- Slides: 36
Morphology of Bacteria-Part 2 2 nd MBBS Ms. Mishra J. K. Asst Lecturer Dept of Microbiology MGM Aurangabad
Morphology of Bacteria-Part 2 Contents – Cytoplasmic membrane Nucleus Flagella Fimbraie Capsule Spore
Structure of Gram positive and Gram negative bacterial cell wall
Morphology of bacteria- Part 2 1. Cytoplasmic membrane: It is 5 -10 um thick elastic semipermeable layer which lies beneath cell wall seperating it from cytoplasm. Act as an semipermeable membrane controlling inflow & outflow of metabolites to & from the protoplasm. Site of numerous enzymes (permease, oxidase, polymerase) involved in the active transport of selective nutrients
2. Cytoplasm: Bacterial Cytoplasm is a colloidal system containing a variety of organic & inorganic solutes in a viscous watery solution. Lacks mitochondria & endoplasmic reticulum. Consists of ribosomes, mesosomes, vacuoles & inclusions.
§ i. Ribosomes: these are centres of protein synthesis, composed of ribosomal RNA (r RNA) & proteins. § Intracytoplasmic Inclusions: sources of stored energy & are § present in some species of bacteria. § They may be present as polymetaphosphate (volutin) lipids, polysaccharide (starch or glycogen) & granules of sulphur. §Special staining techniques like Alberts or Neisser's demonstrate them more clearly. §iii. Mesosomes (Chondroids): Are vesicular, multilaminated or c onvulated structures formed as invaginations of the plasma membrane into cytoplasm § Principle centres of respiratory enzymes.
Nucleus: §No nuclear memb or nucleolus. Cannot be seen by ordinary Microscope (only electron M. s) §DNA does not contain any basic protein. §DNA- is double stranded in form of a circle. §Measures 1 mm(1000 um)- straightened. §DNA is haploid-replicates-simple fission. .
Some-extranuclear genetic material in cytoplasm consisting of DNA-plasmids or episomes. Replicates autonomously. Not essential for life of bacteria. Besides being transmitted to daughter cells during binary fission they can be transmitted from 1 bacteria to another-conjugation, thru' bacteriophages. Confers-properties like drug(R) & toxigenicity thus survival advantage to bacteria.
Flagella: Cytoplasmic appendages protruding thru' cell wall. Thread like long (3 -20 um x 0. 01 um). Made up of Protein i. e flagellin. Organs of Locomotion- All motile bacteria (except spirochaetes) possess 1 or more flagella.
3 parts: 1. Filament, 2. Hook, 3. Basal body. Filament lies external to cell. Connected to hook at cell surface. Hook basal body-embeded in cell envelope. Basal body has-outer & inner rings- by which it is attached to cell membrane. Outer rings absent in gram positive
Structure of Flagellum
Number and arrangement characteristic of different bacteria Perithrichous- flagella arranged all round the cell e. g. Salmonella typhi. Or Polar i. e situated at 1 or both ends of the cell. Polar flagella may be either Amphitrichous-Single flagellum at both the ends e. g. Alcaligenes faecalis. Monotrichous- Single polar flagellum (at one end) e. g. Vibrio cholerae. Lophotrichous- Tufts of flagella at one or both ends e. g. spirilla.
Demonstration – As flagella are less than 0. 02 um in thickness they cannot be seen with light microscope. Seen by. Electron MS or DGI or Special stains which increase thickness(mordanting), indirect method-to show presence of flagella by motility(hanging drop preparation
Arrangement of flagella
Fimbriae or Pili Some gram negative bacteria carry very fine hair like surface appendages called fimbriae or pili. They are shorter, thinner than flagella(0. 5 um long & less than 10 nm thick)& project from cell surface as straight filaments.
8 morphologigal types of pili are known classifiable as either common or sex pili on the basis of their function. They can be seen only under electron microscope. They are unrelated to motility and are found on motile as well as non motile cells.
Functions 1. Adhesion- fimbriae are organs of adhesion. This property enhances the virulence of bacteria. 2. Transfer of genetic material- Sex pili are present in male bacteria. These pili are longer (18 -20 um) and 1 -4 in number. They help the male cells to attach with non-male (female) cells in forming “conjugation tubes” through which genetic material is believed to be transferred from the donor (male) to the recipient (female) cell.
