Microbes in the Environment

Introduction

This exercise introduces the foundational laboratory techniques for isolating and observing microorganisms from environmental and human sources. Students will learn about culture media, aseptic technique, and the process of inoculating and incubating microbial samples to study colony morphology.

Culture Media

Types of Culture Media

Culture media provide the nutrients and environment necessary for microbial growth in the laboratory. Two primary types are used in general microbiology labs:

• Nutrient Broth: A liquid medium where bacteria grow suspended in the fluid. Growth is indicated by turbidity (cloudiness) in the test tube.

• Nutrient Agar: A solid medium made by adding agar powder to nutrient broth. Bacteria grow as visible colonies on the surface, with each colony typically representing a single bacterial species. Used in Petri dishes for isolation and observation.

Additional info: Agar is a polysaccharide derived from seaweed and is not metabolized by most bacteria, making it ideal for solidifying media.

Labeling Petri Plates

Proper Labeling Technique

Accurate labeling is essential for tracking samples and ensuring reliable results. Each student will use two Petri dishes:

• Label the bottom (agar side) of each plate with tape.

• Include: your name, day of the week, date, and the source ("Environment" or "Body").

• Draw a line down the center to divide the plate into two halves for separate samples.

Inoculation Techniques

Environmental Samples

One Petri plate is used to collect two different environmental samples, one on each half:

• Examples: floor, workbench, doorknob, air exposure, phone, backpack, bathroom sink, pen, laptop, shoes.

• Swab Technique:

  1. Wet a sterile swab in sterile water.

  2. Swab the chosen surface.

  3. Swab the corresponding half of the agar plate.

  4. Label that half clearly.

  5. Discard the swab in disinfectant.

Body Samples

The second Petri plate is used to collect two samples from the body:

• Half A – Fingers: Press fingertips onto the agar surface.

• Half B – Hair: Place a single hair directly on the agar.

• Other options: swab skin, press fingernail, cheek swab.

• Label each half clearly and keep the lid on when not inoculating to maintain aseptic conditions.

Incubation of Plates

Incubation Procedure

After inoculation, plates are incubated to allow microbial growth:

• Inverted Position: Plates are placed upside-down to prevent condensation from dripping onto the agar, which could disrupt colony formation or dry out the medium.

• Temperature: Incubate at 37°C, which is optimal for many human-associated bacteria.

• Duration: Plates are incubated until the next lab period, when results will be observed and described.

After observation, contaminated plates must be disposed of in a biohazard bag for autoclaving to ensure safe decontamination.

Describing Microbial Colonies

Colony Morphology

Next week, students will describe colonies using the following characteristics:

• Whole-Colony Appearance: Circular, irregular, biconvex, filamentous, rhizoid

• Margin (Edge): Entire, undulate, lobate, filamentous, curled

• Elevation: Flat, raised, convex, umbonate

• Also note: pigment (color) and texture (shiny, matte, granular)

Additional info: Colony morphology helps in the preliminary identification of bacterial species and is a key skill in microbiology labs.

Laboratory Safety and Waste Disposal

Biohazard Precautions

All contaminated materials must be disposed of properly to prevent the spread of potentially harmful microbes. Used plates are placed in biohazard bags and autoclaved, which uses high-pressure steam to sterilize materials.

Summary Table: Key Steps in Microbial Culture Lab

Preparation for Next Lab

Students should complete the Canvas quiz for Exercises 1 & 2 and read Exercise 4 in preparation for the next session. There may be a pop quiz covering these materials.