Basic Yeast Morphology

Questions about identifying yeast, by their morphology as seen through a microscope, come up frequently in brewing forums. I also receive frequent emails from brewers hoping I can help them identify the yeasts in pictures from their brewery microscope.

Unfortunately, you cannot identify a yeasts species (or even genus) purely from an image. But, an image can be an important first step in identifying your yeast. Where brewers often run into trouble is that there is a lack of visual guides to help them learn the language of yeast morphology. This post is intended as an simple introduction to the basics of yeast morphology, and the language used to describe it.


The term “morphology” may be a new one for some brewers. The term “morphology” means the form and structure of organisms and their specific structural features. For yeast, this generally refers to four key characteristics that we can see through a microscope:

  1. Cell shape – the overall shape of a single cell.
  2. Cell division pattern – the pattern of the positioning of where yeast cells bud, and the shape of the buds themselves.
  3. Cell clustering patterns – the patterns formed when multiple yeast cells stick together.
  4. Vacuolation – the presence, number and size of visible vacuoles in a yeast cell.

I’ll cover each of these below.

1. Cell Shape

Yeast cells come in a range of shapes: circular/ovoid, apiculate and elongated.

circular yeast morphology
Circular/Ovoid: Yeast with a circular or ovoid morphology appear as cells that are round to oval in shape. If oval, the ends are rounded but not pointy. It is not uncommon for a pure strain of yeast to show a mix of circular and ovoid cells, with ovoid cells being those which are about to divide, or which have divided recently. Brewers yeast (Saccharomyces cerevisiae, image above) is a classic example of yeast with an ovoid morphology.
Apiculate yeast morphology
Apiculate: Apiculate yeasts are oval yeast with pointed ends. Often, these are described as appearing similar to the shape of a lemon. Apiculate yeasts often show a mixture of highly ovoid and apiculate morphologies (as in the image above, as the overall shape of the yeast cell changes as the cells age. In fermented beverages, most apiculate yeasts belong to the Hanseniaspora/Kloeckera genus of yeast.
Elongated yeast morphology
Elongated: Yeast with an elongated morphology have cells which are long rod-like cells with straight edges, or highly elongated ovals. Yeast with this morphology often grow in linear or branching chains, rather than as single cells. Brettanomyces is a classical example of this morphology, and are shown above.

2. Cell Division Pattern

With only one exception (Schizosaccharomyces), yeast divide by budding. What this means is that a new yeast cell (called the daughter cell) will be formed as a miniature yeast cell extending off of the mother yeast cell. While all yeast bud, they show two predominant budding patterns: end-budding (technically called polarized budding) and budding (or unpolarized budding).

End-Budding: Many yeasts – especially those which are apiculate and elongated – bud solely from the ends of the cell. This is well reflected in the image above, where (from top-left to middle, to right) cells with a small bud, developed bud, and completely divided bug, can be observed.
Budding Yeast: Circular and ovoid yeasts tend to bud from random positions on the cell. Some yeasts may even form multiple buds off of a single mother cell. Saccharomyces cerevesea is a classical example of a budding yeast.

3. Cell Clustering Patterns

In liquids (e.g. beer) most yeasts grow as single cells, with daughter cells breaking free of their mother as soon as budding is complete. However, some yeasts will undergo incomplete cell division, resulting in the formation of pseudohyphae (chains) of yeast cells.

Single cell morphology
Single Cells: Cells which do not grow as pseudohyphae are easy to identify, as cells will grow without significant clumping or forming into chains. This image is of Voss Kveik, a strain of the single-celled Saccharomyces cerevesea.
Pseudohyphae morphology
Pseudohyphae: Cells which form pseudohyphae can show a range of patterns, ranging from disorganized clumps of cells the long chains (branched or unbranced) of cells. Both of these morphologies can be seen in the above image, which was a Brettanomyces bruxellensis I identified as a contaminant in White Labs Vrai strain.

4. Vacuolation

Vacuolation: Different species of yeast will often display different numbers and sizes of vacuoles.

The final cellular feature we can (sometimes) see are yeast vacuoles – specialized organelles (technically lysosomes) which are acid-filled compartments inside of the yeast cell used to breakdown food and cell wastes. Yeast can have a one or more vacuoles, and they can vary greatly in size. Some yeast’s vacuoles are too small to be seen under a microscope, while others are so large that they occupy most of the interior space of the yeast. Identifying vacuoles can help identify a yeast – but they can also be misleading. The size and number of vacuoles can change in some species of yeast, often in response to the availability of food.

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