3.1: Introduction

Learning Objectives

• Practice inoculating, growing and properly disposing of bacterial cultures.
• Observe the ubiquitous nature of bacteria.
• Practice aseptic techniques in preparing bacterial cultures.
• Familiarize yourself with various features of bacterial colonies and the basic cell shapes of bacteria.

Bacteria represent a diverse collection of organisms which can live in nearly every habitat on the Earth. They are an important group of organisms that have significant impacts on our environment, including playing important roles in nutrient cycling. We encounter bacteria daily in just about everything we interact with. Bacteria have been found to inhabit nearly every niche of our planet, and their microscopic size often makes it difficult to detect their presence. To provide us with insight into the ubiquitous nature of bacteria you will be collecting samples from your surroundings to evaluate their bacterial content. We will also be using our microscopes to get a closer look at the cellular structures and features commonly found in bacteria.

Different bacterial species can be differentiated by a variety of characteristics including the growth pattern they take when growing as colonies on nutrient agar as well as their microscopic features. Bacterial colonies are described based on several factors including form or shape, color, size, margin, elevation, and texture. The form of a bacterial colony reflects the general shape and can be classified using terms such as circular, irregular, filamentous or rhizoid. The elevation is a description of the side view and can be classified as flat, raised, convex, umbonate or crateriform. The margin is the edge of the colony growth. Several margin types are commonly observed including entire, undulate, filiform, curled, lobate, etc. Size of colonies can be influenced by the amount of competition on the agar plate and can be measured with a ruler as the diameter of the colony. Very small colonies are referred to as being punctiform. Some colonies will grow in a spreading manner and may even cover the entire plate or an entire region of the plate in what is referred to as a bacterial lawn. The texture is often observed on the surface of the colony. Common surface textures include smooth, creamy (glistening), waxy (dull), and veined. When examining the color of the colony it is also important to take note of the optical characteristics as well. For example, does the colony appear opaque, cloudy, translucent, iridescent. You will evaluate the colonies grown from the cultures that you collect based on these traits. (See Figure 3.1 below for reference of these patterns).

We will also be examining bacterial samples under the microscope to become more familiar with the diverse bacterial cell shapes including the primary shapes which are: coccus (round), bacillus (rod-shaped) and spirillum (spiral). Additional common bacterial cells shapes include a shorter rod known as a coccobacillus and a curved rod known as a vibrio.

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