Week 7 Bioscience

Question 1

Two cell types make up all living organisms

Answer 1

Eukaryotic cells - True cells
* Membrane-bound nucleus and organelles
* e.g. animals, plants, fungi, protozoa, algae

Prokaryotic cells - Simple cells
* No membrane–bound organelles
* e.g. bacteria

Question 2

Naming microbes

Answer 2

• Binomial system of nomenclature
• 2 names: a genus and a species name
• underlined or italicised
  e.g. Staphylococcus aureus, where Staphylococcus is the genus name and aureus is the species name
• Microbes may also have common names e.g. “golden staph”
• Binomial nomenclature applies to all microbes except viruses

Question 3

Bacteria

Answer 3

• Bacteria (sing. bacterium) are very small, e.g. 1 - 5µm
• Usually single celled
• Three basic shapes:
  
  • rod - bacillus (plural bacilli)
  • spherical - coccus (plural cocci)
  • spiral – spirilla (plural spirillus) (also spirochete, or vibrio)
• Feed on organic material
• derived from either dead organisms or a living host
• release enzymes → digest organic material extracellularly → internalising it in solution

Question 4

Viruses

Answer 4

• Very small (e.g. 20 nm - 100 nm)
• Acellular (or non-cellular)
• Package of genetic material (DNA or RNA)
• No organelles
• Obligate parasite = can only reproduce inside a host cell
• Naked virus = genetic material surrounded by a protein coat (capsid)
• Enveloped virus = naked virus plus an outer membrane or envelope - derived from the plasma membrane of the previous host cell

Question 5

Typical viral infection = Lytic

Answer 5

1. Attachment to target cell
2. Penetration of target cell
   * naked viruses engulfed (endocytosis)
   * enveloped viruses fuse with host cell membrane
3. Replication
4. Maturation and release
   * naked viruses trigger lysis of the host cell → viral exit kills the cell
   * enveloped viruses bud from the host cell, taking part of the cell membrane as an envelope → usually kills the host cell

Question 6

Fungi

Answer 6

• Eukaryotic cells with a cell wall containing chitin
• Feed on organic matter (like bacteria) and recycle organic matter in nature

Yeasts
* 5-10 µm ovoid cells
* Single-celled
* Reproduce by budding

Molds
* Small, multicellular
* Filamentous cells → mesh = mycelium

Mushrooms
* large, multicellular

Question 7

Humans and Microbes

Answer 7

3 classes of microbes humans interact with:
1. Normal flora (resident flora / microbiota / microbiome)
2. Transient microbes
3. Pathogens

3 types of relationship possible between microbes and humans:
1. Commensalism – one partner benefits, the other is unaffected
2. Mutualism – both partners benefit
3. Parasitism – one partner benefits at the expense of the other

Question 8

Normal flora

Answer 8

• At birth humans are colonised by a variety of microbes (mainly bacteria, some fungi/yeasts)
• Colonisation = establishment and growth on a body surface
• Microbes colonise our body surfaces that contact the external environment
• Derived from:
  
  • Vagina (during birth)
  • Diet
  • Environment

Question 9

Transient microbes

Answer 9

• Temporary residents – remain on the body for a short time (hours-days-months)
  
  • Usually acquired by contact
  • Unable to colonise body surfaces:
    
    • Cannot complete with normal flora for living space and nutrients
    • Eliminated by the bodies defences
    • Dislodged by human activity, i.e. hand washing
• Can be found in the same locations as the normal flora

Question 10

Pathogens

Answer 10

• Microbes that can cause an infectious disease
  
  • Infection = invasion of the body by a pathogenic microbe (penetrates body surface to grow in deeper tissues)
  • Infectious disease = disorder of a structure or a function due to damage caused by a microbe
  • Infectious disease is not necessarily transmissible!
• Includes viruses, bacteria, fungi and protozoa, worms and arthropods (insects)

Question 11

Relationships - commensalism

Answer 11

Commensalism – one partner benefits, the other is unaffected
* Normal flora is in ecological balance with the host – benefits:
* Living space
* Nutrition
* Host – not affected/harmed

