General Bacterial Cell Structure - PowerPoint Presentation

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General Bacterial Cell Structure

Lec.No.2 Dr.

Oruba

Cellular Organelles:  

1-

Ribosomes; These are 18 nm. particles, which are scattered in thousands of them in the cytoplasm. Each particle (unit) is of 70S type, which consists of two subunits; large one of 50S, and a small one of 30S. The concentration of Mg++ in the medium plays a very important effect on the aggregation ( forming pairs of 100S)or disaggregation ( separating each unit into 30S and 50S), where both lead to malfunction of protein synthesis. Ribosome is cellular organelle that found is in cytoplasm. Its function is to synthesize proteins for the bacterial cell.

2- The Nucleoid ( the chromosome)

Prokaryotic and eukaryotic cells were initially distinguished on the basis of structure: the prokaryotic

nucleoid, the equivalent of the eukaryotic nucleus is structurally simpler than the true eukaryotic nucleus, which has a complex mitotic apparatus and surrounding nuclear membrane. Under the light microscope, the nucleoid of the bacterial cell can be visualized with the aid of Feulgen staining, which stains DNA. Gentle lysis can be used to isolate the nucleoid of most bacterial cells. The DNA is then seen to be a single, continuous, "giant" circular molecule with a molecular weight of approximately 3 X 10

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. The unfolded nuclear DNA would be about 1 mm long (compared with an average length of 1 to 2 µm for bacterial cells). The bacterial

nucleoid

, then, is a structure containing a single chromosome. The number of copies of this chromosome in a cell depends on the stage of the cell cycle (chromosome replication, cell enlargement, chromosome segregation, etc).

The chromosome carries the genetic information and it duplicates before cell division.

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Plasmids

These are extra chromosomal double stranded circular DNA that are capable of replicating independent of the bacterial chromosome. It helps bacteria to resist antibiotics.

4 - Mesosomes

Thin sections of Gram-positive bacteria reveal the presence of vesicular or tubular-vesicular membrane structures called

mesosomes, which are apparently formed by an invagination of the plasma membrane. These structures are much more prominent in Gram-positive than in Gram-negative organisms. At one time, the mesosomal vesicles were thought to be equivalent to bacterial mitochondria; however, many other membrane functions have also been attributed to the mesosomes. At present, there is no satisfactory evidence to suggest that they have a unique biochemical or physiologic function. Indeed, electron-microscopic studies have suggested that the mesosomes, as usually seen in thin sections, may arise from membrane perturbation and fixation artifacts. No general agreement exists about this theory, however, and some evidence indicates that

mesosomes

may be related to events in the cell division cycle, as well as in

sporulation

.

Surface AppendagesTwo types of surface appendage can be recognized on certain bacterial species: the flagella

, which are organs of locomotion, and

pili (Latin hairs), which are also known as fimbriae (Latin fringes).  FlagellaStructurally, bacterial flagella are long (3 to 12 µm), filamentous surface appendages about 12 to 30 nm in diameter. The protein subunits of a flagellum (flagellins) are assembled in a helical shape to form a cylindrical structure with a hollow core. The ability of bacteria to swim by means of the propeller-like action of the flagella provides them with the mechanical means to perform chemotaxis (movement in response to attractant and repellent substances in the environment). Response to chemical stimuli involves a sophisticated sensory system of receptors that are located in the cell surface and/or periplasm and that transmit information to methyl-accepting chemotaxis proteins that control the

flagellar

motor. The number and distribution of flagella on the bacterial surface are characteristic for a given species and hence are useful in identifying and classifying bacteria.

Typical arrangements of bacterial flagella

Flagella can be sheared from the cell surface without affecting the viability of the cell. The cell then becomes temporarily

nonmotile. In time it synthesizes new flagella and regains retain motility. The protein synthesis inhibitor chloramphenicol, however, blocks regeneration of flagella. Some bacteria have both flagella and pili. The electron micrograph below, shows the characteristic wavy appearance of flagella and two types of pili on the surface of Escherichia coli.

