Observing Microorganisms

Introduction to Microscopy

Microscopy is the study and use of microscopes to observe the vast realm of life known as microorganisms, which are invisible to the unaided human eye. The development of superior lenses and microscopes has enabled scientists to discover microbes in diverse environments, from the human body to extreme habitats.

• Antonie van Leeuwenhoek was the first to observe microbes using a simple microscope, revealing tiny life forms on his teeth.

• Modern microscopes allow the search for microbes in familiar and unexpected habitats, such as the human stomach, deep sea, and peat bogs.

Example: Transmission electron microscopy can reveal internal structures of bacteria, such as intracytoplasmic membranes for methane oxidation.

Size, Shape, and Arrangement of Microorganisms

Size of Microbial Cells

Microorganisms vary greatly in size, and their measurement uses standard metric units:

• Millimeter (mm):

• Micrometer (μm):

• Nanometer (nm):

• Picometer (pm):

Microbes differ in size:

• Eukaryotic microbes (protozoa, algae): typically 10–100 μm

• Prokaryotic microbes (bacteria, archaea): typically 0.4–10 μm

• Viruses: typically 20–800 nm

Additional info: Electron microscopes are required to visualize viruses and subcellular structures.

Shape and Arrangement of Bacteria

Bacterial cell shape is determined by heredity and can be influenced by environmental conditions. The main shapes and arrangements are:

• Bacilli: Rod-shaped

• Cocci: Spherical

• Spiral: Includes vibrio (curved rods), spirilla (rigid, corkscrew-shaped), and spirochetes (flexible, spiral-shaped)

Arrangements of cocci:

• Division in one plane: Diplococci (pairs), Streptococci (chains)

• Division in two planes: Tetrads (groups of four)

• Division in three planes: Sarcinae (cubical packets)

• Division in multiple planes: Staphylococci (clusters)

Arrangements of bacilli:

• Single rods

• Pairs: Diplobacilli

• Chains: Streptobacilli

Additional info: Some bacteria are genetically monomorphic (one shape), while others are pleomorphic (variable shapes), such as Rhizobium and Corynebacterium.

Resolution and Magnification

Resolution of the Human Eye and Microscopes

The ability to distinguish two objects as separate is called resolution. The resolution of the human retina is about 150 μm, which limits direct observation of microbes.

• Resolution: The smallest distance between two objects that allows them to be seen as separate.

• Magnification: The process of enlarging the appearance of an object.

Microscopes are required to resolve cells and their structures beyond the limits of the human eye.

Types of Microscopy

Light Microscopy

Light microscopes use visible light to observe specimens. Types include:

• Bright-field microscopy: Specimens appear dark against a bright background; commonly used with stains.

• Dark-field microscopy: Enhances contrast for unstained specimens.

• Phase-contrast microscopy: Exploits differences in refractive indices to visualize transparent specimens.

• Fluorescence microscopy: Uses fluorophores to label and visualize specific cell components.

Electron Microscopy

Electron microscopes use beams of electrons for much higher resolution than light microscopes.

• Transmission Electron Microscopy (TEM): Provides internal details in 2D.

• Scanning Electron Microscopy (SEM): Provides external details in 3D.

X-ray Crystallography

X-ray crystallography is used to determine the molecular structure of proteins, nucleic acids, and ribosomes by analyzing the interference pattern of X-rays passing through a crystal lattice.

Preparation and Staining of Specimens

Wet Mounts and Flow Cells

Wet mounts involve placing microbes in a drop of water under a coverslip for observation in their natural state. Flow cells allow extended observation by passing medium through the specimen, preventing overheating.

• Advantages: Cells observed in natural state

• Disadvantages: Low contrast, minimal resolution, risk of drying out

Staining Techniques

Staining enhances detection and resolution of cells under bright-field microscopy.

• Fixation: Cells are adhered to a slide using methanol or heat, denaturing proteins to stick to glass.

• Staining: Stains absorb light, giving cells distinct color. Most stains are cationic (positively charged) and bind to negatively charged cell surfaces.

Types of Stains

• Simple stain: Colors all cells the same (e.g., methylene blue).

• Differential stain: Colors one kind of cell but not another (e.g., Gram stain).

Gram Stain

The Gram stain, developed by Hans Christian Gram in 1884, differentiates bacteria into two groups:

Clinical relevance: Gram-positive bacteria are killed easily by penicillin and cephalosporin, while Gram-negative bacteria are generally more resistant due to their outer membrane.

Other Differential Stains

• Acid-fast stain: Detects mycolic acids in Mycobacterium (e.g., tuberculosis, leprosy).

• Endospore stain: Detects spores of Bacillus and Clostridium using malachite green.

• Blood film stain: Uses multiple dyes to distinguish blood cells and parasites.

• Capsule stain: Stains the background, making capsules visible (e.g., India ink).

Immunofluorescence and Molecular Labeling

Antibodies labeled with enzymes or fluorophores can be used to detect specific proteins or cell components, enabling molecular identification and localization.

Summary Table: Types of Microscopy and Staining

Additional info: The textbook referenced for this topic is "Microbiology: An Introduction".