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Food Biotechnology Dr. Kamal E. M. Elkahlout Food Microbiology 1 Characteristics of Predominant Microorganisms in Food
OBJECTIVES To explain the importance of microorganisms in food. To classify bacteria , yeast, mold and viruses based on the taxonomic classification To name and identify microorganisms based on the nomenclature.
INTRODUCTION � 4 major groups � Bacteria � Yeast � Mold � Viruses �Bacteria , yeast and mold are important in food for 3 reasons: � Ability to cause food borne diseases � Food spoilage � To produce food and food ingredients �Many bacterial species and some molds and viruses but not yeast are involved in food borne diseases �Most bacteria , mold and yeast due to their ability to grow in foods (viruses cannot grow in foods) are potentially capable of causing food spoilage
Bacteria forms the largest group due to their ubiquitous presence and rapid growth rate , even under conditions where yeast and mold cannot grow. They are considered the most important in food spoilage, food borne disease , as well as in developing methods to control microorganisms in food.
NOMENCLATURE �Basic taxonomic group – bacteria, yeast and mold is the species �Each species is given a name. 1 st – genus name 2 nd - specific epithet (adjective) Both parts are Latinized ; when written they are italicised (or underlined). e. g Saccharomyces cereviasiae, Lactobacillus acidophillus E. g. Lactobacillus acidophilus
Classification of Microorganisms Bacteria – prokaryotes (before nucleus) Mold , Yeast – Eukaryotes (with nucleus) Viruses – not considered living cells thus not included in classification system.
BACTERIA Are unicellular, most ca 0. 5 -1. 0*2. 0 -10μm Have three morphological forms: spherical (cocci), rod shaped (bacilli) and curved (comma) Can form associations such as clusters, chains (two or more cells) or tetrads. Can be motile or non motile. Cytoplasmic materials are enclosed in a rigid wall on the surface and a membrane beneath the wall Nutrients in the molecular and ionic form are transported from the environment through the membrane by several but specific mechanisms.
Membrane also contains energy generating components and forms intrusions in the cytoplasm (mesosomes) Cytoplasmic material is immobile and does not contain organelles enclosed in a separate membrane Ribosomes are dispersed in the cytoplasm Genetic materials(structural and plasmid DNA) are circular, not enclosed in nuclear membrane and do not have basic proteins such as histones. Cell division is by binary fission
Prokaroyatic cells can also have flagella, capsules, surface layer proteins and pilli for specific functions. Some have endospores (one per cell) A species is regarded as a collection of strains having many common features A strain is the descendant of a small colony (single cell). Among the strains in a species , one is assigned as the type strain , it is used as a reference strain while comparing the characteristics of unknown isolate.
Bacteria Shapes and sizes Typical bacteria measure 1 - 3µm in length and 0. 4 – 1µm in width. Can occur larger or smaller than this. They can occur in associations such as clusters, chains (2 or more) and tetrads.
Bacterial internal cell structure Limited number of cell components. Cell membrane for transport of molecules, energy production and membrane permeability, carry enzymes for metabolism. Cytoplasm consists of mostly water with inorganic and organic molecules, DNA, ribosomes and inclusions.
�Nuclear area contains the DNA of bacterial chromosomes. It can also contain plasmids which are circular, extra chromosomal DNA molecules. �Ribosomes (70 S) consisting of m. RNA and proteins is the site for protein synthesis. �Inclusions are reserve deposits that include polysaccharides granules (glycogen or starch), lipid inclusions, metachromatic granules (inorganic phosphate), sulfur granules, carboxysomes and gas vacuoles. �Endospores – formation of resting spores to allow survival during adverse environmental conditions. Sporulation (formation of endospore). Germination (return of an endospore to germination state following favorable conditions).
Bacterial external cell structure Glycocalyx substances secreted by bacteria that surround the cells. Composed of polysaccharide and polypeptide or both. When firmly attached to the cell wall, it is known as a capsule which act as protective covering and may vary considerably in thickness. Capsules enable the bacteria to adhere to surfaces, prevent desiccation, provide nutrients and protect pathogens from phagocytosis.
