Microbiology: Understanding Prokaryote Size, Shapes, and Bacterial Morphologies - Prof. K., Study notes of Biology

Download Microbiology: Understanding Prokaryote Size, Shapes, and Bacterial Morphologies - Prof. K. and more Study notes Biology in PDF only on Docsity! In library on first floor at reserve desk: MICROBIOLOGY: An Evolving Science- call number is RX52 Circulates 2hrs BUILDING USE ONLY Chapter 2: Observing the Microbial Cell SIZE OF PROKARYOTES  Prokaryotes are generally smaller than eukaryotes  Not always true  Small size allows nutrients to reach all parts of the cell quickly  This is important bc prokaryotes do not have internal membrane structures  Prokaryotic cells can be as small as 0.2μm. The smallest eukaryotic cells are 2μm.  Cocci- typically have diameter of 2 µm  Spherical shape  Some microbiologists have proposed that bacteria smaller than 0.2 μm exist in nature, cells referred to as nanobacteria  If one considers the space needed to house all essential molecules of life, it is unlikely they could exist within a volume available in a cell less than 0.1 μm.  Prokaryotic cells can have a wide variety of cellular morphologies, which are often helpful in identification.  Spherical (cocci)  Rod Shaped (Bacilli)  Sprial Bacterial Shapes 1. Simplest shape is sphere o singular- coccus o plural- cocci  Arrangement: o Coccus- single cells o Diplococcus- 2 cells o Streptococcus- chains of more than 2 cells  Skin and throat infections o Staphylococcus- grapelike cluster of cells o Tetrads- packets of 4 cells 2. Most common shape is rods o Singular- bacillus o Plural- bacilli  Can be very long & thin or short & fat  Occur singly or in chains  Can’t have staphylobacillus because of the way it divides 3. Spirals:  Rigid spiral-shaped bacteria are called spirilla (singular-spirillum)  Flexible spiral-shaped bacteria are called spirochetes  Vibrio – curved rods  Comma shaped Microscopy  Simple microscopes o one lens  If space between specimen and objective is replaced with immersion oil, resolution increases  Oil keeps light from scattering as much as in air  If no immersion oil, light bends  Need to fill in air space 4. Increase contrast - Contrast between bacteria & background is small so bacteria are very hard to see - Increase contrast by staining cells or using special type of microscope - Many stains are basic dyes  Chromophore (pigmented portion) has a positive (+) charge  Cell surface has a negative (-) charge  Examples- crystal violet, methylene blue, safranin  Reason it stains is charges  Simple Stains o Use one dye o Doesn’t differentiate between types of cells or structures • Can tell shape, arrangement, size  Differential stains o Distinguish between different types of bacteria or different structures o Can see different colors o Uses more than one dye/reagent o Gram stain distinguishes Gram positive bacteria from Gram negative bacteria  Stain different colors bc cell wall structure o Acid-fast stain distinguishes Mycobacterium species from other bacteria  Can be diagnostic test, takes 10 min Types of Microscopes Used To Increase Contrast o Microscope we typically use is called bright-field light microscope o See stained cells on bright (white) background **Figure 2.22** o Instead of staining cells, special microscopes can be used to increase contrast between cells & medium  Phase-Contrast Microscopy o Cells slow the speed of light passing through them so cells differ in refractive index from their surroundings o Cells change/bend light as it passes through them o This difference in “phase” is amplified by special ring in lens of phase contrast microscope o Can observe living cells in wet mount o Liquid culture on slide  Not killing or staining it o Can see organelles/internal structures of eukaryotes  Dark-Field Microscopy o Cells appear bright against dark background o Dark-field stop in condenser doesn’t let light pass directly through the specimen o Light reaches specimen from o Only light scattered by sample reaches objective o Very good for narrow objects such as viewing a flagellum or seeing spirochete o Can observe living cells in wet mount o Excellent for studying bacterial motility and very narrow cells o See them moving and narrow structures o Problem- dust particles also scatter light o Hard to tell if bacterial cells or dust  Differential Interference Contrast (DIC) Microscopy o Polarized light passes through specimen  Light ray passes through filter o Sample boundaries bend light o Second polarized lens blocks light o Bent light result in 3D appearance  Fluorescence Microscopy o Some compounds absorb energy from invisible radiation (like short wavelength UV light) then radiate energy back as longer visible wavelength  Fluorophores absorb high-energy light (short wavelength) then emit lower- energy light (longer wavelength)  Can use dyes to stain cell for fluorescence o Used to view cells that make fluorescent compounds such as chlorophyll or cells that have been stained with fluorescent dye o These microscopes use a UV light source to fluoresce objects o Can label molecules of interest in cell  Electron Microscopy (very large) o Resolving power of light microscopes is limited by wavelength of light used o Electron microscopes use beams of electrons instead of visible light and electromagnets instead of lenses o This achieves a much higher resolution o Can magnify over 100,000x o Can be used to view viruses, proteins, nucleic acids o Sample must reflect electrons o Coated with heavy metal o Electron beam and sample are in a vacuum  So air will not interfere  Sample killed http://www.youtube.com/watch?v=fToTFjwUc5M  Transmission electron microscope (TEM) o Sample is sliced very thin  Microtome slices the cells very thinly