Bacteria BSc 1st semester

BACTERIA 

Bacteria- Bacteria are microscopic and single - celled living organisms. Bacteria is discovered by Antony van Leeuwenhoek from rain water. they are omni present. It placed under Protista kingdom.

General Characteristics of Bacteria -

• Bacteria are prokaryotic (primitive nucleus), nucleus not surrounded by nuclear membrane e.g., incipient nucleus or nucleoid.

• Due to the presence of a Rigid cell wall bacteria maintain a definite shape through they varies as shape, size and structure.

• In general bacteria are between 0.2 and 2.0 micrometer - the average size of most bacteria.

• One group of bacteria called the mycoplasma, the measure about 0.25 micron and are the smallest cell known so far, they were formally known as PPLO (Pleuropneumonia like organisms).

• They are omnipresent, found in all possible habitats one can think of.

• Most of the bacteria have heterotrophic mode of nutrition, i.e., they obtain their readymade food directly from external agency, and may be parasitic, saprophytic Or symbiotic. Some bacteria are autotrophic; they possess bacteriochlorophyll—a photosynthetic Pigment.

• The organelles like endoplasmic reticulum and Golgi apparatus are absent.

• Binary fission is the most common method of multiplication.

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

• The motile bacteria may possess one or more flagella, composed of eight parallel chains of flagellin (a protein) molecules.

Classification of Bacteria- 'Bergey's Manual of Determinative Bacteriology' is the standard reference book which deals with the classification and identification of bacteria.

The classification of bacteria given in the first edition of the Manual, published in 1923, was mainly based on phenotypic characterization.

Bacteria have been distinguished from other micro-organisms on the basis of their typical prokaryotic structure and they have been placed in a separate kingdom Procaryote. Further treatment of this kingdom was based on cellular characteristics rather than organismal properties. 

Procaryotes were divided in two divisions—Cyanobacteria and Bacteria.

Bacteria were further subdivided into 19 groups, distinguishable by some readily determined criteria.

 Kingdom Procaryote 

Division I. Cyanobacteria 

• They are photosynthetic procaryotes and their photosynthetic process is similar to that higher plants.
• the photosynthetic pigments are chlorophyll and phycobiliproteins. 
• these bacteria usually occur as simple or branched chain of cells. 
• they reproduce by binary fission, spores or fragmentation. 
• example of cyanobacteria is Anabaena, Nostoc etc.

Division II. Bacteria

• They are unicellular or occasionally show simple arrangements. 
• They are characterized by the presence of a rigid cell wall of peptidoglycans.
• Their photosynthetic process is anaerobic and their photosynthetic pigments are bacteriochlorophylls. 

Distribution of Bacteria- 

• Bacteria are cosmopolitan in distribution, occurring in all habitats. 

• Their wide distribution is primarily due to the fact that they can withstand great extremes of temperature, moisture, acidity and salinity and are adapted to a wide variety of energy sources.

• They occur in the atmosphere to a height of about six kilometers and on the sea floor five kilometers below the mean sea level Some are adapted to a normal aerobic environment and others thrive well in the absence of oxygen (i.e., anaerobic).

• Many species of bacteria are free living, and others are symbiotic parasitic or saprophytic

Size of bacteria -

• Bacteria are very small microscopic organisms. 
• An average bacterial cell ranges from 0.5 (l um =0.001 mm) to 2.0 μum in diameter.
• Their size also varies with the shape.

Shape of bacteria - The bacterial cells show considerable variation in their shape, but all individuals of a species have approximately the same shape. Thus from the point of view of their identification, the shape of bacteria is an important characteristic. 

Three cell forms, characteristic of true bacteria, are described below:

1. Bacillus or Rod shape Bacteria

• This is probably the most common form of bacteria. These are rod-shaped, cylindrical or elongate and are motile or non-motile cells.
•   Examples—Bacillus anthracis, B. fastidious, B. polymyxa, Lactobacillus.

➡The bacillus bacteria are classified into two groups on the basis of their arrangement.

• Diplobacillus- When bacillus bacteria occur in pairs, they are called diplobacillus. e.g., Coryne-bacterium diphtheriae.
• Streptobacillus- The bacilli bacteria of this group occur in long Chains. e.g., Bacillus tuberculosis, B. cereus.

• 

• Coccus or Spherical Bacteria- The spherical or ellipsoidal bacteria are called cocci (singular coccus) They measure 0.5-2.25 um in diameter. They are non-motile.

