Pathogenicity, Virulence, Infection & Disease

• Resident flora are helpful microbes naturally living in the body without causing harm
• Pathogens are harmful microbes causing disease

Pathogenicity and Virulence

• Pathogenicity refers to a microbial agent’s ability to cause disease
• Virulence is the degree to which an organism is pathogenic

Infection and Disease

• Infection is the successful colonization of a host by a pathogenic microorganism and can lead to disease
• Transient flora are temporary microbes on the body that can be removed by washing
• Microbial antagonism refers to good microbes stopping harmful ones from growing
• Systemic infection occurs when an infection becomes disseminated throughout the body

Symptoms, Signs and Syndromes

• Symptoms are felt or experienced by a patient and can include nausea, loss of appetite, chills, and pain
• Signs can be directly observed by a clinician or through laboratory tests and include changes in temperature, heart rate, breathing, blood pressure, and white blood cell count
• A syndrome is a specific group of signs and symptoms characteristic of a particular disease
• Localized infection is confined to a small area of the body, typically near the portal of entry
• Virulence factor determines the degree to which an organism is pathogenic
• Zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human
• True pathogens are germs that cause disease in healthy people
• Opportunistic pathogens are non-pathogenic bacteria that become pathogenic in new environments or when normal environments change
• A reservoir is where a pathogen lives and multiplies, such as in humans, animals, soil or water
• Source is the place or thing from which a pathogen is transmitted to a host
• A carrier is a person or animal that carries and spreads a disease without showing symptoms
• Nosocomial infection is an infection acquired in a hospital or healthcare setting
• Epidemiology studies how diseases spread and affect populations
• Etiology refers to the cause or origin of a disease
• The prevalence rate is the total number of existing cases of a disease in a population at a specific time
• Incidence is the number of new cases of a disease in a population during a specific period
• Mortality rate is the number of deaths caused by a disease in a population over a given time
• Morbidity rate is the number of people affected by a disease (illness) in a population over a given time

Conditions Creating Opportunities for Microbiota to Cause Disease

• A weak immune system
• Injury or surgery
• Antibiotic use
• Flora imbalance due to stress
• Flora imbalance due to diet
• Flora imbalance due to hormonal changes

Beneficial Effects of Resident Flora

• Resident flora prevents infections by blocking harmful bacteria
• Resident flora aids digestion by breaking down food and making vitamins
• Resident flora supports immunity by helping the body recognize threats

Factors Changing Resident Flora

• Antibiotics
• Diet
• Stress
• Illness
• Hygiene practices
• Travel

Virulence Factors and Infection

• Virulence factors are traits which help pathogens cause disease, impacting how serious an infection is and how fast it spreads
• Adhesion factors help bacteria stick to the body
• Invasive factors help bacteria spread inside the body
• Toxins harm the body by damaging cells

Koch's Postulates

• Koch's postulates serve to establish a causal relationship between a pathogen and a disease
• Find a candidate bacterium in every case of disease
• Isolate bacterium form host grow in pure culture
• Show that cultured bacteria causes same disease in healthy subject
• Isolate bacterium from experimental subjects

Types of Epidemics

• A propagated epidemic spreads from person to person over time
• A point source epidemic comes from a single source, affecting many people at once

Routes of Entry For Pathogens

• Skin: Nicks, abrasions and punctures enable pathogen entry
• GI tract: Food, drinks and ingested substances allow pathogen entry
• Respiratory tract: Is achieved predominantly through sneezing and coughing
• Urogenital: Is obtained through the mucosa of the penis, external genitalia, cervix, and urethra
• Pregnancy and birth: Is achieved through microbes that cross the placenta, travel the birth canal

Definitions

• ID50 refers to the infectious dose, or the number of organisms to cause infection in 50% of people
• Phagocytes are white blood cells that destroy germs
• Antiphagocytic factors are tricks bacteria use to avoid being destroyed
• Leukocidins are toxins that kill white blood cells
• Toxogenicity is a germ's ability to produce toxins
• Toxinoses are diseases caused by bacterial toxins
• Toxemias occur when toxins spread in the blood, causing disease
• Intoxications refer to illnesses caused by ingesting toxins, not the bacteria itself

Exotoxins vs Endotoxins

• Exotoxins are toxic in minimal amounts, endotoxins require a larger amount to be toxic
• Exotoxins result in specific damage, dependent and receptor mediated targeting cells
• Exotoxins enact a specific mechanism of actions, endotoxins result in general inflammation and fever
• Exotoxins are heat liable, and endotoxins are heat-stable
• Exotoxins stimulate antitoxins, endotoxins do not slim antitoxins
• Exotoxins are typically secreted from live cells, endotoxins are released during shedding or lysis
• Exotoxins stem mostly from gram+ (primarily) while endotoxins most often stem from gram-

Infection Types

• Localized infection stays in one specific area of the body
• Focal infection starts in one place but spreads to another
• Systematic infection spreads throughout the body via the bloodstream
• Mixed infection is caused by multiple types of microbes at the same time
• Primary infection is the initial infection that occurs before any other infections develop

Stages of Infection

• Incubation is the period between infection and the appearance of symptoms
• The convalescence period is the recovery phase after an infection, when the body heals
• Chronic infections persist over time
• A passive (true) carrier is someone who carries and spreads a pathogen without ever being infected themselves
• Communicable diseases can spread from one person to another
• Noncommunicable diseases do not spread from person to person
• Contagious diseases are highly communicable diseases that spreads easily
• Acute diseases have a rapid onset and short duration
• Chronic diseases have a long-lasting or recurring course

Transmission of Diseases

• Direct transmission is spread through physical contact
• Indirect transmission is spread through contaminated objects or surfaces
• Mechanical vectors involve an insect which carries the pathogen on its body without being infected
• Biological vectors involve an insect or animal which is infected and spreads the disease through biting

More Definitions

• Epidemics are a sudden increase in disease cases in a specific area
• Pandemics are a disease outbreak which spreads across multiple countries or worldwide
• Endemic diseases are those that are always present in a specific region at a steady rate
• Sporadic diseases appear occasionally and irregularly in a population
• Incidence is the number of new cases of a disease in a specific time period
• Prevalence is the total number of existing cases of a disease in a population at a given time
• R₀ (Basic Reproduction Number) is the average number of people an infected person spreads the disease to in a susceptible population
• Nosocomial infections are those acquired in a hospital or healthcare setting