Flagella are long filamentous appendages consisting of a filament, hook and basal body. Prokaryotic flagella rotate to push the cell. Motile bacteria exhibit taxis that is movement towards or away from a stimulus. Positive taxis is movement towards an attraction. Negative taxis is movement away from repellent. Examples Phototaxis (light is the stimuli) Chemotaxis (chemicals are stimuli)
Axial filaments allows movement in spiral cells (Spirochetes) by means of an axial filament (endoflagellum). Axial filaments are similar to flagella, except they wrap around the cells. Fimbriae and Pili are short thin appendages. Fimbriae help bacterial cells to adhere to surfaces and Pili join cells for transfer of DNA from one cell to another during reproduction.
Bacterial cell wall structure Cell wall surrounds the plasma membrane and protects the cell from changes in water pressure. Bacterial cell wall consists of peptidoglygan, a polymer consisting of monosaccharides N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
Gram reaction Reaction to specific staining procedure (gram reaction) is one of the most important features of bacteria. The technique divides the bacteria into two types: Gram positive and gram negative. Gram-positive bacteria retain the crystal violet colour while gram-negative bacteria are decolorized.
Gram reactions of bacteria Gram positive Gram negative
Gram positive bacteria Cell walls consist of many layers of peptidoglycan. Also contain two types teichoic acids. Teichoic acids are negatively charged (due to phosphate groups) and may bind to or regulate the movement of cationic molecules in and out of cell. Some species have a layer over the cell surface called the surface layer protein (SLP).
Gram negative bacteria Complex cell wall containing an outer membrane (OM) and a middle membrane (MM). OM is composed of lipopolysaccharide, lipoprotein and phospholipids. MM is composed of thin layer of peptidoglycan.
Yeasts and Molds Both are eukaryotic Yeasts are unicellular and molds are multicellular Eukaryotic cells are generally larger (20 -100μm) than prokaryotic cells (1 -10 μm)) Have rigid cell walls and thin plasma membranes Cell wall does not have peptidoglycan is rigid and is composed of carbohydrates Plasma membrane contains sterol Cytoplasm is mobile (streaming) and contains organelles (mitochondria, vacuoles) that are membrane bound Ribosomes are attached tot eh endoplasmic reticulum DNA is linear (chromosomes), contains histones, and is enclosed in a nuclear membrane Cell division is by mitosis (asexual reproduction); sexual reproduction when it occurs is by meiosis
MOLD Non motile, Filaments, Branched Cell wall is comprised of cellulose , chitin or both Composed of large numbers of filaments called hyphae – an aggregate of hyphae is called mycelium Hyphae can be nonseptate, septate-uninucleate or septatemultinucleate. Hyphae can be vegetative or reproductive Reproductive hyphae usually extends in the air and form exospores either free (conidia) or in sack sporangium. Shape , size and color of spores are used in taxonomic classification
Zygomycota – common molds The fungal mass of hyphae, known as the MYCELIUM penetrates the bread and produces the fruiting bodies on top of the stalks Mycelia = a mass of hyphae or filaments
YEAST Widely distributed in nature Cells are oval , spherical or elongated (50 to 30 x 2 to 10 um in size) Non motile Cell wall contains polysaccharides (glycan) , protein and lipids. Wall can have some scars indicating the sites of budding Membrane is beneath the wall The cytoplasm has a fairly granular appearance for ribosomes and organelles. The nucleus is well defined with a nucleus membrane
VIRUSES Non cellular entities Bacterial viruses (bacteriophages) important in microbiology and widely distributed in nature. Composed of nucleic acids (DNA and RNA) and several proteins. The proteins form the head (surrounding the nucleic acid) and tail. 2 important viruses implicated in food borne outbreak are hepatitis A and Norwalk-like viruses.