Bacteria

Bacteria are small, microscopic, unicellular organisms that exhibit a great diversity in habitat, morphology, nutrition, metabolism and reproduction. The term bacterium was first used by Ehrenberg which comes from a Greek word bacterion which means little rod or stick. The study of bacteria is known as bacteriology.

A Dutch man, Anton van Leeuwenhoek, was the first to discover the bacteria in 1675, but he thought them to be animalcules. For about two centuries, thereafter, the bacteria did not receive much scientific attention. Later Louis Pasteur in 1864 and Robert Koch in 1876 reported that these microorganisms could cause disease.

Louis Pasteur declared that all putrefaction (decomposition of organic substances) is due to action of microorganisms. He also established that fermentation is also caused by microorganisms and developed the method of preservation of liquids (milk, wine etc) called the pasteurisation. Heating of liquids upto less than 100^o C temperature to kill the microorganisms is called the pasteurisation.

Robert Koch showed that anthrax disease is transmitted through spores. Koch proposed that all bacterial disease must follow some set conditions: i) that the organisms must be present in the lesions of the disease, ii) that it must be possible to grow these organisms in pure cultures outside the host body, iii) that the culture should be able to produce the typical symptoms of the disease, iv) that the same organism must invariably be present in the disease caused by the inoculum.

General Characters of Bacteria:

1. They are omnipresent, found in all possible habitats, which can support life.

2. They are unicellular and morphologically least complex of all living organisms.

3. Cell is prokaryotic – a well organised nucleus is absent.  Membrane bound cell organelles like endoplasmic reticulum, Golgi bodies, Mitochondria etc are absent.

4. Most of the bacteria have heterotrophic mode of nutrition i.e., they obtain their food directly from external source. They may live as saphrophytes, parasities or symbionts. Some bacteria are autotrophic, as they possess bacteriochlorophyll, the photosynthetic pigment.

5. Binary fission is the most common method of reproduction.

6. True sexual reproduction is absent. However, recombination of genetic material occurs by conjugation, transformation and transduction.

Occurrence and Distribution:

Bacteria are among the most numerous of all living organisms being present in almost very conceivable environment. There are about 2,000 known species.

Bacteria are ubiquitous organisms and occur in all natural habitats. They occur in abundance in both fresh waters of rivers, ponds, reservoirs and lakes and alo in saline waters of Oceans and Seas. They are found in all soil and above the earth.

Some thrive well in hot water springs (75^oC) and can also survive below freezing point in the Atlantic ice. Similarly, their tolerance to hydrogen concentration ranges from pH near to 0 to pH 11.

Many species of bacteria are free-living, and others are symbiotic, parasitic or saphrophytic. They are found within and upon the bodies of plants and animals. They flourish well on our skin, mouth and intestine.

The features which contribute to their universal distribution are:

1. Extremely simple structure

2. Small size, cell division occur rapidly.

3. Resistance of vegetative cells to adverse environmental factors

4. Formation of highly resistant endospores

5. Diversity of their modes of nutrition.

Size:

Bacteria are very small microscopic, unicellular organisms. The size of bacteria varies from 0.2µ (1 micron = 0.0001mm) in diameter and less than 2-10 microns in length.

Thiomargarita namibienxsis and Epulopischum fishelsoni are upto ½ a millimetre long and are visible to the unaided eye.

Forms or Shapes of Bacteria:

The bacteria vary in their shape which is usually constant for each species and helps in their identification.

I. Coccus Bacteria

Spherical or ellipsoidal bacteria are called cocci bacteria. Almost all coccal forms lack flagella (non-motile). The spherical cells may occur singly but often tend to stick together and occur in pairs, chains etc. On the basis of arrangement of cells in a cluster, the cocci are classified into the following six groups:

1. Monococcus: single spherical shaped cells. E.g., Micrococcus luteus

2. Diplococcus: a pair of spherical shaped cells (result when cocci divide and remain together to form pairs) e.g., Neisseria, Diplococcus pneumonia

3. Streptococcus: Cocci cells occur in long chains. Result when cells adhere after repeated divisions in one plane. E.g., Streptococcus lactis (milk bacterium).

4. Staphylococcus: Irregular group of many spherical bacteria arranged in bunches(grapes like). Result when cocci divide in radom planes. E.g., Staphylococcus albus (causes abscesses, wound infections).

5. Sarcinia: cuboidal arrangement of eight cocci cells. Results when cocci cells divide regularly in three planes. E.g., Sarcinae lutea

II. Bacillus Bacteria

This is the probably the most common form of bacteria. These are rod-shaped or hyhens (-) like bacteria are called the bacillus bacteria. Some are motile and some non-motile.

