The Probability of Bacteria’s Eternal Survival: Unveiling the Secrets of Petri Dish Life

Life in a Petri dish is a fascinating microcosm of the larger world. The survival of bacteria in such an environment is a subject of great interest to scientists, as it can provide insights into the fundamental principles of life and evolution. One intriguing question that arises in this context is: “A single bacterium is put into an empty Petri dish. Every minute, each bacteria has a chance ‘p’ of duplicating itself into two and chance ‘1-p’ of dying. What is the probability that the bacteria will live indefinitely?” This question, while seemingly simple, opens up a world of complex probabilities and mathematical models that can help us understand the eternal survival of bacteria.

Understanding the Basics of Bacterial Reproduction

Bacteria reproduce asexually through a process called binary fission. In ideal conditions, a single bacterium can duplicate itself into two identical cells. However, the rate of reproduction is not constant and can be influenced by various factors such as nutrient availability, temperature, and presence of toxins. In our scenario, we are considering a simplified model where each bacterium has a fixed probability ‘p’ of duplicating and ‘1-p’ of dying every minute.

The Probability Model

The probability of a bacterium’s eternal survival can be modeled using a branching process, a concept in probability theory. In this model, each bacterium can be considered as a ‘generation’. The next generation can have either 0 (if the bacterium dies) or 2 (if it duplicates) members with probabilities ‘1-p’ and ‘p’ respectively. The process continues indefinitely, creating a potentially infinite tree of generations.

Calculating the Probability of Eternal Survival

Mathematically, the probability of eternal survival ‘P’ can be calculated using the equation P = pP². This equation is derived from the fact that for the bacteria to survive eternally, it must first survive the next minute (with probability ‘p’) and then both its offspring must also survive eternally (with probability P²). Solving this equation gives P = 0 or P = 1 – 1/p. The solution P = 0 corresponds to the case where the bacteria eventually dies out. The other solution gives the probability of eternal survival, provided it is greater than 0.

Implications of the Model

This model, while simplistic, provides valuable insights into the survival of bacteria. It shows that the probability of eternal survival increases with the rate of reproduction. However, even with a high rate of reproduction, the bacteria cannot guarantee eternal survival due to the ever-present chance of death. This model also highlights the importance of the initial conditions, as the survival of the entire lineage depends on the survival of the first bacterium.


While the probability of a bacterium’s eternal survival in a Petri dish might seem like a purely academic question, it has profound implications for our understanding of life and evolution. It underscores the delicate balance between life and death, growth and decay, that all living organisms must navigate. And it reminds us that even in the seemingly simple world of a Petri dish, life is a complex and fascinating dance of probabilities.