Module 3 - Chapter 2 Book Notes

origin of microbial biota

• colonization — the growth of microbiota in or on a body site without the production of damage or notable symptoms

• symbiosis — the association of two organisms living together (symbionts)

  • mutualism: both sides benefit from one another

• commensalism — the relationship where the organism benefits but there is no beneficial or harmful effect on the host

• parasitism

characteristics of indigenous microbial biota

• resident microbiota — microbes that colonize an area for months or years

• transient microbiota — microbes that are present at a site temporarily

  • eliminated by the host immune defenses or by competition with resident biota

  • some pathogenic organisms may establish themselves in a host without manifesting symptoms

    • hosts are carriers and become capable of transmitting the infection (carrier state)

      • the carrier state can be acute or chronic

factors that determine the composition of the usual microbial biota

• location is influenced by nutritional and environmental factors like the amount and types of nutrients available at the site

  • EXAMPLE: bacteria are more likely to inhabit moist areas and are referred to as diphtheroids

• the affinity of microorganisms for a specific site depends on the ability of the organisms to resist the antibacterial effects of substances

  • the composition of the microbial biota is also affected by pH

• opportunistic infection — changes to age, nutritional status, disease states, and drug or antimicrobial therapy use that can predispose an individual to infection by the indigenous biota

composition of microbial biota at different body sites

• use of molecular sequencing strategies to determine which organisms reside in or on the human body

• normal microbiota of the skin

  • mechanisms to prevent infection include:

    • physical separation of microbes from the tissue

    • presence of fatty acids that inhibit many microorganisms

    • excretion of lysozyme by sweat glands

    • desquamation of the epithelium

  • composition of microbiota on the skin depends on the activity of the sebaceous glands

    • microbes concentrate in moist areas

• normal microbiota of the oral cavity

  • Staphylococcus epidermidis

  • Streptococcus mitis

  • Streptococcus sanguinis

  • Streptococcus salivarius

  • Streptococcus mutans

  • Peptostreptococcus spp.

  • Veillonella spp.

  • Actinomyces israelii

  • Bacteroides spp.

  • Prevotella/Porphyromonas

  • Bacteroides oralis

  • Treponema denticola

  • Treponema refringens

• normal microbita of the respiratory tract

  • the upper respiratory tract is composed of the mouth, nasopharynx, oropharynx, and larynx

    • predominantly colonized with viridans streptococci, such as Streptococcus mitis, Streptococcus mutans, Streptococcus anginosus, Streptococcus sanguinis, Moraxella catarrhalis, Neisseria spp., and diphtheroids

    • olbigate anaerobes reside in the gingival crevices (anaerobic env.)

  • the lower respiratory tract is composed of the trachea, bronchi, and pulmonary parenchyma

    • these are protected by the action of ciliary epithelial cells and by the movement of mucus (sterile tissues)

  • microbes found in the nose and nasopharynx

    • Staphylococcus aureus

    • Staphylococcus epidermidis

    • Diphtheroids (Corynebacterium spp.)

    • Haemophilus parainfluenzae

    • Streptococcus spp.

  • microbes found in the oropharynx

    • α-Hemolytic and nonhemolytic streptococci

    • Diphtheroids (Corynebacterium spp.)

    • Staphylococcus aureus

    • Staphylococcus epidermidis

    • Streptococcus pneumoniae

      Streptococcus mutans

    • Streptococcus mitis

    • Streptococcus sanguinis

    • Streptococcus salivarius

    • Moraxella catarrhalis

    • Haemophilus parainfluenzae

    • Bacteroides spp.

    • Prevotella/Porphyromonas

    • Bacteroides oralis

    • Fusobacterium necrophorum

• normal microbiota of the GI tract

  • this tract comprises of the esophagus, stomach, small intestine, and colon

  • GI tract defenses — acidic pH levels

    • the stomach contains gastric juices, acids (pH 2), and enzymes that help to protect the stomach from microbial attack

  • microbe population is lowest in the esophagus

  • microbes found in the stomach

    • Streptococcus, Enterococcus, Prevotella, and the opportunistic pathogen Helicobacter pylori which associate with the stomach lining

  • microbes found in the large intestine

    • gram-positive cocci belonging to the genera Streptococcus and Enterococcus

    • Bacteroides, Clostridium, Prevotella, and Porphyromona (obligate anaerobes)

  • microbes found in the GI tract

    • Bacteroides spp.

    • Clostridium spp.

    • Enterobacteriaceae

    • Enterococcus spp.

    • Eubacterium spp.

    • Fusobacterium spp.

    • Lactobacillus spp.

    • Peptostreptococcus spp.

    • Peptococcus spp.

    • Porphyromonas spp.

