Host–pathogen Interaction

Fungi belong to the eukaryote domain and, like plants, have cell walls and vacuoles, exhibit cytoplasmic streaming, and are immobile. Almost all fungi, however, have cell walls composed of chitin and not cellulose. Fungi do not carry out photosynthesis but obtain their substrates for metabolism as saprophytes (obtain their food from dead matter). Mycosis is an infection caused by fungi.

Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. The replication cycle differs between viruses that infect bacteria (bacteriophages) and viruses that infect eukaryotic cells. Bacteriophages have either a lytic or lysogenic replication cycle, while eukaryotic viruses have a defined 6-step replication process.

Treponema is a gram-negative, microaerophilic spirochete. Owing to its very thin structure, it is not easily seen on Gram stain, but can be visualized using dark-field microscopy. This spirochete contains endoflagella, which allow for a characteristic corkscrew movement. The bacteria are able to avoid immune recognition and phagocytosis by forming a protective coating with fibronectin. Humans are the only reservoir and transmission is through human-to-human contact. The most common species involved in human disease is Treponema pallidum subspecies pallidum, which is the causative agent of syphilis. Other clinically relevant species include T. pallidum pertenue, T. pallidum endemicum, and T. carateum. These are the causative organisms for yaws, bejel, and pinta, respectively.

Transmissible spongiform encephalopathies are diseases caused by prions. Prions differ from viruses in that they are small, infectious pathogens that do not contain nucleic acid. Recognized spongiform encephalopathies include Creutzfeldt-Jakob Disease (CJD), variant Creutzfeldt-Jakob Disease (vCJD), Kuru, fatal familial insomnia (FFI), and Gerstmann-Straussler syndrome (GSS). Common characteristics of these diseases include dementia, ataxia, and myoclonus. Unfortunately, these diseases are associated with long incubation periods (20+ years) and once symptoms occur, rapidly progress to death.

Bacteriology is the branch of microbiology that deals with the morphology, structure, classification, and biochemistry of bacteria. The discipline of bacteriology arose during the 19th century from scientific attempts to prove the “germ theory of disease,” namely that diseases were caused by microscopic organisms invading host cells. Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Management of bacterial disease is generally with antibiotics; however, the choice of antibiotics may vary depending on the bacterial structure and metabolism.