Guardians of the Gastrointestinal System

Instructional Guide

Learners will explore the role of microbes in digestion in different species, aiming in understanding how microbes react to different situation in the GI system.

Intended Grade Level

This lesson is designed for students in grades 9-12. Students should be comfortable with the term symbiotic relationship, understand the general structure and function of the digestive system, and also have an understanding that food provides energy and building blocks for organisms

Learning objectives

• Understand what a microbe is
• Identify which species use microbes for digestion
• Understand how digestion/fermentation work
• Explain why some animals/humans cannot digest plants

Lesson Format

Student page

This can be done in any size group. The material can be printed and disseminated. Activities require access to a computer and internet connection.

• First students will popcorn read an introduction to microbes, and answer two short questions regarding the reading.(5 minutes)
• Next, students will read about ruminant fermentation and the role of microbes. Then they will answer two short questions regarding the reading. (8 minutes)
• Then students will read about equine fermentation and the role of microbes. Then they will answer one short question regarding the reading. (8 minutes)
• Students will read about kangaroos and the role of microbes in their fermentation process. During this section they will answer 3 short questions regarding the reading. (10 minutes)
• The activity involves students hypothesizing how good and bad gut bacteria would respond to different scenarios. More details are highlighted in the activities section. (10 minutes)

Lesson Background

Microbes are microscopic organisms—including bacteria, protozoa, fungi, and archaea—found in nearly all environments. In animal digestion, bacteria and protozoa are the most relevant. Beneficial microbes aid in digestion, protect against pathogens, synthesize vitamins (e.g., B vitamins, vitamin K), and help maintain immune function. Harmful microbes (e.g., certain strains of E. coli, Salmonella) can cause disease or disrupt the gut microbiome.

Many plant-eating animals consume cellulose, a carbohydrate found in plant cell walls. Most animals cannot produce cellulase, the enzyme needed to break down cellulose. Instead, they host microbes that ferment cellulose and release usable energy in the form of volatile fatty acids (VFAs) and other nutrients. .

Fermentation sites vary by species:

• Foregut fermenters (e.g., cows, kangaroos) ferment food before the small intestine

• Hindgut fermenters (e.g., horses) ferment food in the cecum and colon after the small intestine

In ruminants, or foregut fermenters, like cows, goats and sheep, their four stomach system is designed to maximize fermentation, as well as nutrient absorption. Microbes in ruminants break down cellulose and hemicellulose via cellulase, produce VFAs, microbial protein, and vitamins (B-complex, K) and maintain immune balance and pathogen resistance. Microbes significantly impact the pH of the rumen. The ideal rumen pH is around 6.0-6.8, and if the pH drops (e.g., due to high-grain diets), rumen acidosis can occur.

In hindgut fermeters, like horses, microbial fermentation occurs in the cecum and colon – they only possess a one stomach system, unlike ruminants.  The cecum houses bacteria that ferment fiber, producing VFAs. Overall in the horse GI tract, there are up to 1 quadrillion microbes. Imbalances in microbial populations (dysbiosis) can lead to colic or laminitis.

Although foregut fermentation has been discussed in the ruminant, animals like the kangaroo have a unique form of foregut fermentation. They too have a multi-chambered stomach, but not the same as ruminants. Instead, they have the sacciform forestomach, where microbial fermentation occurs,  and the tubiform forestomach, where enzymatic digestion occurs.

Activities

Students will be presented with three scenarios and will be asked to hypothesize what will happen to the good gut microbes and the bad gut microbes. Students will drag the labels of “good microbes” or “bad microbes” into the boxes “these would increase,” and “these would decrease,” to signify their answers.

Common misconceptions and challenge points

• Students may not understand the symbiotic relationship between microbes and animals, where microbes stay inside the animal and both benefit.
• Students may have a difficult time understanding why both horses and cows eat grass, but have different digestive system. remind students of the foregut vs. hindgut fermentation difference, and how that affects the structure and function of their digestive systems.

• Student may have a difficult time understanding why all animals don’t digest plants well, but they all eat them. Remind students that cellulose requires special enzymes, which most animals (including humans) can’t produce themselves.

Assessment

The assessment for this lesson is spread throughout sections of this lesson. After students read a section, they will be prompted to answer one or two questions reflecting what they have just learned.

Further exploration

Other chapters within this textbook may be interesting for students. Sample chapters are linked below:

Why DO Pandas eat Bamboo?

Birds and Horses and Cats, Oh My!

Fore stomach, true stomach, red stomach, blue stomach