Question 12

Relationships - mutualism

Answer 12

Mutualism – both partners benefit
* Normal flora is in ecological balance with the host – benefits:
* Living space
* Nutrition
* Host benefits:
* Colonisation and infection by pathogens is prevented as normal flora
* Take up living space and nutrition
* Maintain an unsuitable environment, e.g. Lactobacillus maintains an acidic environment in the vagina which suppresses the overgrowth of other microbes, e.g. Candida

Question 13

Relationships - parasitism

Answer 13

• Parasitism – one partner benefits at the expense of the other
• Parasites
  
  • Use the host as a resource for growth
  • Cause tissue damage
  • Activate host defences (innate and adaptive defences)
• Parasites are microbes capable of causing infectious diseases, i.e. pathogens

Question 14

Pathogenicity

Answer 14

• Whether or not infection and disease occurs depends on virulence = the capacity of a pathogen to cause disease
• A more virulent microbe:
  
  • Causes a more severe disease e.g. common cold vs. rabies virus
  • Requires smaller number of pathogens to initiate infection
    
    • Infectious dose = minimum number of pathogens which will cause disease
    • Greater the virulence = fewer pathogens required for infection = lower infectious dose

Question 15

Conducive environments

Answer 15

• Environmental conditions affect:
  
  • The health status of individuals, e.g. nutrition, vaccination
  • Reservoir of infection – source of pathogens, , e.g. human host, insects or animals, non-living source (contaminated water)
  • Ease of pathogen transmission between hosts, e.g. sanitation, crowding, food and water sources, vectors
• Conducive environments
  
  • Poverty, famine, natural disasters, conflict
  • Increase the likelihood of infectious disease occurring in a severe form and spreading easily

Question 16

Pathogens and infection

Answer 16

• Infectious disease is usually the result of infection by conventional pathogens
  
  • A microbe which causes disease in previously healthy people with normal defences e.g. measles virus, gonorrhea, influenza
• Infections may be caused by opportunistic pathogens
  
  • Microbes that don’t normally cause infectious disease but can do so under certain circumstances
    
    • Normal flora
    • Environmental microbes
      → Opportunistic infections

Question 17

Opportunistic infections with normal flora

Answer 17

Microbes of the normal flora can cause opportunistic infection and disease when:
1. The ecological balance of the normal flora is upset
2. Microbes of the normal flora are relocated into parts of the body they are normally excluded from
3. Host innate and/or adaptive defences are compromised

Question 18

Upset ecological balance

Answer 18

e.g. Candida albicans (fungus - yeast)
* Occurs in the mouth, parts of the digestive tract, vagina
* Normally low numbers in the vagina due to:
* competition with bacteria * acidic environment (pH 4-4.5) maintained by the bacteria Lactobacillus
* Not affected by treatment with antibiotics kills pathogenic and normal flora bacteria
* Antibiotic treatment reduces competition for living space and nutrients → Candida “population explosion” → painful inflammatory condition, candidiasis (thrush)

Question 19

Relocation of the normal flora

Answer 19

Microbes of the normal flora are relocated into parts of the body they are normally excluded from
* E. coli of the large intestine → urethra (UTI) or vagina
* Perforation of the intestines → normal flora moves into abdominal cavity (normally sterile) → peritonitis
* Staphylococcus bacteria on the skin → enter deeper tissues via a cut (defensive barriers breached) → necrotising fasciitis

Question 20

Compromised host defences

Answer 20

• Compromised host defences (immunocompromised) à inability to mount an effective defence
• May involve:
  
  • Innate defences
    e.g. impaired mucociliary escalator allows normal flora of the mouth and throat to penetrate deeper lung tissues → pneumonia
• Adaptive immunity e.g. AIDS patients have low numbers of TH cells → limited adaptive immunity

Question 21

Opportunistic infections with environmental microbes

Answer 21

• Environmental (transient) microbes
  
  • Low pathogenicity
  • Do not usually interact with humans
  • Do not usually cause infection in healthy people
• May cause opportunistic infectious disease in people with reduced defences (i.e. immunocompromised individuals) e.g. various fungi, such as Aspergillus can cause serious lung infection cancer and death

