Microbiology

A Winogradsky column shows how microbial layers form in mud and water over time. It is a clear, enclosed ecosystem that helps scientists grow sediment microbes that are hard to culture in a test tube or on a petri dish. By keeping conditions close to a microbe’s natural habitat, the column makes it possible to study organisms that help drive Earth’s biogeochemical cycles.

The column is built from pond or wetland sediment mixed with water in a transparent container. Extra sources of carbon,...

Counting bacteria with dilution plates helps microbiologists estimate how many cells are in a sample. Because bacteria can grow quickly and may be hard to count directly, serial dilution and plating make the concentration easier to measure. These methods have been used for more than a century to study bacterial and viral loads in clinical, industrial, pharmaceutical, and academic labs.

Serial dilution reduces a sample in steps. A small volume of the original solution is moved into a fixed...

Selective media help scientists isolate bacteria by supporting some microbes and blocking others. This lab also uses differential media, which reveal a visible chemical trait in the organism being grown. Together, these methods help identify bacteria from mixed samples.

Prokaryotic cells live in almost every environment on Earth. They also show wide metabolic diversity, so the growth medium must supply the molecules they need for energy. Some microbes are extremophiles, meaning they live in...

The streak plate method is used to isolate single bacterial colonies from a mixed sample. It helps separate one bacterial type from another so a pure culture can be grown. This is important in both clinical and research settings, where bacteria often grow together in mixed cultures.

Pure cultures are needed for later genetic and protein studies. They are also used to check whether a sample is pure and to identify infectious agents from clinical samples. Because different strains can affect...

Bacterial identification with 16S rRNA gene sequencing helps scientists tell bacterial species apart. It is used in microbial ecology to study biodiversity in environmental samples and in medical microbiology to help diagnose infected patients. Traditional methods include microscopy, growth on specific media, biochemical and serological tests, and antibiotic sensitivity assays.

Molecular methods have changed how bacteria are identified. 16S ribosomal RNA (rRNA) gene sequencing is often faster...

Bacterial growth stages and cell counts show how a culture changes over time. This page follows a batch culture as it moves through lag, exponential, stationary, and death phases. It also shows how colony forming units, or CFU, and optical density can be used to measure that growth.

Lag phase is the time bacteria need to adjust to a new environment before fast division begins. During exponential phase, cells grow and divide at the fastest rate, and DNA replication, RNA transcription, and...

Measuring bacterial resistance with E-tests helps show how strongly an antibiotic can stop growth. The method is used to find the minimal inhibitory concentration, or MIC, which is the lowest antibiotic level that prevents visible bacterial growth. It is a practical way to study resistance in bacteria and compare how different antibiotics perform.

The E-test uses a plastic strip that contains a built-in antibiotic gradient. The strip is placed on an agar plate that has been inoculated with...

Bacterial stains help scientists see cell walls, capsules, and endospores with a compound light microscope. Many bacteria are hard to tell apart with light alone because they are small, colorless, and can look similar under the lens. Staining makes these hidden features visible and helps identify different bacteria.

A compound microscope uses more than one lens to magnify a sample. Its objective lens collects light near the specimen, and the eyepiece enlarges the image. Many compound...

Bacteriophages, or phages, are viruses that infect prokaryotic cells. The plaque assay is a classic way to count these viruses by measuring viral titer, which is the amount of virus in a sample. It was first described in 1952 and is still one of the most reliable methods for determining viral titer.

Phages were identified in the early 20th century by Twort and d'Hérelle. They have since been recognized for their therapeutic value and for their role in human and global ecosystems. Interest in...

Calcium chloride treatment can help E. coli take up foreign DNA. This chemical transformation method is used in molecular biology to move a plasmid into bacterial cells and then identify the cells that received it. The protocol described here uses chemically competent E. coli DH5α cells and the plasmid pUC19 to measure transformation efficiency.

Bacteria can exchange genetic material through horizontal gene transfer. The three main routes are transformation, transduction, and conjugation.

Bacterial conjugation is a form of horizontal gene transfer in E. coli that moves DNA from one cell to another through direct contact. It was first discovered by Lederberg and Tatum in 1946. Unlike transformation or transduction, conjugation is a natural, one-way transfer of DNA from a donor cell to a recipient cell.

This process can increase bacterial genetic diversity. It is often described as a type of bacterial mating. It has also helped drive the spread of antibiotic-resistant bacteria.

Bacteriophage transduction moves DNA between bacteria by using a phage, or bacterial virus, as the carrier. In this process, a phage can deliver genetic material from one bacterium to another. The method is a form of horizontal gene transfer and is important in both bacterial evolution and laboratory genetics.

Phages were first described in the early 1900s, and transduction was named in 1951 by Norton Zinder and Joshua Lederberg. The transcript describes two main phage classes. Lytic phages...