Morphology and Physiology of Bacteria

Unit II

Morphology and physiology of Bacteria

i. Classification of micro-organisms

ii. Morphology of Bacteria

iii. Bacterial cell wall

iv. Cell wall structures

v. Physiology of bacteria

vi. Bacterial growth and nutrition

Microscopic organisms that cannot be seen by the naked eye are called microorganisms or microbes.

Classification of microorganism

Microscopic organisms that cannot be seen by the naked eye are called Microorganisms or microbes. There are five kingdoms of organisms. These are

1. Monera – Unicellular and prokaryotes

2. Protista – Unicellular and eukaryotes

3. Fungi – Eukaryotes, and heterotrophic with a cell wall (chitin)

4. Plantae – Eukaryotes and, autotrophic with a cell wall (cellulose)

5. Animalia – Eukaryotes and heterotrophic without a cell wall.

Out of five kingdoms, three kingdoms belong to microorganisms. These are Monera, Protista, and Fungi.

1. Monera: Example Bacteria

Bacteria: They are single-celled prokaryotic organisms. They are available in various shapes and sizes. Their cell wall is made up of peptidoglycan. Bacteria include Eubacteria and Archbacteria. Eubacteria are true bacteria. Aracbacteria can survive in extreme environments. Bacteria are present almost everywhere.

Classification of bacteria: Based on their shapes, bacteria are classified into four groups. These are

• i. Coccus or cocci: Spherical in shape.

• ii. Bacillus or Bacilli: Rod shaped

• iii. Spirillum or Spirilla: Spiral shape

• iv. Vibrium or vibrio: Comma shaped.

2. Protista: Example Protozoa, Algae, and Moulds.

• Protozoa: They are single-cell eukaryotic organisms. They are motile. They are mainly found in aquatic environments.

• Classification of Protozoa: Based on locomotion, Protozoa are classified into the following four groups:

  • i. Amoeboid: They have pseudopodia for movement. Example Amoeba.

  • ii. Flagellated: They have flagella for movement. Example Leishmania.

  • iii. Ciliated: They have cilia for movement. Example Paramoecium

  • iv. Sporozoans: They are nonmotile. Example Plasmodium.

• Algae: They are photosynthetic eukaryotic. They may be unicellular or multicellular. They are found in various aquatic environments.

Multicellular Algae are also placed under kingdom plantae. They can be seen through the naked eye.

3. Fungi: They are eukaryotic organisms. They include yeast. They have cell walls made up of chitin. They are heterotrophic and obtain nutrients through absorption.

4. Viruses: They are non-cellular organisms. They require a host to replicate. They consist of genetic materials like DNA and RNA. Genetic materials are enclosed in the protein coat.

MORPHOLOGY OF BACTERIA

The size, shape, and arrangement of bacterial cells are studied under the morphology of bacteria. These characteristics of bacteria are used to identify bacteria and classify bacteria. The details of bacterial morphologies:

Shape of Bacterial cells

• 1. Coccus (Plural: Cocci): Spherical or round-shaped bacteria. Cocci can occur single, in pairs (diplococcus), in chains (Streptococcus), or in clusters (Staphylococcus).

• 2. Bacillus (Plural: bacilli): Rod shaped bacteria. Bacilli can be short or long. They may have round or squared heads.

• 3. Spirillum (Plural: spirilla): Spiral or helical-shaped bacteria with rigid structure. Spirillas have spiral shapes that look like corkscrews.

• 4. Vibrio: Curved or comma-shaped bacteria. Vibrios are more curved than spirilla

• 5. Coccobacillus: Bacteria are oval-shaped or slightly elongated. It resembles both cocci and bacilli.

Bacterial arrangements

Adherence of bacterial cells after cell division is called bacterial cell arrangement. The arrangement of bacterial cells in cocci and bacilli is complex.

A. Arrangement of cocci bacterial cells: Coccus is a round, oval, or spherical-shaped bacterium. Cocci bacterial cells arrangement is as per the below details

• 1. Monococcus: Single coccus bacterium cell.

• 2. Diplococcus: Two cocci bacterial cells.

• 3. Tetracoccus: Four cocci bacterial cells.

• 4. Streptococcus: Cocci bacterial cells in chains.

• 5. Sarcinia: Highly packed cocci bacterial cells

B. Arrangement of bacilli bacterial cells: Bacillus is a straight cylindrical-shaped bacterium that looks like a rod. Both ends of the bacterium cell are flat and round.

