Pathogenicity and Virulence

Pathogenicity and Virulence

• Pathogenicity is a microbe's capacity to cause disease.
• Virulence is the degree to which an organism is pathogenic.
• Avirulent organisms are not harmful, while highly virulent pathogens almost always cause disease.
• Less virulent pathogens may cause mild illness or be asymptomatic.
• Median infectious dose (ID50) is the number of pathogens needed to cause infection in 50% of subjects.
• Median lethal dose (LD50) is the number of pathogens needed to kill 50% of subjects.

Primary vs. Opportunistic Pathogens

• Primary pathogens cause disease regardless of the host's immune status.
• Opportunistic pathogens cause disease when the host is compromised.
• Individuals susceptible to opportunistic infections include the very young/old, pregnant, immunocompromised, or those with breached barriers.
• Enterohemorrhagic E. coli (EHEC) is a primary pathogen producing Shiga toxin, leading to severe diarrhea and renal failure.
• Staphylococcus epidermidis is an opportunistic pathogen, frequently causing nosocomial infections.

Stages of Pathogenesis

• Pathogenesis involves exposure, adhesion, invasion, and infection.
• Pathogens must enter the host, establish infection, evade the immune response, and cause damage.
• The cycle often ends with the pathogen exiting and transmitting to a new host.

Exposure

• Exposure is contact with a potential pathogen.
• Not all contacts lead to infection; pathogens need access to host tissue.
• A portal of entry is an anatomical site allowing pathogen passage into host tissue.
• Mucosal surfaces of the respiratory, gastrointestinal, and genitourinary tracts are key portals of entry.
• Portal specificity is determined by environmental adaptations, enzymes, and toxins.
• The respiratory and gastrointestinal tracts are vulnerable due to inhaled or ingested particles.
• The parenteral route is entry through breaches in protective barriers like skin.
• Pathogens can cross the blood-placental barrier in pregnant women.
• Examples of pathogens that cross placenta: Listeria monocytogenes, Toxoplasma gondii, Treponema pallidum, Varicella-zoster virus, Hepatitis B, Retrovirus, Parvovirus B19, Togavirus, Human herpesvirus 5, and Herpes simplex viruses (HSV) 1 & 2.

Adhesion

• Adhesion is the attachment of pathogens to host cells using adhesion factors after exposure.
• Glycocalyx in biofilms enables bacteria to adhere to host tissues and medical devices.

Invasion

• Invasion is the spread of pathogens throughout local tissues or the body after adhesion.
• Exoenzymes and toxins are virulence factors that colonize and damage host tissues.
• Virulence factors can protect pathogens from immune system defenses.

Infection

• Infection is the successful multiplication of a pathogen after invasion.
• Local infection is confined to a small area.
• Systemic infection is disseminated throughout the body.
• A primary infection is an initial infection caused by one pathogen.
• A secondary infection results from a compromised immune system due to a primary infection.
• Influenzavirus causes primary infection, damaging lung defenses and increasing chances of pneumonia by Haemophilus influenzae or Streptococcus pneumoniae.

Portals of Exit

• Pathogens persist by transmitting to new hosts through portals of exit.
• Common portals of exit include the skin, and respiratory, urogenital, and gastrointestinal tracts.
• Coughing and sneezing expel pathogens from the respiratory tract.
• Secretions and excretions like feces, urine, semen, tears, sweat, and shed skin cells can all transport pathogens.
• Insect vectors transmit pathogens in extracted blood.
• Needles extract blood containing pathogens.

Immunity

• Humans have innate and adaptive immunity.
• Innate immunity is resistance born with
• Adaptive immunity is resistance aquired by the body
• The immune system defends against infection by repelling, killing, and expelling invaders.
• Immunity has nonspecific innate and specific adaptive host defenses.