MBI 111 - Microorganisms and Human Disease

Human Defenses Against Parasitism

Innate Host Defenses: Resistance

Innate host defense factors are non-specific immune (resistance) factors that are present before infection ... they work all the time and effective against many different kinds of microbes

• External resistance factors - those which act on body surfaces
  • Overview
    • Physical
      • barriers - skin, mucus, nasal hair
      • environmental conditions - dryness
      • flushing action - blinking of eyelid, peristalsis, urination
    • Chemical - activities carried out by molecules
      • salts - secreted by sweat glands in skin
      • acids - low pH (HCl in stomach; lactic and other skin acids produced by normal microbiota)
      • lipids - fatty acids secreted by sebaceous glands (sebum) and by gall bladder (bile)
      • enzymes - lysozyme (secretions), digestive enzymes (stomach, small intestine)
    • Cellular - activities carried out by cells
      • normal microbiota compete with pathogens for attachment sites and nutrients as well as producing factors toxic to pathogens
      • phagocytes (macrophages and polymorphonuclear leukocytes (PMNs) are cells that engulf and destroy microbes on mucous membranes by a process known as phagocytosis
  • Details for Microbial Routes of Entry
    • Skin Anatomy and Associated Host Defense Factors
      • epidermis - outer layer of skin (keratinized)
        • physical - barrier, continually sloughing, dry
        • chemical - sebum (fatty acids, lipids); sweat (salt, lysozyme)
        • cellular - normal microbiota make acids (lactic, etc.) that keep pH low
      • dermis - inner layer of skin
        • physical - barrier
        • chemical - sebaceous glands (sebum) and sweat glands (salt)
        • cellular - phagocytes (PMN) and complement from bloodstream
      Respiratory Tract Anatomy and Associated Host Defense Factors
      • Upper respiratory tract - nares (nostrils), oral cavity, pharynx (throat)
        • physical - sneezing, nasal hairs and mucus (filtration), continual sloughing of epithelium, flushing of oral cavity and pharynx by saliva
        • chemical - enzymes in saliva, mucus
        • cellular - phagocytes (PMN), normal microbiota
      • Lower respiratory tract - epiglottis, trachea, bronchi, bronchioles, alveoli
        • physical - coughing, ciliated mucosal epithelium ("mucus escalator")
        • chemical - mucus, enzymes
        • cellular - phagocytes (alveolar macrophages)
    • Gastrointestinal Tract Anatomy and Associated Host Defense Factors
      • Oral cavity (mouth), pharynx (throat), esophagus
        • physical - mucus, sloughing epithelium , saliva, peristalsis
        • chemical - enzymes in saliva, mucus
        • cellular - normal microbiota
      • Stomach
        • physical - mucus, sloughing epithelium, peristalsis
        • chemical - HCl (pH ~2), pepsin (digestive enzyme)
        • cellular - normal microbiota (?)
      • Small intestine
        • physical - mucus, sloughing epithelium, peristalsis
        • chemical - digestive enzymes (pancreas), bile (gall bladder)
        • cellular - normal microbiota
      • Large intestine - includes colon, rectum and anus
        • physical - mucus, sloughing epithelium, peristalsis
        • chemical - anaerobic conditions
        • cellular - normal microbiota (100 billion bacteria per gram of fecal material in large intestine)
    • Genitourinary Anatomy and Associated Host Defense Factors
      • Urinary tract (male and female) - urethra, bladder, ureters and kidneys
        • physical - mucus, epithelium (barrier), flushing with urine
        • chemical - mucus, low pH of urine
        • cellular - normal urethral microbiota
      • External genitalia (male and female)
        • physical - barrier
        • chemical - mucus, specialized sebaceous secretions (smegma, etc.)
        • cellular - normal microbiota
        Genital tract (male) - urethra (see above), seminal ducts
        • physical - mucus; epithelium
        • chemical - mucus
        • cellular - phagocytes; normal urethral microbiota
      • Genital tract (female) - vagina, uterus, fallopian tubes
        • physical - mucus; multi-layered, continually-sloughing epithelium
        • chemical - mucus; low pH (lactic acid produced by normal microbiota)
        • cellular - phagocytes; normal microbiota (live on sloughing epithelial cells, which contain glycogen) maintain pH ~3-4
• Internal resistance factors - those which act within tissues
  • physical - barriers such as connective tissue, blood-brain barrier
  • chemical - activities carried out by molecules
    • enzymes - lysozyme and other enzymes in blood and tissue fluids
    • complement - can be nonspecifically activated by bacterial and fungal polysaccharides or lipopolysaccharides
      • triggers inflammation and enhances phagocytosis (opsonization) by enhancing binding of particles to phagocytes via receptors
      • membrane attack complex (MAC) disrupts (lyses) enveloped viruses and some bacteria
  • cellular - activities carried out by cells
    • inflammation - "early-warning" system
      • histamine released from mast cells at sites of tissue damage
      • increases permeability of capillaries at these sites, causing accumulation of complement, enzymes and phagocytes at sites of infection
      • phagocytes eliminate pathogens and healing is initiated
      • cardinal signs of inflammation - erythema (redness); edema (swelling); fever (heat); pain (soreness)
    • phagocytosis - process by which phagocytes (macrophages and polymorphonuclear leukocytes, aka PMNs) engulf and destroy microbes
      • adhesion (attachment) - phagocyte and pathogen bind to each other
      • ingestion (engulfment) - pseudopodia surround the pathogen, then fuse to form a vacuole called a phagosome
      • killing and digestion - inside phagolysosomes, which are formed by fusion of lysosomes with phagosomes:
        • hydrogen peroxide and related compounds kill microbes
        • enzymes (proteases, polysaccharidases, lipases, nucleases) digest microbes
      • egestion - release of non-digestible material by a process that is essentially the reverse of ingestion

