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Examination and the Use of Yeast and Moulds

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Fundamentals of Microbiology

Report 1 – Exercise 10: examination and the use of yeast and moulds

Introduction

Yeasts and moulds belong to the domain eukaryotes, under the group Opisthokonta, essentially classified under the kingdom Fungi. Even though most of fungi reproduce asexually, some can reproduce sexually. Both yeast and moulds thrive under an optimal temperature of 20-30°C, slightly acidic pH.

Yeast, which is a single-celled organism, can reproduce in both methods but give priority to asexually, in the form of budding. Budding is essentially the formation of an identical daughter cell via mitosis (called a bud) that eventually separates from the parent cell via cytokinesis. The daughter cells mostly stay attached to the parent cells, sometimes causing large chains of the spherical cells to form. Sexual reproduction involves the formation of haploid spores via meiosis. These spores are called meiospores, also known as ascospores (Delfini and Formica, 2001). The spores are extremely thick-walled and are part of the defensive survival mechanisms of most fungal and bacterial cells as they can resist some physical factors such as temperature and pH. The spores eventually germinate and produce a germ tube, which develops into a somatic hypha after growing via mitosis.

Filamentous fungi (moulds), however, are different in that they are multicellular and grow as hyphae. Furthermore, they share a similar characteristic as to a plant cell in that they have a rigid glucose-chitin cellulose cell wall. They reproduce asexually via spores. The type of spores released depends on where they originate (arthrospores, sporangium, conidiospores) and are crucial for fungal reproduction and dispersal. Moreover, these fungi can reproduce asexually via zygospores, ascospores and basidiospores.

The most common method of examining fungal cells under a microscope is LPCB staining, where the substance lactophenol cotton blue (LPCB) is dropped onto the microscopic slide showcasing the fungi. The phenol in LPCB kills off any living organisms, while the lactic acid ensures a preservation of the fungal structures. Cotton blue is used to stain the chitin cellulose cell wall with an intense blue colour.

A common media used to cultivate fungal cells is Malt Extract Agar (MEA) since its pH is around 5.4 and contains digests of animal tissues which is a nutrition source for fungal growth (Manual of laboratory methods in medical mycology, 1948). Furthermore, MEA is usually kept at 20°C to ensure that fungal growth is favoured, so that bacteria does not contaminate the samples. As a result, MEA plates are selective for fungi. However, fungi are also able to grow on most standard bacterial media.

Objectives

The following experiment was conducted to stain via LPCB and understand the morphological characteristics of yeast and fungal cells and to attempt to fulfil the objective of culturing yeast cells.

Materials and Methods

The experiment followed the steps referred to in the Fundamentals of Microbiology Practical Lab Manual’s pages 68-69. However, since sterile toothpicks were not supplied, the use of sterile toothpicks in steps 1.6 and 1.12 were replaced by pouring 2mL of saline solution directly into the yeast.

Results

Three plates were cultured (Saccharomyces cerevisiae, Baker’s yeast, Brewer’s Yeast), its colony morphology was observed before and after incubation at 35°C for 1 week. The following table showcases the colony morphology that was observed after incubation at 35°C for 1 week and observed under the microscope.

Colony Morphology of each fungi

Type of yeast

Observation

Saccharomyces cerevisiae

Beige coloured, round with raised margin, convex

Baker’s yeast

Beige coloured, L form configuration, umbonate elevation, smooth margin.

Brewer’s Yeast

Beige coloured, large, L form configuration, umbonate elevation, smooth margin.

Table 1 – Table of Colony Morphology of each type of Yeast in experiment

Discussion

It was observed that all 3 yeasts had gone through the asexual process of budding. Furthermore, there was a large amount of count of yeast cells after the incubation period. This may have resulted in inaccuracies of the experiment, as further nutrition was not supplied to the plates, causing competition and nutrient availability becoming a limitation factor for the cells. It was observed that Brewer’s yeast contained cells that were larger in size compared to the others, while Baker’s yeast contained cells that were the smallest. Saccharomyces cerevisiae contained cells that did not have an L form configuration, unlike the other two yeast cells, and had a round convex shape with a raised margin. However, the other two yeast samples had umbonate elevation and a smooth margin. Further investigation into the study concluded that both Baker’s yeast and Brewer’s yeast each consisted of a specific strains of Saccharomyces cerevisiae chosen according to their carbon dioxide productivity for Baker’s and alcohol productivity for Brewer’s (Cauvain and Young, 2007). It was concluded that the Saccharomyces cerevisiae used was therefore multiple strains due to having features that are similar to both types of yeasts. It was concluded that different yeasts were used in different scenarios because different strains fermented alcohol, dough and other industrial products in different rates, and the most economically beneficial strains were chosen for each type of industrial production.

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