Based on the number of cells and their arrangement bacillus are classified into following :

1. Monobacillus: single rod shaped cells. E.g., Pseudomonas, Clostridium.

2. Diplobacillus:a pair of rod shaped cells. E.g. Corynebacterium diphtheria, Yersinia pestis (plague bacterium)

3. Streptobacillus: chain of bacillus bacteria appearing like straws. E.g., Bacillus tuberculosis, Lactobacillus.

III. Helical Bacteria : the helical forms are of two types – vibrioid and spirillum..

1. Vibrioid: rod shaped bacteria are slightly curved and look like a comma. A vibrio bears a single flagellum at its tip. E.g., Vibrio cholearae

2. Spirillum: the bacteria having the body twisted spirally or into helix. The spirilla forms usually bear two or more flagella at one or both poles. They occur as single cells or in short chains but do not form colonies. E.g., Spirillum volutans.

IV. Filamentous Bacteria:

Some bacteria are filamentous and branched. E.g., Beggiatoa, Thiothrix.

V. Pleomorphic Bacteria:

Some bacteria are able to change shape and size temporarily in response to changes in the environment. For example, Acetobacter, a group of vinegar bacteria occus as small rods, long rods, ellipsoids or a chain of small rods. Such bacteria are called pleomorphic.

Staining of Bacteria:       

Physician Christian Gram, in 1884 devised a differential staining procedure for identification of bacteria called the Gram staining.

A thin smear of bacterial cell is prepared on a glass slide. To the smear, Crystal Violet solution is applied for 30 seconds – step I. The slide is then rinsed in clean water. Then an Iodine solution is applied for 30 seconds – step II. This, inturn, is rinsed off with clean water.

Examination of the smear with the microscope reveals that all the bacteria have taken the purple colour. The next step in the staining procedure is to apply 95% Ethyl alcohol to the smear until the all the except the thickest layers of the smear have ceased to give off the dye.

Examination of the smear at this stage reveals that many of the bacteria still retain the original purple colour, whereas from others it has been washed off.

The bacteria that retain the original purple stain are called gram-positive bacteria. Those which loss the purple colour and become decolourised are called gram-negative bacteria.

The decolourised gram-negative bacteria are nearly invisible. To study their details counter stains like eosine or safranine are added to the smear.

Structure of Bacterial Cell

Bacteria are unicellular organisms. The ultra structure of bacteria is a typical example for the prokaryotic cell that lacks membrane bound nucleus and other cell organelles.

Like other living plant cells, the bacterial cell comprises of a cell wall and protoplast. Structurally, bacterial cell consists of following components:

1. Structures external to cell wall – flagella, fimbriae, pilli, capsule or slime layer

2. The cell Wall

3. Structures internal to cell wall – cell membrane, mesosomes, ribosomes, nucleoid

4. Special cell structures – storage granules, gas vacuoles, carboxysomes, magnetosomes, endospore

Structures External to Cell Wall

Slime layer:

The gelatinous layer present on the outer surface of the cell wall. It is secreted by the cell protoplast. The slime diffuses through the cell wall and deposits in form of a thin, extracellular, viscous layer external to the cell wall.

It is usually composed of polysaccharides or polypeptides or both. When it is made up of only polysaccharides, which forms a viscous layer and loosely attached to the cell wall, it is called Slime layer.

Under certain conditions of growth, the slime accumulates in form of a well-defined, thick , persistent  layer, firmly attached to the cell wall. It is called the sheath or capsule. E.g., Streptococcus pneumonia, Bacillus anthracis.

The sheath protects the bacteria against dessication and antibodies. The encapsulated pathogenic bacteria can with stand phagocytosis.

Some bacteria produce diffuse slime which spreads into the environment and encloses bacterial colonies. This is known as loose slime.

The bacterial cell with a capsule normally does not bear flagella.

Functions:

Provides protection under unfavourable conditions.

Function in the storage of food substances, and as storage site for the disposal of waste material.

Helps bacterium in attaching to various surfaces in order to survive in its natural environment such as streams, roots, teeth and tissues.

Flagella:

Mobility is universal among the spirillum bacteria, common among the bacillus bacteria but lacking or rare in coccal forms.

The motile bacteria have small whip or hair-like appendages called the flagella. They are about 120A⁰ – 150A⁰ thick and 4 0r 5 µ long. They are cytoplasmic in nature. Each flagellum arises within the cytoplasmic membrane and passes out through the cell wall.

