Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell

FIGURE 1.3 Simplified fungal cell envelope. The cross-linked cell wall is linked to the cell membrane. The cell wall usually consists of innermost fibrils of chitin or cellulose, with outer layers of amorphous, cross-linked glucans.

      1.3.3.3 ALGAE

      Algae are a diverse group that can be found as single, free-living cells, but also as colonies, including multicellular filaments. Algae are phototrophs and therefore derive their energy by photosynthesis (light-mediated energy biosynthesis). Photosynthesis is conducted within special cytoplasmic organelles (chloroplasts), which contain light-sensitive pigments known as chlorophylls. Some bacteria and plants also use photosynthesis. The major habitats of algae are in water (marine or freshwater), and they are commonly encountered as colorful slimes on the water surface and, in particular, on polluted water. Examples are chlorophyta (“green algae,” e.g., Chlamydomonas), rhodophyta (“red algae”), and dinoflagellata (e.g., Gonyaulax). Some species produce toxins, which can be lethal to fish and other marine life, as well as causing mild effects (headaches and respiratory problems) in humans. Structurally, algae are typical eukaryotes. Their cell wall structures vary considerably, including cellulose-, chitin-, and silica-based structures modified by polysaccharides and peptides.

1.3.3.4 PROTOZOA

Protozoa represent one of the most abundant groups of life in the world. Over 60,000 species have been described, but estimates of the total variety that exist are much higher. They are single-celled eukaryotes, but unlike fungi, they lack a cell wall. They can be found in a variety of ecosystems, including water, soil, and as parasites in animals and plants. They are generally mobile and can be classified based on their respective modes of movement and microscopic morphologies (Table 1.6).

Similar to helminths, protozoa produce multiple forms during their respective life cycles, including oocysts and cysts that can survive for extended periods in the environment (Fig. 1.4). The amebas and flagellates mostly reproduce asexually, while the human-parasitic sporozoans are capable of both asexual and sexual reproduction.

      1.3.4 Prokaryotes

      Prokaryotes are a diverse group that show some similarities in their basic structures but are also very distinct. They are generally single celled, range in size from ~0.1 to 10 μm, and unlike eukaryotes, their nucleic acid is free in the cytoplasm (Table 1.3). They can be considered as two general groups, the archaea and eubacteria.

1.3.4.1 EUBACTERIA

Eubacteria (or bacteria) can be subdivided into those that have cell walls and those that do not. The cell wall-free types are known as the mycoplasmas (or mollicutes). Mycoplasmas are a distinct group of prokaryotes containing a small genome and surrounded by a unique cell membrane. The surface structure of a typical mycoplasma cell is shown in Fig. 1.5.

The cytoplasm is surrounded by a lipid bilayer consisting of phospholipids. Phospholipids are molecules made up of fatty acids linked to glycerol and then, via a phosphate group, to an alcohol. Essentially, these molecules form the basic structure of the cell membranes of most bacteria (with the exception of archaea [see section 1.3.4.2]). They contain a hydrophilic (alcohol) end and a hydrophobic (fatty acid) end, which associate to form two layers (known as a bilayer) consisting of an inner hydrophobic core and an outer hydrophilic surface. Mycoplasmas are unique among prokaryotes, as they can also contain other lipids (sterols, such as cholesterol) associated in the lipid core of the membrane; sterols are usually present only in eukaryotes and add rigidity to the cell membrane, which confers greater resistance to extracellular factors than typical bacterial cell membranes do. Proteins may also be present, spanning the membrane or associated with the membrane surface. Glycolipids (polysaccharides linked to surface lipids) have also been reported on cell surfaces, and they are believed to be involved in cell attachment. The reduced genome size is presumably linked to the lack of cell wall metabolism and other biosynthetic pathways (e.g., purine metabolism) that are required in other bacteria. Overall, as mycoplasmas have no cell wall, they are pleomorphic (many shaped) (Table 1.1). They can be commensals or pathogens of plants, humans, and animals (Table 1.7). Mycoplasmas are common contaminants of cell cultures and are also implicated in chronic diseases, such as chronic fatigue syndrome and rheumatoid arthritis. The lack of a protective cell wall may make mycoplasmas more sensitive to drying, heat, and some biocides. Other bacteria (as discussed below) that normally have a cell wall can also be present in a cell wall-free form and are referred to as L or cell wall-deficient forms. These forms have been found in artificial culture media and in infected tissues. They may be stationary forms that can circumvent host defense mechanisms. Examples of described cell wall-deficient forms of bacteria are Helicobacter, Mycobacterium, Pseudomonas, and Brucella. Some are suspected of being involved in autoimmune diseases, such as rheumatoid arthritis (Propionibacterium acnes) and multiple sclerosis (Borrelia mylophora).

TABLE 1.6 Classification of protozoa, based on their motility mechanisms and microscopic morphologies

Classification and organism	Disease(s)	Comments
Flagellates (motility by flagella)
Giardia lamblia	Giardiasis, including dysentery	Trophozoites (~20 μm; shown above) produce cysts, which can survive water chlorination
Trypanosoma gambiense	Sleeping sickness	Transferred in tsetse flies
Leishmania donovani Скачать книгу