Chapter 13


Human-Microbe Interactions



Normal Resident Flora (Biota)

§ Microorganisms routinely found growing on the body of healthy individuals

§ Includes bacteria, fungi, protozoa, viruses

§ Microbes accumulate over time immediately following birth by interaction with the environment

§ Microbes are found almost everywhere on the human body but are more prominent in locations exposed to the environment

§ Some pathogens are found in the normal flora but dont cause problems because competition limits their growth ability

§ only cause problems in a compromised state



Benefits of Normal Resident Flora

§ Block binding sites that pathogens use to infect host cells

§ Create competition for nutrients

§ Stimulate host defenses  

§ Some produce antimicrobial agents:

§ Propionibacterium - live in hair follicles and produce a fatty acid that inhibits the growth of many disease causing species

§ Escherichia coli - produce toxins in the gastrointestinal tract

§ Lactobacillus - produce lactic acid in the vagina causing the pH to be too low for other bacteria to survive



Progress of an Infection: Terms

§ Infection – when microorganisms penetrate host defenses, enter tissues, and multiply

§ Disease – when the infection causes damage to tissues and organs

§ Pathogens- disease-causing organisms

§ Pathogenicity - a microorganisms potential to cause an infection or disease

§ True pathogens – cause disease in healthy individuals with normal immune defenses

§ Opportunistic pathogens – cause disease when host defenses are compromised or are in a part of the body not natural to them

§ Virulence – the degree of pathogenicity of an organism

§ Virulence factors – toxins or enzymes produced by the microbe that cause damage to the host

§ Infectious dose (ID) – minimum number of microbes required to cause an infection

§ those with smaller ID have greater virulence



Becoming Established: Portals of Entry

§ Skin

§ pathogens enter through nicks, abrasions, punctures, or insect bites

§ Gastrointestinal Tract

§ pathogens come from food or other ingested substances and survive stomach acids

§ Respiratory Tract

§ pathogens enter through oral and nasal cavities

§ Urogenital Tract

§ pathogens acquired through sexual contact (STDs), displaced organisms in urinary tract, or overgrowth of normal flora



Becoming Established: Attaching to the Host

§ Adhesion - process by which microbes gain a more stable position in the portal of entry

§ Bacteria can use fimbrae, surface proteins, capsules

§ Worms can use suckers, hooks and barbs

§ Viruses use specific receptors or spikes



Becoming Established: Surviving Host Defenses

§ Antiphagocytic factors

§ leukocidins – kill phagocytes

§    Staphylococcus, Streptococcus

§ slime layer or capsule – difficult to engulf

§    Salmonella, Neisseria, Streptococcus

§ Survive and hide inside phagocytes

§    Legionella, Mycobacterium, Rickettsia



Becoming Established: Causing Disease

§ Extracellular Enzymes (Exoenzymes)

§ Directly inflict tissue damage

§ Break down hosts defenses and allow the microbe to move deeper into the tissue

§ Mucinase – digests the protective coating on mucus membranes

§ Keratinase – digests the primary component of skin and hair

§ Collagenase – digests the collagen fibers of connective tissue

§ Hyaluronidase – digests the component holding animal cells together

§ Exotoxins – released by living bacterial cells into infected tissue

§ Neurotoxins – damage nervous tissue

§    Tetanus, Botulism

§ Enterotoxins – cause intestinal disturbances

§    Cholera, E. coli

§ Hemolysins – disrupt the cell membrane of red blood cells

§    Streptococcus, Staphylococcus

§ Endotoxin (LPS) – released when gram negative bacteria are damaged or destroyed

§ damage is from immune response to lipid A component of LPS                

§ causes endotoxic shock when the infection is systemic



Patterns of Infection

§ Localized infection - microbe enters the body and remains confined to a specific area

§ Boils, fungal skin infections, and warts are examples

§ Focal infection - infectious agent breaks loose from a local infection and is carried into other tissues

§ tuberculosis, streptococcal pharyngitis

§ Systemic infection - infectious agent spreads throughout the body usually in the bloodstream

§ measles, chickenpox, AIDS, diphtheria, typhoid fever, syphilis

§ Characteristics of a systemic infection:

§ bacteremia - bacteria in bloodstream

§ toxemia- toxins in bloodstream

§ viremia - viral particles in bloodstream

§ septicemia - life-threatening illness from infectious agents in the bloodstream

§ Secondary infection - when an initial, primary infection is complicated by infection with a different microbe



Phases of Infection

§ Incubation Period – time from initial contact with infectious agent to appearance of the first symptoms

§ Prodromal Stage – earliest symptoms of infection

§ Vague symptoms such as head and muscle aches, fatigue, malaise

§ Period of Invasion – infectious agent multiplies and becomes established in the host

