Chapter 2: Host-Parasite Interaction

Role of the Usual Microbial Flora

Microbial Flora

  • Symbiosis:

    • is defined as the association of two organisms living together. The organisms are called symbionts.

    • Symbiosis as a biological relationship between two or more organisms where both (host and organism) benefit from one another may be described as mutualism.

    • Example: Lactobacilli in the urogenital tract of women offer a mutual association: the lactobacilli provide the host protection by preventing colonization of pathogenic species at that site while they derive nutrients from the host.

  • Commensalism

    • In the relationship where the organism benefits but there is no beneficial or harmful effect to the host

    • Proteus mirabilis is a commensal species in the gastrointestinal tract of humans.

  • Parasitism

    • one species (microbe) benefits at the expense of the other (host).

    • The parasite Entamoeba histolytica is a pathogenic intestinal ameba that derives nutrients from the host at the expense of the host, causing intestinal ulcers and amebic dysentery.

Characteristics of Indigenous Microbial Flora

Microorganisms that are commonly found on or in body sites of healthy persons are called normal, usual, or indigenous flora.

Microorganisms that colonize an area for months or years represent resident microbial flora

Microorganisms that are present at a site temporarily represent transient flora.

Carriers are hosts that pathogenic organisms established themselves in without manifesting symptoms.

Hosts is called the carrier state in this circumstances, it maybe acute or chronic.

Example:
Chronic: Salmonella typhi that establish themselves in the bile duct
Acute: Neisseria meningtidis which after a few day or weeks the organisms will no longer reside in the hosts

Composition of Microbial Flora at Different Body Sites

Flora of the Skin:

  • Organisms concentrate the most in areas that are moist, such as the armpit, groin, and perineum.

  • The apocrine sweat glands in these areas secrete substances metabolized by the skin bacteria, releasing odorous amines.

  • Aerobic diphtheroid are usually found in moist areas such as the axillae and between the toes

  • Staphylococcus epidermidis and Propionibacterium spp. reside in hair follicles and colonize the sebaceous glands because they are resistant to skin lipids and fatty acids

  • P. acnes colonize the deep sebaceous glands.

Flora of the Mouth:

  • The mouth contains large numbers of bacteria, with Streptococcus being the predominant genus.

  • Bacterial plaques that develop on teeth may contain 10(11) streptococci per gram.

  • Obligate anaerobes reside in the gingival crevices

Flora of the Respiratory Tract

  • Upper Tracts

    • Composed of Mouth, Nasopharynx, and oropharynx

    • Predominantly colonized with viridians streptococci

    • Opportunistic pathogens such as S. aureus, found in approximately 30% of healthy individuals, colonize the anterior nares.

    • Haemophilus influenzae, Streptococcus pneumoniae, and N. meningitidis, all potential pathogens, are also found in the nasopharynx of healthy individuals.

    • Individuals who are hospitalized for several days may become colonized in the upper respiratory tract by gram-negative bacteria, particularly members of the Enterobacteriaceae.

    • The oropharynx contains a mixture of streptococci. Many species of the viridans group can be isolated, including S. mitis, S. mutans, S. milleri, S. sanguis, and Streptococcus salivarius.

  • Lower Tracts

    • The trachea, bronchi, and lungs are protected by the action of ciliary epithelial cells and by the movement of mucus. The tissues of these structures are normally sterile as a result of this protective action.

Flora of the Gastrointestinal Tract

  • The gastrointestinal tract comprises the esophagus, stomach,

    small intestine, and colon.

  • The stomach is usually sterile, especially after ingestion of a meal, when gastric juices, acids, and enzymes, which help to protect the stomach from microbial attack, are produced.

  • Microorganisms prevalent in the colon may produce a count between 10(8) and 10(11) bacteria per 1 g of solid material

  • the facultative anaerobes as being the predominant organisms, obligate anaerobes far outnumber the facultative gram-negative rods, making up more than 90% of the microbial flora of the large intestine.

Flora of the Genitourinary Tract

  • The kidneys, bladder, and fallopian tubes are normally free of

    microorganisms

  • The urethra is colonized in its outermost

    segment by organisms found on the skin.

  • Before puberty and in postmenopausal women, vaginal flora primarily consists of yeasts, gram-negative bacilli, and gram-positive cocci.

  • High estrogen levels in childbearing years cause the vagina to store glycogen, which lactobacilli turn into acid. This acidic environment prevents harmful organisms from growing.

Role of Microbial Flora in the Pathogenesis of Infectious Disease

  • The usual flora are opportunists; they cause disease when their habitat is damaged, disturbed, or changed or when the host’s immune system is weakened or compromised.

