PREDICTING THE GROWTH OF

MICROORGANISMS

Objectives

Students will • Use sterile techniques to collect a sample • Observe the growth of microorganisms on a

nutrient agar plate • Create nutrient jelly ideal for growing microbes

for observation. • Understand the environment for culturing

microbial growth.

SUMMARY

This lab is designed to give students a “hands-on” experience to help answer the question: How can you determine where in

your environment you might find microorganisms?

KEY CONCEPTS

• Microbes are very small and have been around a long time

• Microbes are everywhere; they are extremely abundant and diverse

Growth is in a general manner a process where an individual organism is becoming larger.

Bacteria, however, are so small that it is almost impossible to measure the growth of an individual. Therefore, the growth of bacteria is described through the size of their colonies. In relevant conditions, bacteria are growing so well that the number of bacteria increases twice in an hour or even with a shorter time.

• Bacteria are very small, one celled, microscopic organisms that are found in air, water, soil, and the bodies of other living organisms. If provided with the right conditions they will multiply into a visible colony of bacteria cells.

• The nutrient jelly provides a nutritious environment where the bacteria and fungi can grow into a visible group of cells (colony).

PRECAUTIONS

Most bacteria and fungi collected in the environment will not be

harmful. However, once they multiply into millions of colonies in a

petri dish they become more of a hazard. Be sure to protect open cuts

with rubber gloves and never ingest or breathe in growing bacteria.

Keep growing petri dishes taped closed. When the experiment is

finished, a teacher or demonstrator should safely destroy the fuzzy

bacteria colonies.

BACTERIAL COLONY MORPHOLOGY

Bacteria grow on solid media as colonies. A

colony is defined as a visible mass of

microorganisms all originating from a single

mother cell, therefore a colony constitutes a

clone of bacteria all genetically alike.

Edges (also called margin)

BACTERIAL SHAPES

PETRI DISH

Is a shallow cylindrical

glass or plastic lidded

dish that biologists use

to culture cells – such

as bacteria

AGAR

Is a jelly-like substance, obtained from algae

Where can we find bacteria?

How can you find bacteria? You can’t see it, so how do you find it?• Under the right conditions, bacteria grows very fast,

exponentially• Some bacteria populations can double every 10

minutes• Bacteria grow in colonies – a colony is a visible mass of

bacterial growth on solid medium that developed from 1 or a few cells

• A colony can contain millions of cells• We can find bacteria by growing it in the right

conditions until it is visible to the human eye

How to Sample Bacteria• Get a sterile swab – don’t touch the cotton end• Swab the area to be tested• Open your Petri dish carefully and swab the

cotton in the specified area on the agar• Close the Petri dish quickly• Get a new swab for each sample• When done, close the Petri dish and use scotch

tape to seal the sides• Put the Petri dish upside down in a dark, warm

place