Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell

FIGURE 1.11 Typical viral life cycle. The stages include (1) attachment, (2) penetration into the cell, and (3) multiplication. Depending on the virus type, viral particles can be released by cell lysis (4a) or by budding (4b); alternatively, the virus can remain dormant in the cell (4c).

Viruses have been identified as the causes of a variety of plant, human, and animal diseases, including respiratory, sexually transmitted, neurological, and dermatological diseases (Table 1.14). The traditional difficulty of isolation and identification of viruses limits their study; however, it is thought that many more viruses remain to be identified and implicated in diseases by developing molecular biology and electron microscopy techniques.

Separate families of plant viruses have also been described, including tobamoviruses (nonenveloped RNA viruses; e.g., tomato-tobacco mosaic virus is a significant agricultural and horticultural concern, because it infects vegetables, flowers, and weeds, leading to leaf, flower, and fruit damage), Comoviridae (nonenveloped RNA viruses), and Geminiviridae (nonenveloped DNA viruses). Viruses that infect fungi (e.g., the nonenveloped RNA viruses barnavirus and chryovirus) and bacteria (bacteriophages) (Fig. 1.12) have also been described.

TABLE 1.14 Examples of viral diseases

      Bacteriophages (commonly known as phages) are mostly DNA viruses (e.g., the T3, T7, and lambda [λ] phages are E. coli viruses), although some RNA viruses have been described (e.g., nonenveloped MS2 and enveloped ϕ6 E. coli phages). Bacteriophages have been studied for many years as genetic-engineering tools, but they have other practical applications, including uses in typing of bacteria and as indicators of fecal contamination in water and limited medical applications (such as antibacterials). Lactobacillus phages are a significant contamination concern in the dairy industry. Phages are considered to be resistant to biocides, like other animal and plant viruses, and are therefore used to investigate biocidal activities (e.g., MS2 phage) and modes of action. They can be routinely cultured and purified in most bacteriology laboratories.

FIGURE 1.12 E. coli bacteriophages. The T-phages are complex DNA viruses; MS2 and ϕ6 are RNA viruses, with ϕ6 enveloped.

      Two other groups of infectious agents are also considered “viruses” but have unique morphologies. The first are viroids, which are devoid of protein and appear to consist of naked RNA molecules. The second are proposed to be devoid of a nucleic acid and are termed “prions”; these are discussed in further detail in section 1.3.6. Viroids are known to infect only higher plants and have been identified as the causes of a number of crop diseases. Examples are potato spindle tuber viroid, coconut cadang-cadang viroid, and tomato apical stunt viroid. They consist only of small, circular RNA sequences that range in size from 246 to 375 nucleotides. It is interesting that their sequences do not encode proteins and that they are dependent on the host for replication in the cell nucleus. Although at first it would seem that these agents would not survive well in the environment, their structures are somewhat protected