Bacteria are microscopic and live almost everywhere on Earth. The human body alone naturally contains around 39 trillion bacterial cells, which is more than the 30 trillion human cells that make up the body.
As single-celled organisms, bacteria are prokaryotes. Prokaryote cells differ from eukaryote cells in that their genetic material is not separated from the rest of the cell with a nuclear membrane.
Types of Bacteria
There is a vast diversity of bacteria. Bacteria have adapted to living in places around the earth such as deep-sea vents in the ocean to the cold temperatures of the poles and nearly everywhere in between.
Some bacteria are what we call pathogenic, which means they cause disease when they enter a host body. Other bacteria are nonpathogenic, meaning they are either harmless or beneficial to a host.
Autotrophs generate their own energy sources through photosynthesis or chemosynthesis. Heterotrophs, like humans, can't make their own food, so they obtain energy by eating other organisms.
The morphology of bacteria is highly diverse. Bacteria morphology can be divided into different types based on their shape and cell wall composition. Cell walls can either be Gram-positive and made from peptidoglycan or Gram-negative, which are made from lipopolysaccharide.
The term Gram comes from a test designed by Hans Christian Gram that stains cell walls with dyes and chemicals, resulting in Gram-positive bacteria appearing purple and Gram-negative bacteria appearing pink or red.
When looking at pictures of bacteria, it is evident that there is a variety of unusual shapes such as helical or club forms plus three main shapes. Circular bacteria come in singular, pairs, chains or clusters. Rod-shaped bacteria can appear oval or have an elongated shape. Curved bacteria come in spirals, coils or with a look like bent rods.
Parts of a Microscope
A microscope consists of:
- A stage to place the sample with a light source underneath
- Objective lenses on a revolving turret to magnify the sample
- A focus dial which moves the sample up and down to get it into focus
- An eyepiece to view and magnify the sample
- A condenser which adjusts the amount of light on the sample
The size of bacteria is measured in micrometers. To see bacteria swimming, a 400x magnification is required. A 1000x magnification enables viewing of bacteria in more detail.
Preparing Bacterial Samples for Viewing
Using a clean dropper or inoculating loop, collect and drip a small portion of distilled water onto the slide. Next drip a small amount of bacterial culture next to the distilled water. Sweep the inoculating loop over the glass slide to mix the bacteria with the distilled water.
Place the slide on a drying rack and allow to dry before viewing or place a coverslip over the slide to observe the bacteria in action.
Due to the small size and sometimes transparent nature of bacteria, samples may need to be cultured in advance and Gram-stained. Culturing bacteria increases the concentration of cells in a sample.
To Gram-stain the culture, add either crystal violet, methylene blue or safranin to the bacterial culture for one minute then carefully remove the excess stain with water or an absorbent towel.
Looking at Bacteria Under a Microscope
Viewing bacteria under a microscope is much the same as looking at anything under a microscope. Prepare the sample of bacteria on a slide and place under the microscope on the stage. Adjust the focus then change the objective lens until the bacteria come into the field of view.
Repeat the focus adjustments each time before moving to the next objective lens and continue until desired magnification is reached.
- American Microbiome Institute: How Many Bacteria vs Human Cells Are in the Body?
- Encyclopaedia Britannica: Bacteria
- Archives of Clinical Microbiology: Pathogenic Bacteria
- ThoughtCo: Gram Positive vs. Gram Negative Bacteria
- Medical Microbiology. 4th Edition: Chapter 17 Anaerobes: General Characteristics
- Microscopes for Schools: How To Use a Microscope
- Microscope World: Microscope Magnification
- Microscope Master: Bacteria Under the Microscope: Morphology, Discussion, Types, Habitat
- Cold Spring Harbor Perspectives in Biology: Small but Mighty: Cell Size and Bacteria
- Proceedings. California Academy of Science: Species Numbers in Bacteria