Viruses are obligate intracellular parasites that depend on the host cell’s metabolic machinery for their replication. They consist of a nucleic acid genome surrounded by a protein coat (called a capsid) that is sometimes encased in a lipid membrane. Viruses are classified by their nucleic acid genome (DNA or RNA, but not both), the shape of the capsid (icosahedral or helical), the presence or absence of a lipid envelope, their mode of replication, their preferred cell type for replication (called tropism), or the type of http://ebooksmedicine.net
Table 9.1 Classes of Human Pathogens pathology they cause. Some viral components and particles aggregate within infected cells and form characteristic inclusion bodies, which may be seen with the light microscope and are useful for diagnosis (
Fig. 9.1 ). For example, cytomegalovirus (CMV)-infected cells are enlarged and show a large eosinophilic nuclear inclusion and smaller basophilic cytoplasmic inclusions; herpesviruses form a large nuclear inclusion surrounded by a clear halo; and both smallpox and rabies viruses form characteristic cytoplasmic inclusions. However, many viruses (e.g., poliovirus) do not produce inclusions.
Accounting for a large share of human infections, viruses can cause disease in several ways (
Table 9.2 ). Many viruses cause transient illnesses (e.g., colds, influenza). Other viruses are not eliminated from the body and persist within cells of the host for years, either continuing to multiply (e.g., chronic infection with hepatitis B virus [HBV]) or survive in some latent nonreplicating form, with the potential to be reactivated later. For example, herpes zoster virus, the cause of chickenpox, can enter dorsal root ganglia and establish latency at the site, with periodic reactivation at later times to cause shingles, a painful skin condition. Some viruses are involved in transformation of a host cell into a benign or malignant tumor (e.g., human papillomavirus [HPV]-induced benign warts and cervical carcinoma). Different species of viruses can produce the same clinical picture (e.g., adenovirus and rhinovirus causing upper respiratory infection); conversely, a single virus can cause different clinical manifestations depending on the age or immune status of the host (e.g., CMV causing congenital neurologic damage or gastroenteritis in the immunocompromised).
Bacteria are prokaryotes, meaning that they have a cell membrane but lack membrane-bound nuclei and other membrane-enclosed organelles. Most bacteria are bounded by a cell wall consisting of peptidoglycan, a polymer of long sugar chains linked by peptide bridges surrounding the cell membrane. There are two common forms of cell wall structure: a thick wall that retains crystal-violet stain (gram-positive bacteria) and a thin cell wall surrounded by an outer membrane (gram-negative bacteria) (
Fig. 9.2 ). Bacteria are classified by Gram staining (positive or negative), shape (spherical, called cocci, or rod-shaped, called bacilli) (
Fig. 9.3 ), and their requirement for oxygen (aerobic or anaerobic). Motile bacteria have flagella, long helical filaments extending from the cell surface that rotate and move the bacteria. Some bacteria possess pili, another kind of surface projection that can attach bacteria to host cells or extracellular matrix. Bacteria synthesize their own DNA, RNA, and proteins, but they depend on the host for favorable growth conditions. Many bacteria remain extracellular when they grow in the host, while