Types of Microorganisms
Learning objective
•
Differentiate among the major characteristics of each group of
microorganisms.
BACTERIA (figure 1)
Bacteria (singular: bacterium) are single celled (unicellular) organisms.
Because their genetic material is not enclosed in a special nuclear membrane, they are
called prokaryotes.
Bacteria are enclosed in cell walls that are largely composed of a carbohydrate
and protein complex called peptidoglycan. They reproduce by dividing into two equal
cells; binary fission. Most bacteria use organic chemicals around them. Some
manufacture their own food by photosynthesis, and some derive nutrition from
inorganic substances. Many can “swim” by using appendages called flagella.
ARCHAEA
Archaea consist of prokaryotic cells, but if they have cell walls, they lack
peptidoglycans. They are divided into 3 major groups: methanogens produce methane,
extreme halophiles live in extremely salty environments and extreme thermophiles
which live in hot springs.
FUNGI
Fungi are eukaryotes, which are organisms which have a distinct nucleus
containing cell’s genetic material (DNA) surrounded by a special envelope called the
nuclear membrane. Fungi may be unicellular or multicellular. True fungi have cell
walls composed primarily of chitin. The unicellular forms are larger than bacteria.
The most typical fungi are molds which form visible masses called mycelia which are
composed of long filaments (hyphae) that branch and intertwine. They reproduce
sexually or asexually.
PROTOZOA
Are unicellular, eukaryotic microbes. Amoebas move by extensions of their
cytoplasm known as pseudopods. Others move by flagella, or shorter appendages
called cilia. They live either as free entities or as parasites (organisms that derive their
nutrition from living hosts). They reproduce sexually or a sexually.
ALGAE
Are photosynthetic eukaryotes with both sexual and asexual reproduction. The
cell walls are composed of a carbohydrate called cellulose. Algae are abundant in
fresh and salt water and in soil. They need light, water and carbon dioxide for food
production and growth. They then produce oxygen and carbohydrates.
VIRUSES
They are very different form the other microbial groups. They are so small that
they can be seen only under an electron microscope. They are acellular (not cellular),
structurally very simple with a core that contains only one type of nucleic acid, either
DNA or RNA. The core is surrounded by a protein coat and sometimes encased by an
envelope. Viruses can reproduce only by using the cellular machinery of other
organisms. Thus, on one hand, they are considered to be living when they multiply
within host cells they infect. On the other hand, they are not considered to be living
because outside of living hosts they are inert.
MULTICELLULAR ANIMAL PARASITES
The 2 major groups of parasitic worms, the flatworms and the round worms,
are collectively called helminths.
A bacillus, one of the several shapes
of bacteria
Mucor, a type of fungus
HIV virus budding from a
lymphocyte
Amoeba
Fig 1. Types of microorganisms
A pond alga
Prokaryotic and Eukaryotic Cells
Learning objective
• Compare and contrast the overall cell structure of prokaryotes and
eukaryotes
All living cells can be classified into 2 groups, prokaryotes and eukaryotes,
based on certain structural and functional characteristics (Table 1).
Plants and animals, in addition to some cellular microbes (fungi, protozoa and
algae) are eukaryotes. Bacteria and Archaea are prokaryotes.
Table1. Principle differences between Prokaryotic and Eukaryotic cells
Characteristic
Prokaryotic
Eukaryotic
Size of cell
Nucleus
Typically 0.2-2.0 µm in
diameter
No nuclear membrane or
nucleoli
Membrane-enclosed
organelles
Absent
Flagella
Consist of 2 protein
building blocks
Present as a capsule or
slime layer
Usually present;
chemically complex with
peptidoglycan
No carbohydrates and
generally lacks sterols
No cytoskeleton
Smaller size (70S)
Usually single circular
chromosome; typically
lacks histones
Binary fission
None; transfer of DNA
fragments only
Glycocalyx
Cell wall
Plasma membrane
Cytoplasm
Ribosomes
Chromosome (DNA)
Cell division
Sexual recombination
Typically 10-100 µm in
diameter
True nucleus, consisting of
nuclear membrane and
nucleoli
Present (lysosomes, Golgi
complex, endoplasmic
reticulum, mitochondria,
chloroplasts)
Complex; consist of
multiple microtubules
Present in some cells that
lack a cell wall
When present, chemically
simple
Sterols and carbohydrates
that serve as receptors
Cytoskeleton
Larger size (80 S)
Multiple linear
chromosomes with
histones
Involves mitosis
Involves meiosis