Yeasts are one of the earliest domesticated organisms, with ties to ancient bakeries and breweries. Yeasts are eukaryotic microorganisms that have similarly been found across a multitude of environments. These are primarily water, air, soil, and plant/fruit surfaces. Yeast actively partakes in the decomposition and fermentation of fruit as it ripens and decays. Fruit is the ideal nutrient source for yeast since they provide fermentable sugars, amino acids, vitamins, minerals, and oxygen. Interestingly, yeasts typically reproduce by budding; however, there are some species that will reproduce via cell fission.

Yeasts have high morphological divergence. They are unicellular eukaryotic fungi, with many cellular differences from bacteria. The most significant difference is its size at 5×10 µm in comparison to bacteria at 0.5 x 5 µm. This includes a nucleus, Golgi apparatus, mitochondria, endoplasmic reticulum, vacuole, and a cytoskeleton! All are key characteristics of eukaryotic cells.

Ironically at the visual level, yeast can be easily mistaken for bacteria since their colony morphology can appear very similar. Like bacteria, they are also defined by the characteristics of culture condition, colony shape, colony elevation, colony edge, colony size, colony surface type, colony opacity, colony color, and any other defining characteristics.

Let us look at the image on the left. This organism was successfully identified as Rhodotorula mucilaginosa. Looking at the sample, it is understandable how this can be mistaken for bacteria. If we go through their colony morphologies, they appear to be Sabouraud Dextrose Agar (SDA) 35 °C, circular, raised, smooth, small, glistening, opaque, and pink colony types.

Looking at just the colony morphology description, the similarities between Rhodotorula mucaliginosa and Rhodococcus fasciens are very similar. On an exclusively visual level, it would be very difficult to determine that Rhodotorula was yeast. However, there are some indicators from our morphology description that helps! SDA is a common media type that is used for the subculture of both filamentous fungi and yeast isolates. Pink circular colonies on SDA are a common characteristic of all Rhodoturula species. This is because its distinct color blocks out wavelengths of light (620-750 nm) that can be damaging to the cell itself. Other yeasts have different characteristics or phenotype testing that can help determine if the sample is yeast or not .

MALDI-TOF is an inexpensive option for identifying unknown organisms that we can test to determine an accurate identification regardless of if your sample is a yeast or bacteria. If Biolog Lab Services can get enough growth, our lab can speciate and differentiate between bacteria and yeasts with no change to the sample submission! Click this link for an example report of MALDI for both yeast AND bacteria!