Introduction to Bacteria and Identification Techniques, Slides of Microbiology

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Introduction to bacteria

and

its identification

techniques

Dr. Ronni Joji Dept. of Microbiology, Immunology and Infectious Diseases MICR 201 Nursing AY: 2025 - 26

Objectives

• Types of microorganisms.
• Morphology of a bacterial cell.
• Function of bacterial structures.
• Bacterial growth cycle.
• Difference between Gram-positive and Gram-negative cell wall.
• Laboratory techniques used for bacterial identification.

Eukaryotic Cell Structure Prokaryotic Cell Structure

Eukaryotic cells contain a “true” nucleus, whereas prokaryotic cells do not. Eukaryotic cells possess a complex system of membranes and membrane-bound organelles, whereas prokaryotic cells do not.

Bacterial cell morphology

• Bacteria are divided into three major phenotypic categories:
  • Gram-negative and have a cell wall
  • Gram-positive and have a cell wall
  • Those that lack a cell wall ( Mycoplasma spp.)
• There are three basic categories of bacteria based on shape:
  • Cocci (round bacteria)
  • Bacilli (rod-shaped bacteria)
  • Curved and spiral-shaped bacteria

Morphologic Arrangements of Cocci

Morphologic Arrangements of Cocci

The Domain Bacteria Cell Morphology—(cont.)

• Curved and spiral-shaped bacteria
  • Examples of curved bacteria:
    • Vibrio spp.
    • Campylobacter spp.
  • Examples of spiral-shaped bacteria:
    • Treponema spp.

Bacterial Cell Structure

1. Bacterial cell wall

• A rigid exterior that defines the shape of bacterial cells— chemically complex.
• Main constituent of most bacterial cell walls is peptidoglycan (found only in bacteria).
• Gram-positive bacteria have a thick layer of peptidoglycan; Gram-negative bacteria have a much thinner layer.
• Mycoplasma spp. do not have a cell wall; they are pleomorphic.

Function of cell wall components:

• Peptidoglycan: structural rigidity and shape, preventing osmotic lysis
• Teichoic acids: adhesion, induces septic shock
• Lipoteichoic acids: immune activation and induces septic shock
• Outer membrane : protection from environment, facilitate transportation of nutrients
• Lipopolysaccharides contains lipid portion called lipid A or endotoxin : cause fever and shock

2. Glycocalyx (slime layers and capsules)

• Some bacteria possess glycocalyx, a slimy, gelatinous material produced by the cell membrane and secreted outside the cell wall.
• There are two types of glycocalyx—slime layer (loosely connected to the cell wall) and capsule (highly organized and firmly connected). Capsules protect the bacteria from being phagocytosed by white blood cells (antiphagocytic action)

3. Flagella

• Motile bacteria possess flagella—whiplike appendages composed of threads of protein called flagellin.
• Number and arrangement of flagella are characteristic of a particular species:
  • Peritrichous bacteria—flagella over entire surface
  • Lophotrichous bacteria—tuft of flagella at one end
  • Amphitrichous bacteria—one or more flagella at both ends
  • Monotrichous bacteria—single polar flagellum

4. Fimbriae and Pili

• They are hairlike structures, most often observed on Gram-negative bacteria.
• They are composed of polymerized protein molecules called pilin.
• They enable bacteria to anchor themselves to surfaces.
• Some bacteria possess a sex pilus for conjugation. Fimbriae and pili: enable bacteria to adhere to surfaces

Terminal and Subterminal Spores

Reproduction of bacteria

• Bacteria reproduce by a process known as binary fission —one cell splits in half to become two daughter cells. - Before a prokaryotic cell divides in half, the chromosome must be duplicated.
• The time it takes for binary fission to occur is called the generation time. - Generation time varies from one species to another and depends on growth conditions (under ideal conditions, Escherichia coli has a generation time of about 20 minutes).