Question 22

Attachment

Answer 22

• In order to establish an infection, pathogens must attach themselves to host tissues at their portal of entry
• Many pathogens have adhesins on their surface à bind surface receptors on host target cells e.g. E. coli adhere to epithelial cells in the urethra à UTI
• Some bacteria secrete a sticky substance, a glycocalyx, that cements them to a body surface, e.g. bacteria, involved in tooth decay
• Many viruses adhere to specific surface molecules on their target cells. e.g. human immunodeficiency virus (HIV) attaches to a protein present on TH cells and macrophages
  e.g. influenza has an envelope protein, haemagglutinin, that attaches to many cell surface glycoproteins

Question 23

Portals of entry

Answer 23

• The site at which a pathogen enters the human body to reach deeper tissues
  
  • Most pathogens have a preferred portal of entry
    
    • Portal of entry can be a pre-requisite for disease
    • Example: Streptococcus bacteria cause pneumonia if inhaled but not if swallowed
• Some pathogens can use multiple portals
  
  • Example: Bacillus anthracis (anthrax) can cause disease via multiple portals (broken skin, respiratory and digestive tracts)

Skin
* Hair follicles, sweat glands
* Some pathogens can live in, or bore through, the skin e.g. various bacteria, fungi, papilloma virus, hook worms
* Compromised barriers i.e. penetrated/injured, i.e. puncture, injection, bite, wound, surgery e.g. tetanus, rabies, HIV, hepatitis viruses

Mucous membranes
* Respiratory tract (most common portal) via droplets or particulates, e.g. influenza virus, measles, COVID19
* Gastrointestinal tract via food, water, contaminated hands etc., e.g. Hepatitis A, Salmonella
* Urinary tract, e.g. E. coli
* Reproductive tract (sexually transmitted), e.g. genital warts, chlamydia, HIV, hepatitis viruses
* Conjunctiva of the eyes, e.g. bacteria (conjunctivitis), HIV, hepatitis

Question 24

Multiplication and spread

Answer 24

• The development of infectious disease requires that the pathogens multiply to a critical population size
• The incubation period of an infectious disease = the time during which the pathogen is overcoming early host defences and utilising host resources to multiply to a critical population size
  
  • Extracellular pathogens - the majority of bacteria and fungi obtain their nutrients from the ECF
  • Intracellular pathogens - all viruses, and some bacteria and fungi, obtain nutrients from within host cells
• Spread – involves overcoming host defences

Question 25

Overcoming host defences

Answer 25

• In order to establish an infection, pathogens must penetrate, avoid or inactivate host defences
• The better pathogens are at overcoming host defences the more virulent they are
• Strategies (= virulence factors) include:
  1. Overcoming innate defences:
• Overcoming surface barriers
• Resisting phagocytes
  2. Overcoming adaptive defences
• Degrading antibodies
• Antigenic variation

Question 26

1. Overcoming surface barriers

Answer 26

• Some pathogens release enzymes that break down epithelial/connective tissue barriers to invade deeper tissues e.g. Streptococcus, Clostridium
• Bordetella pertussis (whooping cough) secretes a toxin that inactivates the mucociliary escalator

Question 27

2. Resisting phagocytes

Answer 27

• Some bacteria have slippery capsules (i.e. glycocalyx) that prevent phagocytes adhering and engulfing them e.g. Streptococcus pneumoniae
• Some bacteria release toxins that kill phagocytes or release enzymes that inactive their digestive enzymes
  e.g. Staphylococcus produces
  1. leukocidin → kills neutrophils
  2. catalase → destroys lysosomal enzymes

Question 28

3. Degrading antibodies

Answer 28

• Some pathogens can produce enzymes that can destroy antibodies
  
  • IgA proteases → destroy IgA antibodies in mucus at mucosal surfaces → prevents neutralisation by antibodies
• Examples:
  Neiserria gonorrhoea → gonorrhea
  Neiserria meningitdis → meningitis

Question 29

4. Antigenic variation

Answer 29

• Some pathogens can alter their surface molecules – i.e. antigens that host memory lymphocytes would normally respond to – thus preventing a protective secondary response
  e.g. Neiserria gonorrhoea → gonorrhea
  Influenza virus
  Plasmodium → malaria

Question 30

Damage to the host

Answer 30

• Damage to the host may arise:
  1. Directly from the activities of the pathogen
  