• 1. Bacillus: Single bacillus bacterium cell.

• 2. Diplobacillus: Two bacilli bacterial cells.

• 3. Streptobacillus: Bacilli bacterial cells in the chain.

C. Arrangement of filamentous bacterial cells: Bacterial cells with flagella are called flagellate bacterial cells. Bacterial cells without flagella are called non-flagellate bacterial cells. Bacteria are named as per the presence of flagella as per the below details

• 1. Atrichous: No Flagella in bacillus bacterium cell.

• 2. Monotrichous: Presence of one only one flagellum on bacillus bacterium cell.

• 3. Cephalotrichous: Presence of several flagella at one end of the bacillus bacterium cell.

• 4. Lophotrichous: Presence of several flagella at both ends of the bacillus bacterium cell.

• 5. Peritrichous: Presence of several flagella overall surface of the bacillus bacterium cell.

Bacterial Cell Wall Structure

The bacterial cell wall is a rigid and protective layer to surrounds the cell membrane of bacterial cells. It maintains the structure and shape of the bacterial cells. It holds the internal structure of bacteria.

Based on staining properties, bacteria are classified into two groups. These are Gram-positive bacteria and Gram-negative bacteria. The staining property of bacteria depends upon the structure of the bacterial cell wall. Both Gram-positive bacteria and Gram-negative bacteria have different bacterial cell wall structures.

Gram Positive Cell wall: Gram-positive bacteria cell wall consists of a thick layer of peptidoglycan. Peptidoglycan provides strength and rigidity to the cell wall. Gram-positive cell walls may also have teichoic acid. Teichoic acid stabilizes the cell wall structure. Teichoic acid helps bacterial cells to adhere to the surfaces and host cells.

Gram-negative Cell Wall: Gram-negative cell walls have thinner layers of peptidoglycan. Gram-negative cell walls also have an outer membrane. This outer membrane consists of lipopolysaccharides, lipoproteins, and phospholipids. The outer membrane acts as a barrier. This develops resistance in gram-negative bacteria to certain antibiotics and chemicals. The outer membrane has channels called porins. This allows passage for small molecules.

Bacterial cell (Anatomy of Bacteria):

The internal structure of Bacterial cells includes a Cytoplasmic membrane, cytoplasm, mesosomes, ribosomes, inclusion bodies, nucleoids, and plasmids

1. The cell membrane (Plasma membrane or Cytoplasmic membrane/Plasmalemma): It consists of a double layer of phospholipids and proteins. It surrounds cytoplasm

Functions of cell membrane:

• i. It holds cytoplasm.

• ii. It acts as a semipermeable membrane.

• iii. It allows the movement of nutrients into cells,

• iv. It allows the movement of metabolic waste from cells.

• v. Respiration.

• vi. Lipid synthesis.

• vii. Cell wall components synthesis.

2. Cytoplasm: It is a watery fluid consisting of 80% water, and several soluble and suspended cellular organelles inside the cytoplasmic membrane. Soluble components are nutrients, enzymes, and metabolic wastes. Cellular organelles are nucleoids, plasmids, ribosomes, and inclusion bodies.

3. Mesosomes: Inward growth of cell membrane inside cytoplasm forms mesosomes. There are two types of mesosomes. These are Septal mesosomes and Lateral mesosomes. Septal mesosomes connect cell membranes with DNA. It helps with bacterial cell elongation during cell division. Mesosomes that do not connect cell membranes with DNA are called lateral mesosomes. It carries respiratory enzymes.

4. Ribosomes: Numerous ribosomes are present inside the cytoplasm of bacteria. They develop a granular appearance in the cytoplasm of bacteria. They are made of ribosomal RNA and proteins. Bacterial ribosomes are 70S ribosomes made of 30S and 50S subunits. The main function of ribosomes is to synthesize protein.

5. Inclusion bodies: They remain freely suspended inside the cytoplasm. These are

• i. Inorganic inclusions such as sulfur granules, and iron granules.

• ii. Food reserves such as starch, protein, and lipid granules.

• iii. Gas vacuoles: These are hollow structures surrounded by a non-lipoid membrane. They are impermeable to water and permeable to gases. They help bacteria to float inside a liquid medium. They also protect bacteria from harmful radiation.