    • Prevotella spp.

    • Streptococcus spp.

• normal microbiota of the genitourinary tract

  • consists of the kidneys, bladder, cervix, and fallopian tubes

  • vaginal biota primarily consists of yeasts, gram-negative bacilli, and gram-positive cocci (in puberty & pre-menopausal)

    • during childbearing years heightened presence of lactobacilli which metabolize glycogen from vaginal epithelial cells to maintain low pH

    • low pH encourages colonization of the vagina with lactobacilli, anaerobic gram-negative bacilli, and gram-positive cocci

  • microbes found in the genitourinary tract

    • Lactobacillus spp.

    • Bacteroides spp.

    • Clostridium spp.

    • Peptostreptococcus spp.

    • Staphylococcus aureus

    • Staphylococcus epidermidis

    • Enterococcus spp.

    • Diphtheroids (Corynebacterium spp.)

the role of the microbial biota in the pathogenesis of infectious disease

• surgery patients are immunocompromised to infections caused by organisms that colonize the particular surgical site

  • the host’s immune response can be reduced due to chronic illnesses or suppression of immunosuppressive drugs, chemotherapy, or radiation

role of microbial biota in the host defense against infectious disease

• exposure to microbes builds immune resistance; cell-mediated immunity is developed by microbes presence

• the microbial biota produces conditions at the microenvironmental level that block colonization by extraneous pathogens

  • when indigenous biota composition is altered, another bacterium may swoop in

  • this is troublesome because if that biota is eliminated then resistant or more pathogenic species may be able to establish infection

microbial factors contributing to pathogenesis and virulence

• pathogenicity — the ability of a microbe to produce disease in an individual

• true pathogens — organisms recognized to cause disease in healthy immunocompetent individuals a high percentage of the time

• introgenic infection — an infection that occurs as the result of medical treatment or procedures

• routes of transmission

  • infectious agents gain access to a host through:

    • air (inhalation)

      • coughing, sneezing, and talking transfer aerosols

        • fomites can also transfer if contaminated object touched by someone uninfected

      • pathogens that transmit this way must be resistant to drying and inactivation by ultraviolet light

      • defenses — upper respiratory tract is windy, the lower respiratory tract sweeps microbes upward (from the ciliary epithelium), and the production of IgA, lysozyme, and alveolar macrophages prevent infection

    • via food and water (ingestion)

      • gastric enzymes and juices in the stomach prevent survival of most organisms

        • those which can survive those conditions compete with the resident microbiota & produce damage to the tissues of the GI tract

    • close contact (includes sexual transmission)

      • passage of organisms by salivary, skin, and genital contact

    • cuts and bites

    • via arthropods

      • infection from a mosquito, tick, flea, or mite bite

    • zoonosis

      • animal diseases that can infect humans are transmitted through animal contact

      • diseases can be passed by animal bites (rabies), arthropod vectors (plague), contact with secretions (brucellosis), and contact with animal carcasses and products (tularemia, listeriosis)

• virulence — the relative ability of a microorganism to cause disease or the degree of pathogenicity

  • measured by the numbers of microorganisms necessary to cause infection in the host

  • microbial virulence factors

    • some factors have widespread use such as capsules and toxins but some are specialized

    • virulence factors allow the pathogen to evade or overcome host defenses and cause disease and encompass functions

  • ability to resist phagocytosis

    • a common method to evade phagocytosis is the presence of a polysaccharide capsule on the surface

      • the capsule inhibits phagocytosis primarily by masking the cell surface structures that are recognized by receptors on the surface of the phagocytic cell and in the same manner inhibits the activation of complement by masking structures to which complement proteins bind

    • protein A also protects microbes from phagocytosis

      • this protein binds to the Fc portion of IgG and prevents opsonization and ultimately phagocytosis by turning the antibody around on the surface

        • interference with the binding of the host’s antibodies to the surface of the microbe

    • excretion of potent materials to kill phagocytes

      • pathogenic staphylococci release leukocidins that cause lysosomal discharge of white blood cells into the cytoplasm

      • some microbes can inhibit chemotaxis

• surface structures that promote adhesion to host cells and tissues

  • microbes typically require adherence to host cells before infection and disease can progress

    • adhesins — the microbial surface structures that mediate attachment; pili

    • host cells must have necessary receptors for the adhesins

• ability to survive intracellularly and proliferate

  • host factors work to prevent microbe proliferation

    • secrete lactoferrin antibody and lysozyme to protect against infection

  • invasion — the process of pathogens to penetrate and grow in tissues

    • necessary depth can vary

  • dissemination — the ability of the microbe to spread to distant sites

• ability to produce extracellular toxins and enzymes

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