48-hour cultures of three bacterial types prepared previously by each lab team.
STUDENT INSTRUCTIONS
Retrieve your 48-hour cultures from the incubated rack. Take care not to jostle the broth cultures. Examine the broth tubes. Observe and record your findings as to degree of turbidity and position of growth (top, bottom, throughout, etc.). Classify the growth as to oxygen requirements. .
Sometimes the top growth of a strict aerobe becomes heavy enough to fall to the bottom of the tube and appear as though it grew there in the manner of strict anaerobic growth. Strict anaerobic growth may not occur at all under the circumstances of an aerobic inoculation. Therefore, special procedures for inoculation and growth must be followed if one expects a clinical lab specimen to demonstrate anaerobic metabolism of microorganisms.**
Examine the agar slants. Look for characteristics such as border pattern,
texture, shape, presence of dome, color, and size of colonies. There is
usually too little surface available on an agar slant to show isolated colonies.
Observe the characteristics as best you can with the dense growth. Refer to this
exercise once again after you have learned the streak plate technique and are
able to observe growth of isolated colonies on the agar surface in a Petri dish
(next lab).
Growth Patterns Determined by Oxygen Requirements
| TYPE OF ORGANISM | APPEARANCE OF BROTH AFTER INCUBATION |
| Control: sterile broth (no organism, no growth) | Clear Broth (no turbidity) |
| Facultative anaerobe | Turbid broth throughout tube |
| Microaerophil | Slightly turbid, particularly a few mm below surface |
| Strict aerobe | Growth only at surface |
| Strict anaerobe | Growth at bottom only** (refer to italicized note above regarding common errors in interpretation here; specialized media is required to ensure isolation and observation of anaerobic organisms) |