→The cocci are classified into the following six groups.

• Micrococci- when cocci occurs singly, they are known as micrococci; e.g., Micrococcus cerolyticus, M. cyrophilus, M. luteus.

• Diplococci- When cocci occur in pairs, they are called diplococci.

  e.g., Diplococcus pneumoniae.

• Streptococci- Spherical  bacteria when occur in long chains, they are called streptococci e.g., Streptococcus lactis, S. pyogenes.

• Tetracocci-  When they form groups of four cells, they are said to be Tetracocci.

  E.g., Pedicoccus cerevisiae, Neisseria.

• Staphylococci-  An irregular group of many spherical bacteria is known as staphylococcus; e.g., Staphylococcus albus, S. aureus.

• Sarcinae- When spherical bacteria divide in three planes in a regular pattern producing a cuboidal arrangement of cells, they are said to be sarcinae. e.g., Sarcinae lutea, S. verticuli.

 

• Vibrio and spirillum shape of bacteria- vibrio commonly known as comma bacteria because of its shapes. 

Structure of Bacterial cell- The conventional light microscope provides only a views notion of the internal structure of the bacterial cell. Therefore the cytological details of these organisms could be possible only after the discovery of electron microscope and development of staining techniques.

[A] Slime layer- It is a gelatinous layer present on the outer surface of the cell wall, usually composed of polysaccharides and /or polypeptide chains of amino acids. when its constituents are only polysaccharides, which form a viscous layer, it is said to be slime layer. it is a protecting layer.

[B] Cell wall- the cell wall is characteristics of plant kingdom. in higher plants and algae, it is composed of cellulose and shows a fibrillar structure. In contrast, the bacterial cell wall has a granular structure but is tough and rigid. the three main constituents of cell wall are :

• N-acetylglucosamine (NAG)
• N-acetyl muramic acid (NAM)
• A peptide chain of four or five amin acids.

these together form a polymer called peptidoglycan or mucopeptide.

The function of bacterial cell wall appears to be wholly mechanical, giving the cell its shape and rigidity.

 [C] Cytoplasmic Membrane- Inner to cell wall, a semipermeable cytoplasmic membrane is present, which is about 75Å thick. it is composed of a double layer of phospholipid molecules. 

Phospholipids are of two types- Hydrophobic and hydrophilic. the hydrophilic phospholipid molecules are present towards the inner side. 

cytoplasmic membrane is the bounding layer of the cytoplasm and its center of multifarious activities of the cell.   

Difference between Gram-positive and Gram-negative bacteria   

Gram-positive bacteria	Gram-Negative bacteria
Cell wall single layered. Cell wall is very rigid due to high proportions of peptidoglycans. It does not contain lipid. Relatively complex in many species. Ex- Bacillus subtilis	Cell wall triple layered. Cell wall is elastic due to the plastic nature of lipoprotein and polysaccharide. It contains lipids. Relatively simple. Ex- Rhizobium, E coli

 Flagellation in Bacteria or Locomotion in Bacteria 

Some bacteria are self motile. they are swim through the liquid in which they live. they can't crawl over dry surface or fly through the air. 

The organ of the locomotion is small whips or hair like appearance called flagella.

Flagellation - the flagella are distributed over the surface of the bacterial cell in a characteristic manner. their number, position and arrangement varies with the species. on the basis of the flagellation and arrangement the bacterial cell can be classified as:-     

[A] Atrichous - These bacteria possess flagella, hence they are motile. the flagella are distributed over the surface of bacteria in a characteristics fashion. 

• Polar flagellation - It is usually found in Gram-negative bacilli and spirilla. the following four types of polar flagellations are recognized. 

1. Monotrichous- Only one flagella present at one end. e.g., vibrio cholerae 
2. Amphitrichous- Only one flagella present at the both side of end. e.g., Spirillum.
3. Cephalotrichous- Many flagella present present at the both end. e.g., Pseudomonas fluorescens.
4. Lophotrichous- Many flagella is present at one end. e.g., Spirillum volutans. 

• Non-Polar flagellation- In non-polar or peritrichous flagellation the flagella are evenly distributed throughout the surface of the cell. e.g., Proteus vulgaris, Bacillus typhosus, Salmonella and Clostridium. 