Electron micrograph of negatively stained E coli showing wavy flagella and numerous short, thinner, and more rigid

hairlike

structures, the pili. (B) The long sex pilus can be distinguished from the shorter common pili by mixing E coli cells with a specific bacteriophage that binds specifically to sex pili.

PiliThe terms

pili

and fimbriae are usually used interchangeably to describe the thin, hair-like appendages on the surface of many Gram-negative bacteria, and the proteins of pili are referred to as pilins. Pili are more rigid in appearance than flagella. As it is easily recognized in strains of E coli, pili are of two types: short, abundant common pili, and a small number (one to six) of very long

pili

known as sex

pili.The

sex

pili

attach male to female bacteria during conjugation.

Pili

in many enteric bacteria confer adhesive properties on the bacterial cells, enabling them to adhere to various epithelial surfaces, to red blood cells (causing

hemagglutination

), and to surfaces of yeast and fungal cells. These adhesive properties of

piliated

cells play an important role in bacterial colonization of epithelial surfaces and are therefore referred to as colonization factors.

Capsules and Loose Slime

Some bacteria form capsules, which constitute the outermost layer of the bacterial cell and surround it with a relatively thick layer of

viscous gel. Capsules may be up to 10 µm thick. Some organisms lack a well-defined capsule but have loose, amorphous slime layers external to the cell wall or cell envelope. The hemolytic Streptococcus mutans, the primary organism found in dental plaque, is able to synthesize a large extracellular mucoid glucans from sucrose. Not all bacterial species can produce capsules, however, the capsules of encapsulated pathogens are often important determinants of virulence. Encapsulated species are found among both Gram-positive and Gram-negative bacteria. In both groups, most capsules are composed of high molecular-weight viscous polysaccharides that are retained as a thick gel outside the cell wall or envelope. The capsule of Bacillus anthracis

(the causative agent of anthrax) is unusual in that it is composed of a g-

glutamyl

polypeptide. The exact functions of capsules are not fully understood, but they do

confer resistance to

phagocytosis

and hence provide the bacterial cell with protection against host defenses to invasion

.

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Other Intracellular ComponentsOther granular inclusions randomly distributed in the cytoplasm of various species include; metabolic reserve particles such as;

poly-b-

hydroxybutyrate (PHB), polysaccharide and glycogen-like granules, and polymetaphosphate or metachromatic granules, and others. They serve as energy and nutrient reservoirs. They may be rich in lipids and carbohydrates.EndosporesEndospores are highly heat-resistant, dehydrated resting cells formed

intracellularly

in members of the genera Bacillus and Clostridium.

Sporulation

, the process of forming

endospores

, is an unusual property of certain bacteria. The series of biochemical and morphologic changes that occur during

sporulation

represent true differentiation within the cycle of the bacterial cell. The process, which usually begins in the stationary phase of the vegetative cell cycle, is initiated by depletion of nutrients (usually readily utilizable sources of carbon or nitrogen, or both). The cell then undergoes a highly complex, well-defined sequence of morphologic and biochemical events that ultimately lead to the formation of mature

endospores

.

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Endospores

These are small thick walls formed in the cytoplasm that helps to resist the unfavorite environmental conditions like depletion of nutrients, heat, and dryness. The spores formation occurs by the development of an ingrowth of the cytoplasmic membrane cutting off a portion if the cytoplasm. Then a thick cortex and a tough spore coat are formed. Endospores Spores are commonly found in the genera Bacillus and Clostridium.

Where it finally consisting of the following layers;

1- The central core

: contains the minimized contents of the spore plus a single strand of DNA and high content of Calicium dipicolinate.2- The cortex layer: which contains a double layer of peptidoglycan differs from the original one, which is more resistant.3- The spore coat; contains special hard resistant protein.4- The keratinized exosporium; contains keratinized resistible

Electron micrograph of a thin section of a Bacillus

megaterium

spore

References: 1- Jawetz, Melnick, & Adelberg’s.( 2013). Medical Microbiology (Twenty-Sixth Edition).

2- Kenneth

Todar. (2008).Todar’s Online Textbook of Bacteriology ,University of Wisconsin. 17