Infectious Disease and Microorganisms

Infectious Disease

• Infectious disease occurs when microorganisms invade the human body, causing harmful and potentially lethal consequences.
• Microorganisms can be pathogens or non-pathogens, and all microbes are microorganisms, but not all microorganisms are microbes.

Microorganisms and Terminology

• Microorganisms are organisms so small they are invisible to the naked eye and can only be seen with a microscope.
• Infection: the presence and multiplication within a host of another living organism with subsequent injury to the host.
• Infectivity: the ability of an organism to enter, multiply, and survive in a host.
• Host: any organism capable of supporting the nutritional and physical growth requirements of another organism.
• Colonization: the act of establishing a presence within a host.
• Virulence: the disease-producing potential of an invading organism.
• Pathogen: an organism so virulent that it is rarely found in the absence of disease.
• Microflora: a multitude of non-harmful bacteria inhabiting the internal and external exposed surfaces of the human body.
• Saprophytes: free-living organisms that obtain nutrition from dead or decaying organic materials in the environment.
• Opportunistic Pathogen: an organism that produces an infectious disease only when the health and immunity of the host have been severely compromised.

Types of Interactions Between Host and Colonizing Microorganisms

• Commensalism: the multitude of non-harmful bacteria inhabiting the internal and external exposed surfaces of the human body acquire survival needs from the host, but the host is not adversely affected by this relationship.
• Mutualism: an interaction in which colonizing organism and host both derive benefits.
• Parasitic: an interaction in which a colonizing organism benefits and the host sustains injury.

Agents of Infection Disease

• Prions: protein particles that lack any kind of demonstrable genetic material, can affect other normal proteins and alter them, and aggregate in the brain to form plaques.
• Rickettsia, Chlamydia, Coxiella, Mycoplasma: these organisms combine the characteristics of viruses and bacteria, are obligate intracellular pathogens like viruses, and most produce a rigid peptidoglycan cell wall, reproduce asexually, and contain RNA and DNA like bacteria.

Modes of Transmission

• Airborne transmission: occurs when microbes are transmitted by air currents across long distances (more than one meter).
• Vector-borne transmission: animals, or biting arthropods transmit microbes to humans.
• Vehicle-borne transmission: occurs when infection is transmitted by a contaminated source (vehicle).
• Portal of Entry: the process by which a pathogen enters the body and gains access to susceptible tissue.
• Penetration: any disruption in the integrity of the body's surface barrier (skin or mucous membrane) is a potential site for invasion.
• Direct contact: pathogens are transmitted directly from infected tissue or secretions to exposed, intact mucous membranes.
• Ingestion: entry of infectious agent occurs through oral cavity and GI tract.
• Inhalation: entry of infectious agents through respiratory tract.

Bacterial Defences

• Biofilm: some bacteria secrete a substance that helps them to attach to other bacteria, cells, or body tissues.
• Capsule: some bacteria are enclosed in a protective capsule that helps to protect them from ingestion by macrophages.
• Outer membrane/cell wall: gram-negative bacteria have an outer membrane that protects them from the effects of antibiotics.
• Spores: some bacteria produce spores that enable them to survive when environmental conditions are difficult.
• Flagella: long thin filaments that project from a cell wall and enable bacteria to move from one place to another.
• Antibiotic resistance: bacteria can develop resistance to drugs because their genes can mutate.

Bacterial Infections

• Salmonella infection: can lead to typhoid fever, a sometimes deadly disease more common in developing countries.
• Shigella infection (a.k.a. Shillosis): a common infection in settings where hygiene is poor, spread via the fecal-oral route.
• Bacterial infections of the urinary tract: affect the urethra, urinary bladder, ureters, and kidneys, commonly caused by E. coli, Pseudomonas aeruginosa, and Klebsiella.

Viral Infections

• Viruses are the smallest obligate intracellular pathogens.