Bacteria Under the Microscope

Morphology, Discussion, Types, Habitat


Like archeans, bacteria are prokaryotic cells. This means that they are single-celled organisms without a nucleus membrane (nuclear envelope). While they are very small, they are diverse and vary in shape and size.


Microscopy


Before preparing for microscopy, bacteria are grown in culture media. This helps enhance cell division and thus increase their numbers.

Some of the media used include:

Basal Media (e.g. nutrient broth and peptone water) - This is used for such bacteria as Staphylococcus that do not require special enrichment for growth

Selective Media (e.g. Lowenstein-Jensen media) - This type of media is used to grow specific types while prohibiting others.

Transport Media (E.g. Amies medium) Transport media are particularly useful in instances where the sample has to be preserved. This also prevents the bacteria from overgrowing before it is transported to the desired destination (laboratory etc).

Enriched Media (e.g. with added blood) - Enriched media contain a special component added to the media to favor the growth of given bacteria. For instance, the addition of blood in a given media supports the growth of Streptococci.


Slide Preparation 



Requirements

Procedure

Smear

  • Using the marking pencil, mark a circle at the center of a clean slide to mark a spot for the smear
  • Using either a clean dropper or the inoculating loop (make sure to flame the loop using the Bunsen burner) place a drop of distilled water in the marked spot on the slide (if the media being used is broth, then distilled water is not necessary)
  • Pass the inoculating loop through the flame again and allow to cool before scooping (the surface of the culture) a small amount of the sample from the tube or Petri dish with the culture (to prevent contamination of the remaining sample, pass the tips of the tube through the flame before covering it with the lid)
  • Gently mix the sample with the drop of water on the slide to create a smear
  • Place the slide on the drying rack and allow it to dry completely (air dry)
  • Fix the smear by passing the slide over the flame several times (about 3 times) - Fixing enhances stain penetration into the cells


Bacteria Staining Procedure




  • Place the slide on a staining rack and cover with either of the following stains (gram stains) for one minute:
  1. Crystal violet
  2. Methylene blue
  3. Safranin
  • Run a gentle stream of water along the slide to remove excess stain
  • Wipe away excess water and stain on the edges of the slide using a absorbent paper
  • View under the microscope starting with low power (for high power, add immersion oil)


Observation (Discussion)


Depending on the sample under investigation, students will have the opportunity to observe and identify the size and shape of the bacteria. They are categorized according to their shape (Morphology) and the how they stain (gram positive and gram negative bacteria).

Morphology

There are several types based on their general appearance (shape) including:

Coccus Bacteria

Cocci (coccus) bacteria are some of the most common bacteria. They are spherical (or ovoid at times) in shape and are divided into; diplococcus (occur in pairs such as Neisseria spp) streptococcus (occur as a long chain or cells such as Streptococcus pneumoniae) and staphylococcus where they occur in clusters (e.g. staphylococcus saprophyticus). Cocci may also occur in tetras or in packets of 8 to form a structure that appears like a cube such as the sarcina bacteria.



Bacillus

Unlike coccis bacteria, bacillus will appear as elongated rods (rod-like) when viewed under the microscope. In most cases, the bacilli  occur as single cells (e.g. Mycobacterium tuberculosis), but may occur in pairs (diplobacillus) or form chains commonly refered to as streptococcus (e.g. Bacillus cereus).

Spirilla and Virus

Whereas vibrio bacteria appear comma shaped, spirilla are the type that appear spiral in shape. While some people may confuse the two when viewed under the microscope, they are different when students compare them under high magnification.

Example of vibrios includes vibrio vulnificus and Vibrio harveyi while some examples of Spirilla include members of the Campylobacter species.

Spirilla have flagella (on both ends), which they use to move in water or aquatic environment. This allows them to move faster in such environments compared to other bacteria. On the other hand, Vibrio have a flagellum on one end that allows them to move about.

Some of the other types of bacteria based on morphology include:

  • Spirochetes (flexible spiral bacterial) e.g. Spirochaetales – Spirochetes have an axial filament that allow them to move in a type of motion that has been described as “corkscrew motion"
  • Actinomycetes (branched) e.g. Streptosporangineae
  • Mycoplasmas (lack cell wall and may appear as elongated cells with a thicker central section or spherical) e.g. Mycoplasma hominis


Gram-Positive and Gram-Negative Bacteria


Bacteria can also be grouped according to how they stain.

Gram-Negative Bacteria

Gram negative are the type of bacteria that do not retain the primary stain. During decolorization, these bacteria lose the crystal violet stain (primary stain) because they have a thin Peptidoglycan layer. However, they take up the counter stain (safranin) and will appear reddish or pink when viewed under the microscope. Common examples of gram-negative include Salmonella spp, Escherichia coli (E. coli) and the Enterobacteriaceae spp.

Gram-Positive Bacteria

Unlike gram-negative, gram positive bacteria have a thick peptidoglycan layer that allows them to retain the primary stain/dye (crystal violet stain). Fixing with heat allows the stain to penetrate the layer, which is then retained even when the cells are washed using alcohol.

When viewed under the microscope, gram-positive appear purple or bluish in color. Examples of gram-positive include; Staphylococcus epidermidis, Bacillus cereus and Clostridium botulinum among others.

Check out more on Gram Stain


Reproduction


The main methods of reproduction among bacteria include:

Binary fission - Binary fission is the main method of reproduction. The process starts with the cell growing in size and then splitting into two separate cells.

Before the cell splits into two, the genetic material has to be copied and separated into two copies that move to the polar ends of the cell before the cell cytoplasm separate followed by cell splitting.

Budding - When viewing under the microscope, students may be able to see a small protrudence on the surface of some of the cells (bud). For some bacteria, budding is a means of reproduction.

This involves the formation of a bud on one end of the cell surface followed by replication of the genetic material. A copy of the material gets in to the bud as it enlarges and ultimately breaks off and separates from the parent bacterial cell.

Formation of endospores - This is a form of asexual reproduction among gram positive bacteria particularly during extreme, unfavorable conditions.

During such conditions, part of the protoplast is transported near to the chromosome and ultimately surrounded by a touch cell wall forming the endospore. This allows the endospore to survive extreme conditions.

In favorable conditions (presence of moisture, nutrients and the right temperature) the tough wall is broken down allowing a new bacteria cell to grow from the protoplast.


Habitat




Bacteria can be found virtually everywhere on earth. This means that they can be found in the soil, in oceans and other water bodies, rocks, on plants and even in the artic.

More recent studies have shown that they are even found in some of the most extreme environments such as the Dead Sea and other extremely hot areas.

  • Soil - E.g. Denitrifying bacteria (responsible for converting nitrate to nitrogen) and Actinomycetes which grow as hyphae and decomposed a wide range of substrates in soil
  • In ocean and sea water -Vibrio bacteria are the most common type of bacteria found in oceans. Also, the Dead Sea has been shown to contain Halobacterium sp. 
  • Plants - Rhizobacteria is an example of bacteria found in various plant roots where they enhance nitrogen fixation and increasing nutrient availability.


  • Like E. coli live in the intestines of animals where they help in producing the vitamin k2 in addition to preventing pathogenic bacteria from thriving in the large intestine. See Also E.Coli Under the Microscope.


See Also: Microscope Experiments like Yogurt under the Microscope

Return to Prokaryotes Main Page

Return to Unicellular Organisms Main Page

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References

Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.

http://generalbacteriology.weebly.com/culture-media.html

https://drinc.ucdavis.edu/dairy-food-sciences/introduction-bacteria


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