Role of the Microbial flora in the Host Defense Against infectious Disease

  • Microbial flora stimulates and supports the immune system, crucial for health.

  • Germ-free environments weaken immunity, making organisms more susceptible to infections.

  • Flora prevents harmful pathogens from colonizing by maintaining balance.

  • Disrupting this balance, like with antibiotics, can lead to infections.

  • Flora is essential but can cause disease if its balance is disturbed.

Extra Information

  • Human milk creates a pH level of 5.0 to 5.5 s which promotes the environment of Bifidobacterium spp. that gives great gut health for an infant

  • Candida albicans is a fungus that is a common cause of opportunistic infections in the genitourinary tract, especially in women of childbearing age. It can overgrow when the natural balance of the vaginal flora is disrupted, often leading to yeast infections.

Discussion Extra Information

Nutritional Factors:

  • Antibacterial substances

    • Bile, lysozyme, fatty acids

  • Environmental factors

    • Moist or dry

    • Low pH

Composition of Microbial Flora

  1. Usual flora of the Skin

  2. Usual flora of the Mouth

    1. Streptococcus spp.

  3. Usual flora of the Respiratory Tract

  4. Usual flora of the Gastrointestinal

    1. Most of the

    2. Colon and Large intestine many obligate anaerobes

    3. Pseudomonas non fermenter bacteria

  5. Usual flora of the Genitourinary Tract

    1. As the woman grows older they are more susceptible to yeast infection

    2. Good bacteria of vagina is Lactic acid bacteria

Pathogenesis of Infection

  • Pathogenicity

    • Ability of a microbe to produce in a susceptible individual

      • True pathogens - can cause disease in immunocompetent individuals

        • Example: Bacillus anthracis cause anthrax and Yersinia pestis cause bubonic plague

      • Opportunistic Pathogens - Can cause disease when they’re not in their normal habitat

    • Latrogenic Infection

      • Infection as the result of medical treatment or procedures

  • Virulence

    • Relative ability of a microorganism to cause disease or the degree of pathogenicity

    • Low infective dose: More virulent they only need a few to infect. Example: Salmonella

  • Microbial Virulence Factors:

    • Ability to resist phagocytosis

      • Presence of Capsule

        • Examples: Haemophilus influenzae and Streptococcus pneumoniae

      • Protein A

        • Found in the cell wall or Staphylococcus aureus.

        • Prevents Opsonization & Phagocytosis

      • Hemolysin

        • Lyse red blood cells

        • Example: Streptococci

      • Leukocidin

        • Lysosomal discharge in the cytoplasm

    • Surface structures that promote adhesion

      • Adhesins

        • Fimbriae

          • Examples: Escherichia coli and Neisseria (Tulo)

    • Ability to survive intracellularly & proliferate

      • Host Factors:

        • Lactoferrin & Lysozyme

      • igA Protease

        • Degrades IgA found in mucosal surface

        • Enzyme that cleaves igA (An antibody)

        • H. influenzae, N. Gonorrhaeae, N. meningtidis

      • Shifting Key cell surface Ags

        • Borrelia spp.

    • Ability to produce Extracellular & Enzymes

      • Invasion

        • Pathogens ability to penetrate and grow in tissues

      • Dissemination

        • Ability of an organism to spread in distant site

      • Ability to produce Extracellular Toxins & Enzymes

        • Table 2-4 Exotoxins and Endotoxins

        • Exotoxins is more toxic

Steps in Phagocytosis

  • 1. Chemotaxis

    • Migration of the phagocyte to the area of infection

  • Attachment

    • Opsonization (Speeds up phagocytosis)

      • Coating of bacterium with Antibodies or complement

  • Inflammation

    • Body’s responses to injury or foreign body

    • Cardinal signs

      • Rubor -Redness

      • Calor -Heat sensation

      • Dolor - Pain

      • Tumor - Swelling

      • Functio laesia - Loss of function

Host Resistance Factors

  • Natural Immunity (Non-specific immunity)

    • Resists infections by normally present body functions

    • No prior exposure is required

    • Response doesn’t change w/ subsequent exposures

    • Lacks memory

  • Adaptive Immunity (Specific immunity)

    • Specificity for each

    • Individual pathogen

    • Ability to REMEMBER a prior exposure

    • Result in an INCREASED response upon repeated exposure

    • Has memory

Vaccine create pseudo virus that creates an antibody that remembers that virus.

Natural Immunity

  • Check asdasda

Acquired Immunity

  • Types of Acquired Immunity

Routes of Transmission

  1. Airborne (Can travel the air)

  2. Droplets (They need to be carried by moist)

  3. Transmission by Food and Water

  4. Close Contact

  5. Cuts and Bites

  6. Arthropods

  7. Zoonoses