Bacterial Cell Structure Basic Bacteriology Part-1-1 (Second Semester 2016-2017) Bacterial Cell Structure

Classification Schemes of Cellular Organisms: Based on comparing the ribosomal RNA of organisms, the origin of cellular organisms can be classified into three main domains: Bacteria/ Eubacteria (true bacteria) (single-celled) Archaea (single-celled) Eukarya (Eukaryotes) (can be unicellular or multicellular)

Eukaryotes versus Prokaryotes: 1- Bacterial cells (prokaryotic cells) are smaller than Eukaryotic cells. Bacteria range in size from about 0.2 to 5 um in diameter 2- The eukaryotic cell has a true nucleus, which consists of the genetic material surrounded by a membrane known as nuclear membrane. That is to say, the genetic material is separated from the cytoplasm. Although the prokaryotic cell has genetic material, the genetic material is not separated from the cytoplasm by a membrane as in the case of the eukaryotic cell. Most bacteria have only one chromosome that is known as the Nucleoid.

2- Eukaryotic cells contain membrane-bound organelles (such as mitochondria and endoplasmic reticulum). On the other hand, bacterial (prokaryotic cells) do not have membrane-bound organelles. 3- Both eukaryotic cells and prokaryotic cells contain ribosomes. However, the eukaryotic ribosome is larger (size 80S) than the bacterial (prokaryotic) ribosome (size 70S). 4- Most bacteria have a rigid cell wall that is mainly made of peptidoglycan. Among eukaryotic cells, only fungal and plant cells have cell walls. The plant cell wall is made of cellulose The plant cell wall is made of chitin

Microbial Taxonomy Taxonomy is the science of Biological Classification: The main goals of this science are: 1- Classification of organisms into into groups or Taxa ( Taxon means a group). 2- Nomenclature: Assignment of names to Taxa 3- The development of an identification scheme: to determine to which taxon a newly identified organism is belong to.

Taxonomic Ranks: Microbes that are placed at a particular level share common features

Definitions: A Genus: it is a collection of species Two bacterial strains are said to be belonged to the same species , if they share at least 70% of their Genome Example: All Staphylococcus aureus species are supposed to share at least 70 % of their genes so as to belong to the same genus (Staphylococcus). Regarding the rest of their genes ( 30% ), these genes may vary among Staphylococcus aureus species, This will give them some variations properties such as: Variation in their surface antigens Variation in their pathogenesis Variation in their antibiotic resistance Each Staphylococcus aureus species that has a particular property is known as a strain

Terminology: Bacteria, fungi, protozoa, and helminths are named according to the Binomial System of Nomenclature (known also as the Linnaean Binomial Nomenclature System). This binomial system uses the last two levels of taxonomic ranks, which are the genus and species. For example, regarding the name of the well-known bacteria Escherichia coli, Escherichia is the genus and coli is the species name. Similarly, the name of the yeast Candida albicans consists of Candida as the genus and albicans as the species. Notice that the genus name starts with a Capital letter, while the species name starts with a small letter and that both the genus and the species are written in an Italic font. Note: nomenclature of viruses is not subjected to this system

Structure and Genome of Bacterial Cells Bacterial Cell Morphology/Shape The morphology/shape of the bacterial cell is mainly determined by its cell wall and cytoskeleton. The most common shapes of bacterial cells are: 1- Coccus, which means round or spherical 2- Bacillus, which means rod. A bacillus bacterium can be: Straight rod Curved S-shaped Spiral-shaped 3- Spirally-shaped: Spirellum and Spirochetes Notes: Some bacteria may have an oval shape. These are named as coccobacilli Some bacteria have cells that are variable in their shapes. Such kind of bacteria called pleomorphic

Arrangement of bacterial cells: Some types of bacteria, when they undergo cell division, the newly formed daughter cells remain attached to each others so that, the newly formed cells may remain in pairs, form long chains or clusters. Examples: Diplococci Tetrads Streptococci Staphylococci V- shape L- Shape Chinese letters

Bacterial Cell: Surface area to volume ratio: In contrast to the eukaryotic cell, bacterial cell has a large surface area to volume ratio. This, implies that bacterial cells are more efficient in up taking nutrients from the surrounding environment than eukaryotic cells as more surface area is available for each particular portion of the cell volume. Why it is important that the bacterial cell has a small size? Unlike eukaryotic cells, the bacterial cell lacks an advanced an intracellular trafficking system, so, substances spread through out its cytoplasm by diffusion, which usually a slow process. Accordingly, in order to guarantee distribution of substances as fast as possible through out its cytoplasm by diffusion, the bacterial cell MUST have a small cytoplasm (in terms of its volume)

1- Bacterial Cytoplasm: Under the Electron microscope, bacterial cytoplasm shows two distinct areas: An outer amorphous region, which has a granular appearance. This granular appearance is attributed to ribosomes , nutrient granules, and plasmids. An inner dark region, which represents the Nucleoid region (which is the super-coiled / folded bacterial chromosome)

The bacterial ribosomes: As in eukaryotic cells, bacterial ribosomes mediate the translation process. Similar to the eukaryotic ribosome, the bacterial ribosome consist of two subunits, a small subunit and a large subunit. Despite the similarity in its structure and function, the total size of the bacterial ribosome is smaller that the eukaryotic ribosome. The bacterial ribosome is about 70S The eukaryotic ribosome is about 80S In addition, there are some variations between both of ribosomal RNA and ribosomal proteins between prokaryotic ribosome and eukaryotic ribosome.

Cytoplasmic Granules (Inclusions): The cytoplasmic granules that can be seen in the bacterial cytoplasm under the electron microscope represents accumulation of organic or inorganic substances, which can serve nutrients storage for future utilization upon nutrient shortage in the surrounding environment.