Cellular Structure of Bacteria

The basic components of bacterial cells are:-

• Cell wall
• Plasma membrane
• Cytoplasm
• Chromosome
• Ribosome

The outer most structure of bacteria is the cell wall a semi rigid envelope that maintains the integrity of the cell in the same way that the skin maintains the integrity of the human body.

The cell wall helps protect the cell against environmental changes for example heat, cold and drugs that would otherwise damage or destroy it.

It allows most molecules to pass through it. This layer is composed of molecules called Peptidoglycans; this is often called Peptidoglycans layer.

Just inside the cell wall is a second less rigid envelope the plasma membrane sometimes called the inner membrane or cytoplasmic membrane which encloses the cell contents; in bacterial cells the plasma membrane has two primary functions:-

First it serves as a selective barrier to molecules that penetrating the cell wall; allowing some - such as water or oxygen - to flow easily into the cell enteria and restricting the passage of another such as proteins.

Second it contains enzymes; proteins that cause chemical reactions to occur as vital to the life functions of the cell.

Bacteria can be divided into two main groups based on differences in the cell walls structure; this difference was first noticed as differences in staining with a dye called gram’s stain.

Gram positive organisms have the structure defined so far.

But Gram negative bacteria have an additional membrane outside the peptidoglycans layer called the outer membrane; this which contains openings called channels some them formed by protein called Porins which allows nutrients, waste products, fluids to flow into and out of the cell.

Some of these channels are non-specific they allow any molecule up to certain size to flow through them; other channels are specific; only certain particular molecules can pass through them while prohibiting the entry of other molecules including antibiotic drugs into the cell; the cell wall channels protect the cell contents and enhance the likelihood of the cells of survival.

Cytoplasm is a viscid, thick, sticky substance that serves as the matrix fully interior contents of the cell; cytoplasm is made up primarily of water accounting for eighty percent of the total content; it also contains enzymes and nutrients such as carbohydrates, lipids, fats and other molecules.

In most bacteria the genetic material is contained in one single circular chromosome and contains all hereditary information required for bacteria; the chromosome consists of genes which made up of deoxyribonucleic acid (DNA)

DNA is a material that is composed of long and twisted strands of compounds called nucleotides arranged in a double helix pattern; DNA Carries the codes of reproducing specific proteins which determine characteristics or functions of the organism; in the higher organisms such as humans and plants chromosomes are enclosed in the cell nucleus; therefore these organisms are called eukaryotes; in contrast bacterial cells are prokaryotes since they lack of cell nucleus in addition to the existence of a single chromosome.

In addition to single chromosome; many bacterial cells it contains circular molecules of DNA called plasmid; it may carry genes the codes helps for resistance to antibiotics and for production of toxins both of which are important to bacterial survival; however the DNA contained in the plasmid is not for reproduction or other of basic cell function.

There are more than one type of ribonucleic acid RNA; ribosomal RNA and other proteins complex to form the ribosome; ribosomes are structured to serve as protein factories for the cell; because the cell cannot survive without proteins; ribosomes are vital to cell life like DNA; RNA is a component of all living cells and is composed of nucleotides and arranged in long strands that unlike the double stranded DNA; RNA is a single strand of nucleic acid and the primary function of RNA is the protein synthesis; the genetic code contained in the DNA is transcribed into messenger RNA which then travels to the ribosomes;  the genetic code is then translated into proteins.

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Pathogenic Sub-Type

In a human body normal flora a normally harmless bacteria that lives primarily on the skin and in the eyes, mouth, upper respiratory system, the gastrointestinal tract mainly in the large intestine (colon) and urogenital tract; however under certain conditions some of the normal flora may be become pathogenic then by causing opportunistic infections

The co-existence of bacteria and the human hosts is called symbiosis; normal flora existing one of three symbiotic relationships with the human hosts.

In commensalism the bacteria benefit from the relationship and the human host is neither harm nor benefit it; examples of commensals include the corynebacterium species that inhabit the tissues surrounding the eye and mycobacterium species that normally lives in the ear and external genitals; these bacteria use normal body secretions and died cells as nutrients and cause no harm to the host.

In mutualism both the bacteria and the host benefit; examples of mutualistic bacteria include the E. coli species that inhabit the large intestine; these bacteria live on the nutrients that pass through intestine and in return they produce essential vitamins use such as K and B for the human host.

In parasitism the bacteria benefit at the expense of the host; parasites in the human body are pathogenic bacteria that responsible for many diseases; the important bacterial sub-types that frequently cause diseases in humans are:-

• Cocci and Bacilli.
• Mycoplasmas.
• Mycobacteria.
• Spirochetes.
• Rickettsias.
• Nocardia Forms.

