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HISTORY
A V Leeuwenhoek in 1676 first observed bacteria and called 'animalcules'.
Ehrenberg in 1838 coined the term bacteria (singular Bacterium).
Louis Pasteur is known as father of Bacteriology and together with F. Cohn and Robert Koch, is regarded as one of the three main founders of bacteriology.
Robert Koch in 1890 demonstrated the role of bacteria as pathogens and given Koch’s Postulates. Robert Koch’s postulates are the four criteria designed to assess whether a microorganism causes a disease. The four criteria are
1) The microorganism must be found in diseased individual but not in healthy ones.
2) The microorganism must be cultured from the diseased individual.
3) Inoculation of a healthy individual with the cultured microorganism must recapitulate the disease.
4) The microorganism must be reisolated from the inoculated diseased individual and matched to the original microorganism.
INTRODUCTION
Bacteria may be defined as microscopic, unicellular, prokaryotic, and least complex living organisms (Fig. 1A).
Membrane bound cell organelle such as mitochondria, endoplasmic reticulum, golgi apparatus, plastids are absent in bacteria.
Bacteria also lack nucleus and nucleolus and contain nucleoid (naked circular DNA without histone) as a genetic material.
An average bacteria cell ranges from of 0.5–6.0 µm in length.
Bergeys’s manual of determinative bacteriology first published in 1923 by David H Bergey deals with the classification and identification of bacteria. The 9th edition of the manual, published in 1994.
Bacteria possess several forms and shapes and generally are of 4 different types
Coccus (Round shaped).
Bacillus (Rod shaped).
Vibrio (Comma shaped).
Spirillum (Spiral-shaped).
There are different ways of grouping bacteria.
On the basis of gaseous oxygen, they may be aerobic bacteria, anaerobic bacteria and facultative anaerobes.
On the basis of mode of nutrition, they may be heterotrophic (Bacteria) and autotrophic (Cyanobacteria)
BACTERIAL CELL STRUCTURE AND FUNCTION
A bacterial cell is prokaryotic in nature and shows following structures (Fig 2).
1. Glycocalyx
2. Flagella
3. Pili
4. Cell wall
5. Cell membrane
6. Cytoplasm
7. Nucleoid
8. Plasmid
9. Ribosomes
10. Mesosomes
1. Glycocalyx (Capsule & slime layer)
Glycocalyx is a surface coating or additional layer outside cell of bacteria. In general, this additional layer can come in one of two forms:
Capsule: The glycocalyx is considered a capsule when it is more firmly attached to the cell wall. It is composed of polysaccharides and nitrogenous substances (amino acids)
Slime layer: A glycocalyx is considered a slime layer when it is loosely associated with the cell wall. It is usually composed of polysaccharides.
Glycocalyx generally composed of 98% water and 2% polysaccharide or glycoprotein/ polypeptide or both. It is approximately 0.2 µm thick viscous layer outside the cell wall.
Capsule protects the cell from phagocytosis by larger microorganisms. It gives sticky character and enables bacteria to adhere to their substrate and protect against dehydration and antibodies.
Capsule is most important virulence factor of bacteria. It has been found that non-capsulated mutants of E. coli and S. pneumoniae are avirulent.
Capsulated bacteria are usually atrichous (non-flagellated).
2. Flagella
Flagella (singular, flagellum) are long hair like thin structure. Each flagellum is about 3 to 12 µm long and is helical.
A flagellum is composed of flagellin protein (globular) called H antigen and has 9+2 fibrillar structure.
A flagellum consists of three parts (Fig 3).
The long filament outside cell wall.
Hook at the end of the filament.
Basal body (blepharoplast) which imparts motion to the flagellum.
The main function of flagella is to provide motility to the bacteria (towards or away from stimulus) and it is brought by clockwise or anticlockwise spins of flagellum around its own axis.
Flagella can be found at either end or both ends of a bacterium or all over its surface.
Sometimes in some species flagella is absent and called as Atrichous.
The number, position and arrangement of flagella vary in different species and are of following types (Fig 4).
Monotrichous: A cell with single flagellum at one end only e.g., Vibrio cholerae, Pseudomonas
Amphitrichous: A cell with a single flagellum at each end. e.g., Nitrosomonas
Cephalotrichous: A cell with tuft of flagella at one end only e.g., Pseudomonas fluorescens
Lophotrichous: A cell with a cluster of flagella at one or both ends. e.g., Spirillum volutans
Peritrichous: A cell with flagella all over the surface. e.g., Salmonella, Clostridium
3. Pili or Fimbriae
Many species of bacteria have small hair like projections called pili (singular, pilus) and are non- helical structure, emerge outside cell surface.
