Chapter 11 - Interactions b/w Microbes & Humans

Colonization

Our resident microbiota colonize us for the long term & do not cause disease

Infection

Microbes get past host defenses, enter tissues, & multiply

Disease

Deviation from health that results when cumulative effects of infection damage or disrupt tissues & organs

Infectious disease

A pathologic state caused directly by microorganisms or their products

When colonization leads to disease

• Pathogen: a microbe whose relationship w/ its host is parasitic & results in infection & disease

• Pathogenicity: an organism’s potential to cause disease

  • True pathogens: capable of causing disease in healthy people w/ normal immune defenses

  • Opportunistic pathogens: cause disease when the host’s defenses are compromised or when the pathogens become established in a part of the body that is not natural to them

Virulence

• Relative severity of disease caused by a particular microbe

• Degree of pathogenicity

• Virulence of a microbe is determined by its ability to…

  • Establish itself in a hot

  • Cause damage to host

• Virulence factor: any characteristic or structure of the microbe that contributes to its ability to establish itself in the host & cause damage

Polymicrobial infections

• Majority of infections a polymicrobial, w/ contributions from more than 1 microbe

• Influenza infection frequently leads to pneumonia

Steps involved when a microbe causes disease in a host

1. Finding a portal of entry

2. Attaching to host cells

3. Surviving host defenses

4. Causing damage (disease)

5. Exiting host

**1st three steps = establishing infection**

Step 1 - Becoming Established: Portal of Entry

• Portal of entry: the route that a microbe takes to enter the tissues of the body to initiate an infection

• **Intact skin is NOT a portal of entry → only when skin is open/cracked**

• Exogenous: organisms coming from outside the body

• Endogenous: organisms coming from somewhere in the same human host

Specific or more than 1 portal of entry

• The majority of pathogens have adapted to a specific portal of entry, one that provides a favorable habitat for further growth & spread

• If certain pathogens enter the “wrong” portal, they will not be infectious:

  • Inoculation of the nasal mucosa w/ the influenza virus will result in the flu, but if the virus contacts the skin, no infections occurs

• Occasionally, an infectious agent can enter by more than one portal:

  • Mycobacterium tuberculosis can enter through respiratory & GI tracts

  • Streptococcus & Staphylococcus can enter through the skin, urogenital tract, & respiratory tract

Quantity of microbes in the Inoculating Dose

• Infectious dose: the minimum number of microbes necessary to cause an infection to proceed

  • Microorganisms w/ smaller infectious doses have greater virulence

  • ID for Q fever = 1 cell

  • ID for TB, giardiasis, & coccidioidomycosis = ~10 cells

  • ID for gonorrhea = 1,000 cells

  • ID for typhoid fever = 10,000 cells

  • ID for cholera = 1,000,000,000 cells

Step 2 - Becoming Established: Attaching to Host Cells

• Adhesion: process by which microbes gain a more stable foothold on host tissues

  • Dependent on binding b/w specific molecules on both the host & pathogen

  • Pathogen is limited to only those cells (& organisms) to which it can bind

  • Firm attachment is almost always a prerequisite for causing disease, since the body has so many mechanisms for flushing microbes from tissues

Adhesion mechanisms

• 1. Bacterial, fungal, & protozoal pathogens attach by…

  • Fimbriae (pili), surface proteins, adhesive slimes/capsules

• 2. Viruses attach specialized receptors (spikes)

• 3. Parasitic worms fastened by suckers, hooks, & barbs

Step 3 - Becoming Established: Surviving Host Defenses

• Phagocytes: white blood cells that engulf & destroy pathogens

• Antiphagocytic factors:

  • 1. Leukocidins: kill phagocytes outright

  • 2. Extracellular surface layer (slime or capsule) makes it difficult for the phagocyte to engulf the pathogen

  • 3. Some captured bacteria survive inside the phagocyte

Step 4 - Causing Disease

• Three ways that microorganisms cause damage to their host:

  • 1. Directly through the action of enzymes or toxins (both endotoxins & exotoxins)

  • 2. Indirectly by inducing the host’s defenses to respond excessively or inappropriately

  • 3. Epigenetic changes made to host cells by microbes (expression of genes)

Exoenzymes

• Enzymes secreted by microbes that break down & inflict damage on tissues

• Dissolve the host’s defense barriers to promote the spread of disease to other tissues

• Examples:

  • 1. Mucinase: digests the protective coating on mucous membranes

  • 2. Hyaluronidase: digests the substance that cements animal cells together

  • 3. Coagulase: cause clotting of blood or plasma

  • 4. Kinase: dissolves fibrin clots

Bacterial toxins

• Bacterial toxins: a potent source of cellular damage

• Toxin: a specific chemical product of microbes that is poisonous to other organisms

• Toxins are named according to their target:

  • Neurotoxins = act on nervous sytem

  • Enterotoxins = act on intestines

  • Hemotoxins = lyse RBCs

  • Nephrotoxins = damage kidneys

Endotoxins

• Lipopolysaccharide (LPS): part of the outer membrane of gram-negative cell walls

• Has a variety of systemic effects on tissues & organs

  • Causes fever, inflammation, hemorrhage, & diarrhea

• Blood infections by Salmonella, Shigella, Neisseria meningitidis, & Escherichia coli are particularly dangerous & can lead to fatal endotoxic shock

