Bacteria Discovery & Characteristics

1. BACTERIA
By: Dr. Taswar Ahsan
(Microbiology & Pathology)

2. He was a Dutch Biologist
First time discovered bacteria in 1676
Reported this discovery to the Royal Society of London in
the year 1683
He described these microscopic organisms as “Streaks and
Threads”. Later on some workers gave them the name
“Animalcules”.
The name bacteria were given by a German
biologist Christian G. Ehrenberg in 1828
No doubt, the word Bacteria appeared from
time to time in scientific writing, but it was
not until the fermentation experiments of
Louis Pasteur
C hrist ian G.
Ehrenberg

3. Louis Pasteur that the bacteria
took a renewed significance
He also discovered that bacteria
were cause of some diseases
This fact was called “Germ
Theory of Disease”.
The famous “Koch’s Postulates” of the German
Physician Robert Koch (1843-1910) were based on the
studies on disease causing bacteria.
Later bacteria came to be regarded as plants by many
biologists and were classified under the name
“Schizomycetes”.

4. Avery, Macleod and Mc Carty in 1944
established the role of DNA as
genetic material by using bacterial
transformation as evidence.
Later on, Lederberg and Tatum in 1946 described genetic
recombination through their experiment on bacterial
conjugation.

5. • 1.Bacteria (Eubacteria) are the smallest and structurally the simplest, not only of all
plants, but of all living organisms. They occur in all places where ever life is possible.
• 2.Since they possess rigid cell wall, they are considered to be plants.
• 3.They are typically unicellular, rarely exceeding 3 to 5 microns in length.
• 4.Since they lack distinct nucleus, nucleolus and nuclear membrane, they are
prokaryotes like blue green algae.

6. • 5.They also lack membrane bound organelles like endoplasmic reticulum, Golgi bodies, mitochondria,
plastids et. Because of these features, bacteria are now grouped with blue green algae in their own
kingdom Monera or Cyanobacteria.
• 6.Majority of bacteria lack chlorophyll and hence are heterotrophic. Those, who possess chlorophyll, are
autotrophic. They are mostly parasites or saprophytes.
• 7.Genetic material is present in the form of histone-free DNA.
• 8.They mostly reproduce vegetatively or asexually. However, some sort of Genetic Recombination has
also been reported in some cases.
• 9.Bacteria can exist even in the absence of oxygen, and withstand temperature as low as -17 0C and as
high as 80 0C.
• 10.Bacteria have been found at depths of many meters in soils and also in ooze beds, far below the
surface of the sea.

7. 1.Coccus form (GK. Kokkos=berry)
These are spherical or ovoid. When they occur singly they are called
Micrococcus (monococci), when in two-Diplococci, when in four-
Tetracocci, in chains Streptococci, in irregular clusters Staphylococci
and when in three dimensional forms they are referred as Sarcinae.
1
Bacillus form
They are cylindrical or rod-shaped. The bacilli may occur singly or
in groups. Depending upon the form of grouping they may be
called Diplobaccillus, Palisadebacillus and stertobacillus. The
stretobacilli may be branched (e.g. Cladothrix) or unbranched (e.g.
B. subtilis).
2
Spirillum form
They are spiral in shape. Mostly they look like a corkscrew. They
do not aggregate or form colonies.
3

8. Filament form
They are thread- like in form. They take various
morphological forms, such as:
Vibrio, Mycelia, Stalked, Budding
1
Pleomorphism
Some bacteria occur in multiple forms, and are called
plemorphic bacteria. For instance Rhizobium
leguminosarum occurs in three forms, viz, rod, oval
swarmers and irregularly branched (bacteriods).
2

9. Gram Positive Bacteria
Gram Positive Bacteria take up Gram stain have
thicker cell wall (100-200 A); their cell membrane
becomes invaginated inside the cytoplasm and
form mesosomes; they lack pili or fimbriae.
1
Gram Negative Bacteria
Gram negative bacteria do not take up Gram stain,
have thinner cell wall (70-1220A); cell wall is more
complex and contains about 20% lipids;
mesosomes are not formed; and they bear pili.
Majority of the pathogenic bacteria are Gram
negative.
2

10. Property Gram-Positive Bacteria Gram-Negative Bacteria
Gram Staining Takes stain Does not take stain
Cell wall Thicker cell wall(100-200 A) Thinner cell wall(70-120 A)
Cell membrane Forms mesosomes Mesosomes not formed
Pili or fimbriae Absent Present
Streptomycin susceptibility Less susceptibility More susceptibility
Susceptibility to lysozyme More susceptibility Less susceptibility
Types of toxin produced Exotoxin Enaotoxin
Amount of cell wall lipid Low High
Mesosome in cytoplasm Present Absent
Nutritious requirement Relatively complex Relatively simple
Typical bacteria Spore forming rods Few cocci