Detection of fimbriae 1. Electron microscopy 2. Haemagglutination- Many fimbriated bacteria (e. g. E. coli) strongly agglutinate RBCs of guinea pigs, fowl, horses & pig etc.
Bacterial Capsule & Slime Layer: It is the amorphous viscid bacterial secretion which surrounds some bacteria as their outer most layer Slime layer: when this secretion diffuses into the surrounding medium and remains as a loose undemoarcated secretion it is known as slime layer. e. g. Leuconstoc. .
Capsule: §When this secretion is organised into a sharply defined structure it is known-capsule e. g. in Streptococcus pneumoniae (pneumococci) Both slime layer & capsule present in streptococcussalivarius. Klebsiella secretes large amount of slime and produces mucoid growth on agar.
Functions: 1. Capsule enhances bacterial virulence by inhibiting phagocytosis. 2. Acts as a protective covering against antibacterial substances such as bacteriophages, phagocytes, enzymes. 3. Capsular antigen is specific for bacteria and can be used for identification & typing of bacteria.
Methods for demonstration of capsule: 1. India Ink staining (Negative staining)- Capsule appears as a clear halo around bacterium as the ink cannot penetrate the capsule. 2. Serological methods- Capsule swelling reaction or Quellung or Neufeld 3. Special Capsule Staining- Copper salts are used as mordants for staining capsule.
Pneumococci stained with India ink to show capsule
Bacterial Spore: Spores are highly resistant resting stage formed in unfavorable environmental contitions (depletion of nutrients). e. g. Bacillus & Clostrium Sporulation is not method of reproduction. Each bacterium- 1 spore-germination (favorable conditions)form single vegetative cell. Endospores (form within parent cell.
Morphology of spore: §cell membrane grows in wards & forms spore wall around the core (fore spore). Inner most layer of spore wall forms spore membrane from which C. W. Of future veg cell develops. Outside spore membrane is thick layer cortex and a multi layered tough spore coat. Some spores additional loose outer covering Exosporium
Sporulation is initiated by the appearance of a clear area, usually near one end of the cell, which gradually becomes more opaque to form the forespore The fully develop spore has at its core the nuclear body surrounded by the spore wall, a delicate membrane from which the cell wall of the future vegetative bacterium will develop. Outside this is the thick spore cortex which in term is enclosed by multilayer spore coat. Some spores may have outer covering called exosporium.
The fully develop spore has at its core the nuclear body surrounded by the spore wall, a delicate membrane from which the cell wall of the future vegetative bacterium will develop. Outside this is the thick spore cortex which in term is enclosed by multilayer spore coat. Some spores may have outer covering called exosporium
Structure of bacterial spore
Young spores are seen attached to the parent cell. Shape & position of the spore and its size relative to the parent cell are species characteristics. Shape & Position: Bulging/Nonbulging Terminal, subterminal, central-position Oval or spherical-shape. E. g. terminal spherical- Clostridium tetani
Resistance: R- boiling, disinfectants, drying. But most pathogenic spores- autoclaving 121*c-15 min (destroyed) Demonstration: 1. gram stain-spores seen as unstained refractile body within cell. 2. Modified ZN stain-(0. 25 -0. 5% sulphuric acid as decolourizer)-appear pink/red (Acid fast) Uses- Some spores e. g. B. stearo thermophilusindicators for proper sterilization
Types of bacterial spores
Pleomorphism and involution forms Some species of bacteria exhibit great variation in the shape & size of individual cells. This is known as pleomorphism. Certain species (for example, plague bacillus, gonococcus. Pleomorphism and involution form are often caused by defective cell wall synthesis).
Kleineberger-Nobel, studying cultures of streptobacillus moniliformis, observed swollen cells and other aberrant morphological forms and named them L forms, after Lister Institute, London, where the observation was made. L forms are seen in several species of bacteria, developing either spontaneously or in the presence of penicillin or other agents that interfere with the cell wall synthesis.
L forms may be unstable in that the morphological abnormality is maintained only in the presence of penicillin or other inducing agents, or stable, when the aberrant form becomes the permanent feature of the strain and is retained in serial subcultures. L forms resemble mycoplasma in several ways, including morphology , type of growth on agar and filterability. It is possible that mycoplasmas represent stable L forms of as yet unidentified parent bacteria.