Adaptive Host Defenses: Immunity

Adaptive host defense factors are immune factors that are triggered by antigens, substances produced by microbes during infection, and are specific for those pathogens to which one is exposed

• Antibody-mediated immunity (AMI)
  • antibody responses
    • antibody is produced by B cells (lymphocytes) stimulated by T cells in response to antigens made by infectious agents
      • Phagocytes called antigen presenting cells (APCs) engulf, process and present fragments of antigen molecules on their surfaces
      • Lymphocytes called T helper (Th) cells recognize (using antigen receptors on their surfaces) antigen fragments, then proliferate and differentiate to form a large population of cytokine-producing Th cells
      • Lymphocytes called B cells bind antigen (using antigen receptors on their surfaces) and bind cytokines (using cytokine receptors on their surfaces), then proliferate and differentiate to form a large population of antibody-producing plasma cells
    • antibody binds specifically to the antigen that induced its formation
  • mechanisms of action (ways in which AMI functions)
    • neutralization - inhibition of toxin function, viral infectivity, microbe attachment due to antibody binding to surface antigens of pathogens
    • antibody/complement-mediated lysis - complement is activated by binding to antibody molecules that have bound to antigens, and causes lysis of microbes via formation of a membrane attack complex (MAC)
    • opsonization - antibody and complement both enhance attachment of pathogens to phagocytes via receptors that bind them
• Cell-mediated immunity (CMI)
  • cellular responses
    • cytotoxic T lymphocytes (CTLs) are activated by T helper cells in the presence of viral antigens on virus-infected cells
      • Phagocytes called antigen presenting cells (APCs)engulf, process and "present" fragments of antigen molecules on their surfaces
      • Lymphocytes called T helper (Th) cells cells recognize (using antigen receptors on their surfaces) antigen fragments, then proliferate and differentiate to form a large population of cytokine-producing Th cells
      • Lymphocytes called Tc cells (CTL precursor cells) bind antigen (using antigen receptors on their surfaces) and bind Th cell-produced cytokines (using cytokine receptors on their surfaces), then proliferate and differentiate to form a large population of CTLs
    • macrophages are also activated by T helper cells
      • APCs engulf, process and "present" fragments of antigen molecules on their surfaces
      • Th cells recognize (using antigen receptors on their surfaces) antigen fragments, then proliferate and differentiate to form a large population of cytokine-producing Th cells
      • macrophages bind Th cell-produced cytokines (using cytokine receptors on their surfaces), then differentiate to form activated macrophages
  • mechanisms of action (ways in which CMI functions)
    • CTLs kill "target" cells by inducing them to destroy themselves via programmed cell death (apoptosis) ... apoptosis video
    • activated macrophages engulf, kill and digest intracellular bacteria and fungi much more efficiently than normal macrophages do, partially because they starve these microbes (by destroying nutrients the microbes need to grow)

Coping Strategies

Vaccines are killed or attenuated (weakened) preparations containing microbial antigens that are used to stimulate immune responses without causing disease