Cocci are round bacteria and the bacilli are rod shaped; pathogenic cocci and bacilli include gram positive, gram negative, aerobic or anaerobic sub-types.

Staphylococcus aureus which are gram positive aerobe cocci cause skin and lung infections; Pseudomonas aeruginosa which are gram negative aerobic bacilli cause numerous respiratory and urinary tract infections.

In contrast Mycoplasmas are very small bacteria that lake cell walls as a result they are irregularly shaped, they atypically facultative anaerobe; mycoplasmas such as Mycoplasma pneumoniae are primarily associated with respiratory infections.

Mycobacteria are a group of aerobic non-motile, non-endospore-forming rods; mycobacterium tuberculosis is responsible for tuberculosis and Mycobacterium leprae causes leprosy; the Mycobacterium avium, Mycobacterium intracellulare complex a group of microorganisms that rarely infect humans but it has now classified as one of the leading opportunistic infections associated with AIDS.

Spirochetes are motile bacteria with coiled shapes since they lack flagella they moved by rotating in a corkscrew fashion spirochetes can be an aerobic or anaerobic; the pathogenic species of spirochetes cause syphilis and Lyme disease.

Rickettsias are gram negative and non motile and can be either the rods or cocci; they also parasites since they most inhabit another organism to survive a and cannot reproduce outside a host cell; rickettsias are almost always transmitted to humans via insect and tick bite; vehicles diseases such as epidemic typhus and Rocky Mountains spotted fever

Chlamydia trachomatis are coccoid rickettsia is that can cause blindness and two types of sexually transmitted diseases (STDs) one of which Non-Gonococcal Urethritis is probably the most common STD in the U.S.

Finally Nocardia forms are aerobic rods; Nocardia asteroides can cause chronic tuberculosis like respiratory infection.

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Bacterial Transmission

The sites which bacteria live and multiply are called reservoirs; the three main reservoirs of the pathogenic bacteria are humans, animals and non-living matters; the human body is considered to be a reservoir both in terms of the normal flora that inhabit it and the bacteria that can enter the body from exogenous sources; Diphtheria and Streptococcal infections are some of the disease that spread from human reservoirs and such diseases are called communicable.

Infectious diseases can also be transmitted from animals to humans; such diseases called zoonosis include cat scratch disease (CSD) also called cat scratch fever and rocky mountains spotted fever; pathogenic bacteria such as those that cause botulism can also thrive in the non-living reservoirs of soil and water; to transmit a disease from one organism to another; bacteria must first escape from the reservoir; each pathogen in a human reservoir has a specific area of the body from which it is shared; this area called a portal of exit is usually located at the area of infection; the most common portals of exit from human reservoirs are the openings of respiratory and gastrointestinal system;

Bacteria also leave the body via urogenital tract, wounds in the skin and blood; the respiratory system includes the nose, trachea and lungs; when a person coughs of sneezes microbes are expelled violently from the mouth and nose; examples of diseases caused by pathogens that exit via the respiratory tract are tuberculosis and pneumonia; components of the gastrointestinal system include the mouth, esophagus, stomach and large intestine and small intestines; pathogens depart the body from the gastrointestinal tract by means of saliva or feces; typhoid fever, cholera and polio can be transmitted from the host’s gastrointestinal tract to other hosts.

The urogenital system includes the kidneys, uritis, bladder, urethra the tubes through which urine is transported from the bladder out of the body and genital organs; bacteria can be carried out of the urogenital track through urine, semen and vaginal secretions; diseases caused by pathogens that leave through the urogenital tract include the sexually transmitted diseases (STDs).

When damaged; this skin can function as a portal of exit; since bacteria can escape from the body through open wounds; additionally infected blood may contain and transmit many pathogens; diseases caused by pathogens carried in blood include yellow fever and meningitis.

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Classification System of Bacteria

Based on differences in the cell wall structure the kingdom prokaryote is divided into four of divisions:-

·         Gram positive cell walls.

·         Gram negative cell walls.

·         Cells without walls.

·         Cells with unusual walls.

The bacterial sub-types are next classified into genera related to genus and species.

Members of a genus share the same heredity; however they different from each other in the certain morphological and behavioral ways; for example the early human and modern human belong to the same genus; other level of species; classification of bacteria and humans differs; in humans and other animals the terms species refers to a natural population that cannot successfully breed with another group like (Neanderthal) and modern humans; in contrast bacteria within a species are similar to another as one human is similar to another human; finally some bacterial species may be divided further into strains that share a particular characteristic but it is not common to the rest of the species; individuals of the same strain are equivalent to identical twins since their old descendants of the same parental cell; in clinical medicine the shared characteristic that define the particulars strain is often resistance to an antibiotic to which the species was one susceptible; the relationship of individual bacteria to another is often described by a system of binomial nomenclature in which the name for a species of bacteria consists of two parts both of them usually in Latin or Greek; in the bacterial species named “Streptococcus pneumoniae”; thus “Streptococcus” is the genus and “pneumoniae” represents the species; in stand of usage; the genus is always capitalized while the species is not; additionally the genus name is written note as first time it appear in a document that is an abbreviated as a single initial thereafter “Streptococcus pneumoniae” becomes “S. pneumoniae”.

