Introduction to Bacteria: Structure, Classification, and Morphology

Introduction to Bacteria

Overview

Bacteria are a major group of microorganisms that play essential roles in ecosystems, human health, and disease. This section introduces the domains of life, the principles of microbial classification, and the basic features that define bacteria.

The Domains of Life

Classification of Cellular Life

• Domain Bacteria: Unicellular prokaryotes, including Bacteria and Cyanobacteria.

• Domain Archaea: Unicellular prokaryotes, often extremophiles (organisms living in extreme environments).

• Domain Eukarya: Eukaryotes, including Kingdoms Protista, Fungi, Plantae, and Animalia.

Viruses

• Virus: An infective agent consisting of nucleic acid in a protein coat, too small to be seen with a light microscope, and only able to multiply within a living cell (host).

Microbes and Microorganisms

Taxonomy and Nomenclature

• Taxonomy: The science of classifying organisms; groups in the hierarchy are called taxa.

• Classification involves identifying new organisms or reclassifying existing ones.

• Binomial nomenclature: Scientific naming using two words (genus and species), italicized or underlined. The genus name is capitalized (e.g., Pseudomonas aeruginosa).

• Names are assigned in Latin.

• Classification is increasingly based on DNA sequencing, which is subject to change as new data emerges.

Ubiquity of Bacteria

Distribution and Normal Flora

• Ubiquity: Bacteria are found everywhere in the biosphere (soil, water, air, etc.).

• Normal flora: Microorganisms living on and in the human body that do not cause harm.

• Bacteria are absent only in areas purposely made sterile, such as clean rooms and sterilized/autoclaved equipment.

Viewing Bacteria

Microscopy and Culture

• Bacteria are grown on nutrient media such as Tryptic Soy Agar (TSA) plates.

• Individual bacterial cells must be observed under a microscope due to their small size.

Defining Features of Bacteria

Cellular Structure

• Bacteria are defined by their lack of a nucleus and nuclear membrane.

• They lack complex organelles (e.g., mitochondria, endoplasmic reticulum).

• Most possess a cell wall containing peptidoglycan.

• Typical diameter is approximately 0.2 μm (compared to 6–8 μm for red blood cells).

Handling Bacterial Cultures

Aseptic Technique

• Aseptic technique: Procedures to prevent contamination by unwanted microorganisms.

• Disinfect work area before and after handling cultures.

• All tools (loops, needles) must be sterilized, typically by flaming in a Bunsen burner.

• Tubes, plates, and other equipment are autoclaved to kill existing bacteria.

• Keep cultures covered until use; if contamination is suspected, restart the procedure.

Characterizing Microorganisms

Growth and Replication

• Bacteria replicate asexually by binary fission.

• Doubling time: The time required for a bacterial cell to divide and double in number.

• Standard incubation for cultures is 24 hours, after which colonies become visible to the naked eye.

Colony and Cell Morphology

• Colony morphology: Includes form (surface view), elevation (height), margin (edge), color, and smell.

• Cell morphology: Includes size, shape, and Gram stain reaction.

Shapes of Bacteria

Coccus (plural: cocci)

• Spherical, ovoid, or round bacteria.

• Examples:

  • Staphylococcus: Cocci in clusters.

  • Streptococcus: Cocci in chains.

Bacillus (plural: bacilli)

• Rod-shaped bacteria.

• Examples:

  • Pseudomonas: Short bacilli.

  • Bacillus: Long bacilli and bacilli with endospores.

Spiral Forms

• Vibrio: Curved/comma-shaped rods.

• Spirillum: Helical-shaped, rigid bodies.

• Spirochete: Helical-shaped, flexible bodies (e.g., Borrelia).

Laboratory Identification of Bacteria

Microscopy and Staining

• Bacterial morphology is observed under high magnification (typically 1000X) using stains.

• Correct spelling and nomenclature are essential for scientific communication.

Examples of Bacterial Species and Morphology

Species	Gram Reaction	Shape	Arrangement
Pseudomonas aeruginosa	Negative	Bacillus/rod	Simple stain
Bacillus subtilis	Positive	Bacillus/rod	Chains
Vibrio cholerae	Negative	Comma-shaped rod	Single/comma
Neisseria gonorrhoeae	Negative	Coccus	Diplococci
Streptococcus pneumoniae	Positive	Coccus	Chains
Streptococcus pyogenes	Positive	Coccus	Chains
Staphylococcus aureus	Positive	Coccus	Clusters
Borrelia burgdorferi	Not specified	Spiral	Flexible bodies

Additional info:

• Gram staining is a key method for differentiating bacterial species based on cell wall structure.

• Binary fission is described by the equation: $N = N_0 imes 2^n$, where $N$ is the final number of cells, $N_0$ is the initial number, and $n$ is the number of generations.