• Types of vaccines
  • toxoid vaccines - a chemically inactivated toxin that is used to induce production of neutralizing antibody
  • killed vaccines - chemically inactivated microbe used to induce antibody production
    • whole microbe vaccines - include the entire microbe
    • split microbe vaccines - include only the antigens needed to induce antibody responses
  • live attenuated vaccines - living, but less virulent, microbe causes a transient infection that leads to generation of both AMI and CMI
  • recombinant vaccines
    • vector vaccines are those generated by incorporating antigen genes isolated from pathogens into non-harmful bacteria or viruses
    • food vaccines are those generated by incorporating genes for microbial antigens into foods such as bananas or tomatoes
  • DNA vaccines - injection of DNA containing genes that code for microbial antigens can lead to transient expression of those genes, resulting in immunization of the host
• Childhood immunizations (vaccinations)
  • Hepatitis B - vaccinate within 2 months after birth; boost 1 month later; boost again 2 months after the second dose (and 4 months after the first dose); if not vaccinated earlier, assess immunity, then vaccinate as needed at 11-12 years of age
  • DTaP (diphtheria, pertussis, tetanus) - vaccinate with DTaP vaccine at 2, 4, 6,15-18 months, then boost at age 4-6 years; Td vaccine (no pertussis component) is used as a booster at age 11-16, then every 7-10 years after that
  • hepatitis A - vaccinate at 2-12 years of age in areas where high risk of infection occurs; especially recommended for children living in communities with high rates of hepatitis A (vaccination helps prevent carrier state as well as outright disease)
  • Hib (Haemophilus influenza b) - vaccinate at 2, 4, 6, and 12-15 months
  • poliomyelitis - injectable polio vaccine (IPV) at 2, 4 and 6-18 months; boost at 4-6 years (the oral polio vaccine (OPV) is no longer recommended in the US because it causes the very small number of cases (~4 cases from 1997-1998) of polio in this country
  • MMR (measles, mumps, rubella) - vaccinate at 12-15 months; boost at 4-6 years of age; if not vaccinated earlier, assess immunity, then vaccinate as needed at11-12 years of age; populations that don't get vaccine have Congenital Rubella Syndrome
  • varicella (chickenpox) - vaccinate at 12-18 months; if not vaccinated earlier, assess immunity, then vaccinate as needed at 11-12 years of age (or up to 18)
• Other immunizations (vaccinations)
  • Menomune induces immunity against major strains of Neisseria meningitidis, the major cause of meningitis in college-age people; currently recommended for first-year college students, especially those living in dorms
• Prevnar - induces immunity to the 7 major strains of Streptococcus pneuminiae, which causes middle ear infections and meningitis in children and pneumonia in all age groups; it is given as a four dose series, starting at two months of age (with a minimum of 6 weeks of age); subsequent doses are given at age four months and six months of age, with a fourth dose at age 12-15 months (and at least two months after the third dose); recommended that all children under age 23 months be given this vaccine; adults 65 and over are recommended to get this vaccine as well
• Rotashield is a newly-formulated oral vaccine aimed at preventing diarrheal disease (rotavirus causes at ~55,000 hospital admissions and 20-40 infant deaths each year in the US, and is a major cause of death in developing nations, where it causes >600,000 deaths each year)
• Smallpox vaccine, used to eradicate smallpox worldwide in 1977, is no longer used outside of research environments, but millions of doses have been generated since September 11, 2001, as a possible "hedge" against bioterrorist attack

• sterilant - agent that kills all microorganisms
• germicide - a chemical agent that kills harmful microorganisms
  • disinfectant - used only on environmental surfaces because it is too harsh to be used on human skin
  • antiseptic - can be used on human skin because it is rather mild
• incineration - direct flame burns microorganisms; destructive, but sterilizes
• hot-air oven - dry heat, doesn't penetrate well; takes a long time to sterilize; works only on heat-stable objects
• boiling - moist heat disinfects, but takes a long time to penetrate
• autoclaving - moist heat under pressure (15-30 minutes at 121C and 15 psi) sterilizes solids or heat-stable liquids
• pasteurization - moderately high heat and for short exposure time disinfects liquids (milk, beer, wine, etc.) that would be destroyed by other methods; heating to 72C for 15 seconds or 82C for 3 seconds is effective
• radiation
  • ultraviolet (UV) light - disinfects only on thin films or surfaces
  • ionizing radiation (X-rays, gamma rays) - sterilizes many substances (plastics, cloth, etc.)
• filtration - disinfects liquids or gasses 

• origin
  • antibiotics are naturally occurring substances made by certain bacteria and fungi (these can be modified chemically, as well)
  • antimetabolites are synthetic substances made from chemicals by humans in factories
• mechanisms of action - CTAs work by damaging:
  • metabolites (required for energy-generating reactions in cells) - sulfa drugs, INH (isoniazid)
  • membranes - nystatin, polymyxin B
  • cell walls - penicillin, cephalosporin, bacitracin, vancomycin
  • protein synthesis - streptomycin, tetracycline, chloramphenicol
  • nucleic acid synthesis - naladixic acid, novobiocin, rifampin
• Considerations for use of CTAs
  • spectrum - indicates the variety of microbes killed; broad-spectrum CTAs kill many different microbes and may be preferred because of this
  • selective toxicity - a selectively toxic agent is harmful to one living thing (e.g., microbe) but not to another (e.g., host)
  • effectiveness - CTAs must be administered by an appropriate route so they can reach infected sites in an appropriate concentration to act against the organism(s) within their spectrum
  • susceptibility of microbe - if a microbe is killed or its growth inhibited by an antimicrobial CTA, it is said to be sensitive to that CTA; microbial resistance to an antimicrobial CTA is an undesirable lack of sensitivity to that CTA:
    • resistance can be acquired by a microbe via a mutation or acquisition of resistance genes (on plasmids)
    • testing bacterial and fungal cultures for antibiotic sensitivity is very important for proper therapy of infected persons due to the possibility of resistance
  • allergenic potential - the CTA must not induce allergic (hypersensitivity) reactions in the host because this can be uncomfortable, and might even be life-threatening in some cases

  Did ya know...? More than 133 million antibiotic prescriptions are written by US doctors each year to ambulatory patients and ~190 million doses of antibiotics are administered each day