Bacteremia, Septicemia, and Symbiotic Relationships

• Bacteremia involves the presence of bacteria in the blood.
• Commensalism is a symbiotic relationship where one organism benefits without harming the other.
• Contamination is when unwanted materials are present that may or may not be harmful.
• Disease results from an undesirable relationship between host and pathogen, disrupting normal body functions.
• Infection is the invasion of the body by pathogenic microorganisms, which isn't always synonymous with disease.
• Mutualism is a symbiotic relationship where both organisms benefit.
• Parasitism is a symbiotic relationship where one organism benefits while harming the other.
• Pathogenicity is the ability of an organism to cause disease.
• Pollution is the presence of contaminants causing adverse effects on humans and communities.
• Pyemia is the presence of pus-producing bacteria in the bloodstream.
• Septicemia is when bacteria actively multiplies in the blood, often leading to sepsis.
• Symbiosis involves prolonged interaction between different species.
• Toxemia is the presence of toxins in the blood.
• Viremia is the presence of viruses in the blood.
• Virulence is the degree of pathogenicity an organism has.

Koch's Postulates

• Robert Koch proved microorganisms cause specific diseases, later formalized as Koch's postulates
• A suspected organism must be absent in healthy individuals, but present in the diseased.
• Isolate the suspected organism from the infected host and grow in pure culture.
• Organisms from pure culture must produce the original disease when inoculated into a susceptible animal.
• The same organism must be re-isolated from the experimentally infected host.
• The postulates' validity lies in growing pathogens in artificial culture, but viruses require living cells.
• Not everyone infected develops disease, and reactions to pathogens vary among individuals.
• Cultured organisms must be inoculated into a susceptible animal, but some pathogens are species-specific.
• Ethical issues surround testing on animals, and pathogens can alter or lose pathogenicity in artificial media.

Factors Influencing Infection: The Chain of Infection

• Infectious disease development involves an etiologic agent, the host, and the environment.
• Transmission begins when an organism leaves its host/reservoir (portal of exit).
• A susceptible organism gets infected through transmission, entering its body (portal of entry).
• The chain of infection starts when the organism multiplies and causes disease.

Reservoirs

• Reservoirs are the origin of disease-causing microorganisms, where infectious agents reside and multiply.
• Reservoirs provide the survival and multiplication conditions that lead to chances of transmission to a susceptible host.
• Animals, humans, and the environment all act as reservoirs.
• Zoonotic infections are transmissible from animals to humans, where humans are incidental hosts.
• Anthrax, plague, and rabies are zoonotic infections.
• Humans can be reservoirs; these pathogens transmit directly from one person to another through respiratory or sexual routes.
• Some infected humans harbor the organism but have subclinical disease, while others are carriers.
• Asymptomatic/healthy carriers don't manifest symptoms.
• Incubatory carriers transmit during the illness' incubation.
• Chronic carriers harbor the organism for months, while convalescent carriers transmit after recovery, some carriers transmit infectious agents unknowingly.

Environmental Reservoirs and Portals

• Environmental reservoirs include water, soil, and plants that contain certain infectious organisms.
• The portal of exit is how an infectious agent leaves its host, often the site where it's located.
• Routes vary by organism and site of localization such as in blood flukes, they exits via urine
• Transmission mode categorizes as direct or indirect contact.

Direct Contact

• Most infectious agents are transmitted through direct contact, including contact with pathogens in the environment.
• Person-to-person contact involves transmission through skin, kissing, or sexual contact
• Droplet spread occurs when respiratory infections transmit via coughing or sneezing.

Indirect Contact

• Airborne transmission occurs through dust or aerosols containing infectious agents.
• Vehicle transmission involves pathogens spread through contaminated food, water, fomites and biologic substances.
• Vector transmission uses insects to transmit infectious agents mechanically or biologically.

Portal of Entry, Host Susceptibility

• A portal of entry is how infectious agents enter a host, also where they can multiply.
• Infected organisms that leave through respiratory tract will also enter another host through this route
• The final infection link is the host's susceptibility, which is affected by individual constitutional and genetic factors.
• The body has natural barriers and defense mechanisms against pathogenic organisms and they can be dampened by many factors.

How Organisms Produce Disease

• Invasiveness: Organisms cause disease by damaging tissues or body surfaces through colonizing, evading host defenses, and generating extracellular substances.
• Adhesins like pili or fimbriae in Gram-negative bacteria promote adhesion, aiding invasion via bacteria's contact to bodily surfaces.
• Capsules help evade phagocytosis, while coagulase promotes hiding.
  • Some bacteria survive and multiply inside macrophages by inhibiting phagosome-lysosome fusion.
• Some microorganisms produce substances promoting invasion:
  • Neisseria gonorrhoeae can enter, multiply, and spread, using enzymes to invade target cells.
  • Clostridium perfringens produces collagenase to break down collagen, contributing to gas gangrene.
• Toxin Production: Toxins cause disease and are either exotoxins or endotoxins.
  • Endotoxins are integral components of Gram-negative bacteria, such as Salmonella, Shigella, and Escherichia coli.
  • The endotoxin activity stems from lipopolysaccharide (LPS), specifically lipid A with toxic effects

Characteristics of Exotoxins

• Exotoxins come from some bacteria during their growth and metabolism and are released into the medium
• Gram positive bacteria mostly produces it.
• Three types of exotoxins include cytotoxins, neurotoxins, and enterotoxins.
• Produced by exotoxin caused bacterial infection are related to the impacts from exotoxin, and not the bacteria themselves.
• Diphtheria, botulinum, and tetanus toxins are important examples.

Immunologic Reaction to Infectious Disease

• Disease caused by immune response rather than invasion or toxin production.
• Damage to the liver from hepatitis viruses from immune response rather than viral effect.
• Antibodies are produced, and cytotoxic T cells are activated.
• Rashes from measles and German measles are due to the body's specific immune response.

Classification of Diseases

• Communicable diseases spread directly/indirectly (measles, tuberculosis, typhoid fever)
• Contagious diseases spread easily (measles, chickenpox)
• Fulminant infections cause quick death (meningococcemia)
• Non-communicable diseases don't spread (Clostridium tetani)
• Exogenous infections come from outside the body (cholera)
• Endogenous infections come from inside the body (Escherichia coli)
• Sporadic diseases occur occasionally, endemic diseases occur constantly at low levels.
• Epidemics occur greatly and rapidly in areas.
• Pandemics have a global effect.

Duration

• Acute diseases develop quickly and occur for a short time.
• Chronic diseases develop slowly and occur for a long time.
• Some diseases have both forms.
• Latent diseases are inactive, but reactivate later creating repeat symptoms.
• Infections are localized with limited spread, and systemic/generalized when spread through out bloodstream and lymph.
• Focal infections in specific area that arose from another already existing infection.
• Primary infections cause initial illness.
• Secondary infections by opportunistic pathogens weaken the body.
• Subclinical/inapparent infections have no noticeable illness.

Stages of Infectious Disease

• Incubation Period: The time between agent entry and initial symptoms; length varies.
• Prodromal Period: A short time of mild, nonspecific symptoms in measles for example.
• Period of Illness: Disease is most acute; signs/symptoms are distinctive
  • Outcomes include resolution, fulminant, chronic, or carrier state.
• Period of decline, or defervescence, where the signs and symptoms decreases
• Period of convalescence marked as recovery of patient from a disease.