Overview
This lecture covers essential aseptic techniques for transferring bacterial cultures among different types of microbiological media, emphasizing sterilization, labeling, and observation of bacterial growth characteristics.
Microbiological Media Types
- Petri dishes use solid agar, a gel-like growth medium.
- Slants are test tubes filled with solidified agar at an angle for increased surface area.
- Broths are liquid media where bacteria grow and turn the liquid cloudy.
Transfer Tools and Sterile Technique
- Inoculating loops (with a looped end) and needles (straight end) are used to transfer bacteria.
- Sterilize inoculating tools by holding them in a flame until red hot before and after use.
- Work within 6–8 inches of a flame to maintain a sterile environment.
Transfer Procedures
- Avoid overheating the loop before touching bacteria to prevent killing the cells.
- Use a small amount of bacteria; visible clumps are too much.
- When transferring, minimize exposure of plates and tubes to air to prevent contamination.
- For petri dishes, streak bacteria gently without fully opening the lid.
- Flame the mouth of test tubes before and after inserting the loop.
Incubation and Labeling
- Always incubate petri dishes upside down to prevent condensation from interfering with colony growth.
- Label media with organism name, date, initials, and type of medium, preferably on the bottom of plates or tube sides.
- Incubate cultures at room temperature or 37°C depending on experimental needs, for 24–48 hours.
Interpreting Bacterial Growth
- Observe colony color and growth patterns for identification clues.
- Serratia marcescens appears reddish-pink at room temperature, beige at 37°C.
- Note pigmentation and coverage as key identification factors.
Key Terms & Definitions
- Agar — Gel-like substance used as a solid growth medium for microbes.
- Inoculating Loop — Wire tool with a loop used to transfer microbes.
- Aseptic Technique — Methods to prevent contamination during microbial transfer.
- Incubation — Storing cultures under optimal conditions to encourage bacterial growth.
- Slant — Test tube with solidified agar set at an angle to maximize surface area.
Action Items / Next Steps
- Label all cultures clearly according to guidelines given.
- Incubate transferred samples at room temperature for 24–48 hours as instructed.
- Observe and record growth patterns, color, and other characteristics for each culture.