This exercise will introduce you to the “streak plate for isolation” technique used to obtain isolated colonies. You will also transfer a sampling of a pure culture to a sterile agar slant. During your next lab period, you will observe the media you inoculated for visible growth and your broth culture for turbidity or cloudiness which indicates growth. This sampling of a bacterial culture is called the inoculum. In this exercise, you will transfer a sampling of a pure culture (single strain of bacteria to new, sterile media. It does not contain any inhibitory substance. This means it contains a readily utilizable carbohydrate source (such as glucose), vitamins (often provided by beef or yeast extracts), and mineral salts.

TSA contains all the nutrients necessary for successful isolation of the most frequently cultured types of bacteria. In this lab, you will be using a basic nutrient media called Trypticase Soy Agar or TSA. Media can be in liquid form, known as broth or a solid form called agar, referring to the solidifying ingredient. Nutrient material suitable for the cultivation of microorganisms is called media. Crowded colonies rapidly deplete the nutrients available in the media and thus do not grow as large or exhibit the typical characteristics of those unencumbered by dense growth. from the adjacent colony) when studying the characteristics of a colony such as the form, elevation, margin, size, or color.

It is important to observe a well-isolated colony (>5 mm. At least a million bacteria must be present in a colony for the unaided human eye to see the colony!Ĭolonies come in a variety of types. Since all the cells in a colony descend from a single parent that reproduced asexually, they are expected to be identical to each other. When we spread bacterial cells on plated media to achieve separation of the cells, each cell reproduces many times and gives rise to a colony. If the culture contains a single species of microbe, it is called a pure culture, whereas the isolation of more than one species of microbe is called a mixed culture. Since specimens taken from the human body, animals, soil, water or food seldom contain just one species of microbe, it is essential that we separate each individual type of microbe to observe its cultural, morphological, and physiological characteristics, as well as determine the effect of that microbe on its environment. If you always use aseptic technique in the laboratory, you should have only the desired organism growing on your cultures, and you should not contaminate yourself or others around you as you work. Bacteria may unwittingly be introduced from the surface of our skin, from coughing or sneezing onto media, from using an inoculating loop that has been insufficiently sterilized, from swabs that are not sterile, or exposing the media to the air. Aseptic techniques (procedures used to avoid contamination) have been devised to prevent these contaminating microbes from entering our cultures.

Therefore, it is essential to handle microbes in such a manner to prevent contaminating laboratory media and to prevent infecting ourselves or classmates. Microorganisms are ubiquitous, meaning they are everywhere.

Aseptic technique types