presence of free oxygen.
Anaerobes are organisms
that do not reproduce in the presence of oxygen, and
obligate anaerobes are organisms that grow only in
the absence of free oxygen and are killed if exposed to
free oxygen. Facultative organisms are organisms
that grow in the presence of free oxygen and in an
o x y g e n - f r e e a t m o s p h e r e .
organisms are organisms that grow only in low
amounts of free oxygen.
requirements for the various types of bacteria depends
on what their particular environment provides.
Autotrophic bacteria are self-nourishing, and
heterotrophic bacteria are not self-sustaining.
MOISTURE REQUIREMENTS.Moisture is
indispensable for bacterial growth.
The structural (or morphologic) characteristics of
bacteria are based on three distinct shapes or
Coccus (pl. cocci)spherical, appears singly, in
pairs, chains, clusters, or packets.
Bacillus (pl. bacilli)rod-shaped, appears
singly, in chains, or in palisades.
Spirillum (pl. spirilla)spiral-, corkscrew-, or
comma-shaped, appearing singly only.
Three special structures, present on some bacteria,
aid in the classification process of bacteria.
special structures are the capsule, the spore, and the
flagellum. The capsule is a gummy, gelatinous, or
mucoid structure surrounding certain bacteria. The
spore is an inactive, resting, and resistant form
produced within the organism, usually as a result of
unfavorable environmental conditions. The third and
final special structure is the flagellum, a hairlike
structure that provides motility.
A colony is a cohesive mass composed of many
millions of bacterial cells, growing on or in a medium
(such as blood agar, a gel enriched with blood that is
used in the preparation of solid culture media for
microorganisms) as a result of the multiplication and
division of a single cell. The size, color, shape, edge,
topography, consistency, and odor of the colony vary
with each organism.
Generally, toxins produced are waste products of
metabolism in a bacterial cell. Some bacteria produce
toxins that attack red blood cells in a culture medium
such as blood agar. Examples of toxins produced by
bacteria are listed below:
Alpha hemolysinproduces partial hemolysis
(the disruption of the integrity of the red cell
membrane causing release of hemoglobin) and
changes the medium to a green color.
Beta hemolysincompletely lyses the RBC,
leaving a clear zone of hemolysis.
Endotoxin (low potency)comprises part of
the cell wall and is released as the bacterial cell
self-generated enzymes (a process known as
Exotoxin (high potency)derives from the
bacteria during its growth but is found outside
the bacterial cell in the surrounding medium.
Exotoxins are highly poisonous, soluble, and
protein in nature.
Grams Stain Reaction
To differentiate and identify bacteria, you must
make them visible by staining.
procedure, devised by Dr. Hans Christian Joachim
Gram, stains microorganisms such as bacteria with
crystal violet, treats them with 1:15 dilution of strong
iodine solution, decolorizes them with ethanol or
ethanol-acetone, and counterstains them with a
contrasting dye, usually safranin.
that retain the crystal violet stain (a dark blue-black
color) are said to be gram-positive, and those that lose
the crystal violet stain by decolorization but stain with
counterstain (a deep pink or reddish color) are said to
Bacteria are named by genus and species. The first
word (capitalized) indicates the genus; the second
word (not capitalized) indicates the species, a
subdivision of the genus. For example: