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MEDICAL MICROBIOLOGY > L2 Host Parasite Relationship and L3 Bacterial Pathogenesis > Flashcards

L2 Host Parasite Relationship and L3 Bacterial Pathogenesis Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

Define Symbiosis

A

relationship between 2 different organisms to survive

  • commensalism
  • Mutualism
  • Parasitism
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2
Q

Mutualism

A

Both organisms benefit from each other e.g. humans and normal flora (NF)

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3
Q

Commensalism

A

one benefits and host is unharmed e.g. barnacles on whales, psuedomonasl

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4
Q

Parasitism

A
  • parasite benefits & the host is harmed
     All pathogens are considered to be parasites, not just protozoan and helminth infections of humans (worms)
     Must have some kind of balance.
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5
Q

Define a parasite

A

A parasite is an organism that lives on or within the host organism and are metabolically dependent on the host.
Often refers to protozoan or helminths
There are two types of parasites:
- Ectoparasite: Lives on surfaces of host (e.x Lice, fleas)
- Endoparasite: Lives within host (e.x malaria)

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6
Q

Name and explain the types of Hosts

A
  1. Final Host: Host on or in which a parasite gains sexual maturity or reproduces
    mosquito- Plasmodium
  2. Intermediate Host: Serves as a temporary but essential environment for some stage of parasite development
  3. Transfer Host: Is not necessary but serves as a vehicle for reaching the final host.
  4. Reservoir Host: non-human organism that is infected with a parasite which can infect a human.
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7
Q

Define an infection

A

Is the growth and multiplication of microorganisms on or within the host

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8
Q

Define an infectious disease

A

A disease resulting from an infection

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9
Q

The Relationship between Normal Microbiota and the Host

A
  • Usually mutually beneficial
     normal microbiota often prevent colonization by pathogens
     bacterial produces, e.g., vitamins B and K are beneficial to the host
  • Opportunistic pathogens
     members of normal microbiota that produce disease under certain circumstances
  • Compromised host
     debilitated host with lowered resistance to infection
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10
Q

_Normal Microbiota (flora)

A 
Normal Flora derives from us 
-	Supply of nutrients
-	Stable environment
-	Constant temperature
-	Protection
-	Transport 
Host obtains – we obtain
-	Nutritional benefits
-	Stimulation of the immune system
-	Exclusion of potential pathogens at colonisation sites
-	Antagonism of pathogens
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11
Q

Benefits of Normal Flora

A

Provide essential nutrients
- Enteric bacteria secrete Vit K and Vit B12
 Half of body’s Vit K made by gut flora
 Vit K used in the liver to make clotting factors
 Vit B12 used for cell replication & nervous function
- Other secretions: riboflavin (Vit B2), pyridoxine (Vit B6), thiamine (Vit B1)
Prevent colonisation by pathogens
- Compete for attachment sites
- Compete for essential nutrients

Antagonise other bacteria
- Produce non-specific fatty acids and peroxides
- Produce highly specific bacteriocins
 Protein/peptides that are toxic to other bacteria
 Usually affect closely related organisms/strains

Stimulate development of immune system

  • Contact with the intestinal lymphatic tissues
  • Production of cross-reactive antibodies
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12
Q

Tissue Tropism

A
  • Means - Normal flora specifically colonise a particular tissue or surface
     e.g Staphylococcus auricularis – only in outer ear
  • Specific interaction between surface ligands (on bacteria) and receptors located at the colonisation site (tissue)
     Capsules, fimbriae, cell wall components
  • Tissue Tropism facilitated by:
     Host provides an essential growth factor(s)
     Host excludes many bacteria by establishing an inhospitable environment
     i.e. stomach acids, bile salts and lysozyme (eyes, sweat glands)
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13
Q

Skin

A
  • biggest organ
  • Approximately 2 m2 of skin -mechanically strong barrier
  • Inhospitable environment
     slightly acidic pH
     high concentration of NaCl
     many areas low in moisture
  • Inhibitory substances (e.g., lysozyme, cathelicidins)
  • Bacterial numbers are usually low, 100s or 1000s per cm2
  • Commensal microbes include both resident and transient microbiota
  • High moisture content of the axilla, groin, and areas between the toes supports high densities of bacteria
  • Low numbers where ↓pH, dry, ↑NaCl
  • Mixed flora at the entrance to an orifice
  • Consist largely of micrococci (Staphylococcus epidermidis and Micrococcus sp.) and corynebacterial
     These are classed as commensals (? mutalism)
     S.epidermidis makes up 90% of skin organisms
     Anaerobes live below the skin in hair follicles, sweat & sebacious glands e.g. Propionibacterium acnes
  • Staphylococcus aureus sometimes found on the face and hands, particularly of nasal carriers (20-40% of the population)
     Opportunistic pathogen, especially in healthcare institutions
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14
Q

Define the term ‘commensalism’.

A

When one organism benefits as a parasite and the host is not harmed e.g. barnacle growing on a whale or scallop shell

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15
Q

List a member of the normal human microbial flora (full genus and species) that is a major cause of human infection and list where it is normally found?

A

Staphylococcus aureus. Most common pathogen isolated form wounds and skin lesions

Staphylococcus aureus in nasal membranes (opportunistic in clinical areas)
OR
Staphyloccocus epidermidis on skin

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16
Q

A patient with a suspected urinary tract infection is asked to provide a midstream urine specimen. Why?

A

To see if there are WBC in urine = prove there is an infection in urinary tract

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17
Q

What is a nosocomial infection?

A
  • Acquired during hospital stay

* Frequently caused by members of normal flora

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18
Q

Why is lower respiratory tract sterile? (4 reasons)

A
  • Cleansing action from ciliated epith. & mucociliary blanket
  • Sneezing, coughing, swallowing
  • phagocytic action of alveolar macrophages
  • Lysozyme in mucus
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19
Q

What is lysozyme and its role in tears?

A

Lysozyme, a secretion with tears. breaks down bacterial walls. It works on gram-negative microorganisms (thinner walls)

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20
Q

Which bacteria (full genus and species) is the most common cause of urinary tract infections?

A

Escherichia coli

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21
Q

Name 1 pathogen (genus species) that is found in Upper respiratory tract

A
  1. Streptococcus pneumoniae
  2. Haemophilus influenzae
  3. Streptococcus pyogenes
  4. neisseria mengitidis
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22
Q

Describe steps of Koch’s postulates

A
  1. Specific causative agent must be found in every causing disease
  2. Disease organism isolated in pure culture
  3. From culture, introduce a sample of disease to healthy, susceptible animal => produce same disease
  4. Disease organism recovered from animal. Should be the same to prove that specific microbe = specific infectious disease
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23
Q

define ammensalism

one organism

A

inhibits/destroys a diff. organism

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24
Q

Define predation

A

Predator eats prey

25
Q

Name 4 bacteria on the skin

A

staphylococcus aureus
staph epidermidis
propionibacterium aureus
candida sp

26
Q

Where do microbes on the skin mostly lie and why?

A

In hair follicles, sebaceous glands and sweat glands bc skin is slightly acidic, Lo moisture, Hi [NaCl], inhibitory substances (e.g. lysozyme)

27
Q

Types of Pathogenic Microbes

A
  • Bacteria including chlamydia, rickettsia and mycoplasma
  • Fungi (yeasts and molds)
  • Parasites (protozoa, multicellular parasites – helminthes, insects)
  • Viruses
  • Sub-viral agents including prions
28
Q

Pathogenicity

A

– The ability to cause disease

29
Q

Infection

A

 invasion by pathogenic organisms resulting in a host response, usually inflammation

  1. Redness
  2. Swelling
  3. Pain - Loss of function
  4. Heat
30
Q

latent, chronic, acute infection

A
  • Latent
     dormant infection that flares at a later date
     e.g varicella-zoster (chicken pox in children, shingles in adults)
  • Acute
     rapid in onset and often more severe
  • Chronic
     slow, progressive infection
31
Q

Endogenous infection

A
  • infection by member of your own normal flora
32
Q

Exogenous infection

A
  • infection by an organism that is not a member of your normal flora
33
Q

local, focal. systemic infection

A
  • Local infection - an infection restricted to a confined area
  • Focal infection - a localised site of infection from which pathogens and/or their products are disseminated to other parts of the body • e.g. tetanus
  • Systemic infection - an infection spread throughout the body
34
Q

primary and secondary

A
  • Primary - initial infection caused by one pathogen
  • Secondary - infection that follows a primary infection as a complication
  • —- not referring to prim and sec response -> infection leading to another infection/ hinder immune system – new..
35
Q

Infection v Disease

A
  • Disease – any deviation from or interruption of the normal structure or function of any body part, organ, or system that is manifested by a characteristic set of symptoms and signs whose aetiology, pathology and prognosis may be known or unknown.
  • Infectious Disease – a disease caused by pathogenic microorganisms that causes significant overt damage to the host
  • Infection may imply
     Colonisation
     Multiplication
     Invasion
     Persistence
  • Inflammation is the most frequent symptom
     Acute: swelling, pain, redness, heat, loss of function (immobility)
     Chronic: mainly marked by new connective tissue formation
36
Q

Define pathogenicity

A

The ability of a parasite to cause disease

37
Q

Pathogen types

A
  1. Opportunistic pathogens
    - Infection by an organism that ordinarily doesn’t cause disease but becomes pathogenic under certain circumstances (i.e. if an organism gains entry to a sterile site)
    - Can be transmitted from one host to another without having to cause disease
    - Cause infection in hosts with reduced defenses (immunocompromised)
     the elderly, the young, those with underlying chronic illnesses (diabetes, cancer) or on immunosuppressive drugs (organ transplant recipients)
    - Much of the normal flora is opportunistic re:endogenous infection
  2. Obligate pathogens always cause disease in order to be transferred from one host to another and infect the host to survive
    - Mycobacterium tuberculosis
    - Treponema pallidum
  3. Overt Pathogen – when isolating and identifying organisms in a clinical microbiology laboratory, an organism that is ALWAYS regarded as being pathogenic i.e. it is always reported to the requesting physician
38
Q

What are the factors that have an impact on the outcome of the host-parasite relationship

A
  1. number of organisms present
  2. Degree of virulence of pathogen
  3. Hosts defenses or degree of resistance

Infection = ( no of organisms X Virulence ) / Host resistance

39
Q

State examples for virulence factors

A
  1. Endotoxin in LPS layer
  2. Antiphagocytic proteins
  3. O antigen (inhibits phagocyte killing)
  4. Flagellum (motility)
  5. H antigen ( adherance, inihibits phagocyte killing)
  6. VI capsule antigen ( inhibits complement binding)
  7. Cytotoxin (Inhibits host cell protein synthesis)
  8. Type 1 fimbriae (adherance)
  9. Siderophores
  10. Enterotoxin
40
Q

Define Virulence

A 
Degree or intensity of pathogenicity
determined by:
1. invasiveness
2. infectivity
3. pathogenic potential
41
Q

How is virulence measured?

A

Its measured by determining Lethal Dose 50 or Infectious Dose 50

42
Q

Define Toxigenicity

A

The ability to produce toxins

Pathogenicity caused by toxins produced by pathogens

43
Q

Define Immunopathology

A

Ability to trigger exaggerated immune responses upon a second exposure or chronic exposure to a microbial antigen

44
Q

What is ID50 and what does it measure?

A

Number of organisms required to cause infection in 50% of the hosts exposed. measure of virulence

45
Q

Tuberculosis may progress very slowly over a number of years. What type of infection is this?

A

Chronic infection

46
Q

Why is tetanus described as a focal infection?

A
  • a localised site of infection from which pathogens and/or their products are disseminated to other parts of the body
  • e.g. tetanus
  • tetanus exotoxin blocks inhibitory nerve reflex so all muscles contract and body suffers rigid paralysis
47
Q

What is a glycocalyx and how can this contribute to the virulence of a pathogen?

