In microbiology, a pure culture is a laboratory culture that contains a single species of microorganism. This population of cells is grown in a controlled environment, free from other species. The development of pure culture techniques in the 19th century by scientists like Robert Koch was a foundational moment for microbiology, allowing for the systematic study of individual microbes. By isolating a single species, researchers can investigate its specific characteristics, such as growth patterns and metabolic processes. This is fundamental for diagnosing infectious diseases, developing antibiotics and vaccines, and for applications in biotechnology.
Understanding Mixed Cultures in Nature
In nearly every natural environment, microorganisms exist in complex, mixed communities. A single teaspoon of soil, a drop of pond water, or a swab from human skin contains a vast and diverse population of bacteria, fungi, and other microbes. These organisms live in intricate relationships; some may compete for nutrients, while others might produce substances that help their neighbors grow. This natural state is referred to as a mixed culture.
These environments are dynamic, with the types and numbers of microorganisms constantly shifting based on factors like temperature, pH, and nutrient availability. The gut microbiome in humans is a dense mixed culture where different bacteria play roles in digestion and health. Because of this immense diversity and the interactions between species, finding a naturally occurring pure culture is exceptionally unlikely.
The nature of these environments promotes diversity. One organism might break down a complex substance, creating food for another. Another might alter the environment by changing the pH, which allows a different species to thrive. This interdependence means that any sample taken directly from the environment will contain a mixture of many microbes, making it an unreliable source.
The Most Reliable Source for a Pure Culture
The most dependable source for a pure culture is a sample from an existing, previously isolated culture within a laboratory. This reliability comes from the culture having already undergone a deliberate separation process and being maintained under sterile conditions to prevent contamination. While the original organism may have come from a mixed source, the isolation process in the lab is what creates the pure culture.
An isolated colony on a streak plate is a prime example. This colony is a visible mound of cells that originated from a single microbial cell physically separated on the plate’s surface. Because this single cell multiplied in place, all its descendants are genetically identical, forming a clonal population. Transferring a small part of this single colony to a new, sterile growth medium will result in a new pure culture.
Methods for Isolating Pure Cultures
To create a pure culture from a mixed sample, scientists use laboratory techniques designed to separate individual microbial cells. The most common of these is the streak plate method. This technique mechanically dilutes a concentrated sample of microbes across the surface of a solid growth medium in a Petri dish. The goal is to deposit individual cells far enough apart so they can grow into distinct, isolated colonies.
The process begins by sterilizing an inoculating loop. A small amount of the mixed culture is then picked up on the loop, which is then gently streaked across one section of the agar surface. The loop is sterilized again and used to drag some microbes from the first section into a second, and then a third and fourth, sterilizing the loop between each step. This sequential streaking progressively thins out the bacteria.
By the final streaks, individual bacterial cells are deposited on the agar surface at a significant distance from one another. The plate is then incubated under conditions suitable for microbial growth. After a period of 24-48 hours, visible colonies will appear on the plate. In the areas of the final streaks, these colonies will be well-separated, and each represents a pure culture derived from a single cell.
Confirming the Purity of a Culture
After performing an isolation technique, it is necessary to verify that the resulting culture is pure. This confirmation involves both macroscopic and microscopic examination. The first step is to visually inspect the colonies on the agar plate based on their morphology—features like size, shape, color, and texture. A pure culture will produce colonies that are uniform in these aspects, while colonies of different appearances indicate a mixed culture.
For a more definitive confirmation, a sample from a single, isolated colony is examined under a microscope. A small amount of the colony is placed on a glass slide, stained with a dye like the Gram stain, and viewed at high magnification. If the culture is pure, all of the cells observed will have the same shape, size, and staining characteristics. The presence of cells with different morphologies or staining reactions confirms that the culture is contaminated.