Nutrition in Bacteria 

On the basis of their mode of nutrition two groups of bacteria recognized are- Autotrophic bacteria and heterotrophic bacteria. 

[A] Autotrophic Bacteria - Those bacteria which are capable for synthesizing their food by themselves from organic and inorganic substances are called autotrophic. The autotrophs can be divided further into two subgroups:  phototrophs, and chemotrophs, on the basis of energy source they use. The division may also be based on the nature of substances oxidised; lithotrophs utilizing inorganic substances and organotrophs oxidizing organic matter for their energy.

• Photosynthetic bacteria - These are also known as photoautotrophic or photolithotrophic bacteria. Like higher plants, they are capable of converting radiant energy into chemical energy. In photosynthetic bacteria photosynthetic pigments like bacteriochlorophyll and chlorobium chlorophyll are present. During this no oxygen released so its called anoxygenic photosynthesis.   

• Chemosynthetic bacteria- Photosynthesizing bacteria represent only a small fraction of Schizophyta, but chemosynthetic or chemotrophic bacteria are most abundant and are important geochemical agents. These are non photosynthetic but autotrophic bacteria. They derive energy from ammonia, nitrate, nitrite, ferrous, iron, hydrogen sulphide and other inorganic compounds for the synthesis of their food.

[B] Heterotrophic Bacteria- These bacteria obtain their readymade food from any organic source. They can be distinguished into three major nutritional categories.

• Parasitic bacteria- Those bacteria which feed on living organisms are known as parasitic bacteria. The organism from which parasitic bacteria obtain their food is known as host. Some parasitic bacteria are known to cause diseases in plants (e.g., citrus canker)and animals (e.g., pneumonia, typhoid), and such bacteria are called pathogenic bacteria.

• Saprophytic bacteria- These bacteria grow on dead and decaying organic matter. They obtain their food by decomposing the complex organic molecules into simple inorganic constituents. The decomposition of carbohydrates and proteins by saprophytic bacteria is technically known as fermentation and putrifaction respectively.

• Symbiotic bacteria- Those bacteria, which grow in close (beneficial)association with other living organisms, are known as symbiotic bacteria. In terms of their association with the host, symbiotic bacteria may be ectosymbionts (when bacteria live on the surface of the host) or endosymbionts (when bacteria live inside the host tissue). The bacteria inhabiting the intestines of man and animals are good examples of symbiotic bacteria. The enzymes secreted by these bacteria are helpful in the digestion of cellulose and in return they obtain their food from the host. Similarly, root nodule bacteria (Rhizobium), present in the roots of leguminous plants, fix atmospheric nitrogen to augment their nitrogen supply and in return the plants provide them shelter and carbohydrates.

REPRODUCTION

Bacteria reproduce mainly by asexual method and therefore they have a dominant haploid phase in their life-cycle. They do not have sex organs or gametes but they definitely ’show genetic recombination, i.e., exchange of genetic material.

1. ASEXUAL REPRODUCTION- Asexual reproduction in bacteria takes place by the following methods.

• Binary Fission- It is the simplest and most common method of multiplication in bacteria. Under favorable conditions.

• Budding- In this type of multiplication the bacterial cell gives out many outgrowths. These outgrowths are filled with cytoplasm and other cell organelles also move into them. These outgrowths, called buds eventually get detached from the parent cell by a constriction and each grows into a new bacterial cell.

• Conidia- Many filamentous bacteria (e.g., Streptomyces)form chains of small, spherical spore-like conidia at the tips of the filaments. A conidium develops by the formation of a transverse wall at the tip of the filament. The filaments bearing conidia are known as conidiophores. After liberation each conidium gives rise to a new filamentous bacterium, provided conditions for germination are favourable.

GENETIC RECOMBINATION

(SEXUAL REPRODUCTION)

Sexuality in bacteria was first demonstrated by Tatum and Lederberg (1947) in Escherichia coli. In bacteria sexual reproduction does not involve fusion of gametes as in other organisms but each heritable character is determined by a portion of its genetic material (DNA) as in other organisms.

 

• Conjugation - Bacterial conjugation is a process of horizontal gene transfer where a bacterial cell (donor) directly transfers genetic material, typically a plasmid or part of its chromosome, to another bacterial cell (recipient) through direct cell-to-cell contact facilitated by a sex pilus. This process, often called "bacterial sex," enables bacteria to acquire new traits, such as antibiotic resistance, and is a key mechanism for the spread of genetic information.