MORPHOLOGY AND PHYSIOLOGY OF BACTERIA

BACTERIA

· Bacteria are microscopic single-celled organisms that are found virtually everywhere on Earth.

· Bacteria shapes including cocci (oval or spherical), bacilli (rod-shaped), and spirilla (spiral-shaped), etc.

· Some bacteria are harmful causing diseases while some are beneficial & play essential roles in ecosystems.

· Bacteria have versatile metabolisms enabling them to obtain energy through photosynthesis, respiration & chemosynthesis.

· They reproduce asexually through binary fission.

MORPHOLOGY OF BACTERIA

On the basis of shape, size & arrangement bacteria are classified as.

1. Shape:

a. Cocci (coccus):

· Spherical or round-shaped bacteria.

· Examples: Streptococcus, Staphylococcus.

b. Bacilli (bacillus):

· Rod-shaped bacteria.

· Examples: Escherichia coli (E. coli)

c. Spirilla (spirillum):

· Spiral or helical-shaped bacteria.

· Examples Spirillum, Helicobacter pylori.

d. Vibrios:

· Comma-shaped bacteria.

· Example: Vibrio cholerae

e. Coccobacilli:

· Bacteria that are between cocci & bacilli in shape appearing somewhat oval or short rods.

· Example: Haemophilus influenzae.

2. Size:

· Size ranging from as small as 0.2 micrometers (μm) to as large as 2-3 μm in diameter.

· Some bacteria may be visible under a light microscope while others require electron microscopes to be observed due to their small size.

3. Arrangement:

a. Diplo:

· Bacteria arranged in pairs.

· Example: Neisseria gonorrhoeae.

b. Strepto:

· Bacteria arranged in chains.

· Example: Streptococcus pyogenes.

c. Staphylo:

· Bacteria arranged in clusters.

· Example: Staphylococcus aureus.

d. Tetrads:

· Bacteria arranged in groups of four.

· Example: Micrococcus luteus.

e. Sarcinae:

· Bacteria arranged in cubical packets of eight or more.

· Example: Sarcina ventriculi.

4. Additional Morphological Features:

a. Filamentous:

· Some bacteria may appear filamentous, forming long chains or filaments.

· Example: Actinobacteria.

b. Pleomorphic:

· Bacteria that exhibit variations in shape & size, lacking a distinct, fixed form.

· Example: Mycoplasma species.

ANATOMY OF BACTERIA

· Anatomy of bacteria based on cellular components such as:

1. Cell Wall:

· The cell wall is a tough & rigid.

· It weight about 20-25% of the dry weight of the cell.

· The thickness of Gram positive cell wall is 16-80 nm while thickness of gram negative cell wall is 2-10 nm.

Difference between gram positive & negative cell wall

a. Gram-Positive Bacteria:

· Peptidoglycan Layer: Gram-positive bacteria have a thick layer of peptidoglycan (16-80).

· Teichoic Acids: Gram positive bacteria has significant amount of teichoic acids. The teichoic acids constitute major surface antigens of Gram positive bacteria. They are water soluble polymers. Teichoic acids are of two types, cell wall & membrane acid.

· Other components: Certain Gram positive cells also contain antigens such as protein and polysaccharides.

· Staining: Gram-positive bacteria retain the crystal violet stain due to the thick layer of peptidoglycan in their cell wall. They appear purple or blue-purple under the microscope after staining.

b. Gram-Negative Bacteria:

· Peptidoglycan Layer: Gram-negative bacteria have a thinner layer of peptidoglycan (2-16) compared to Gram-positive bacteria. This layer is located in the periplasmic space between the inner and outer membranes.

· Outer Membrane: Gram-negative bacteria possess an outer membrane external to the peptidoglycan layer. This outer membrane contains certain protein named as outer membrane protein (OMP).

· Lipopolysaccharide (LPS): This layer consists of lipid A, to which is attached a polysaccharide. LPS constitutes the of gram-negative bacteria.

· Periplasmic space: It is the space in between the inner and outer membranes. It contains various binding proteins for specific substrates. Peptidoglycan.

· Staining: Gram staining procedure, the thinner peptidoglycan layer of Gram-negative bacteria does not retain the crystal violet stain. Instead, they are counterstained with safranin, appearing pink or red under the microscope.

Function:

· Provides size, shape, and protection against osmotic pressure changes.

· It takes part in cell division.

· It possesses target site for antibiotics,

2. Cytoplasmic Membrane (Plasma Membrane):

· It is semipermeable membrane

· Detected through electron microscope

· It consists of lipids and protein molecules.

· Cytoplasmic membrane act as an osmotic barrier.

Function:

· It acts as a semipermeable membrane controlling the inflow and outflow of metabolites to & from the protoplasm.

3. Cytoplasm:

· The bacterial cytoplasm is a colloidal system containing a variety of organic and inorganic solutes in a viscous watery solution.

· It lacks mitochondria & endoplasmic reticulum of eukaryotic cell.

· It contains ribosomes, mesosomes, vacuoles & inclusions.

Function:

· Site of metabolism, protein synthesis, DNA replication & transport of nutrients within the cell.

A. Ribosome:

· Small, spherical structures found in the cytoplasm.

· Composed of ribosomal RNA (rRNA) & ribosomal protein.

· Site of protein synthesis where mRNA is translated into proteins.

· Size – 10-20

B. Mesosomes:

· They are the principle centres of respiratory enzymes & are analogous to mitochondria of eukaryotes.

c. Inclusion Granules:

· Storage granules found in the cytoplasm.

· Source of stored energy present is same species of bacteria

· Present as poly-metaphosphate, lipid, polysaccharide & granules of sulphur.

4. Nucleus:

· Bacterial nucleus has no nuclear membrane or nucleolus

· The nuclear DNA doesn’t appear to contain any basic protein

· Genomic DNA is double standard

· It serves as the site for DNA replication, transcription, and genetic regulation

5. Capsule:

· Capsule is the outermost layer of bacterial cell.

· Composed of gel like substance made of polysaccharides & proteins

· Cannot demonstrate through electron microscope.

Capsulated organism:

• Streptococcus pneumonia

• Hemophilus influenza

• Klebsiella species

• Bacillus anthrax

Demonstration of capsule

Capsule has little affinity for basic dye it cannot stain with gram stain, i.e the demonstration can be done by various method.

· Indian ink (negative stain )

· Serological method – Quelling phenomenon

6. Flagella:

· Flagella are organ of locomotion

· Thread like structure composed of protein

· They are primarily involved in bacterial motility

· Size- 5-20 micrometer in length, 0.01-0.02 micrometer in width.

· All motile bacteria posses one or more flagella.

Parts and composition:

Filament

Hook

Basal body

Types of Flagella:

• Monotrichous: single polar flagella. Example- vibrio cholera

• Amphitrichous: single flagella at both end. Example- alcaligenes faecalis.

• Lophotrichous: tuft of flagella at one or both end. Example- spirilla

• Peritrichous: flagella arranged all around cell.