  • Damage caused by enzymes or toxins
  • Cytopathic effects
  • Use of host as a food source
    2. Indirectly from the host’s innate and/or adaptive defences
  • Inflammation – swelling, phagocytic enzymes, complement, scarring
  • Antibodies and cytotoxic cells

Question 31

Pathogen-induced damage: cytopathic effects

Answer 31

• Cytopathic effect → microbe activity damage → kills host cells
• Viruses → lytic infections → host cell death
• Bacteria can kill host cells to gain nutrients, e.g. liver cells to gain essential iron
• Microbial toxins → kill host cells (many damage cell membranes), e.g. cytotoxins

Question 32

Pathogen-induced damage: enzymes & toxins

Answer 32

• Enzymes → tissue damage
  
  • Collagenase - breaks down the collagen fibres connective tissue , e.g. Streptococcus causes necrotising fasciitis
• Toxins → fever, cardiovascular and nervous system disturbances, shock, diarrhoea → death
  
  • Toxins that kill phagocytes, e.g. leukocidin
  • Tetanus toxin acts in the CNS → sustained muscle contractions (tetany)
  • Botulinium toxin (“botox”) blocks the release of ACh at the NMJ → flaccid paralysis

Question 33

Host defences cause tissue damage

Answer 33

• Damage to the host may arise from the activities of the host’s own defences (see previous week’s content):
  
  • Innate defences, e.g. phagocyte and complement activation, inflammation, natural killer (NK) cell activity
    
    • Inflammation
      
      • Universal feature of infectious disease – triggered by tissue damage
      • Process enhanced by immune complexes and complement
      • Can damage healthy tissue (next slide)
• Adaptive defences
  
  • Antibodies (immune complexes) and cytokines – promote phagocyte activity and inflammation
  • Cytotoxic T cells – perforins and granzymes may kill both infected (target) cells and bystander cells

Question 34

Portals of exit

Answer 34

• Pathogens leave the body via portals of exit to spread to a new host
• The most common: the respiratory and digestive tracts
• Damage to body tissues may assist in exit, e.g. coughing
• The more pathogens that are shed, the greater the chance of reaching a new host
• Body fluids/wastes provide excellent vehicles for exit

Question 35

Host defences cause tissue damage

Answer 35

• Damage to the host may arise from the activities of the host’s own defences (see previous week’s content):
  
  • Innate defences, e.g. phagocyte and complement activation, inflammation, natural killer (NK) cell activity
    
    • Inflammation
      
      • Universal feature of infectious disease – triggered by tissue damage
      • Process enhanced by immune complexes and complement
      • Can damage healthy tissue (next slide)
• Adaptive defences
  
  • Antibodies (immune complexes) and cytokines – promote phagocyte activity and inflammation
  • Cytotoxic T cells – perforins and granzymes may kill both infected (target) cells and bystander cells

Question 36

The course of infectious disease

Answer 36

The course of the infection depends on how effectively the host defences deal with the pathogen as it multiplies:
* If the pathogen is eliminated quickly
* the disease follows an acute course
* recovery is generally rapid
* If the pathogen is eliminated slowly, or not at all
* The infection may be
* Chronic (persistent) – very slow or no recovery
* Latent – apparent recovery interrupted by recurrent illness

Question 37

Chronic infection

Answer 37

• An initial acute, symptomatic disease (red line) period is followed by apparent recovery (appears to be acute infection), but …
• The pathogen is not completely eliminated and continues to multiply slowly and cause increasing amounts of damage (green line)
• Depending on the pathogen, it is eliminated/cleared very slowly (long term infection) or never eliminated (lifelong infection) and subsequent acute episodes may occur
• The host acts as a carrier, transmitting the pathogen even while appearing healthy (as the pathogen is multiplying it can be transmitted)
• Example: Hepatitis B

Question 38

Latent Infection

Answer 38

• The initial acute, symptomatic disease (red line) period is followed by apparent recovery without complete elimination of the pathogen
• The pathogen does not multiply but becomes “dormant” as it “hides” inside cells from body defences – it cannot be transmitted as it is not multiplying (and thus host is asymptomatic)
• This latent period may last for many years
• The pathogen may be “reactivated”, grow and cause acute disease (blue line) – however, the symptoms may be quite different from the symptoms of the initial disease
• Example: Herpes viruses, i.e. chickenpox/shingles or cold sores