Fundamentals of Microbiology and Taxonomy

Binomial Nomenclature

Under the system of binomial nomenclature, every organism is given a two-part scientific name. The first part is the Genus, which must be capitalized, and the second part is the species, which is lowercase. Both terms must be written in italics (or underlined if handwritten).

Example: Streptococcus pneumoniae.

The Three Domains of Life

By comparing 16S rRNA sequences, scientists have divided all life into three distinct cellular domains:

• Bacteria: Often referred to as "true" bacteria, these are prokaryotic organisms.
• Archaea: These are also prokaryotes, but they are genetically and biochemically distinct from bacteria.
• Eukarya: This domain includes fungi, protozoa, animals, and plants. Unlike the other two, eukaryotes possess a membrane-bound nucleus and membrane-bound organelles.

Prokaryotic vs. Eukaryotic Cells

The most fundamental distinction in biology is the difference between prokaryotic and eukaryotic cell structures.

Prokaryotic Cells

Prokaryotes (Bacteria and Archaea) are typically smaller, ranging from $0.5–3\ \mu\text{m}$ in size. They lack a defined nuclear membrane; instead, their DNA exists as a single circular chromosome located in an irregular region called the nucleoid. They also lack membrane-bound organelles such as mitochondria or chloroplasts. In bacteria, the cell wall is often composed of peptidoglycan.

Eukaryotic Cells

Eukaryotes are generally larger, typically $> 5\ \mu\text{m}$. They are characterized by a true nucleus that houses multiple linear strands of DNA. They possess complex internal structures, including mitochondria for respiration and, in plants and algae, chloroplasts for photosynthesis.

Comparative Summary

Ribosomes and Svedberg Units

Ribosomes are the essential sites of protein synthesis (translation) in all living cells. Because all cells possess ribosomes, they are a universal feature of life.

The size of a ribosome is expressed in Svedberg (S) units, which measure the sedimentation velocity of a particle during centrifugation rather than its linear size. Because S units depend on both mass and shape, they are not additive when subunits combine.

• Prokaryotic Ribosomes (70S) are composed of a $50\text{S}$ large subunit and a $30\text{S}$ small subunit.
• Eukaryotic Ribosomes (80S) are composed of a $60\text{S}$ large subunit and a $40\text{S}$ small subunit.

Exam Tip: Do not perform simple arithmetic on S units! $50\text{S} + 30\text{S}$ does not equal $80\text{S}$; it equals $70\text{S}$ because the sedimentation coefficient is not a linear measure of size.