- the foundations of food microbiology
- the role of microorganisms in nature
- the six basic microbial groups – bacteria, yeasts, molds, protozoa (multicellular and unicellular), viruses and prions
- the basics of microbial biota in soils, water, on plants, on animals, on food handlers, and in air or dust
Food Microorganisms Food microbiology encompasses the study of microorganisms that have either a beneficial or deleterious effect on the quality and safety of foods. Food microbiology focuses on the microorganisms that are found in and around foods. It includes studies of microbial growth characteristics, identification, and prevention. Specific areas of interest are: foodborne illness, food spoilage, food preservation, sanitation, and food legislation. These interest areas comprise this course. The microbiological world consists of bacteria, yeasts, molds, viruses, parasites, algae and prions. All except algae are considered food microorganisms. Let’s look at each group briefly. Bacteria Bacteria are prokaryotes because they do not possess nuclei. They have a normal double stranded DNA genome. Sometimes there may be extracellular DNA fragments known as plasmids. Plasmids can often encode proteins that make the cell enhanced
such as antibiotic resistance. Plasmids can be shared with other cells usually of a similar genus or species. The DNA clusters in the center of the cell forming a nucleoid. (If it had a membrane around it, it would be a nucleus). Closely associated with the nucleoid are the ribosomes. Ribosomes translate DNA into amino acids which are the building blocks of proteins. Discuss plasmids and food microbiology The main interior is called the cytoplasm. This is where water, enzymes, and nutrients all work to power the cell. There are three main functions, energy production, metabolism and catabolism. Energy naturally is in the form of ATP generated from metabolic reactions. Metabolism also consists of enzymes that build up the components of the cell. Catabolism consists of enzymes that break down nutrients. All three
Figure 1. Diagram of bacterial structures.
Discuss catabolic enzymes and food microbiology The cytoplasm is surrounded by a cytoplasmic membrane which determines what goes in and out of the organism. Water, dissolved gases (e.g. CO2 and O2) and lipid-soluble molecules simply diffuse across the phospholipid bilayer. Water-soluble ions generally pass through small pores in the membrane. All other molecules require carrier molecules to transport them through the membrane. Discuss membrane function and food microbiology Immediately outside of the cytoplasmic membrane is the cell wall. This is the component that stains differently in the Gram stain. Red is gram negative a thinner peptidoglycan cell wall. Blue or purple is gram positive usually a much thicker peptidoglycan cell wall. Its main function is structural. The cell wall provides the bacteria shape: cocci, rods, spirals, etc.
External to the cell wall in SOME bacteria is a capsule layer. This is a polysaccharide/protein layer that can protect the cell from desiccation. There are other benefits. Similar to a capsule, some cells produce a slime layer. The major difference in the two is the rigidity of the polysaccharide. Pili are the next structures external to the cell wall and capsule. These are hair-like structures that assist in attaching to other cells and surfaces. Some specialized pili are used for passing plasmids between bacterial cells.
Discuss functions of slime layers and pili in food microbiology
Flagella are long hair-like structures that are used mostly for motility. Not all bacteria possess flagella and most that do are motile. The last structure we’ll cover is the spore. Spores are produced by some species making them resistant to hostile conditions such as heat and drying. They serve as survival mechanisms when environmental conditions are not suitable for growth and replication.
Figure 2. Diagram of cell wall and cell membrane. The cell wall is above and the cell membrane below in the diagram. Above the cell wall is the external environment and below the membrane the cells’ cytoplasm.
Figure 3. Slime layer (above) and capsule (below).
Yeasts are eukaryotic microorganisms classified in the kingdom Fungi (along with molds) There are over 1500 known species. Most reproduce asexually by budding. There are no known food pathogenic (illness causing) yeasts. There are a few medically pathogenic yeasts such as Candida. Most yeast are fermentative, meaning they ferment carbohydrates. As they ferment these carbohydrates they produce byproducts. In some cases the byproducts are desired, e.g. wine and beer. In other cases the byproducts are considered spoilage. The genus Saccharomyces (sugar fungi) is one of the most common yeasts responsible for wine, beer, and bread. Yeast are common in the environment and can be found in higher numbers where there are high levels of sugars.
Molds are also eukaryotic fungi. They differ from yeasts in that they are multicellular. There are thousands of species of molds. They grow structures called hyphae and reproduce by producing spores. Molds secrete catabolic enzymes from their hyphae to “dissolve” organic matter and make the byproducts available for their growth. Molds can grow in extreme conditions, including dry and acidic. Different molds are able to degrade starch, cellulose and lignin into sugars. Molds also secrete other molecules that can have functions related to food microbiology. On the beneficial side some secrete antibiotics. On the negative side a few secrete mycotoxins. Mycotoxins are chemicals that make humans ill.
Parasites are organisms that derive nourishment and protection from other living organisms known as hosts. Like yeasts and molds, there are thousands of species. Parasites that use humans as hosts are considered pathogenic. Unlike bacteria, yeasts or molds, they
have no cell wall, but instead have a cytoskeleton. In that respect they function more like animal cells. Parasites that are present in foods that cannot infect a
Figure 4. Yeast cells in scanning electron microscopy. Note the cell buds (new yeast cells).
Figure 5. Mold. Hyphae at the bottom and spores at top.
Figure 7. Giardia (protozoan) trophozoite.
Figure 6. Giardia cyst.
human are considered adulteration. There are no food “spoilage” parasites. The harm they do may be small to quite significant.
Parasites are of different types and range in size from tiny, single-celled, microscopic organisms (protozoa) to larger, multi-cellular worms (helminths) that may be seen without a microscope. The protozoa typically have a life cycle where they go from an active unicellular microbe (trophozoite) to a spore-like cyst. The cyst form is dormant and can withstand harsher environmental conditions. Most often a host is required to transform from cyst to active form creating a life cycle.
The helminthes are probably the scariest of the food microbes and the definition of “micro” may no longer apply. Some of these worms grow to 1-2 meters in humans and one species reaches 12 meters in cows. The worms can be flat (a tape worm) or round.
Viruses are “parasitic” organisms that can only reproduce inside a host. They are extremely small and cannot be seen in normal microscopy. There are no food “spoilage” viruses. Viruses unlike all of the rest of the food microbes above can have either DNA or RNA. It’s genome is surrounded by a protein coat. Some steal lipid layers from their host to have a third layer. Basically, the virus must invade a host cell, take over its reproduction system, produce new viruses, then finally lyse the host cell to start the process over. Fortunately, viruses do not survive well outside of hosts. The main pathology of viruses is due to this cell lysis. However, there are some newer theories looking at the virus genetic mutations and cancers. Human papillomavirus is one such virus. Viruses can use bacteria as hosts. When the bacteria are desired cultures, such as in cheese-making, this can become a major problem.
Prions are unique microorganisms, if they can even be called microorganisms. They are made up of only protein, no nucleic acids. Prions are believed to induce misfolding of native (targeted) proteins that normal cells make. These abnormal proteins become prions themselves. Prions are responsible for several spongiform encephalopathy’s. As the number of misfolded proteins increases a localized area or gap forms in brain tissue. The progression of the disease is dependent on the creation rate of the native host protein that is targeted by the prion. Some prion diseases take many years to decades to show pathologies. SUMMARY Food microbiology encompasses the study of microorganisms that have either a beneficial or deleterious effect on the quality and safety of foods. This food microbiological world consists of bacteria, yeasts, molds, viruses, parasites, and prions. A basic understanding of each microbial group is required to later examine their good or bad effects on food and human health.
Write a one to two pages summary of the learning objectives for (Taxonomy). Be sure and include information from the textbook ( see attachment files)