CHAPTER 1 Classification of Microorganisms and Food Microbiology Generalities
ALFREDO IVANOE GARCÍA-GALINDO,1 LEONARDO SEPÚLVEDA2 and CRISTÓBAL NOÉ AGUILAR
1Center for Interdisciplinary Studies and Research, Autonomous University of Coahuila Saltillo, México
2Bioprocesses and Bioproducts Research Group, Food Research Department, Autonomous University of Coahuila Saltillo, México, E-mail:
[email protected] (C. N. Aguilar)
ABSTRACT
Each life form is part of one of the five known kingdoms: Plantae, Animalia, Protista, Fungi, and Monera. The structural unit of life is the cell, in which genetic information is replicated and stored, cellular components are synthesized, and energy is generated, also there are cells which have mechanisms for movement. Plants and animals are multicellular beings. Plants have cell walls composed of cellulose, hemicellulose, pectin, and lignin, while animal cells have no cell wall. In plants, the energy process known as photosynthesis is carried out, while animals must ingest and digest food. To perform each specific function, the cellular material of each plant and animal is differentiated into tissues, organs, and systems. All members of the protist kingdom are unicellular forms, which perform the functions with no specialization, however, some members of this kingdom form well-developed aggregates (colonies) that reassemble multicellular structures. The fungal kingdom is formed by unicellular (yeast) and multicellular structures or filamentous masses (filamentous fungi). Unicellular life forms belong to the Monera kingdom, such as bacteria and blue green algae [1].
These five kingdoms can be divided into two groups: eukaryotes (protists, fungi, animals, and plants) and prokaryotes (Monera). The two groups differ by the nucleus. In eukaryotic cells, the nucleus is a discrete body surrounded by a membrane. The nucleus is divided according to the classic Mendelian description. Each cell has vacuoles, Golgi apparatus, and mitochondria. Some plant cells and protists also have plastids [2]. Prokaryotic cells do not contain defined structures of the organelles, their nucleus is diffuse, sectioned, and has no membrane. During the cell division, the nucleus separates into two parts, each capable of generating a new cell. Within prokaryotes, the organisms of greatest interest to the food microbiologist are bacteria, while eukaryotes are fungi [3].
1.1 FUNGI
Multicellular fungi are known as filamentous fungi, while unicellular fungi as yeasts. However, there are dimorphic microorganisms capable of growing as unicellular or multicellular, and their growth strongly depends on the conditions and culture time. Many fungi grow by accommodating their cells one after the other, forming filaments or hyphae, which branch out and together form the mycelium. Some fungi can gather their hyphae so much that they form firm structures called fruiting bodies, like mushrooms. Some fungi have divisions between each cell, while others do not, the former are known as septate, while the latter are known as non-septate [4].
Fungi differed by their ways of reproduction. All fungi, except those belonging to the Mycelia Sterila class, can produce spores asexually by simple cell division without nucleus fusion. Members of the Phycomycetes, Basidiomycetes, and Ascomycetes classes also reproduce sexually by fusion of two similar or non-similar cells, but asexual spores occur more than sexual. Because it is extremely difficult to induce forms of sexual or perfect reproduction, they are considered within a class called Fungi imperfecti or deuteromycetes [5].
1.1.1 PHYCOMYCETES
This class of fungi is divided into three subclasses: Archimycetes, Oomycetes, and Zygomycetes. Members of the Archimycetes subclass are primitive aquatic fungi, which are not of considerable relevance to food microbiologists. The members of the Oomycetes subclass are also considered primitive aquatic fungi; however, these can be isolated from soil and some fruits where they can rot. The most important members of this subclass are the genera: Pythium, Saprolegnia, Phytophthora, and Plasmopara. A large part of the fungi of interest in food microbiology belong to the Zygomycetes subclass and the genera Mucor, Rhizopus, Mortierella, Thamnidium, Choanephora, Cunninghamella, and Syncephalastrum, capable of growing in fruits, vegetables, meats, bread, and cereals [6].
1.1.2 BASIDIOMYCETES
Some members of the basidiomycetes class are of great importance in the food area, since they form fruiting bodies that are generally edible, such is the case of Ustilaginales, Tremalla, and Urocystales. Examples of them are fungi commonly known as mushrooms and huitlacoches [7].
1.1.3 ASCOMYCETES
The class with the highest number of members is the Ascomycetes, comprising yeasts to filamentous fungi capable of forming fruiting bodies. Ascomycetes do not have much importance for the microbial in food [8].
