Microorganisms and Humans: a Dynamic Relationship

Chapter 9 This chapter discusses the relationship between humans and microorganisms, focusing on the role of microorganisms in causing infections. It explores different types of infections caused by bacteria, viruses, fungi, and parasites, as well as the challenges of antibiotic resistance. The chapter highlights the importance of vaccines and immunotherapy in protecting against infections. Chapter Outline: Infection Microorganisms and Humans: a Dynamic Relationship Process of Infection Stages of Infection Infectious Disease Bacterial Infection Viral Infection Fungal Infection Parasitic Infection Antibiotic/Antimicrobial Resistance Vaccines and Protection Against Infection Active Immunization Passive Immunotherapy Overview: Modern healthcare has made progress in preventing and treating infectious diseases Emergence of new infections, reemergence of old infections, and antibiotic resistance still pose threats to human health Endemic diseases, chronic hepatitis, HIV infection, sexually transmitted infections, and foodborne infections remain challenges in the US Most deaths from infections occur in individuals with compromised immune systems (young children, elderly, and those with chronic diseases) Microorganisms and Humans: a Dynamic Relationship Human body provides a hospitable environment for many microorganisms Only a small number of microorganisms can cause disease Human microbiome refers to the resident microorganisms found in different parts of the body Symbiotic relationship between the human body and microbiome Maintained by physical barriers and interaction with inflammatory and immune systems Process of Infection Infectious diseases caused by pathogenic microorganisms Process of infection includes encounter and transmission, colonization, invasion, dissemination, and cellular/tissue damage by pathogens Classes of Microorganisms Infectious to Humans: Virus: 20-300 nm, examples include human immunodeficiency virus, hepatitis A and B, chicken pox, measles Bacteria: 0.8-15 mcg, examples include Staphylococcal wound infection, Cholera, Streptococcal pneumonia, Tuberculosis Fungi: 2-200 mcg, examples include Tinea pedis (athlete's foot), Candidiasis (e.g., thrush), Sporotrichosis, Histoplasmosis Protozoa: 1-50 mm, examples include Giardiasis, African trypanosomiasis (sleeping sickness) Helminths: 3 mm to 10 m, examples include Trichinosis, Filariasis Transmission of microorganisms: Direct transmission or contact: vertical transmission from mother to child, horizontal transmission from person to person, zoonotic infections Indirect transmission: contact with infected materials, inhalation or droplet infection, ingestion of contaminated food or water, inoculation Colonization and mechanisms used by pathogens to resist immune defenses: Colonization is the ability of a pathogenic microorganism to survive and multiply on or within the human environment Pathogens can destroy or block components of the immune system, produce toxins, prevent phagocytosis, produce surface molecules that mimic self-antigens Minimum infective dose: The minimum number of microorganisms needed to cause infection varies with the specific pathogen Examples include Vibrio cholerae (103-108 microorganisms), norovirus and rotavirus (10-100), Giardia lamblia parasitic diarrhea (10), Mycobacterium tuberculosis (<10) Adherence and its effect on immunity: Adherence helps protect the microorganism from removal by mechanical forces Adherence occurs between receptors on the microorganism and on the surface of cells, which can localize the infectious agent to particular sites Coccus antigens M antibodies can be formed against self-antigen Can lead to red blood cell hypersensitivity disease and rheumatic heart disease Antibodies can also react with Mycoplasma antigens Biofilms Consist of mixed species of microorganisms in an extracellular matrix Become resistant to antibiotics and immune responses Associated with chronic infections and can form on implanted medical devices Invasion and dissemination of infection Invasion is the ability of pathogens to cross surface barriers Direct extension through surrounding tissue or through the blood or lymphatic vessels Tissue damage or cellular alterations can occur directly or indirectly Examples of mechanisms used by pathogens to cause tissue damage Direct production of exotoxins or endotoxins Direct damage through invasion Indirect damage through immune complexes or cell-mediated immunity Factors influencing the capacity of a microorganism to cause infection Communicability Immunogenicity Infectivity Mechanism of action Pathogenicity Steps involved in the process of infection: Incubation period: Initial exposure to the infectious agent and onset of first symptoms Prodromal stage: Occurrence of initial mild symptoms Invasion or acute illness period: Pathogen multiplies, invades tissues, triggers immune and inflammatory responses Convalescence: Immune and inflammatory systems successfully remove the infectious agent, symptoms decline Mechanisms by which pathogens can cause tissue damage: Invasion: Pathogens invade tissues and damage them Toxigenicity: Production of soluble toxins or endotoxins that greatly influence virulence Portal of entry: Route through which microorganism infects the host (direct contact, inhalation, ingestion, etc.) Virulence: Capacity to cause severe disease