How to make a bacterial flowchart

Written by jillian o'keeffe Google
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How to make a bacterial flowchart
The first step towards identifying a bacterial species is to isolate it on an agar plate. (Bacteria Colonies image by ggw from

Bacterial flowcharts are visual aids to quick identification of bacterial species. In order to create a flowchart, you need to know the characteristics of the different groups of bacteria, and which characteristics are most common. This will help you exclude non-relevant groups and figure out what bacteria are present in the sample more efficiently. Software such as PowerPoint can be used to create the flowchart and present it neatly.

Flowcharts generally deal with one group at a time, by Gram stain and shape, as there are a wide array of characteristics and tests which do not fit neatly into a single flowchart. For an example, we shall follow a flowchart through to Staphylococcus aureus, a relatively common species of skin bacteria.

Skill level:

Things you need

  • Characteristics of bacteria
  • Bacteriology study book
  • Pen and paper
  • Computer
  • Flowchart software such as PowerPoint

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  1. 1

    Begin the flowchart by specifying that the culture to be tested is a pure culture. In this section, you can also enter where the sample came from, if you wish, in order to narrow the range of the particular flowchart and direct it for use in particular circumstances.

  2. 2

    Enter "Gram Stain" as the primary test and create two offshoots from this box. The offshoots will be "Gram Positive" and "Gram Negative." Gram Positive stains, such as Staphylococcus, will appear purple and Gram Negative stains, such as E.coli, will be red. Bacteria are split into these two groupings and Gram staining is the first test performed on an unknown sample, as it is a relatively uncomplicated and cheap procedure and allows a large percentage of possibilities to be discarded.

  3. 3

    Enter the next options, which will be the shape of the bacteria. This is obtained from microscopic analysis of the Gram stain. Types of shapes are rods, cocci (spheres or ovoids), spirillia (spiral twisted rods), spirochaetes (very thin long spiral twisted rods), budding and appendaged (shapes with extra appendages) and filamentous (threadlike). Rods and cocci are most common. Staphylococcus will be a coccus.

  4. 4

    Split the following steps into biochemical tests. These will differ for the various shapes of the Gram groupings. The primary test for Gram Positive cocci is the catalase test, and the resultant options will be catalase positive or catalase negative. For example, Micrococcus and Staphylococcus species will be positive and Streptococcus will be negative.

    Examples of other next steps for groups outside of the non-cocci, non-Gram Positives include lactose fermentation or non-fermentation for Gram Negative rods.

  5. 5

    The next step will analyse the fermentation properties of the bacteria. The bacteria will either ferment or oxidise. Staphyloccoccus aureus will be fermentative as opposed to micrococcus, which is oxidative.

    Fermentation analysis is a common step for both Gram Positive and Gram Negative bacterial identification.

  6. 6

    The next step will be a biochemical test called coagulase. Staphylococcus aureus produces the coagulase enzyme, and when tested with a suitable medium, will result in a jellylike clot. All other staphylococci will be coagulase negative.

    Alternate steps for other groupings of bacteria include oxidase testing for Gram negative rods, or fast or slow lactose fermentation.

  7. 7

    A final optional step can be a DNA test using polymerase chain reaction. DNA steps are expensive and are not usually performed in laboratories for this reason, but a DNA step will definitively confirm the result of the identification.

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