Medical brain notes 30

  PATHOGENS AND VIRULENCE INTRODUCTION The body is exposed to many pathogenic microorganisms and multicellular parasites. Most microorganisms found associated with the body are harmless and live as commensals or symbionts but others cause disease and are known as  pathogens .  Infection , from the Latin ‘inficere’ (to put in), is the successful persistence and/or multiplication of the pathogen on or within the host. In this respect,  pathogenesis  may be defined as the molecular and biochemical mechanisms that allow pathogens to cause diseases. Mummies preserved in ancient Egypt and elsewhere display evidence that infectious diseases have always been a threat. The emergence of new pathogens and the development of resistance to current treatments for existing pathogens means that infectious diseases will probably always be with us.   Many antibiotic drugs are available to treat infectious diseases and, in many cases, will effect a cure. It is, however, possi...

  TYPES OF PATHOGENS Infectious diseases are caused by pathogens that have the ability to infect humans. They may be subcellular, such as prions and viruses, single-celled prokaryotic bacteria, single-celled eukaryotic protozoa and yeasts, or multicellular organisms such as, fungi, certain worms, such as nematodes and flukes (generally referred to as  helminths ) and arthropods, such as mites. The term  parasite , an organism that lives at the expense of another, is often applied to viruses, protozoa and helminths, although the terms pathogen and parasite are virtually interchangeable. PRIONS Spongiform encephalopathies or prion diseases are all fatal diseases for which there is no cure. They include Creutzfeldt-Jacob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS) and fatal familial insomnia (FFI). These diseases generally develop slowly over 10 to 20 years in older individuals. Prion diseases occur sporadically, or they can be familial, that is genetic, or ...

  VIRULENCE FACTORS Evolution has provided pathogens and parasites with a wide range of factors that allow them to invade and colonize their host while at the same time avoiding and/or neutralizing host defense mechanisms. Many virulence factors of pathogens have been identified; some are relatively nonspecific in action: some microorganisms, for example, possess specialized iron uptake systems. Microorganisms require iron for oxygen transport, mitochondrial energy metabolism, electron transport, the synthesis of nucleic acids and gene expression. However, although an essential element, iron is often only available in limited quantities and microorganisms that possess a variety of iron uptake systems are able to grow in regions of the host that would otherwise be expected to be sterile since little iron is available. Other virulence factors have rather more defined defensive or offensive actions.   DEFENSIVE VIRULENCE FACTORS Numerous pathogens evade the host’s defenses by pro...

  COURSE OF INFECTION The course of an infection can be considered to follow up to four major stages namely: adhesion, entry, localized infection and generalized infection. A virulent pathogen is one that is well adapted to establish an infection. Most pathogens are adapted to adhere to cells, usually epithelial, that line the site of entry. Adherence is the first stage of infection, given that it prevents the pathogen being swept away and eliminated from the body. Pathogens enter the body through one of a number of so-called  portals of entry . These include the skin and conjunctiva, respiratory tract, gastrointestinal tract (GIT), urogenital system and, in the case of fetuses, the placenta. Once entry has been gained, conditions for growth, such as temperature, nutrients, must be favorable, but the pathogen must also be able to overcome the local defenses. Pathogens are often adapted to enter their hosts through a single portal of entry and do not cause infectious diseases i...