1.1.4 FUNGI IMPERFECTI
The members of the fungi imperfecti class are a special group of fungi for food microbiologists, many of them produce primary or secondary metabolites of industrial interest, and because many of them are pathogenic to humans through food consumption. The genera Aspergillus, Penicillium, Fusarium, Talaromyces, and Gibberella belong to this class [9].
1.1.5 MYCELIA STERILIA
The members of this class live in soils, generally of rice. They are not important for food microbiologists, but it is important to note that some members of the class Fungi imperfecti such as Fusarium and Helminthosporium, when losing their ability to produce conidiospores, erroneously they are placed within the Mycelia Sterila class [10].
1.2 CHARACTERISTICS OF FILAMENTOUS FUNGI
Filamentous fungi grow best on solid surfaces, the granular form of soil causes fungi to grow with a very branched filament. On food, filamentous fungi grow invasively without allowing other microorganisms to grow. The filamentous fungi associated with food are: Aspergillus, Penicillium, Rhizopus, Mucor, Absidia, Aurebasidium, Epiccocum, Alternaria, Helminthosporium, Monilia, Botrytis, Scopulariopsis, Cephalosporium, and Fusarium. Each of these fungi grows on defined types of food and under specific conditions [11].
1.3 YEAST
True yeasts are single-celled organisms belonging to the Ascomycetes and Fungi imperfecti classes. They have spherical, ellipsoidal, or elongated forms. Some produce a pseudomycelium and usually reproduce by budding. Yeasts are the simplest eukaryotic microorganisms, and much of their metabolism has been studied. Yeasts are widely used in the food industry, mainly in the production of bread and beer, and in the production of organic acids, enzymes, ethyl alcohol, ketone, and biomass. The most important genera are Saccharomyces, Torulopsis, and Candida [12].
1.4 BACTERIA
The Monera kingdom comprise Bacteria and Cyanobacteria, which are prokaryotes. Their cells have a three-dimensionally linked peptidoglycan cell wall, which produced very compact rigid structures. The amount of peptidoglycan and lipopolysaccharide material is used to taxonomically classify bacteria. Bacteria have a membrane, and, in some cases, they have a capsule structure. All bacteria associated with food are heterotrophic, they hydrolyze carbohydrates, proteins, lipids, and other food ingredients [13].
1.5 MICROORGANISMS AS FOOD PRODUCERS
Since the most remote historical times, microorganisms have been used to produce food. Microbial processes give rise to alterations in them that give them more resistance to deterioration or more desirable organoleptic characteristics like taste and texture. A lot of food manufacturing processes involving microorganisms are based on fermentation processes, mainly lactic fermentation of starting materials. This fermentation is usually carried out by lactic bacteria [14]. As a result, the pH decreases reducing the survival of undesirable bacterial species (mainly enteric bacteria), organic acids of short chain accumulate in the food that, in addition to their antibacterial effect, confer a pleasant taste, and, in certain cases, antibacterial compounds accumulate reducing the microbial food load by increasing its half-life or prevent the germination of spores of possible Gram-positive bacteria that cause food poisoning.
1.6 MICROORGANISMS AS FOOD SPOILAGE AGENTS
Impaired food is food damaged by microbial, chemical, or physical agents that made them unacceptable for human consumption. Food deterioration can cause important economic losses, approximately 20% of the fruits and vegetables collected are lost because of microbial deterioration caused by any of the 250 market diseases. The causative agents of deterioration can be bacteria, molds, and yeasts; being bacteria and molds the most important. Of all the microorganisms present in food, only some are able to multiply on the food, so being selected over time so that the initial heterogeneous population present in the food, is being reduced to more homogeneous populations and finally to an only type of microorganisms that manage to colonize all the food displacing others [15]. Therefore, during the deterioration process, a predominant population or type of microorganisms is selected so that the initial variety indicates little deterioration and reflects the initial populations.
There are several factors that determine the resistance to colonize food and these factors are:
- Intrinsic Factors: They are the derivatives of the food composition: water activity (aw), pH, redox potential, nutrients, food structure, and antimicrobial agents present.
- Technological Treatments: Factors that modify the initial flora because of food processing.
- Extrinsic Factors: These derived from the physical conditions of the environment in which the food is stored.
- Implicit Factors: The relationships established between microorganisms because of factors above [16].
Food can be attacked by a variety of microorganisms. Thus, each food type is damaged by the action of a specific type of microorganism, establishing a specific association between the altering microorganism and the altered product, for example, meat is th...