A

• Glycocalyx-capsule/slime layer made up of glycoproteins and/or polysaccharides
• sticky (adherent, resist cough reflex)
• resists phagocytosis
The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (e.g. prevents phagocytosis). The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages. … Capsules also help cells adhere to surfaces

48
Q

How may collagenase act as a virulence factor?

A

Proteolytic enzymes that breakdown peptide bonds in collagen in connective tissue

49
Q

What is the basis of the vaccine for tetanus?

A

Tetanus vaccine, also known as tetanus toxoid (TT), is an inactive vaccine used to prevent tetanus

50
Q

What is an endogenous infection?

A

Infection caused by a member of one’s own normal flora

51
Q

Why does a single molecule of diphtheria toxin subunit A lead to the death of a host cell?

A

• A cytotoxic exotoxin that can cause organ damage (i.e. heart, liver, throat)
• Produced by toxigenic strains of Corynebacterium diphtheriae that are infected with a lysogenic βphage (not lytic)
The bacteria produce a toxin that kills cells in the throat. These cells then join to form the grey-white membrane that is typically seen in cases of diphtheria. The toxin can also spread via the bloodstream and cause damage to the nervous system and heart. The toxin causes the disease in humans by gaining entry into the cell cytoplasm and inhibiting protein synthesis

52
Q

Direct contact

A
  • Cough
  • Sneeze
  • touch
53
Q

Indirect contact

A
  • food
  • water
  • soil
54
Q

Vectors

A
  • insects
55
Q

Fomites (inanimate objects)

A
  • needles
  • splinters
  • blankets/sheets
  • clothing including ties
  • door knobs
  • furniture surfaces
  • mops, cleaning cloths
56
Q

Establishment of Disease: Entry

A
  • No entry required
     V. cholerae – consumed in contaminated food/water
     N. gonorrhoeae – sexual activity (intercourse, oral, anal)
  • Passive entry – without a defined entry mechanism would not cause disease
     Injury
     insect bites
     surgery
  • Active entry – depending on the microorganism, can occur following inhalation, contact with an open wound or following contact with intact skin
     Enzymes destroy ground substance and basement membrane of mucosal linings, destroy cell connections, destroy epithelial cells
     Invade phagocytic cells that wander through tissues disseminate pathogen
     Invasins stimulate host cells to ingest bacteria (see next slide)
57
Q

Virulence Factors:

A

Extracellular Enzymes - Collagenase

  • Hyaluronidase
  • Lecithinase – breaks down the phospholipid layer of cell membranes and causes extensive cell & tissue damage
  • Coagulase – produced by S.aureus, causes fibrinogen to form a fibrin clot around bacteria and prevent phagocytosis
  • Fibrinolysin (kinase) - has the opposite effect to coagulase by dissolving a fibrin clot and is thought to assist the pathogen by preventing localisation of the infection by the inflammatory response
  • IgA Protease
  • Nuclease – breaks down viscous nucleic acids released by the lysis of WBC during pus formation allowing bacteria to continue to spread
  • Catalase, peroxidase, superoxide dismutase – these enzymes inhibit the reactive oxygen species produced during the oxidative response following phagocytosis by neutrophils, monocytes and macrophages
58
Q

Other Virulence Factors

A
  • Protein A
     binds to Fc portion of IgG molecules
     Antibodies are not recognised by phagocytes
  • Hemolysins - depending on the organism may have the following properties:
     Leucotoxins – kill PMN and WBC
     activity against alveolar lung tissue & endothelial cells
     activity against endothelial cells of CNS
     induces apoptosis of liver cells
  • Pigments (carotenoids) - depending on the organism may have the following properties:
     Toxic to other organisms and to some human tissues
     Inhibit immune function
  • Siderophores for Fe capture and transport
     low molecular weight molecule
     high affinity for iron
     Iron is an essential inorganic growth factor for all cells

MEDICAL MICROBIOLOGY (3 decks)

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