Bacteria

Bacteria

Middle School

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Learning Objectives

• Define bacteria and describe their three common shapes.

• Compare Gram-positive and Gram-negative bacteria based on their cell walls.

• Identify the main parts of a bacterial cell and their functions.

• Distinguish between helpful and harmful bacteria by providing examples of each.

• Explain how bacteria reproduce and the concept of antibiotic resistance.

Key Vocabulary

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Pathogen

A bacterium that causes disease by producing poisons called toxins, which can make you feel sick.

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Binary Fission

A form of asexual reproduction where a parent cell divides into two genetically identical daughter cells.

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Plasmid

A small, circular molecule of DNA that contains helpful genes but is not considered crucial.

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Endospore

A spore that forms inside a cell to help it survive stressful conditions for long periods.

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Gram Staining

A method used to classify bacteria into two groups based on their cell wall composition.

Common Shapes of Bacteria

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Spherical (Cocci)

• ​Cocci are bacteria that have a round or spherical shape.

• ​​They can be found alone, in pairs, chains, or clusters.

• ​Some types of Cocci can cause different infections in humans.

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Rod-Shaped (Bacilli)

• ​Bacilli are bacteria that are shaped like small rods or cylinders.

• ​​They can also be found as single cells or joined together.

• ​Many Bacilli are helpful, but some can cause serious diseases.

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Helical (Spirilli)

• ​Spirilli are bacteria with a spiral or twisted corkscrew shape.

• ​​Their unique shape helps them to move around very efficiently.

• ​They are mostly found in aquatic environments like freshwater ponds.

Multiple Choice

Which of the following best describes bacteria?

• Microscopic, single-celled prokaryotes that come in spherical, rod, or helical shapes.

• Multicellular organisms that can only survive in the human intestine.

• The rarest organisms on Earth, all of which are rod-shaped.

• Single-celled eukaryotes that always live in clusters.

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Classifying Bacteria: Gram Staining

Gram-Positive

• ​These bacteria stain purple because they have a thick cell wall.

• ​​Their cell wall lacks an outer membrane, which helps them retain the dye.

• ​An example of a gram-positive bacterium is a species of Streptococcus pneumoniae.

Gram-Negative

• ​These bacteria stain red because they have a very thin cell wall.

• ​​Their cell wall has an outer membrane that prevents dye retention.

• ​An example of a gram-negative bacterium is a species of Salmonella typhi.

Multiple Choice

A scientist discovers a bacterium with a thin cell wall and an outer membrane. What color would it stain in a Gram stain test?

• Purple, because it is Gram-positive.

• Red, because it is Gram-negative.

• Blue, because it is a type of Cyanobacteria.

• It would not stain, because it has an outer membrane.

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Key Structures of a Bacterial Cell

• A cell wall surrounds the plasma membrane, which controls entry and exit.

• The cytoplasm contains ribosomes and a circular DNA plasmid having essential genes.

• Some have flagella for movement or a capsule to stick to surfaces.

• To survive stress, some bacteria form a protective endospore.

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Multiple Choice

What is the function of a plasmid in a bacterial cell?

• It contains helpful accessory genes.

• It controls what enters and exits the cell.

• It encloses the DNA during times of stress.

• It is used for movement.

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Bacterial Reproduction and Genetic Variation

Binary Fission

• Bacteria reproduce asexually through a process called binary fission.

• A single parent cell divides to create two identical daughter cells.

• This process does not create new genetic variation in the offspring.

Genetic Transfer

• Bacteria can introduce genetic variation by exchanging DNA with other cells.

• One way this happens is by sharing a small piece of DNA called a plasmid.

• They can also absorb foreign DNA from their surrounding environment.

Multiple Choice

How do bacteria primarily reproduce, and what is the outcome?

• Through binary fission, creating two genetically different daughter cells.

• Through genetic transfer, creating one new cell from two parent cells.

• Through sexual reproduction, creating genetically diverse offspring.

• Through binary fission, creating two genetically identical daughter cells.

How Bacteria Live

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Food Producers

• ​Cyanobacteria make their own food from the sunlight.

