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Learning goals

Note some basic facts about bacteria

Learn the structure of bacteria

See the shapes of bacteria

Discover practical uses of bacteria

How they reproduce

How bacterial sex leads to evolution


Bacteria (plural) – Bacterium (single)

I. Basic facts

They are simplest organisms on Earth.

They have existed for billions of years.

Most bacteria live in the ground or in water.

Some live deep in the Earth, in cracks between rocks.

Strata Quebrada de Cafayate, Salta, Argentina

Photo of Quebrada de las Conchas, Salta, in Argentina. Wikimedia.

Many bacteria live inside, or on the skin of, other organisms – even humans.

They are very small – can only be seen through a microscope.

A thousand bacteria lined up would be just one millimeter long.

II. Structure

They are prokaryotes: they have no nucleus, and very few organelles.


Image from Wikimedia

They do have DNA strung together into a chromosome

They have a cell membrane (lipid bilayer) and cell wall

Ribosomes float freely in the cytoplasm.

III. Shapes/types of bacteria

IV. Uses of bacteria

1. Gut bacteria help us digest our food. Without them we’d often have constipation or other problems, and it would more difficult for us to digest food completely.

2. Some bacteria help us ferment food.

Fermentation is using bacteria or yeast to convert carbohydrates to other chemicals.

This only happens in anaerobic conditions (very low amount of oxygen)

Through fermentation, bacteria converts carbs into alcohols and carbon dioxide gas, or other organic acids.

3. Examples

Fermentation of milk makes cheese and yogurt.

Fermentation of cacao (chocolate) beans, and coffee beans, helps them achieve their desired flavors.

Fermentation of finely cut cabbage is sauerkraut.

Fermentation of ground, spiced meat creates salami.

Salami is a type of sausage — ground meat stuffed into a casing — which is not cooked, but is instead allowed to ferment with bacteria, before being dried and made edible. (This includes pepperoni)

Uses of bacteria

V. Reproduction of bacteria

Through the process of binary fission (splitting into two) one cell can divide into two.

Also called mitosis.

Each daughter cell is identical in shape to the parent.

bacterial binary fission

Image from naef-co.weebly.com

VI. Bacterial sex and evolution

Bacteria have no gender.

They do not have sexual intercourse as animals do, yet nonetheless they can exchange DNA with other bacteria cells.

Bacteriophage Conjugation Transduction Transformation

This exchange allows bacteria to change their genes.

Therefore, when this happens, each daughter cell is different from the parent.

Why do this? If the environment that they live in changes, then some bacteria may die. In fact, if they all had the same genes, then all could die!

But if their genes are different, then even though some may die, others may live.

The cells that live pass on their genes to their offspring. These new (“mutant”) genes become more common.

For the cell that die do not pass on their genes; those genes become less common, and may disappear altogether.

Therefore, as time goes by, the genes of the remaining bacteria change. This is evolution.

All forms of life on Earth have changes in their DNA, as time goes by. All forms of life undergo evolution.


Lichens and bacteria

Lichens, and how they are related to bacteria



How antibiotics work

External resources

What are bacteria? This exhibit is the best place to start.

Not all microorganisms are bacteria.  Microorganisms that are not bacteria are briefly described here.

What is bacteriology? The study of bacteria is explained here.

Applied bacteriology how bacteria are applied in industry.

Bacteriophages are viruses that prey on bacteria. So bacteria can be ill, too.

Bacteria and our senses describes how we can see, taste, feel and hear bacteria. Or not.

Worksheets on bacteria (MS Word format)
Archaea and Bacteria

Sample questions

MCAS: The illustration below shows the external features of a prokaryotic organism. Which of the following can be concluded about the internal cellular contents of this prokaryote?

A. The cell does not contain ribosomes.
B. The cell does not contain a nucleus.
C. The cell contains mitochondria.
D. The cell contains a vacuole.


Feb 2016 MCAS.

Which of the following statements best compares reproduction in viruses with reproduction in single-celled protists?

A. Viruses reproduce by releasing spores; single-celled protists reproduce by dividing in half.

B. Viruses require large colonies to reproduce; single-celled protists reproduce as individual cells.

C. Viruses require the cellular machinery of host cells to reproduce; single-celled protists reproduce on their own.

D. Viruses form haploid gametes when they reproduce; single-celled protists form diploid gametes when they reproduce.


Learning Standards

Massachusetts Science and Technology/Engineering Curriculum Framework

Life Science (Biology), Grades 6–8.
Classify organisms into the currently recognized kingdoms according to characteristics that they share. Be familiar with organisms from each kingdom.

Biology, High School
5.2 Describe species as reproductively distinct groups of organisms. Recognize that species are further classified into a hierarchical taxonomic system (kingdom, phylum, class, order, family, genus, species) based on morphological, behavioral, and molecular similarities.

Benchmarks for Science Literacy, American Association for the Advancement of Science

Students should begin to extend their attention from external anatomy to internal structures and functions. Patterns of development may be brought in to further illustrate similarities and differences among organisms. Also, they should move from their invented classification systems to those used in modern biology… A classification system is a framework created by scientists for describing the vast diversity of organisms, indicating the degree of relatedness between organisms, and framing research questions.

SAT Biology Subject Area Test

Evolution and diversity: Origin of life, evidence of evolution, patterns of evolution, natural selection, speciation, classification and diversity of organisms.

Teaching About Evolution and the Nature of Science, National Academy Press (1998)

Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.

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