Structure

Bacterial flagellum is composed of three parts – basal body, the hook and the filament.

The outermost, long, hair-like, cylindrical, hollow structure of the flagellum is called filament or shaft. It has a constant diameter of 120-200A⁰. It is made up of globular proteins, the flagellin.

The flagellum is composed of several chains of flagellin subunits forming a hollow cylindrical thread. Unlike the Eukaryotes, the filaments are not covered by a membrane or sheath.

Hook is short, curved, thicker segment of the flagellum. It is made up of a single type of protein. It connect the basal bodies to the filament of the flagellum. It passes out through the cell wall.

The basal body attaches the flagellum to the cell wall and plasma membrane. It is composed of a small central rod inserted into a series of rings. In Gram negative bacteria two pairs of rings, the proximal ring and the distal ring, are connected by a central rod.

The outer pair of rings i.e., L-ring (Lipopolysaccharide ring) and P- ring (Peptidoglycon ring) are attached to respective polysaccharide and  peptidoglycan layers of the cell wall. The inner pair of rings i.e., M-ring (Membrane ring) and S-ring (Super membrane ring) are attached with the cell membrane.

In Gram positive bacteria only inner pair of rings are present. The S ring is attached to the peptidoglycan layer and M-ring is attached to the cell membrane.

Flagellation:

The number, position and arrangement of flagella varies with the species. The flagella may be restricted to one or both ends of the bacterium cell (polar flagellation) or may be distributed uniformly all over the body surface (non-polar flagellation).

On the basis of number and arrangement of flagella, bacteria are classified into the following types:

Polar Flagellatoin:

This type of flagellation is found in bacilli and spirillum bacteria. They are all gram-negative. Polar flagellation are following types:

I. Monontrichous: bacteria with a single flagellum located at one end or pole. All Vibrios are monotrichous. Eg., V.cholerae

II. Amphitrichous: bacteria with one flagellum at each pole or end of the cell.

III. Cephalotrichous : two or more flagella at one pole of the bacterium cell. Eg., Pseudomonas fluorescenes. The flagellum shows more than two curves.

IV. Lophotrichous: Bacteria with two or more flagella at both the poles of the cell. Eg., Spirillum volutans.

Non-Polar Flagellation:

Flagella are found all over the cell surface. It is of one type only.

Peritrichous: bacterial cell with flagella evenly distributed all over the cell surface. Eg., Bacillus tylphosus, Proteus vulgaris.

Atrichous: all bacteria which lack flagella are called atrichous. Eg., Diptheria bacilli, Lactobacillus.

Pili:

Minute, straight, hair-like, non-flagellar appendages found on the cell wall of many gram negative bacteria. They are rigid and composed of a protein called pilin. They may be several microns long but are smaller in diameter around 30-50 A⁰.

There may be 100-400 pilli distributed over the surface of the bacterium cell.

The structures which are shorter than the pili are called the fimbriea.

Function:

Some pili are used for the attachment.

Pathogenic bacteria use them to identify and to attach their host cells.

F or sex pili are involved in the transfer of the DNA during bacterial conjugation.

Cell Wall:

Enveloping the protoplast of bacterial cell, there is a thin, firm cell wall. It gives rigidity and shape to bacterial cell.

The bacterial cell wall is composed of  4 layers. The outer most layer is L₄, it is wavy and electron dense. Within this is a lighter layer called the L₃. Next is the inner dense layer – L₂ made up of mucopeptide. The inner most layer is the L₁.

The most important component of bacterial cell wall is peptidoglycon or murein (mucopeptide or muramic acid, Gr. Murus=wall). This polymer is made up of chains of alternating molecule of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM). NAG and NAM are linked by 1, 4, β-glycosidic bonds.

The cell wall of Gram +ve bacteria (Clostridium, Bacillus) is thick, homogenous, mainly consists of peptidoglycon (90%). The remainder being made up of proteins, polysaccharides and teichoic acid. Lipid content is very less in Gram +ve bacteria. The cell wall of Gram +ve bacteria is more sensitive to pencillin.

The cell wall of Gram –ve bacteria is chemically more complex and heterogenous. Peptidoglycan comprises up to 10% of the cell wall. The outer layer that surrounds peptidoglycan is two layered and consists of phospholipids, proteins and lipopolysaccharide mixture. Gram –ve bacteria possess more lipid. Teichoic acid is absent. The cell wall is less sensitive to pencillin.

The protoplast:

Within the cell wall is the living liquid called the protoplast. The protoplast is a clear water or slightly viscous substance.