The major characteristics used to classify bacteria into genus and species are:-

·         Cell shape and arrangement.

·         Gram stain reaction.

·         Oxygen requirements.

·         Nutritional and metabolic characteristics.

Differences in cell shape and arrangement provide an initial clue to the identity of the bacterial genus and species.

The three basic shapes of bacterial cells are:-

• Coccus (round).
• Bacillus (rod shaped).
• Spirochetes (spiral).

Some coccoid bacteria or cocci exist as single cells; however the when other cocci divide and reproduce; individual bacteria remain attached to the each other; these group include pairs of  a cells (Diplococci); chains of cells (Streptococci) and clusters of cells (Staphylococci);

A major characteristics for classifying bacteria is the gram stain reaction which distinguishes two types of bacterial cell walls; grams stain dye process stain gram positive bacteria dark violet and gram negative bacteria red.

Due to several layers of peptidoglycans network which is made up of amino acids and simple sugar molecules linked by chemical bridges; the cell walls of gram positive bacteria are more rigid and stronger than those gram negative bacteria; in contrast gram negative cell walls are more vulnerable to breakage than the thicker, strong walls of gram positive cells.

However a gram negative cells are quite complex; between the cell wall and outer membrane and the cytoplasmic membrane of gram negative cells is an enzyme filled gel compartment called periplasm; impeded in the gel a small peptidoglycans backbone to provide some strength to the wall; more importantly gram negative cell walls contain Lipopolysaccharides molecules made of sugars and fats that is not found in most gram positive cell walls; these molecules are involved in the formation of toxins that can be harmful to the human upon their release.

Bacteria differ in their need for and tolerance of oxygen on the basis of whether they can use oxygen for their growth and metabolic processes; bacteria are classified generally as:-

·         Aerobic or aerobes which use oxygen.

·         Anaerobic or anaerobes which do not use oxygen.

Aerobic bacteria are fragile since they require oxygen for life and the loss of oxygen in their environment kill them; some aerobes are classified as Microaerophilic because they require oxygen in a concentration lower than that present in the atmosphere; in contrast many anaerobes are less fragile in particular Facultative anaerobes which use oxygen when it is available; and can survive without it (most adaptive bacteria).

Anaerobes also include Aerotolerant and Obligate bacteria; aerotolerant anaerobes cannot use oxygen but they usually not damaged by oxygen; obligates anaerobes are unable to use oxygen and are damaged by oxygen.

Carbohydrate metabolism is another distinguishing factor used in classifying bacteria; for example among number of similar bacteria that inhabit the human intestinal tract; only certain species metabolize carbohydrates such as lactose - a milk derived sugar -; therefore the presence of intestinal gas a byproduct of lactose metabolism indicates the presence of these specific bacteria.

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External Structure of Bacteria

A capsule is gel like compound  that surrounds the exterior of some bacterial cells; the gel compound is produced inside the bacterial cell and is excreted outside it and it attached firmly to the cell exterior; in some bacteria this material is not organized and is more loosely attached; in this case it is called a slime layer; a capsule can help bacterium to survive from harmful environmental changes and provide effective protection against attack by the host defense system; in addition of the sticky capsule helps the bacterium to adhere to the cell surfaces inside the host tissues.

Flagella are work like tales that specialized extensions of bacterial cell walls; a bacterium may have one, several or numerous flagella; Flagella can be located anywhere on the bacterium and give bacterial motility - the ability to move on her own-  motility allows the bacterium to move into more favorable environment; the response of senses located in the cell wall by specific stimuli produces taxis - movement toward or away from the stimulants - the majors responses of motile bacteria are chemotaxis (the response to chemicals stimuli) and phototaxis the (response to the light); these responses control the speed and direction of the flagellum rotation to create different movement paths; the cells can move ahead in constant rate or they can tumble, turn and continue on another path.

The cell walls of some bacteria have hale-like appendages called Fimbria and Pili these appendages are shorter and thinner than flagella and more like bristles and function in cellular attachment rather than cell movement; fimbria present in multiple numbers and found either in each end of the cell or distributed eventually along the entire cells surface; fimbria helps the bacterium to adhere to tissues surfaces within the host.

Pili are longer than fimbria and they are present either singly or in pairs on a single bacterium;

Their propose is to enable one bacterium to adhere to another during a specialized process called conjugation in which DNA is transferred between the cells.

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