Читать книгу Antisepsis, Disinfection, and Sterilization - Gerald E. McDonnell - Страница 18

Multicellular eukaryotes include microscopic (or, indeed, macroscopic) arthropods and helminths (or worms). These are higher forms of life, composed of eukaryotic cells that are specialized into organs and structures. Arthropods are not considered further here, but some introduction is given to the helminths. Helminths are a diverse group of multicellular parasites and can be further classified as nematodes (roundworms) and platyhelminthes (flatworms). Over 20,000 species of nematodes have been defined, although it is estimated that 10 to 100 times this number possibly exist. The flatworms can be further separated into trematodes (flukes) and cestodes (tapeworms). Many helminths can be free-living in water environments (in particular, the roundworms), but most are parasitic in nature. Some of the key diseases caused by helminths are summarized in Table 1.4. Helminths reproduce sexually and have a typical associated life cycle (Fig. 1.1).

Externally, the adult forms (including worms and flukes) are protected by a rigid proteinaceous (collagen) cuticle, which can resist the effects of biocidal processes; however, parasitic forms will not survive in the environment without their respective hosts (including, in some cases, intermediate hosts). Further, during their respective life cycles they produce dormant forms (including ova [eggs] or cysts) that can survive under harsh environmental conditions. Little work has been published on the structure of these eggs and their relative resistances to biocides, but microscopically they are diverse, consisting of various proteins and carbohydrates and having a variety of thicknesses (see section 8.11).

TABLE 1.1 Examples of various types of microorganisms

Microorganism	Typical structuresa	Size (μm)	Nucleic acid	Cell wall
Prions		<0.01	None	No
Viruses		0.01–0.4	DNA or RNA	No Envelope may be present
Chlamydias, rickettsias		0.3	DNA	Minimal or simple cell wall
Mycoplasmas		0.1–0.3	DNA	No
Bacteria		0.3–0.8	DNA	Yes
Fungi		Yeast, 8–10 Fungi, >0.5 (wide), >5 (long)	DNA	Yes
Algae		1 – >1,000	DNA	Yes/no (some)
Protozoa		10–200	DNA	No
Helminths		>1,000	DNA	NAb

^aNot to scale; simplified structures are shown. In addition, the basic structures of microorganisms can vary considerably based on their type, environmental conditions, and growth (or life cycle) phase.

^bNA, not applicable.

TABLE 1.2 Some advantages and disadvantages of microorganisms

Advantage or disadvantage	Example(s)
Advantages
Food and beverage production	Saccharomyces cerevisiae: bread and beer production Saccharomyces ellipsoideus: wine fermentation
Antibiotic production	Bacillus licheniformis: bacitracin Penicillium chrysogenum: penicillin
Vitamin metabolism	Pseudomonas spp.: vitamin B12 production Escherichia spp.: vitamin K synthesis in the gut
Genetic engineering	Agrobacterium tumefaciens: plasmids used for generating transgenic plants (e.g., herbicide or pathogen resistance)
Disease prevention	Bacteroides, Enterococcus spp.: prevention of pathogen colonization of the intestinal tract
Bioremediation	Desulfotomaculum spp.: arsenic detoxification
Disadvantages
Animal/human diseases	Mycobacterium spp.: tuberculosis HIV:AIDS Plasmodium spp.: malaria
Plant diseases	Phytophthora: potato blight Corynebacterium: vegetable infections
Surface damage	Pseudomonas spp.: biofilm development and surface corrosion
Food spoilage	Rhizopus: bread mold Streptococcus: milk souring
Allergic reactions	Fungal spores, including Aspergillus spp.
General product contamination	Bacterial and fungal spores, including Bacillus spp.