It is differentiated into the cell or cytoplasmic membrane, cytoplasm and chromatin or nuclear body.

Cell or Cytoplasmic or Plasma Membrane:

At the surface of protoplast is a differentiated, delicate, fine membrane known as cell membrane or cytoplasmic membrane. It is about 75A⁰ thick. It lies close to the cell wall. Chemically it consists of phospholilpids with proteins and polysaccharides.

It is a selective permeable membrane which controls the passage of dissolved substances in an out of the cell. The cytoplasmic membrane also contains enzymes involved in enzymes and synthesis of capsule and cell wall material. It is also the site of ATP synthesis and the centre for control of flagellar motility.

Mesosomes

The plasma membrane of bacteria, at certain places, forms infoldings to form vesicle or tubular structures called the mesosomes. The term mesosomes was coined by Fitzjames. They increase the surface area of the membrane. They have respiratory enzymes, but they are not analogous to mitochondria. During division, they play a role in the DNA replication, septa formation, cell wall synthesis and spore formation.

Cytoplasm:

The bacterial cytoplasm is a complex mixtures composed of 70-80% of water and proteins, carbohydrates, lipids, inorganic ions. Organic matter is in the colloidal state.

The cytoplasm shows no streaming movement and contain no vacuoles. Cytoplasm is dense and transparent.

Bacterial cytoplasm contains nucleoid, plasmid, and ribosomes. Other membrane bound cell organelles like mitochondria, Endoplasmic reticulum, Golgi bodies are absent. Although a well organised chloroplast is absent, the photosynthetic bacteria have chromatophores.

The non-living inclusions are the storage granules of volutin, glycogen, lipid globules, poly-β-hydroxybutyrate (PHB).

Ribosomes:

In electron micrographs the bacterial cytoplasm is granular and contains numerous, minute, nearly spherical, hollow bodies called the ribosomes. They are composed of rRNA and proteins. Their number varies from 10,000 to 15,000 in a cell, which account upto 30% of total dry weight of the cell.

Bacterial ribosomes are of 70S type and is made up of two sub-units – 50S and 30S. Ribosomes are functional only when the two subunits are together. The association and dissociation of two subunits of ribosomes depends on the concentration of Mg²⁺ ions.

The 50S subunit has one molecule of 23S rRNA (3200 nucleotides), one molecule of 5S rRNA (120 nucleotides) and 34 different proteins. 30s subunit contains one molecule of 16s rRNA (1600 nucleotides) and 23 different proteins.

The ribosomes lie free in the cytoplasm but sometimes they occur in small groups called polyribosomes held by a strand of messenger RNA.

The ribosomes are sites for protein synthesis. Several antibiotic like streptomycin, neomycin and tetracycline inhibit protein synthesis on the ribosomes.

Nucleoid:

Area in the bacterial cytoplasm where the genetic material or DNA is present is called as nucleoid or genophore. Usually it is centrally located in bacterial cells and is equivalent to the nucleus in eukaryotes. However, it lacks nuclear membrane and nucleoli.

The shape of the nucleoid varies, often it is irregular, sometimes spherical or oval but elongated. The nucleoid is composed of about 60% DNA, 30% RNA and 10% proteins.

The DNA is single, long, circular, double stranded approximately 5X10⁹ base pairs and molecular weight of about 3X10⁹ daltons. The nucleoid can be stained with Feulgen stains.

Plasmids:

In addition to the nucleoid, bacterial cytoplasm normally contains one or more molecules of circular, double stranded DNA. Small, circular, self replicating and double stranded DNA molecule present in bacterial cell, in addition to nucleoid are called as plasmids. Lederberg coined the term plasmid.

They replicate autonomously, and inherited by both the daughter cells during the cell division. Some plasmids get integrated  into the bacterial DNA or chromosome. They are called episomes by Jacob Schaeffer and Wollman.

The number of plasmids ranges from one to hundred or more per bacterial cell. A plasmid contains 5-100 genes that govern several biological functions. Plasmids are not essential for the growth and survival of the bacterial cell, but they confer special characteristics like drug resistance, toxigenecity to the bacteria.

On the basis of functions, plasmids are divided into several types:-

i. F-factor (fertility) plasmids,

ii. R-factor (Resistance) plasmids

iii. col-factor (calicinogeny) plasmids

iv. Virulance plasmids conferring pathogenecity to mammals

v. Ti- plasmids of Agrobacterium tumifacines

vi. Ri-plasmids (root inducing) if Agrobacterium rhizogenes