Most colonies on streak plates grow from isolated colony-forming units (CFUs). O

Question

Most colonies on streak plates grow from isolated colony-forming units (CFUs). On rare occasions, however, a colony can be a mixture of two different organisms. If a culture is started from this colony (thinking it is pure), correct identification will be next to impossible because the extra organism could confound the identifying test results. How could you verify the purity of a colony? (The answers may vary depending on what experience you have had prior to performing this exercise.) If you found the colony to be a organisms, what could you do to purify it?

Explanation / Answer

A mixture of colonies can be found only after performing the gram staining procedure.Before that the colony characteristics like margins, elevation, color and appearance can be observed with the naked eye by carefully observing the streaked plates. During the staining process the colony used for smear preparation must be marked. Two or more different smears are prepared from the mixed culture. After the staining procedure, the smear must be carefully observed for the colony morphology (bacilli or cocci or streptococci or staphylococci or diplococi etc). The color of appearance (purple or pink) must be observed to identify whether the smeared culture contains gram positive or gram negative bacteria. Once The given culture was found as gram positive or negative, then biochemical characteristics are studied. These include a wide variety of tests like fermentation of different carbohydrates, IMVIC tests, H2S reduction, catalase and Oxidase tests, Growth at different temperatures, pH and salt concentrations; Nitrate reduction gelatin liquifaction and antibiotic sensitivity tests. Finally, the results observed are compared with the characteristics given in Bergy's manual to obtain an idea about the organisms present in the mixed culture. Further, they can be confirmed through 16SrDNA sequencing to particularly know about which species is present in the culture.

Related Questions

Navigate

• Browse (All)
• Browse M
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.