Know entire exotoxin & endotoxin table

Definitions of infection types - table

Signs vs. Symptoms

• Sign: objective evidence of disease as noted by an observer

  • Ex. edema, lymphadenitis, fever, redness

• Symptom: subjective evidence of disease as sensed by the patient

  • Pain, soreness, malaise, nausea

• Syndrome: a disease identified by a certain complex of signs & symptoms

Signs of infection in the blood

• Leukocytosis; increase in WBC levels

• Leukopenia: decrease in WBC levels

• Septicemia: a general state in which microorganisms are multiplying in the blood & are present in large numbers

• Bacteremia or viremia: bacteria or viruses are present in the blood but not multiplying

Infections that go unnoticed

• No noticeable symptoms

• Microbe is active in host tissues

• Host does not seek medical attention

• These infections are known as asymptomatic or subclinical (inapparent)

Stages in the course of infection & disease

1. Incubation (actively multiplying, but no symptoms)

2. Prodromal stage (vague symptoms)

3. Acute phase (height of infection, severe symptom/damage, large # of pathogen)

4. Convalescent period (sharp decline of s/s, still infectious)

1. Incubation period

• Time from initial contact w/ infectious agent to appearance of symptoms

• Agent is multiplying at portal of entry but has not caused enough damage to elicit symptoms

• Varies according to host resistance, degree of virulence, & distance b/w target organ & portal of entry

• Ranges from several hours → several years

• Majority of infections range 2-30 days

2. Prodromal stage

• 1-2 day period when earliest notable symptoms of infection appear

• Vague feeling of discomfort → head & muscle aches, fatigue, upset stomach, general malaise

3. Acute phase

• Infectious agent multiplies at high levels, exhibits its greatest virulence, & becomes well established in its target tissue

• Marked by fever & other prominent & specific signs & symptoms

• Extremely variable in length of this period

4. Convalescent period

• Patient begins to respond to the infections & symptoms decline

• Patient’s strength & health gradually return due to the healing nature of the immune response

• Many patients stop taking antibiotics during this period, even though pathogens are still in their system, leading to antibiotic resistance

Continuation phase

• Only some infections have this phase

• Either the organism lingers for months, years, or indefinitely after the patient is well OR the organism is gone but symptoms continue

Reservoirs

• Where pathogens come from

• Primary habitat in the natural world from which a pathogen originates

• Living reservoirs:

  • 1. Humans (ill patients or carriers)

  • 2. Animals

• Non-living reservoirs:

  • Soil & water (can also be plants)

Zoonosis

• An infection indigenous to animals but naturally transmissible to humans

  • Some zoonotic infections have multi-host involvement

  • Some have complex life cycles in the wild

  • Zoonotic spread of disease is promoted by close associations b/w humans & animals

  • Make up full 70% of all new emerging diseases worldwide

  • Impossible to eradicate w/out also eradicating the entire animal reservoir

Transmission of infectious agents - communicable

• A disease in which an infects host can transmit the infectious agent to another host & establish infection in that host

  • Contagious: a disease that is highly communicable, especially through direct contact

    • Influenza & measles are highly contagious

    • Hansen’s disease (leprosy) is only weakly communicable

Transmission of infectious agents - noncommunicable

• An infectious disease that does not arise through transmission of the infectious agent from host to host

  • 1. Compromised person is invaded by their own microbiota

  • 2. Individual has accidental contact w/ a microbe that exists in a nonliving reservoir

  • 3. Infected people do not become a source of disease to others

Patterns of transmission in communicable disease - Horizontal transmission

• Disease is spread through a population from one infected individual to another

• 3 major modes:

  • 1. Direct contact: close contact b/w people, droplets

  • 2. Indirect transmission: an object or substance carries the agent

    • Fomite: inanimate object (ex. door knobs, phones, pens)

    • Vehicle: air, water, soil, food

  • 3. Vector transmission: arthropods that harbor an infectious agent & transfer it to a human

    • Mechanical: arthropod carries microbes on outside surface of its body

    • Biological: microbes enter body of arthropod

Patterns of transmission in communicable disease - Vertical transmission

Disease transmitted from parent to offspring via ovum, sperm, placenta, milk

Healthcare-Associated Infections

• Also called nosocomial infections

• Infectious diseases are acquired or developed during a hospital or health care facility stay

• 0.1-20% of all admitted patients, w/ an average of 4%

• 750,000 cases per year, resulting in 75,000 deaths

Circumstances that make HAIs unavoidable

• Compromised patients

• Collection point for pathogens

• Lowered defenses permit normal biota to enter the body

  • **Opportunistic infections**

• Infections acquired directly or indirectly from fomites, medical equipment, other patients, medical personnel, visitors, air, & water

Health-care processes that make HAIs occur

• Treatments using reusable instruments such as respirators & thermometers

• Indwelling devices such a catheters, prosthetic heart valves, grafts, drainage tubes, & tracheostomy tubes form read portals of entry

• High proportion of hospital population receives antimicrobial therapy, so drug-resistant microbes are selected at a much higher rate

Most common HAIs

• Pneumonia

• GI illness

• UTIs

• Bloodstream infections

• Surgical site infections

Control of HAIs

• Infection control officers: implement proper practices & procedures, tracks potential outbreaks, identifies breaches in asepsis, & trains health car workers in aseptic techniques

• Training for nurses & caregivers: regularly exposed to needle-sticks, infectious secretions, blood, & physical contract w/ patients → they need to be especially aware of infection control

• Most hospitals have adopted universal precautions → all secretions from all people in the clinical setting are potentially infectious & transmission can occur in either direction