensing is critical—this phase demonstrates how microorganisms assess nutrient availability and stress conditions before committing to division

Exponential (Log) Phase

• Population doubles at a constant rate—this is when you calculate generation time ($g = \frac{t}{n}$, where $t$ is time and $n$ is number of generations)
• Cells are most vulnerable to antibiotics—rapidly dividing cells with active cell wall synthesis and DNA replication are prime targets for antimicrobial agents
• Metabolic activity peaks—this phase is ideal for harvesting cells in biotechnology applications because enzyme production and protein expression are maximized

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Phases of Resource Limitation and Decline

As nutrients deplete and waste products accumulate, microbial populations shift from growth mode to survival mode. These phases reveal how microorganisms cope with environmental stress.

Stationary Phase

• Growth rate equals death rate—the population plateaus as new cell production balances cell death, creating a dynamic equilibrium
• Survival strategies emerge—some species form endospores (like Bacillus and Clostridium), while others activate stress-response genes and produce secondary metabolites
• Quorum sensing intensifies—high cell density triggers coordinated behaviors including biofilm formation, virulence factor production, and competence for DNA uptake

Death (Decline) Phase

• Viable cell count drops exponentially—death rate exceeds reproduction as nutrients are exhausted and toxic byproducts (acids, alcohols) accumulate
• Not all cells die simultaneously—some enter viable but nonculturable (VBNC) states or persist as dormant forms, complicating sterilization efforts
• Critical for applied microbiology—understanding death kinetics informs food preservation methods, autoclave protocols, and predicting how long pathogens remain infectious

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Quick Reference Table

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Self-Check Questions

1. A bacterial culture is transferred from nutrient broth to minimal media. Which phase would be extended, and why?

2. Compare the metabolic activity of cells in lag phase versus stationary phase. What are cells "doing" in each, and how do their goals differ?

3. You're producing a recombinant protein in E. coli. Which growth phase would you harvest cells from, and what's the reasoning?

4. An FRQ asks why penicillin is ineffective against non-growing bacteria. Which phases would show reduced antibiotic susceptibility, and what mechanism explains this?

5. How does the transition from stationary to death phase differ from the transition from exponential to stationary phase in terms of what's happening at the cellular level?