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Chapter 5 Instructor Resources​

Single-Celled Pathogens: Bacteria

Bacteria are single-celled prokaryotic (without nuclei) organisms, which are very different from eukaryotic and mostly multicellular fungi. Bacteria can cause plant diseases in addition to their roles in important human and animal diseases. For example, fire blight of apple and pear and citrus canker have ravaged fruit tree groves in the United States and elsewhere. Another important bacterial plant pathogen is Agrobacterium tumefaciens, which causes a disease called crown gall on a wide variety of plants. The bacterium is able to alter the genetic information in plant cells and is thus called a natural genetic engineer.

  • Chapter 5 Podcast

    Chapter 5 Podcast

    Listen to the Podcast (mp3)

    The short podcast provided for each chapter includes a review of a major concept or issue, clarification of an important point that can be confusing to students, and questions for students to think about. This podcast discusses the properties of plant-pathogenic bacteria and explains how the term inoculum is used in plant pathology.ween a disease and an injury.

  • Demonstrations and Desktop Lab Exercises

    Gram Stain Without the Mess

    Prepare plates of Gram-negative bacteria. Use a streaking method, as shown in Figure 5.4, to demonstrate how pure colonies are obtained. Explain to students that they are seeing the gelled surface of a nutrient agar and that bacteria are the superficial, sticky growth on top of that surface.


    • Wooden toothpicks (one for each student)
    • Glass slides (one for each student)
    • 3% KOH (several small squeeze bottles to be shared among students)
    • Petri plates of recently streaked Gram-negative bacteria

    Squeeze a drop of 3% KOH onto a glass slide. Scoop up a large mass of bacteria from the plate onto the tip of a toothpick. (Avoid digging the toothpick into the agar.) In tight circles, swirl the bacteria in the KOH to dissolve the cell walls and release the viscous DNA from the cells. In less than a minute, students should be able to slowly draw up visible strands of DNA. Because of the structure of the cell walls, Gram-positive bacteria will not lyse in 3% KOH, and the mass will remain watery.


    • This method is the equivalent of doing a Gram stain that requires several stains and a microscope.
    • Students are usually quite impressed by the amount of DNA that they can string up from the bacteria.
    • Besides allowing students to see living bacteria, this activity provides a nice introduction to genes and genetic engineering, the subject of the next chapter.

  • Group Discussion

    Symptoms and Signs

    Break the class into small groups of 3–5 students, and have them work together for 10–15 minutes to complete the grid below. Ask each group to provide at least one answer in each grid square:

    Late Blight
    Downy Mildew
    Coffee Rust
    Fire Blights
    Citrus Canker
    Crown Gall

    Then, have the whole class fill in the grid with contributions from different groups. Review different kinds of symptoms, clarify the difference between symptoms and signs, and explain that not all diseases have visible signs (e.g., crown gall).

  • Short Writing Assignments

    Notes: These assignments require each student to write a paragraph (introductory sentence, body, concluding sentence) and can be completed in 10–15 minutes in class. They provide a good way to check student comprehension and to improve student writing skills. See Chapter 1 for a simple grading system.

    1) What are Bacteria and How do they Compare to Fungi?

    Last week, you learned about bacteria and how they can affect plants. Briefly define what a bacterium is, and compare and contrast plant pathogenic bacteria and fungi. List at least one way in which bacterial and fungal plant pathogens are similar and one way in which they are different. Items for discussion may include cellular structures, life cycles, methods of dissemination, methods of infection, methods of reproduction, and management strategies.

    2) Eradication of Bacterial Plant Diseases

    Last week, you learned about bacterial diseases of plants, including citrus canker and fire blight of apples and pears. Although both diseases are caused by bacteria, citrus canker was managed successfully in the United States for decades using eradication, while efforts to eradicate fire blight in Europe failed. Widespread eradication of fire blight in the United States was never even attempted. In paragraph form, discuss two differences between these disease situations that resulted in eradication being a successful management strategy for one disease but not the other.

  • Longer Writing Assignment

    The Use of Antibiotics to Manage Bacterial Plant Diseases

    Many bacterial diseases are difficult to manage, including fire blight, a damaging disease of apples and pears. Fire blight has eliminated production of these high-value crops in some areas of the United States and the world. Several management strategies are used for fire blight, including spraying orchards with the antibiotics streptomycin and oxytetracycline. Antibiotics also are used to manage many bacterial diseases of humans and animals. However, a bacterium can develop resistance to one or more antibiotics, and this resistance can move to other bacteria. For this reason, some people are concerned about the use of antibiotics to manage plant and animal diseases.

    Should the use of antibiotics be one of the management options for fire blight? Should the state or federal government regulate the use of antibiotics more stringently or eliminate their use for some purposes altogether? Write a short essay (~ 350 words) about this topic.

    Here are some questions to consider:

    • What causes fire blight disease of apples and pears?
    • How much yield loss can fire blight cause, and what is the cost to growers and consumers?
    • What management strategies are available for fire blight?
    • What is an antibiotic?
    • What are the relative amounts of antibiotics used to manage diseases of humans, animals, and plants?
    • How does a bacterium become resistant to an antibiotic, and how can resistance move to other bacteria?


    • You can make the audience for the assignment someone other than yourself—for example, a legislator who is drafting a position statement on the use of antibiotics in agriculture. The selection of audience should dictate the style of writing students use.
    • Opinion papers such as this can be written in several drafts: Draft 1 (evaluated but not graded) summarizes the facts; draft 2 (also evaluated but not graded) incorporates the student’s opinion, supported by the facts; and draft 3 is a polished version of draft 2, which is finally graded. Students appreciate the opportunity to get feedback on their writing and thinking before being assigned a grade. This system also provides an opportunity for students to learn to separate facts from opinions and to evaluate the biases of different sources of information.
    • You can provide students with a reference list (including links), so that this is an exercise in evaluating and using information, rather than finding it. Extra credit can be given for sources with new information that students find and use in their papers.

    Evaluation of Paper:

    Criteria used in evaluation can include the following:

    • Accuracy and completeness of factual information
    • Development and support of opinion
    • Overall organization and clarity
    • Mechanics (e.g., spelling, grammar, sentence structure)
    • Use of supporting references

    Important Note: This paper has no “right” or “wrong” conclusion. The student can take either side of the issue (pro–antibiotic use or anti–antibiotic use), include the pertinent facts, and use them to support his or her position.