Microbial and Fungal Diversity

Microorganisms are defined as life forms too small to see with the naked eye. But while we don't typically see them, microbes are everywhere, doing good things like helping our digestion or making yogurts and wine, to not-so-good things like causing infection or disease.

Speaking of infections, back in 1928, Alexander Fleming returned to his laboratory from vacation, and noticed a strange fungus contamination on one of his bacterial culture plates. Interestingly, the bacterial colonies immediately around the contamination had been destroyed. This observation led to much further research, eventually culminating in the development of penicillin and other antibiotics that could treat bacterial infections. Yet, although they may live side-by-side in the microscopic world, we know from studying their forms and phylogenies that bacteria and fungi are vastly different and diverse organisms. Let's walk through some of the defining characteristics of these two major groups briefly.

Bacteria are unicellular prokaryotes, meaning that that they are single-celled organisms without a nucleus. All bacteria have a cell wall, but its composition and thickness vary. Gram positive bacteria have a cell wall with a thick peptidoglycan layer, which readily retains crystal violet molecules in gram staining. However, gram negative bacteria have a thin cell wall surrounded by an outer lipopolysaccharide membrane, and they only retain the counterstain, the red-colored safranin. Gram staining allows observation of bacterial shapes as well. Round bacteria are called cocci. Rod-shaped bacteria are called bacilli. And spiral-shaped bacteria are called spirilla. Furthermore, bacterial colonies can also be characterized with the naked eye, using traits such as color, shape and size of the colony.

Now, let's take a look at the fungi. Unlike the bacteria, fungi are eukaryotes, and thus their cells have membrane-bound organelles and a nucleus. They are commonly classified into groups based on their reproductive strategies. For instance, Ascomycota, or sac fungi, which includes morels, penicillium, and truffles, have fruiting bodies called ascocarps. Morels are filled with these structures. If we look at these more closely, they contain many smaller structures called asci, which in turn contain many ascospores formed during sexual reproduction. However, Ascomycota often reproduce asexually by budding. The second major fungal group we will examine is the Basidiomycota, or club fungi. Most edible and poisonous mushrooms in the forest are actually reproductive structures of the Basidiomycota. In this group, what we would refer to as the mushroom cap is technically known as the basidiocarp. In gilled fungi, gills under the basidiocarp are lined with tiny structures know as basidia, and these produce basidiospores, microscopic reproductive spores, during sexual reproduction. These spores are typically disbursed by wind, similar to many plant seeds. Here, we have examined several major bacteria and fungi groups, and the characteristics that define them in a very simplified fashion. However, due to the vast numbers of microbial species, upwards of one trillion by many estimates, scientists often identify and group them using additional methods, including examination of their DNA.

In this lab, you will identify different bacteria species using gram staining, and examine colony characteristics. You will also examine fungi species to identify their reproductive structures.