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Bacteria in the Human Digestive System

In this worksheet, students will learn about friendly bacteria that live in our digestive system and help us break down food.

Key stage:  KS 3

Curriculum topic:  Biology: Structure and Function of Living Organisms

Curriculum subtopic:  Nutrition and Digestion

Difficulty level:  

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QUESTION 1 of 10

Bacteria live everywhere around us, on our skin, even inside our body. There are bacteria that cause disease, which we call pathogens, but there are also good bacteria that live inside us and help us by taking up space, so pathogens cannot multiply inside us. Quite a few good, friendly bacteria live in our digestive system and help with the process of digestion. Below is a diagram of the human digestive system:

 

 

Friendly bacteria in the digestive system live mainly in the colon (the large intestine) and in the part of the small intestine which is further away from the stomach. The rest of the small intestine, the oesophagus and the stomach are bacteria-free. Digestive enzymes and hydrochloric acid in the stomach make it impossible for bacteria to survive there.

It is estimated that around 100 trillion friendly bacteria live in our gut. If we were to extract them, they would have a mass of 1 kg. There are between 300 and 1000 different species, with 50 of them the most common. About a third of all bacteria in the gut belong to the Bacteroides species. Other species are Lactobacillus, the bacteria commonly used in probiotic foods such as live yoghurt, Escherichia species, such as E. coli, some Clostridium bacteria, the Fusobacteria, the Eubacteria, members of the Bifidobacteria group and the less common Peptostreptococcus, Ruminococcus and Peptococcus.

Friendly gut bacteria have evolved in order to withstand the hard conditions of our digestive system. Bacteroides species coat themselves with a layer of sugar that they remove from the cells of the intestinal wall. They disguise themselves, so our immune system does not fight them.

 

The main advantage of having friendly bacteria in the gut is protection against harmful bacteria that would otherwise cause infection and invade the cells of the intestinal wall. Additionally, bacteria help with the digestion of materials that we are unable to digest, like hard plant material. A lot of the vitamins from vegetables would be wasted if it wasn't for the friendly bacteria that digest them for us. Some species produce vitamins K and B that is hard for us to obtain from food. They also maintain the acidity balance and break down some drugs, hormones not needed anymore and toxins that would be potentially dangerous to our health.

The population of bacteria in our gut is renewed regularly; about half the dry mass of our faeces consists of bacteria.

It is possible that some friendly species of bacteria may acquire extra genetic material that would make them turn harmful and produce toxins. An example is the E. coli. If such an infection is established, we get food poisoning. It is very important that we cook food very well to ensure no bacteria in it survive before we eat it. 

What parts of our digestive system do friendly bacterial live in?

Tick two answers.

oesophagus

small intestine

colon

stomach

Why is it impossible for friendly bacteria to live in our stomach?

Tick two answers.

Pathogens take up most space.

They cannot survive hydrochloric acid in the stomach.

Our immune system would fight them.

They would be digested by digestive enzymes.

How many bacteria live in our gut?

100 000 000

100 000 000 000

100 000 000 000 000

What would be the mass of all the bacteria that live in our gut?

1 g

100 g

1 kg

What is the range of species that live in our gut?

50 to 300

50 to 1000

300 to 1000

What is the most common species of gut bacteria?

Bacteroides

Clostridium

Peptococcus

How do Bacteroides species disguise themselves so the immune system does not recognise them?

They coat themselves with a layer of acid.

They coat themselves with a layer of antibodies.

They coat themselves with a layer of sugar.

What materials do friendly bacteria help us digest?

hard plant material

red meat

glucose

What vitamins are hard to obtain from food, but we manage to absorb with the help of gut bacteria?

Tick two answers.

K

A

B

C

How could friendly bacteria turn harmful?

Tick two answers.

by digesting more food than needed

by acquiring extra genetic material which would turn them harmful

by releasing toxins which would cause food poisoning

by attacking pathogens

  • Question 1

What parts of our digestive system do friendly bacterial live in?

Tick two answers.

CORRECT ANSWER
small intestine
colon
EDDIE SAYS
Friendly gut bacteria live mainly in the colon (large intestine) and the part of the small intestine furthest from the stomach.
  • Question 2

Why is it impossible for friendly bacteria to live in our stomach?

Tick two answers.

CORRECT ANSWER
They cannot survive hydrochloric acid in the stomach.
They would be digested by digestive enzymes.
EDDIE SAYS
Friendly bacteria cannot survive hydrochloric acid in the stomach and they would be digested by digestive enzymes.
  • Question 3

How many bacteria live in our gut?

CORRECT ANSWER
100 000 000 000 000
EDDIE SAYS
The last option is 100 trillion. This is the estimated number of friendly bacteria in our gut.
  • Question 4

What would be the mass of all the bacteria that live in our gut?

CORRECT ANSWER
1 kg
EDDIE SAYS
The estimated mass of the friendly bacteria in our gut is 1 kg.
  • Question 5

What is the range of species that live in our gut?

CORRECT ANSWER
300 to 1000
EDDIE SAYS
There are between 300 and 1000 species of friendly bacteria in our gut.
  • Question 6

What is the most common species of gut bacteria?

CORRECT ANSWER
Bacteroides
EDDIE SAYS
The Bacteroides species are the most common gut bacteria.
  • Question 7

How do Bacteroides species disguise themselves so the immune system does not recognise them?

CORRECT ANSWER
They coat themselves with a layer of sugar.
EDDIE SAYS
The Bacteroides species coat themselves with a layer of sugar they take from the cells of the lining of the intestinal wall.
  • Question 8

What materials do friendly bacteria help us digest?

CORRECT ANSWER
hard plant material
EDDIE SAYS
Gut bacteria help us digest hard plant material and keep many vitamins that would otherwise be wasted.
  • Question 9

What vitamins are hard to obtain from food, but we manage to absorb with the help of gut bacteria?

Tick two answers.

CORRECT ANSWER
K
B
EDDIE SAYS
We absorb more vitamins K and B with the help of gut bacteria.
  • Question 10

How could friendly bacteria turn harmful?

Tick two answers.

CORRECT ANSWER
by acquiring extra genetic material which would turn them harmful
by releasing toxins which would cause food poisoning
EDDIE SAYS
Friendly bacteria could turn harmful by acquiring extra genetic material which would make them release toxins that cause food poisoning.
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