A tetrad appears as a square of four cocci (arrows).It is difficult with a conventional light microscope to tell a tetrad arrangement (square of four cocci) from a sarcina arrangement (cube of eight) so in our lab, anytime you see a square of four cocci, say it is either a tetrad or a sarcina arrangement.

Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
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