A pure culture is a population of microorganisms consisting of a single species or strain with no contaminating organisms. It is derived from a mixed population by isolating individual cells and allowing them to multiply under controlled conditions. Pure cultures are a cornerstone of microbiological research and diagnostic practice because they allow the characteristics of a single species to be studied without interference from others.
Isolation techniques and purpose
Natural samples such as soil, water, food and clinical specimens typically contain diverse microbial communities. To obtain a pure culture, microbiologists employ techniques that physically separate individual cells and provide conditions favoring their growth. The streak plate method uses an inoculating loop to spread cells across the surface of an agar plate so that single cells are deposited and grow into discrete colonies. Pour plate and spread plate techniques dilute samples in molten or liquid agar before plating to achieve similar isolation. Selective and differential media may suppress unwanted organisms while enhancing the target species. Once a colony arises from a single progenitor, it can be picked and re‑cultured to establish an axenic culture. Aseptic technique throughout the process is essential to prevent contamination.
Applications and examples
Pure cultures enable detailed characterization of microorganisms, including their morphology, metabolic capabilities, antibiotic susceptibility and genetic composition. In clinical microbiology, isolating the causative agent of an infection in pure culture is necessary for accurate identification and determination of appropriate treatment. Pure cultures of industrial organisms such as Saccharomyces cerevisiae, Lactobacillus species and Bacillus subtilis underpin fermentation processes producing bread, yogurt, antibiotics and enzymes. In molecular biology, defined strains like Escherichia coli K‑12 serve as model systems for gene cloning and expression. Maintaining purity requires periodic subculturing and verification because airborne microbes or handling errors can introduce contaminants.
Pure cultures provide a controlled framework for studying and utilizing microorganisms. By separating a single species from a complex community, researchers and clinicians can attribute observed properties to that organism alone.
Related Terms: Streak plate, Colony, Selective medium, Axenic culture, Contamination