IC5 - II

• Infection/infectious disease occurs when an infectious agent overcomes the body's defenses to cause tissue/organ damage or dysfunction, leading to disease
• Presence of organisms alone is not enough to diagnose infection
• Most micro-organisms living in/on the human body do not cause disease
• Microbes become a problem when they cause disease, depending on the virulence of the organism and the response of the host
• Clinical infection is more likely when local defenses are undermined or when organisms develop strategies to overcome barriers
• Environmental factors play a role in infection, with terms like endemic, epidemic, outbreak, and pandemic used to describe infection rates
• For every pathogen, there is a reservoir where the pathogen can survive and spread, which can be living things or contaminated non-living things
• Hands are involved in the spread of many infections, emphasizing the importance of social distancing, mask-wearing, and handwashing
• Different environments pose different risks for individuals, leading to different organisms causing nosocomial (hospital-acquired) and community-acquired infections
• New and emerging infectious diseases have been recognized, with infections crossing species barriers and becoming more common due to medical treatments, transportation, travel, environmental changes, and drug resistance
• Host barriers like the skin, GI tract, and respiratory tract play a crucial role in preventing pathogen entry and infection
• SARS-CoV-2 infects human cells by binding to the angiotensin-converting enzyme 2 (ACE2)
• SARS-CoV-2 infects human cells by binding to angiotensin-converting enzyme 2 (ACE2) highly expressed on epithelial cells of the respiratory tract and endothelial cells
• Less virulent organisms can infect when there is damage to the epithelium/cilia, such as bacterial pneumonia following influenza infection
• Urinary tract is usually sterile as it is filtered fluid and flushed many times a day
• Vagina is protected by low pH produced with the help of commensal lactobacilli
• Innate immune responses are the first line of defence and do not adapt to repeated attacks, including phagocytic cells, complement, and acute inflammatory response
• Adaptive immune responses are mediated by T and B cells, stimulated by each exposure, and improve with successive contacts
• Compromise of normal barriers to infection or the immune system can result in increased chance of infection by "usual" organisms and possibility of infection by less virulent organisms
• Genetic differences can result in differences in susceptibility to infection in apparently immunocompetent hosts
• Bacteraemia is the presence of bacteria within the bloodstream
• Septicaemia is infection due to bacteria within the bloodstream, usually a larger number of organisms no longer held in check by the immune system and begin replicating
• Sepsis is the result of having septicaemia, with symptoms being a combination of damage done by the organisms and the host response
• After the initial exposure, patients typically develop symptoms within 5-6 days (incubation period)
• Infection is more likely to result in abscess formation, fever, and severe bacterial infection may lead to features of septic shock
• Symptoms of infection may be caused by a wide range of pathogens, such as food poisoning and diarrhoea caused by viruses, bacteria, or protozoans
• Community-acquired bacterial pneumonia is mainly caused by Gram-positive bacteria of high virulence, while Gram-negative bacteria are largely responsible for hospital-acquired infection
• Severe pneumonia is rare in both influenza and Covid-19, with the majority of people with Covid-19 being asymptomatic or having mild disease
• Tuberculosis should be considered if pneumonia is chronic and difficult to treat
• It is important to identify the correct infectious agent before starting antibiotics
• Different types of immune compromise may affect different cells of the immune system and result in a different spectrum of infection
• Immune compromise results in increased likelihood of 'usual' infections as well as infection by organisms of low virulence (opportunistic infection)
• Deficiency in antibody production or in neutrophils increases susceptibility to extracellular bacterial infection, while deficiency in T cell mediated immunity increases susceptibility to intracellular bacteria and viruses
• Infections in immunocompromised hosts may not show classic symptoms of infection and may spread more quickly
• Infections which are usually confined to specific organs may spread systemically in immunocompromised hosts
• Three general mechanisms of microbial pathogenesis:

  • Directly cause cell death or dysfunction
  • Local or systemic release of bacterial products (e.g. toxins)
  • Induce immune responses that cause tissue damage
• Disease pattern 1 (exotoxin-mediated):

  • Cell and tissue necrosis
  • Example: Necrotising fasciitis caused by Group A Streptococcus, resulting in skin blistering and tissue turning black
• Disease pattern 2:

  • Abscess formation (acute suppurative inflammatory reaction)
  • Example: Bacterial conjunctivitis caused by Staphylococcus aureus
• Staphylococcus aureus:

  • Some strains cause disease by producing potent exotoxins
  • Can cause food poisoning symptoms, blistering of the skin, and staphylococcal scalded skin syndrome
• Disease pattern 3:

  • Antigens from bacteria cause chronic inflammatory responses, including granulomatous inflammation
  • Seen in mycobacterial and fungal infections