“Aseptic” means free from contamination caused by any
microorganism that is not of interest to the researcher. Aseptic technique
means using practices and procedures to prevent contamination from
microorganisms, including the use of sterile media and equipment, as well as proper
handling procedure. Aseptic technique is used in microbiology because it is
essential for identification of a microbe; in order to study an organism, it
must first be isolated.
Media, or singular medium, is a broth-like liquid or gel-like
solid designed to support the growth and/or selection of cells. Most commonly,
agar petri plates or tubes of liquid broth are used. Specialized media are
sometimes required for microorganism and cell culture growth. Media are used to
make microbial cultures, which are used to determine the type of organism, its
abundance in a sample, and for selective or differential growth. Media can also
be utilized, such as in the streak-plate method, to obtain pure cultures. If
media are not kept sterile, microorganisms other than the microbe of interest
can be introduced; if aseptic technique is not maintained, there is no
guarantee that the culture/colonies grown are from the species of interest.
Good scientific method requires that experiments should be designed to minimize
possible errors, especially through the use of appropriate controls, such as
aseptic technique to isolate the independent variable.
The lid of a petri dish or culture tube should never be laid
on the table or benchtop when inoculating because contamination may be
introduced to the culture.
The bottom of a petri dish should be labeled, rather than
the lid; lids can accidentally be switched. Labelling on the bottom ensures
that the written information stays with the intended culture, even if the lid
gets accidentally exchanged with another plate.
Petri dishes are incubated in an inverted position to
prevent condensation that can disturb a culture, and to lessen the risk of
contamination from airborne particles settling on them.
An inoculating loop is an instrument used to transfer bacteria
from one medium to another.
Visible bacterial growth in a culture tube is referred to as
A colony is a visible, discrete clump of bacteria growing on
the surface of solid media. Each colony contains the offspring of a single
cell, which are all genetically identical (barring random mutations).
Colony morphology is a method that scientists use to
describe the characteristics of an individual colony of bacteria growing on
agar in a petri dish; it can be used to help to identify them. Colonies are
described according to size, shape, texture, opacity, elevation, pigmentation,
effect on growth medium, etc.