Microbiological culture

A microbiological culture is a specialized method used to multiply microorganisms by letting them reproduce in a predetermined culture medium. This process occurs under controlled laboratory conditions to ensure the organisms grow as intended.

These cultures are foundational tools in molecular biology and serve as primary diagnostic methods in microbiology. Scientists use them to determine the specific type of organism present in a sample. They can also measure the abundance, or how many organisms are there, within that sample. By allowing an agent to multiply in a controlled medium, researchers can identify the exact cause of an infectious disease.

The mechanism of culturing depends heavily on the type of medium used. In liquid cultures, organisms are suspended in a nutrient-rich liquid called a broth. This is often done in an upright flask, such as with Luria broth.

To encourage uniform growth, scientists may use a mechanical shaker to mix the liquid. For solid growth, scientists use a gelling agent like agar, which is a gelatinous substance derived from seaweed.

In an agar plate, the medium provides a stable surface for stationary growth. To isolate individual microbes, scientists may use the streak plate method. This involves spreading the sample back and forth across the agar using an inoculating loop. This physical separation allows single cells to grow into distinct, visible colonies.

Microbial cultures are categorized by the type of organism being grown. Prokaryotic cultures typically involve bacteria, as archaea are difficult to culture in a lab setting. To achieve a pure culture, which is a population of cells growing without any other species, one must start from a single cell or colony. Because prokaryotic colonies are asexual offspring of one cell, the resulting cells are genetically identical clones.

Viral cultures are different because viruses and phages require living host cells to multiply. For bacteriophages, scientists infect bacterial cells and then isolate the phage from the resulting plaques. Eukaryotic cell cultures provide controlled environments for studying more complex organisms. This includes single-celled eukaryotes like yeast, algae, and protozoans, as well as multicellular microscopic organisms like C. elegans.

The history of microbiology was shaped by several key inventions. In 1860, Louis Pasteur designed the first culture media, which were liquid media. This remained the standard until 1881, when Robert Koch developed solid media. This was a major advancement for the field. In 1887, Julius Richard Petri improved this by creating a round box for the solid media. This device is known today as the Petri dish. These foundational discoveries allowed for the diverse array of modern media and methods used by scientists today.

Scientists use different specialized media to reveal specific biological traits. Selective media is designed to distinguish organisms by allowing one type to grow while inhibiting others. For example, eosin methylene blue (EMB) medium can select for Gram-negative bacteria by hindering the growth of Gram-positive bacteria.

Differential media, however, are used to reveal biochemical characteristics. MacConkey agar (MAC) is a common example that uses a pH indicator. This indicator changes color when acids are produced during lactose fermentation, helping to identify specific bacteria. There are also agar-based dipsticks, which are miniature, cost-effective versions of plates used for rapid diagnosis at the point-of-care.

Different growth formats serve specific laboratory needs. Stab cultures involve inoculating a test tube of solid agar with a needle or pipette tip. These are frequently used for short-term storage or for shipping cultures. They can also reveal if a bacterium is motile, meaning it can move, or non-motile. For organisms that live in extreme heat, known as thermophiles, scientists use different gelling agents. While agar is common, guar gum or low acyl clarified gellan gum can be used to maintain these organisms at temperatures between 50 and 70 degrees Celsius.

Microbiological cultures are essential to the broader scientific community through culture collections. These are specialized repositories that focus on the acquisition, authentication, and distribution of standard reference microorganisms.

Major national collections exist all over the world to support research in microbial systematics. Examples include the American Type Culture Collection (ATCC) in Virginia and the Institut Pasteur (CIP) in Paris. These collections act as vital hubs, allowing researchers to access specific cell lines and type strains for study. This global network ensures that scientists can maintain and share the biological tools necessary for modern medicine and biology.