General Microbiology (Micr300) Lecture 2 Overview of Microbial Diversity Prokaryotic and Eukaryotic Cells Taxonomy and Nomencluture (Text Chapters: 2; 11)

Cell Structure All microbial cells share certain basic structures in common, such as cytoplasm, a cytoplasmic membrane, ribosomes, and (usually) a cell wall. Two structural types of cells are recognized: the prokaryote and the eukaryote. Prokaryotic cells have a simpler internal structure than eukaryotic cells, lacking membrane-enclosed organelles (Figure 2.1).

Prokaryotic Cell

Eukaryotic Cell

Viruses Viruses are not cells but depend on cells for their replication (Figure 2.3c). Ribosomes—the cell's protein-synthesizing factories—are particulate structures composed of RNA (ribonucleic acid) and various proteins suspended in the cytoplasm. Ribosomes interact with several cytoplasmic proteins and messenger and transfer RNAs in the key process of protein synthesis (translation) (Figure 1.4).

Genetic Materials Genes govern the properties of cells, and a cell's complement of genes is called its genome. DNA is arranged in cells to form chromosomes. In prokaryotes, there is usually a single circular chromosome; whereas in eukaryotes, several linear chromosomes exist. excellent models for understanding cell function in higher organisms, including humans. Plasmids are circular extrachromosomal genetic elements (DNA), nonessential for growth, found in prokaryotes.

Genetic Materials The nucleus is a membrane-enclosed structure that contains the chromosomes in eukaryotic cells. The nucleoid, in contrast, is the aggregated mass of DNA that constitutes the chromosome of cells of Bacteria and Archaea

The Tree of Life Evolution is the change in a line of descent over time leading to new species or varieties. The evolutionary relationships between life forms are the subject of the science of phylogeny.

The Tree of Life Comparative ribosomal RNA sequencing has defined the three domains of life: Bacteria, Archaea, and Eukarya. Molecular sequencing has also shown that the major organelles of Eukarya have evolutionary roots in the Bacteria and has yielded new tools for microbial ecology and clinical microbiology. Although species of Bacteria and Archaea share a prokaryotic cell structure, they differ dramatically in their evolutionary history.

Microbial Diversity All cells need carbon and energy sources. Chemoorganotrophs obtain their energy from the oxidation of organic compounds. Chemolithotrophs obtain their energy from the oxidation of inorganic compounds. Phototrophs contain pigments that allow them to use light as an energy source. (Figure 2.8)

Microbial Diversity Autotrophs use carbon dioxide as their carbon source, whereas heterotrophs use organic carbon. Extremophiles thrive under environmental conditions in which higher organisms cannot survive. Table 2.1 gives classes and examples of extremophiles.

Prokaryotic Diversity Several lineages are present in the domains Bacteria and Archaea, and an enormous diversity of cell morphologies and physiologies are represented there. Retrieval and analysis of ribosomal RNA genes from cells in natural samples have shown that many phylogenetically distinct but as yet uncultured prokaryotes exist in nature.

Prokaryotic Diversity The Proteobacteria is the largest division (called a phylum) of Bacteria (Figure 2.9). The Cyanobacteria (Figure 2.12) are phylogenetic relatives of gram-positive bacteria and are oxygenic phototrophs.

Prokaryotic Diversity There are two main lineages of Archaea, the Euryarchaeota and the Crenarchaeota (Figure 2.18).

Eukaryotic Microorganisms Microbial eukaryotes are a diverse group that includes algae, protozoa, fungi, and slime molds (Figure 2.22).

Eukaryotic Microorganisms Collectively, microbial eukaryotes are known as the Protista. Some protists, such as the algae, are phototrophic. Cells of algae and fungi have cell walls, whereas the protozoa do not. Some algae and fungi have developed mutualistic associations called lichens.

Comparison of 3 Domains

Three Domains Although the three domains of living organisms were originally defined by ribosomal RNA sequencing, subsequent studies have shown that they differ in many other ways. Table 11.3 summarizes a number of other phenotypic features, physiological and otherwise, that can be used to differentiate organisms at the domain level.

Classical Taxonomy Conventional bacterial taxonomy places heavy emphasis on analyses of phenotypic properties of the organism (Table 11.4).

Species Concept in Microbiology The species concept applies to prokaryotes as well as eukaryotes, and a similar taxonomic hierarchy exists. Groups of genera (singular: genus) are collected into families, families into orders, orders into classes, classes into phyla (singular: phylum), and so on up to the highest-level taxon, the domain.

Species Concept in Microbiology Domain HIGH Phylum Class Order Family Genus Species. LOW

Species Concept in Microbiology It has been proposed that a prokaryote whose 16S ribosomal RNA sequence differs by more than 3% from that of all other organisms (that is, the sequence is less than 97% identical to any other sequence in the databases), should be considered a new species. Bacterial speciation may occur from a combination of repeated periodic selection for a favorable trait within an ecotype and lateral gene flow

Nomenclature Following the binomial system of nomenclature used throughout biology, prokaryotes are given descriptive genus names and species epithets. Escherichia coli Staphylococcus aureus