Bacterial Structure and Morphology

Bacterial Morphology

• Bacteria range from 0.1 to 10µm in length, with spherical bacteria having diameters of 0.5 to 2µm, and rod-shaped cells being 0.2 to 2µm wide and 1 to 10µm long.
• There are three basic shapes of bacteria: cocci (round or oval), bacilli (rod-shaped), and spiral-shaped bacteria.

Cocci

• Cocci can be arranged in pairs (diplococci), chains (streptococci), or clusters (staphylococci).

Bacilli

• Bacilli can have rounded, tapered, square, or swollen ends.
• Short rods that resemble cocci are called coccobacilli.
• Some rod-shaped bacteria have tapered ends and are termed fusiform.

Spiral-Shaped Bacteria

• Spiral-shaped bacteria are flexible, coiled, and motile, and can be classified as spirilla (rigid) or spirochetes (flexible and undulating).

Cell Wall

• The cell wall is the outermost component of bacteria, protecting them from lysis and allowing them to grow in a wide range of osmotic pressures.
• The main constituent of bacterial cell walls is peptidoglycan.
• Gram-positive bacteria have thick layers of peptidoglycan combined with teichoic acid, while Gram-negative bacteria have a thinner layer of peptidoglycan covered with an outer lipid membrane.

Plasma Membrane

• The plasma membrane is 5-10 nm thick and consists of proteins embedded in a bilayer of phospholipids.
• Functions of the plasma membrane include selective permeability, energy generation, synthesis and assembly of cell wall components, secretion of enzymes and toxins, and protection against osmotic rupture.

Flagella

• Flagella are long, whiplike appendages that move bacteria toward nutrients and other attractants through a process called chemo-taxis.
• Flagella are made of chains of flagellin and are anchored to the cell wall and membrane.
• The system enables bacteria to detect changes in concentration of certain chemicals and to move toward or away from the substance.

Fimbriae or Pili

• Fimbriae or pili are short, rigid rod-like appendages composed of a single protein pilin.
• Functions of fimbriae or pili include facilitating adherence of bacteria to other bacterial cells, red blood cells, or the lining of the intestine.

Prokaryotic Cells

• Prokaryotic cells, such as bacteria, possess a naked DNA molecule without associated basic proteins, divide amitotically by binary fission, and have a semi-rigid cell wall.
• Characteristics of prokaryotic cells include a circular DNA molecule, no nuclear membrane, and 70S ribosomes.

General Features of Bacterial Cells

• Bacteria are the smallest free-living microorganisms that are visible only with the aid of a microscope.
• They are able to carry out their own life processes, such as growth, energy generation, and reproduction, independent of other cells.
• In general, bacteria are typical prokaryotic cells that contain both DNA and RNA, can grow in artificial media, replicate by binary fission, have a rigid cell wall, and are sensitive to antimicrobial agents.