6. Nucleoid: It is an irregularly shaped structure inside the cytoplasm that contains circular chromosomes of bacteria. It does not have a nuclear membrane. The bacterial chromosome is a single, double-stranded, helical DNA. The main function of bacterial nucleoids is transcription and translation during protein synthesis.

7. Plasmid: It is an extrachromosomal double-stranded DNA molecule inside the cytoplasm. It carries additional genetic information. Plasmids and nucleoids form the total bacterial genome.

Physiology of bacteria

Various biological processes and functions within bacterial cells are the physiology of bacteria. Bacterial physiology includes metabolism, energy production, cell growth, reproduction, response to the environment, etc.

1. Metabolism and Energy Production: Bacteria produce energy through a metabolic process. Bacteria that utilize oxygen to produce energy through metabolic processes are called aerobic bacteria. Bacteria that produce energy through metabolic pathways in the absence of oxygen are called anaerobic bacteria. There are four types of bacteria based on energy sources.

• i. Phototrophs: Utilises light energy for photosynthesis

• ii. Chemotrophs: Obtain energy from chemical compounds

• iii. Autotrophs: Synthesise own organic molecules from inorganic sources.

• iv. Heterotrophs: Depends upon external organic sources for their nutrition.

• 2. Cell growth and reproduction: Increase in bacterial cell size and their population is bacterial cell growth. Most of the bacteria reproduce asexually by binary fission. One cell divides to form two daughter cells. The bacterial growth phase consists of the lag phase, exponential phase, stationary phase, and death phase.

3. Cellular structure and functions: Bacterial cell consists of cell walls, cell membranes, ribosomes, and genetic materials. Genetic material is a single circular DNA molecule. It transfers genetics to daughter cells. Ribosomes synthesize proteins, cell membrane acts as a semipermeable membrane to transport nutrients into cells and excrete waste products from the cells. The cell membrane also participates in respiration and lipid synthesis.

4. Nutrient uptake and transport: Bacteria receive nutrients from the environment. Mechanisms of nutrient transport into bacterial cells are passive diffusion, active transport, and facilitated diffusion. Nutrients are essential for growth, metabolism, and energy production.

5. Response to Environment Changes: Bacteria locate itself to favourable environment in response to environmental change.

6. Endospore formation: Under unfavorable conditions, bacteria form endospores. Endospores are dormant stages of bacteria. It can survive in high temperatures, dry conditions, and other unfavorable environments.

Bacterial growth and nutrition

Nutrients: Bacterial growth and nutrients are closely related with each other. Nutrients are essential for cell growth, reproduction, and metabolism. These nutrients are classified into two groups. These are macronutrients and micronutrients. Macronutrients are carbon, nitrogen, and inorganic ions such as potassium, magnesium, calcium, phosphate, etc. Micronutrients (Trace elements) are iron, zinc, copper, and manganese.

Bacterial growth: An increase in several bacterial cells with time is called bacterial growth. There are four phases in bacterial growth.

1. Lag Phase: There is no cell division in the bacterial phase. Bacterial cells prepare for growth.

2. Exponential phase (Log phase): Bacterial cells have rapid division. Bacterial cells shall be metabolically very active.

3. Stationary phase: Decrease in nutrients in the surrounding environment and accumulation of wastes in the surrounding environment slow down the cell division rate. There will be an increase in cell death. Cell division and cell death stop bacterial growth.

4. Death phase: The bacterial death rate is more than the bacterial cell growth rate. There will be a decrease in number of live bacterial cells.

Factors affecting bacterial growth: The following factors affect the bacterial growth rate.

1. Temperature: The temperature at which bacterial growth is highest is called optimum temperature. Bacterial growth rate decreases above and below optimum temperature. Bacteria showing high growth rates at high temperatures are considered thermophiles.

Bacteria showing high growth rates at moderate temperatures are considered mesophiles.

Bacteria showing high growth rates at cold temperatures are considered psychophiles.

2. pH: The pH at which bacteria show the highest growth rate is called the optimum pH for bacterial growth. Bacteria showing high growth rates in acidic pH are considered acidophiles.

Bacteria showing high growth rates in alkaline pH are considered alkaliphiles.

Bacteria showing high growth rates near neutral pH are considered neutrophils.

3. Oxygen Availability: The oxygen requirement to survive bacterial cells determines the type of bacterial cells. Bacteria that grow in the presence of oxygen are called obligate aerobes. Bacteria that grow in the absence of oxygen are called obligate anaerobes.

Author: Dr. Pramila Singh