Common Terms and Definition in Microbiology Laboratory

Photo by Chokniti Khongchum on
aerosol – a spray of live microbes ejected from sputtering
brightfield microscope – a microscope which the general background or field of view is bright while dense specimens appear darker objects; used for stained materials
capsule – thick layer of substance found on the surface of some bacteria that inhibits phagocytosis by white blood cells
capsule stain – involves the process of negative staining the slide and staining the cytoplasm of the bacteria making the capsule only the colorless structure which appears as a light halo around the bacteria
carbol fuchsin – the primary stain used in acid fast staining
citrate test – detects species of bacteria that survive on citrate as its carbon source
coagulase – a bacterial enzyme which brings about the coagulation of blood or plasma and is produced by disease-causing forms of staphylococcus
colony – visible mass of microorganisms originating from a single mother cell
compound microscope – microscope with two magnifying lens
conclusion – logical judgment or analysis we make from our results
condenser – lens system in microscope whose function is to focus the light onto the specimen
conjugation – joining of two bacteria or unicellular organism for the transfer of genetic material by direct cell to cell contact or by bridge like connection called sex pilus
contamination – the presence of undesirable microorganism that accidentally gets into a culture
control group – the group in the experiment that does not receive treatment; used to compare how the experimental group do
controlled variable – the factors in the experiment that we control and keep the same
crystal violet – primary stain used in Gram staining
darkfield microscope – creates the effect of a negative image; field of view is dark while cells and other objects are lit up; used to observe live, motile cells
decolorizer – removes the primary stain from the gram-negative bacteria making it colorless
dependent variable – the variable being tested, observed, and measured
differential medium – a medium with diagnostic test built into it that changes color with different species of microbes
differential staining – procedure that will dye different kinds of bacteria in contrasting color
double blind – a type of experiment which neither the subjects nor the experimenters know which subjects are in the test and control groups
electron microscope – a microscope that uses beam of electron rather than beam of light to observe a specimen
endemic – when a disease prevalence is fairly stable in a location
epidemic – occurs when a disease is spreading rapidly in a particular population
ethyl alcohol – the decolorizer used in Gram staining
fluorescence – the ability of a substance to absorbed UV light and emit back the light as a visible color
Gram’s iodine – mordant used in Gram staining
HCG – is a hormone that prevents menstruation from occuring during pregnancy
Hfr cell – a bacterial cell with conjugative plasmid integrated into its chromosomal DNA
immunocompromised – means more susceptible to infections
independent variable – also called the experimental variable; anything that can be changed or manipulated
lysozyme – an enzyme that dissolves the chemical bond between the NAG and NAM within the backbone of the peptidoglycan molecule in gram-positive bacteria
malachite green – primary stain in spore staining
methylene blue – counterstain used in acid fast staining
microbial hot spots – warm and moist areas where the number of microorganisms is usually the highest
minimum inhibitory concentration – lowest concentration of drug which prevent visible growth of bacteria
mordant – a substance which causes the primary stain to become more tightly bound to the cell
negative stain – a stain which involves staining the background of the slide
nosocomial infection – infectious disease acquired during hospitalization
nutrient agar – a general purpose medium that will support growth of many common bacteria
ocular lens – also called eyepiece; the top lenses of the microscope which you look through
ocular micrometer – also called the microscopic ruler inscribed into the eyepieces on the microscope
opsonin – an antibody which binds to foreign microbes making them susceptible to phagocytosis
pandemic – occurs when a disease is spreading globally or over one continent
parfocal – means when one lens is focused, the others are also focused
phase contrast microscope – a microscope that increases the contrast between cells or portions of cells that vary only slightly in density; used to observe unstained bacterial cells
pour plate – is made by inoculating melted agar with bacteria and then pouring the agar into an empty petri plate to harden, thus the microbes are distributed evenly throughout the agar; creates an evenly distributed lawn of bacteria to be use for phage typing
pseudopodia – extension of cytoplasm use for movement; also called false feet
pure culture – a medium growing with only one intended species of microorganism
refraction – the bending of light
resident or normal flora – microorganisms that live naturally and permanently in various areas of the human body
resolving power – defined as the closest distance two objects can be where you can still see them as separate objects
results – refers to the actual data that you collect such as the number of colonies
R-plasmid – is a plasmid in bacteria that contains the antibiotic resistant gene
Sabouraud agar – medium that are slightly acidic and have extra glucose; also used for fungal growth; used to check fungal spores in the air
safranin – the counter-stain used in Gram staining
scanning electron microscope – a microscope that reflects beam of electrons off the exterior of the specimen; produces 3D view of specimen’s surface
selective medium – a medium that only allows certain species of microbes to grow and inhibits others
simple microscope – microscope with single magnifying lens
simple staining – defined as a procedure that stains all cells with the same color
Snyder test – measures the amount of acid produced by normal flora in a medium containing sugar
sporadic – occurs when a disease is infrequent and in scattered location
stage – the platform below the objective lenses of a microscope used to hold the slide
stage micrometer – term typically referring to a slide that comes with a known scale on its surface
sterile – a term which means free of bacteria or living things
streak plate – is performed by spreading an inoculum of bacteria across the surface of an agar plate in such a way as to produce isolated colonies
Streptococcus mutans – oral bacteria involve with the development dental caries or tooth decay
super-infection – second infection with microbial agent that is resistant to the treatment used against the first infection
synthetic medium – medium made from scratch with every single ingredient defined and listed separately
thermal death point – minimum temperature required to kill a bacteria with a given amount of time
thermal death time – time required to kill a bacteria at a particular temperature
transduction – is the process by which foreign DNA is introduced into a cell by a virus or viral vector
transformation – is a process of horizontal gene transfer by which some bacteria take up foreign genetic material from the environment
transient flora – microbes that are temporary and can be removed by handwashing
transport medium – is a medium used to transport microbes from one place to another and limits the overgrowth of microbes


Alderson, Gary D. Microbiology Experiments and Lab Techniques 14th Edition. Palomar College. Fountainhead Press. Accessed November 26, 2019.


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