§ May include fever and more specific signs and symptoms

§ Convalescent Period – symptoms decline and patient recovers




Acquisition and Transmission of Disease

§ Reservoir – the natural habitat of the microorganism



Human Reservoirs

§ Carriers – shelter a pathogen and spread it to others unknowingly

§ Asymptomatic carriers – infected but show no symptoms of disease

§ Incubating carriers - spread pathogens during symptomless incubation period

§ Convalescent carriers - continue to shed pathogens after recuperation

§ Chronic carriers - shelter the pathogen for a long time after recovery because of latency

§ Passive carriers – pick up pathogens and accidentally transfer them to patients



Animal Reservoirs

§ Zoonotic diseases – transmitted to humans but exist primarily in animals

§ Poultry are a reservoir for Salmonella and Campylobacter

§ Racoons, skunks, and bats harbor the rabies virus

§ Rodents are a reservoir for plague and Hantavirus

§ Vector – any living organism that transmits an infectious agent from one host to another

§ Mechanical vector – carries the microbe on their body from one place to another

§ Biological vector – actively participates in the pathogen’s life cycle

§ Include fleas, mosquitos, flies, ticks



Environmental Reservoirs

§ Pathogens with environmental reservoirs are the hardest to eliminate

§ Soil

§ agents for anthrax & tetanus

§ Water

§ agent for Legionnaires disease



Acquisition of Diseases

§ Communicable diseases – can be transmitted from one host to another

§ contagious – easily transmitted

§ Non-communicable diseases – transmitted by means other than host to host

§ ex. from normal flora or soil



Patterns of Transmission

§ Horizontal transmission – disease is spread from one infected individual to another

§ may involve contact, ingestion of food or water, or via a living agent such as an insect

§ Vertical transmission – disease is spread from parent to offspring

§ via ova, sperm, placenta, or milk



Modes of Transmission

§ Contact

§ Direct contact – person to person transmission by touching, kissing, sexual intercourse, etc.

§ Indirect contact – transfer of the pathogen via inanimate objects (fomites)

§    clothing, door knobs, drinking glasses, keyboards

§ Food and water

§ Food-borne pathogens - originate from animal reservoirs or contamination during food preparation

§    Fecal-oral route – food/water contaminated with fecal matter

§    Cross-contamination – bacteria from one food product is transferred to another food product and then ingested

§ Waterborne pathogens - frequently originate from sewer contamination

§ Air

§ Droplet nuclei – dried microscopic residues from respiratory secretions

§    can be spread up to 1 meter

§ Aerosols – suspensions of fine dust or moisture particles that contain live pathogens

§ Airborne transmission of pathogens is difficult to control



Nosocomial Infections

§ Infections acquired by patients during their hospital stay

§ Conditions in a hospital make an ideal situation for the contraction of disease

§ High-density population with many reservoirs/carriers

§ Patients tend to be immunosuppressed

§ Many antibiotic-resistant microbes present

§ Large number of non-living reservoirs

§ Reservoirs of infectious agents:

§ Other patients

§ Hospital environment/medical devices

§ Health care workers

§ Patients normal flora

§ Examples of Nosocomial infections:

§ Klebsiella

§    normal intestinal flora; cause UTIs, pneumonia, and septicemia

§ E. coli

§    normal intestinal flora; commonly cause UTIs

§ Pseudomonas sp.

§    can grow on humidifiers and mechanical ventilators; cause hospital-acquired pneumonia, UTI, burn and wound infections

§ Staphylococcus and Streptococcus sp.

§    normal skin flora; cause septicemia, pneumonia, and surgical wound infections

§ Universal Precautions (UPs) – based on the assumption that all patient specimens may harbor infectious agents





§ Study of the factors influencing the frequency and distribution of diseases within a community

§ Diseases are reported and monitored at the local, state, national, and international levels

§ Epidemiologists rely on case reporting by physicians and hospitals; however, many infections go undiagnosed and unreported

§ Iceberg effect



Epidemiological Statistics

§ Prevalence – the total number of existing cases in a given population at risk

§ Incidence – the number of new cases over a certain time period in a given population at risk

§ Morbidity rate – number of cases of illness over a certain time period in a given population at risk

§ Mortality rate – total number of deaths in a population due to certain disease

§ Index case – refers to the first patient found in an epidemiological investigation

§ Endemic disease – exhibits a steady frequency over a long time period in a specific area

§ ex. common cold, Lyme disease

§ Sporadic disease – occasional cases are reported at irregular intervals in random areas

§ ex. Tetanus and Diptheria

§ Epidemic disease – when an endemic or sporadic disease increases beyond what is expected for that population

§ point-source – the infectious agent came from a single source

§    ex. a contaminated food item

§ common-source – common exposure to a single source of infection that can occur over a period of time

§    ex. a carrier of Hepatitis A

§ propagated – results from an infectious agent that is communicable from person to person 

§    ex. Influenza

§ Pandemic – the spread of an epidemic across continents

§ ex. AIDS