Pili are numerous and shorter than flagella and made of special type of protein called pilin.
They assist the bacteria in attaching to other cells and surfaces. It also acts as receptor for bacteriophage.
In addition to normal pili, there are some specialized pili e.g., sex pili are used in conjugation for exchange of plasmid DNA
4. Cell Wall
Each bacterium is enclosed by a rigid cell wall, composed of peptidoglycan and a protein-sugar (polysaccharide) molecule.
Cell wall gives shape to the organism and surrounds the cytoplasmic membrane protecting it from the environment as well as from bursting and collapsing due to osmotic change.
Hans Christian Gram in 1884, uses a staining and washing technique to differentiate two major group of bacteria i.e., Gram positive and Gram-negative bacteria.
The performance of the gram stain on any sample requires four basic steps that include applying a primary stain (crystal violet) to a heat-fixed smear, followed by the addition of a mordant (Gram’s Iodine), rapid decolorization with alcohol, acetone, or a mixture of alcohol and lastly, counterstaining with safranin.
When exposed to a gram stain, gram positive bacteria retain the purple color of the stain because the structure of their cell walls traps the dye.
In gram negative bacteria, the cell wall is thin and releases the dye readily when washed with an alcohol or acetone solution.
Gram negative cell wall is made of thin peptidoglycan (murein), outer membrane and lipopolysaccharides (Fig 5A) while gram positive bacterial cell wall mainly consist of thick peptidoglycan (murein) and teichoic acid (Fig. 5B).
Peptidoglycans also known as murein consist of glycan backbone formed by repeated unit of N acetyl glucosamine and N acetyl muramic acid. It is the component of both but gram positive have thick layer.
Teichoic Acids: Teichoic acids is water soluble polymer of glycerol or ribitol phosphate bearing a strong negative charge. It is the major surface antigen of gram-positive bacteria.
Outer membrane is the additional layer present in gram negative bacteria and is composed of lipid and protein.
Lipopolysaccharides is complex molecule consisting of a lipid, a polysaccharide core, and chains of carbohydrates present in gram negative bacteria.
Bacteria devoid of cell wall are called as Mycoplasma species.
5. Cell Membrane
Cell membrane lies inner to the cell wall and encloses the cytoplasm.
It is composed of phospholipids bilayer with a mosaic of embedded proteins.
Prokaryotic cell membrane is characterized by the absence of sterols.
It is about 80 nm thick and selectively permeable.
It performs many functions, including transport, biosynthesis, and metabolic reactions.
6. Cytoplasm
The bacterial cytoplasm is gel-like matrix composed of water (70-80%), carbohydrates, enzymes, nutrients, mineral salts, wastes and gases.
Cytoplasm stores organic material in the form of glycogen and volutin.
It contains large number of ribosomes, a chromosome and plasmids.
It serves as solvent for materials used in cell functions.
7. Nucleoid
Nucleoid (Genophore) is not a membrane bound but simply an area of the cytoplasm where the strands of DNA are found.
The DNA in bacteria is single, circular (Fig. 6), double stranded without histone proteins and measures bout 1000 µm in length and usually referred as bacterial chromosome.
It controls all activities inside the cell.
8. Plasmid
In addition to main chromosome many bacteria contain extra chromosomal DNA called plasmids.
The term plasmid was first time coined by Lederberg and found in the cytoplasm.
Plasmids replicate independently of the bacterial chromosome while not essential for survival. They appear to give bacteria a selective advantage.
It contains important gene like fertility factor (F factor) and resistant factor (R factor).
Plasmid have wide application in genetic engineering.
Episomes are plasmids that are able to exist freely in the cytoplasm or can be integrated into main bacterial chromosome.
9. Ribosomes
Ribosomes are rounded granules found freely floating in the cytoplasm.
It is made of RNA (60%) and proteins (40%)
Bacterial ribosome is of 70s type and consists of two subunits i.e., 50 S and 30 S (Fig 7).
Their number varies from 10000-15000. They occur singly or in small groups called polyribosomes.
It is universal cell organelle and helps in protein synthesis.
10. Mesosomes
Mesosomes are round, sac like tubular membrane structures (Fig 8) which are formed by an invagination of the plasma membrane.
These structures are much more prominent in gram-positive than in gram negative bacteria.
Earlier they are thought to be equivalent to bacterial mitochondria but at present, there is no satisfactory evidence as enzymes like ATPase, dehydrogenase are absent.
Some evidence indicates its role in septa formation during cell division and replication of DNA.