Microbiology Glossary

A gelatinous substance derived from red algae used as a solidifying agent in microbiological culture media. Agar remains solid at incubation temperatures and is not degraded by most microorganisms, making it ideal for growing bacterial colonies.

A chemical substance produced by or derived from microorganisms that kills or inhibits the growth of bacteria. Antibiotics target essential bacterial processes such as cell wall synthesis, protein synthesis, DNA replication, or metabolic pathways.

A set of laboratory practices designed to prevent contamination of cultures, sterile media, and equipment by unwanted microorganisms. Techniques include working near a flame, sterilizing instruments, and using laminar flow hoods.

A device that uses pressurized steam (typically 121 degrees Celsius at 15 psi for 15-20 minutes) to sterilize laboratory equipment, media, and waste by killing all microorganisms including endospores.

A virus that infects and replicates within bacteria. Phages are the most abundant biological entities on Earth and play critical roles in bacterial population control, horizontal gene transfer (transduction), and are being explored as alternatives to antibiotics.

A well-organized layer of polysaccharides or proteins that surrounds the cell wall of some bacteria. Capsules protect bacteria from phagocytosis by immune cells and desiccation, and they are an important virulence factor in pathogens like Streptococcus pneumoniae.

A visible cluster of microorganisms growing on a solid culture medium, typically originating from a single cell or group of cells. Colony morphology (shape, color, texture, elevation) is used as a preliminary identification tool.

A lipopolysaccharide (LPS) component of the outer membrane of Gram-negative bacteria that is released upon cell lysis. Endotoxins trigger strong inflammatory responses and can cause fever, septic shock, and organ failure at high concentrations.

An organism whose cells contain a true nucleus enclosed by a nuclear membrane, along with other membrane-bound organelles such as mitochondria and endoplasmic reticulum. Fungi, protozoa, algae, plants, and animals are eukaryotes.

A potent toxic protein secreted by living bacteria that damages host tissues or disrupts normal cellular processes. Exotoxins are highly specific in their action and include neurotoxins (e.g., botulinum toxin), enterotoxins, and cytotoxins.

An anaerobic metabolic process in which microorganisms convert sugars into products such as ethanol, lactic acid, or other organic acids without using an electron transport chain. Fermentation is widely used in food production and industrial biotechnology.

A long, whip-like appendage used by many bacteria for motility. The bacterial flagellum is powered by a proton motive force-driven rotary motor and enables chemotaxis, the directed movement toward or away from chemical stimuli.

A form of viral replication in which the bacteriophage genome integrates into the host bacterial chromosome as a prophage and is replicated with the host DNA during cell division, remaining dormant until induced to enter the lytic cycle.

The entire community of microorganisms (bacteria, archaea, fungi, viruses) living in a particular environment, along with their collective genomes and the conditions of the habitat. The human gut microbiome is one of the most intensively studied examples.

A functional unit of bacterial DNA consisting of a cluster of genes under the control of a single promoter, transcribed together as a polycistronic mRNA. The lac operon and trp operon in E. coli are classic models of gene regulation.

A microorganism capable of causing disease in a host organism. Pathogens include certain bacteria, viruses, fungi, protozoa, and prions, and they employ various virulence factors to invade tissues, evade immunity, and cause damage.

A hair-like appendage on the surface of bacteria involved in attachment to surfaces or other cells. Type IV pili assist in twitching motility, while sex pili (F pili) are essential for the conjugation process of DNA transfer.

A small, extrachromosomal, circular DNA molecule that replicates independently within a bacterial cell. Plasmids frequently carry genes encoding antibiotic resistance, virulence factors, or catabolic enzymes and can be transferred between bacteria.

A unicellular organism that lacks a membrane-bound nucleus and other membrane-bound organelles. Bacteria and archaea are prokaryotes, characterized by circular DNA, 70S ribosomes, and cell division by binary fission.

A laboratory culture containing only a single species or strain of microorganism, free from contamination by other organisms. Pure cultures are essential for studying the characteristics and behavior of individual microbial species.

A mechanism of bacterial cell-to-cell communication that relies on the production and detection of signaling molecules called autoinducers. When autoinducer concentration reaches a threshold, coordinated gene expression changes occur, regulating processes like biofilm formation and virulence.

The complete elimination or destruction of all forms of microbial life, including endospores. Methods include autoclaving, dry heat, filtration, chemical agents, and irradiation. Sterilization is critical in medical and laboratory settings.

A close and long-term biological interaction between two different organisms. Forms include mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits at the expense of the other).

The degree of pathogenicity of a microorganism, determined by its ability to invade host tissues, evade immune responses, and cause damage. Virulence factors include toxins, adhesins, capsules, and enzymes that facilitate infection.

An infectious disease that is transmitted from animals to humans. Zoonotic diseases include rabies, Ebola, influenza, and COVID-19, and they represent a significant proportion of emerging infectious diseases worldwide.