The Study of Microbial Structure: Microscopy and Specimen Preparation Contd...

Types  of light microscopes(contd....)

3. Phase Contrast Microscope
     It is useful for examining intra-cellular component of living cells at relatively high resolution. It is used to observe unstained living microorganisms.  It is based on principle that cells slow the speed of light passing  via them thus differ in the refractive index from their surroundings. This result in a difference in phase  between cells & its's surroundings.This subtle difference is a  amplified by a special ring called phase plate in the objective lens which leads in the formation of dark image on light background.  




















 

 Examples of Dark-Field and Phase-Contrast
Microscopy.  Treponema pallidum, the spirochete that causes
syphilis; dark-field microscopy ( 500).  Volvox and Spirogyra;
dark-field microscopy ( 175). Note daughter colonies within the
mature Volvox colony (center) and the spiral chloroplasts of Spirogyra
(left and right).  Spirillum volutans, a very large bacterium with
flagellar bundles; phase-contrast microscopy ( 210).  Clostridium
botulinum, the bacterium responsible for botulism, with subterminal
oval endospores; phase-contrast microscopy ( 600). Paramecium
stained to show a large central macronucleus with a small spherical
micronucleus at its side; phase-contrast microscopy ( 100).

Comments

Post a Comment

Popular posts from this blog

Stains/ Dyes

STAINS/ DYES      They are colored organic compounds used for staining microorganisms. Chemically, Stains= Benzene ring+ Chromophore+ Auxochrome According to nature of stain, it can be classified into: 1. Acidic Dyes:   It is dye which has negative charge so they bind to positively charged cell structures like some proteins. Acidic dyes are not very often used in Microbiology lab.except to provide background staining like Capsule staining. Examples: Nigrosine, Picric acid, Eosin, Acid fuschin, India ink etc.  2. Basic Dyes:  This dye have positive charge & bind to negatively charged molecules(nucleic acid, -COOH -OH). Since, surface  of bacterial cells are negatively charged(due to Teichoic acid), basic dyes are most commonly used in bacteriology. Examples: Crystal Violet, Methylene Blue, Safranin , basic fuschin. 3. Neutral Dyes:  They are usually formed from precipitation in which are produced when aqueous acidic & basic stains are combined. Neutral dyes stains nuc
Read more

CLINICAL MICROBIOLOGY

Image
3)   Triple Sugar Iron(TSI) Agar Test:                         The TSI agar test  is used to differentiate among the members of the family Enterobacteriaceae and between Enterobacteriaceae and other intestinal bacteria. The authors have chosen three common bacteria in the family Enterobacteriaceae: Escherichia coli, Proteus vulgaris, and Shigella flexneri .  All three are facultatively anaerobic gram-negative rods. In a TSI tube, E. coli produces an acid butt, an acid or alkaline slant, and no H2S, but does produce gas. P. vulgaris produces an acid butt, an acid or alkaline slant, H2S, and gas. S. flexneri produces an acid butt, an alkaline slant, no H2S, and no gas. For the other intestinal bacteria, the authors have chosen Alcaligenes faecalis and Pseudomonas aeruginosa. Both of these intestinal bacteria are gram-negative aerobic rods. In a TSI tube, A. faecalis produces an alkaline butt, alkaline slant, H2S, and gas; P. aeruginosa, an acid butt, alkaline slant, H2S,
Read more

Fixation

Fixation    The stained cells in a microscope should resemble living cells as closely as possible. Fixation is process by which the internal & external structures of cells & microorganisms are preserved & fixed.  It inactivates enzymes that might disrupt cell morphology & toughens cell structures so that they don't change during staining & observation. A microorganism is usually  killed & firmly attached  to microscopic slide during fixation.        There are two fundamentally different types of fixation.  Bacteriologists heat fix bacterial smear by gentle  & an air dried film of bacteria. This adequately overall morphology  but not structures within cells. Chemicals fixation must be used to protect  fine cellular structures & the morphology of larger, more delicate microorganisms. Chemical fixatives penetrate cells & react with cellular  components, usually proteins,lipids to render them inactive, insoluble & immobile.  Common fixatives  
Read more