The Essential Characteristics of Prokaryotic Bacteria

The Fundamental Characteristics of Bacteria

Bacteria are prokaryotes. The absence of compartments separated by membranes means metabolites are disseminated throughout the cytoplasm. Despite this structural simplicity, bacteria are complex and diverse creatures from a biochemical viewpoint, which has enabled them to adapt to varied conditions.

Size and Structure

Most bacterial cells are very small: their volume is about one thousandth the size of the smallest eukaryotic cells. They are typically about 1 micrometer ($\mu$m) in diameter and 0.2 to 3-4 $\mu$m long. Most bacteria are unicellular organisms, but some form colonies or filaments, sometimes exhibiting cell specialization.

Basic Bacterial Shapes

Although many species have irregular shapes, bacteria generally adhere to some basic forms:

• The cocci (spherical)
• The bacilli (cylindrical)
• Spirochetes (spiral)
• Vibrios (comma-shaped)

Cocci Groupings

Cocci may be separated in some species, while in others, groups of cells remain independent. These groups can be:

• Diplococci: Two individuals.
• Streptococci: Many individuals forming long chains.
• Staphylococci: Forming irregular masses resembling bunches of grapes.

Metabolism and Reproduction

Bacteria have various mechanisms of nutrition: some feed by absorbing nutrients, while others are autotrophic, performing either photosynthesis or chemosynthesis. All bacteria reproduce asexually.

Genetic Material (Chromosome and Plasmids)

The genetic material of bacteria is found in the cytoplasm, forming a single circular DNA molecule, which corresponds to its chromosome. Some bacterial species also possess episomes and plasmids. This extrachromosomal DNA is also circular and grants the bacteria specific characteristics, such as resistance to antibiotics.

Internal and External Structures

Bacteria do not possess a cell cytoskeleton or membranous organelles. They lack:

• Nucleus
• Mitochondria
• Chloroplasts
• Endoplasmic reticulum
• Golgi complex
• Lysosomes

They do, however, have a distinctive, relatively thin, and stiff cell wall with a chemical composition very different from that presented by the cell walls of plant cells and fungi. The dense cytoplasm of a bacterial cell contains ribosomes and storage granules (glycogen, lipids, or phosphate compounds).

The enzymes needed for metabolic activities are usually located in the cytoplasm. Those involved in cellular respiration and photosynthesis can be attached to the plasma membrane or to folds that occur toward the interior of the bacterial cell (mesosomes).

Constant vs. Accessory Structures

Bacterial structures can be classified into constant structures and accessory structures.

Constant structures are those essential to the life of the bacteria and include the cytoplasm, the bacterial chromosome, the membrane, and the cell wall.

Accessory structures (such as the capsule, flagella, and fimbriae) are present only in some bacteria. Although not essential for life, they grant extraordinary adaptive advantages to those that possess them.