Bacteria Characteristics

Bacteriology: It is a branch of microbiology that deals with the study of bacteria. It deals with the identification and cultivation of bacteria and their application in medicine, biotechnology, agriculture, and industry. Bacteria are metabolically active single-cell microorganism without nuclear membranes that divides by binary fission.

General characteristics of bacteria- morphology:

A. General characteristics of gram-positive bacteria:

• 1. Gram-positive bacteria appear purple or blue after gram staining. It takes up the color of a crystal violet stain.

• 2. Gram-positive bacteria cell wall is made of multiple layers of proteins.

• 3. Gram-positive bacteria have a thicker polypeptidoglycan layer in the cell wall than gram-negative bacteria.

• 4. Gram-positive bacteria cell wall has low lipid contents.

• 5. Gram-positive bacteria cell wall has more teichoic acid than lipid.

• 6. Gram-positive bacteria have thick cytoplasmic lipid membranes.

• 7. Gram-positive bacteria do not have an outer membrane.

• 8. Cilia and flagella help in the locomotion of gram-positive bacteria.

• 9. Gram-positive bacteria have different shapes. Such as spherical shape (cocci), rod-shaped (bacilli), and thread-like shape (branching filaments). Staphylococcus, streptococcus, clostridia.

B. Morphology of gram-positive bacteria:

Morphology of gram-positive bacteria is the study of the anatomy, shape, and arrangement of bacterial cells.

• Anatomy: The anatomy of gram-positive bacteria is the study of the external and internal structure of the gram-positive bacterial cell.

• External Structure: The external structure of gram-positive bacterial cells includes bacterial cell envelopes, flagella, and pili.

• Bacterial cell envelopes: Gram-positive bacteria do not have a cell envelope or outer layer on the cell wall.

• Bacterial Cell Wall: It is made of polypeptide glycan and protein. Peptide provides rigidity to bacterial cell walls. The cell wall of gram-positive bacteria is thicker than the cell wall of gram-negative bacteria. The gram-positive bacteria cell wall has teichoic acid along with mucopeptide.

• Functions of bacterial cell wall: The cell wall provides rigidity and shape to bacterial cell

• Flagella (Sing. Flagellum): It is a long unbranched curved filament. It is a proteinous structure that originates from the cytoplasm. It consists of a basal body, filament, and hook. The basal body is rod-shaped and present inside bacterial cell envelopes. The filament is a long tubular structure present outside the bacterial cell wall. The curved end of the filament is called a hook. Flagella are not an essential part of bacterial cells. Functions: It helps in the locomotion of bacterial cells inside the liquid medium. .i.e. swimming.

• Pilli: Some bacteria have very short unbranched, very fine fibre-like tubular structures on the external surface of bacterial cell wall. One bacterial cell has one to four pili. They act as conjugation tubes. They donate plasmid (genetic material) from one bacterium to another bacterium during conjugation.

• Fimbriae: The external surface of bacteria has 300 to 400 fimbriae. They are solid and short structures. They help in the adhesion of bacteria to the host surface.

• Internal Structure: The internal structure of Bacterial cells includes the Cytoplasmic membrane, cytoplasm, mesosomes, ribosomes, inclusion bodies, nucleoids, and plasmids.

• The cell membrane (Plasma membrane, or Cytoplasmic membrane or Plasmalemma): It consists of a double layer of phospholipid and protein. It surrounds cytoplasm

• Functions of cell membrane:

  • i. It holds cytoplasm

  • ii. It acts as 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

• 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.

• Mesosomes: Inward growth of cell membrane inside cytoplasm forms mesosomes. There are two types of mesosomes. These are Septal mesosomes and Lateral mesosomes. The septal mesosome connects the cell membrane with DNA. It helps with bacterial cell elongation during cell division. A mesosome that does not connect cell membranes with DNA is called a lateral mesosome. It carries respiratory enzymes.

• 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 the ribosome is to synthesize protein.

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

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

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

  • iii. Gas vacuoles: These are hollow structures surrounded by nonlipoid membranes. 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.

• 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.

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

1. Size and shape of bacterial cells:

The size of bacteria is measured in micrometers (microns or µm) or in millimicrons or in nanometres or in angstrom (Aͦ). Details of size and shape of bacteria

• i. Rod-shaped bacteria: Bacilli

• ii. Spherical or oval-shaped bacteria: Cocci

• iii. Spiral bacteria: Spirillum

• iv. Peer-shaped bacteria: Posteuria

• v. Lobed-shaped bacteria: Sulfolobus

• vi. Comma shaped bacteria: Vibrios

• vii. Flexuous spiral-shaped bacteria: Spirochetes

• viii. Filamentous bacteria: Actinomycetes

• ix. No stable shape: Mycoplasm

Arrangement of bacterial cells:

Adherence of bacterial cells after cell division is called the arrangement of bacterial cells. The presence of flagella on bacterial cells is also studied under the arrangement of bacterial cells. The arrangement of a bacterial cell is more complex in cocci than in bacilli and filamentous bacterial cells.

Arrangement of cocci bacterial cells:

Coccus is an oval, spherical, or round-shaped bacterium. Cocci are arranged and named as per the below details:

• i. Monococcus: Single coccus bacterium cell

• ii. Diplococcus: Two cocci bacterial cells

• iii. Tetracoccus: Four cocci bacterial cells

• iv. Streptococcus: Cocci bacterial cells in chain

• v. Sarcinia: Highly packed cocci bacterial cells,

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.

• i. Bacillus: Single bacillus bacterium cell

• ii. Diplobacillus: Two bacilli bacterial cells.

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

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:

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

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

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

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

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

Dr Pramila Singh