Introduction
Most of the microorganisms naturally present in the human body are harmless commensals. However, there also exists other disease causing microorganisms known as pathogens and the mechanism by which they cause disease is known as pathogenesis. These pathogens persist or multiply within the host causing infection. Some pathogens like Pseudomonas aeruginosa are considered as opportunistic pathogens that cause disease only in patients with compromised immunity and only under certain conditions. The infectious diseases may be treated with antibiotic drugs. However, it is important to know the different types of pathogens and the virulence factors by which they cause disease to develop a cure.
Types of pathogens
Pathogens causing infectious diseases may be subcellular or single-celled prokaryotic or eukaryotic organisms or multicellular organisms.
Prions
Prions are degenerate host proteins. The usually present α helical conformation of the protein is harmless. However, this may at times refold to a β-rich form that after aggregation forms deposits in the brain. This leads to the formation of holes in the brain. The diseases caused by Spongiform encephalopathies or prion are all fatal diseases. The examples include Creutzfeldt-Jacob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS) and fatal familial insomnia (FFI). These diseases may occur sporadically, or may be familial or may be acquired. These diseases are incurable. The prions can enter the body by various means like through diet or through surgery or blood transfusions.
Viruses
These are obligate intracellular parasites. A viral particle or virion consists of nucleic acid that can be either DNA or RNA enclosed by a complex of proteins called the capsid. Some viruses may also have an envelope surrounding the capsid. Viruses enter a target cell by binding to specific proteins or carbohydrates on the cell surface known as receptors. Examples of these receptors include CD4 (HIV), ICAM 1 (Human rhinovirus 91 and Coxsackie A), and Hepatitis B virus (HBV binding factor). After infection, the viral particles undergo replication in the host cell. Replication may include an inactive latent state in some viruses like Varicella zoster causing chickenpox while, in others like hepatitis B virus and human papillomavirus, the viral nucleic acid gets integrated into the host genome leading to cell transformation and cancer.
Bacteria
Bacteria are microorganisms without well defined nucleus. Hence, they are known as prokaryotes. They can cause a large number of infectious diseases that can be treated by antibiotic therapy. The differences in the metabolism of prokaryotes and eukaryotes makes the bacteria are susceptible to chemical agents like antibiotics. Bacteria can be extracellular pathogens or intracellular parasites like Mycobacteria and Listeria.
Protozoa, fungi and helminths
These are eukaryotes that are responsible for a large number of infectious diseases. Protozoa is single-celled eukaryote. Examples of protozoan infection are malaria caused by Plasmodium sp., amebiasis, leishmaniasis, toxoplasmosis, and trypanosomiasis.
Fungi are heterogeneous group of microorganisms. They vary from unicellular yeasts to elongated chains of cells called hyphae. Microsporum, Trichopython species cause athlete’s foot and nail infection, yeasts and other fungi can cause candidiasis and aspergillosis. AIDS is another fatal disease caused by Human Immunodeficiency Virus (HIV).
Helminths (worms) can be divided into three groups: nematodes (roundworms), cestodes (flatworms) and trematodes (flukes). Helminths affect different hosts including humans. The examples include the pork tapeworm Taenia solium and Schistosoma causing bilharzia.
Arthropods and vertebrates
Arthropods like the mite Sarcoptes scabiei that lives in outer layers of the skin are parasites while other arthropods like tick transmitting Borrelia burgdorferi that causes Lyme disease act as vectors of the pathogens. Rabies virus uses mammals as vectors.
Virulence factors
Pathogenic microorganisms show the presence of different virulence factors. These virulence factors help the pathogens to enter the host and overcome the host defense mechanism. Virulence factors may be non-specific like iron uptake system or specific like defensive or offensive virulence factors.
Defensive virulence factors
Pathogens survive the host defense mechanism by producing slime layers or with the help of the capsule. Both protect the bacteria by holding large amount of water. Some bacteria produce pathogenicity factors called modulines or microkines that inactivate or suppress the immune system. Some microorganisms like HIV and influenza show hypermutability. Thus, due to the antigenic variation, the population is no longer immune to the virus.
Offensive virulence factors
Bacterial offensive mechanism includes adhesions, invasins and toxins. Adhesins are cell surface proteins that bind to specific sites on the host cell. Example includes pili of Vibrio cholerae and Escherichia coli. Invasins are proteins that allow pathogens on the host to be internalized thus preventing the bacteria from being washed away. Toxins are of two types: endotoxins are lipopolysaccharides produced by and embedded in the outer membrane of Gram negative bacteria while exotoxins are the proteins secreted by both Gram-positive and Gram-negative bacteria. Exotoxins are further divided into three groups – those that damage membranes, those with specific host target and superantigens.
Infection caused by a microorganism can be divided into four stages: adhesion, entry, localized infection and generalized infection. In the first stage, pathogens adhere to the cells that protect them from being swept away and thus prevent elimination from the body. Once adhered, pathogens enter the body through one of the portals of entry like skin, conjunctiva, gastrointestinal tract and the placenta. In localized infection, pathogens are prevented from spreading in the body by host defenses. However, if the pathogens can spread throughout the body and infect other tissues or organs through blood or lymphatic system causing generalized infection. However, the pathogen may also be eliminated from the body at any stage during the course of infection.