• ​​This process, called photosynthesis, releases oxygen into the air.

• ​They helped add oxygen to the Earth’s early atmosphere.

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Scavengers

• ​Scavenger bacteria get their food from their surroundings.

• ​​For example, some live in your stomach and help digest food.

• ​They are a very important part of many different ecosystems.

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Pathogens

• ​Pathogens are harmful bacteria that attack other living things.

• ​​They make you sick by producing poisons called toxins.

• ​These are the types of bacteria we often call germs.

Multiple Choice

What is the role of Cyanobacteria?

• They consume dead organic matter for energy.

• They produce their own food through photosynthesis and release oxygen.

• They are scavengers that live in the human stomach.

• They are pathogens that produce toxins to make other organisms sick.

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Helpful vs. Harmful Bacteria

Harmful Bacteria

• Pathogenic bacteria make you sick by producing poisons called toxins.

• Symptoms of these toxins can include fever and diarrhea.

• Examples include Salmonella in raw meats and Listeria in deli foods.

Helpful Bacteria

• Most bacteria are harmless, and many are very beneficial.

• As decomposers, they help recycle important nutrients in ecosystems.

• We use helpful bacteria to make foods like cheese and yogurt.

Multiple Choice

Which of the following is an example of a helpful role of bacteria?

• Toxins from Pathogenic E. coli causing a fever.

• Getting sick from Listeria in deli foods.

• Using Lactobacillus to make yogurt.

• Infection by Salmonella from raw meat.

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What Is Antibiotic Resistance?

• Antibiotics are medicines that are used to fight bacterial infections.

• Overuse and misuse of antibiotics can lead to antibiotic resistance.

• Some bacteria have evolved, or changed, to resist the antibiotics.

• Resistant bacteria make infections much harder to treat effectively.

Multiple Choice

What is the primary cause of antibiotic resistance?

• The overuse and misuse of antibiotic drugs.

• Helpful bacteria like Lactobacillus.

• The use of bacteria to make vaccines.

• The process of binary fission in bacteria.

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Common Misconceptions About Bacteria

Misconception	Correction
All bacteria are harmful and cause disease.	Most bacteria are harmless, and many are even helpful.
Bacteria are not living things.	Bacteria are living organisms that eat, grow, and reproduce.
Bacteria have a nucleus.	Bacteria are prokaryotes, so they do not have a nucleus.
Antibiotics can cure any sickness, including colds.	Antibiotics only work on bacterial infections, not viruses.

Multiple Choice

Why are some bacteria, like those in your stomach, considered scavengers?

• Because they attack other living things to get nutrients.

• Because they share and consume the food from the environment around them.

• Because they make their own food using sunlight.

• Because they only consume dead organisms.

Multiple Choice

How does the cell wall of a Gram-positive bacterium affect its result in a Gram stain test?

• Its thin cell wall allows the purple dye to be washed away.

• Its thick cell wall retains the purple dye.

• Its outer membrane prevents any dye from entering the cell.

• Its lack of a cell wall causes it to stain red.

Multiple Choice

A patient with a bacterial infection stops taking their prescribed antibiotics after two days because they feel better. Analyze the most likely negative outcome of this action.

• It will cause the helpful bacteria in their gut to grow faster.

• It could lead to the development of antibiotic-resistant bacteria.

• The infection will be completely cured.

• The patient will become immune to all future bacterial infections.

Multiple Choice

Based on the methods of bacterial reproduction and genetic variation, what is the most likely way for antibiotic resistance to spread from one bacterium to another?

• Through the transfer of a plasmid containing a resistance gene.

• During binary fission from a parent cell to a daughter cell.

• By taking up random DNA from a dead, non-resistant bacterium.

• Through the formation of an endospore.

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Summary

• Bacteria are diverse, single-celled prokaryotes classified by shape and Gram stain.

• Their cells have a cell wall, plasmids, and sometimes flagella or capsules.

• They reproduce by binary fission but can also exchange genetic information.

• Bacteria can be helpful or harmful; antibiotic misuse creates resistant bacteria.

Poll

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