Introduction to microbial infection

Question 1

What are the four elements of Koch’s Postulates?

Answer 1

• bacteria must be present in every case of the disease
• bacteria must be isolated from the host with the disease and grown in pure culture
• the specific disease must be reproduced when a pure culture of the bacteria is inoculated into a healthy susceptible host
• bacteria must be recoverable from the experimentally infected host

Question 2

What does the inmate immune system consist of? (4)

Answer 2

• normal microbiota
• physical barriers
• chemical barriers
• phagocytic cells

Question 3

How do normal microbiota help protect against microbial infection?

Answer 3

• offer protection by competing with pathogens for colonisation sites
• produce antibiotic surfaces suppressing growth of competing organisma
• may produce toxic metabolic products to inhibit other microorganisms
• may alter pH
  ( - surpassed by antibiotics)

Question 4

What are normal microbiota?

Answer 4

microorganisms that reside on the surface and deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts of every human being.
• These microbiota are not harmful to humans; some are even beneficial and most help maintain our health.

Question 5

How do physical barriers help protect against microbial infection?

Answer 5

• skin - secretes sebum and fatty acids to inhibit growth (- microbes have evolved ways to penetrate)
• mucomuciliary clearance - particles settle on sticky mucus of respiratory epithelium - debris transported by cilia to oropharynx where it is swallowed
• flushing - urinary tract
• peristalsis - GI tract

Question 6

How do chemical barriers help protect against microbial infection?

Answer 6

• mucus
• antimicrobial proteins - lysosome (tears, saliva), lactoferrin (breast milk, tears, saliva) - defensives
• gastric acid - pH 2
• plasma proteins - complement cascade, C-reactive protein(CRP), mannose-binding lectin (MBL), transferrin

Question 7

How do phagoytotic cells help protect against microbial infection?

Answer 7

• process used by the human body to destroy dead or foreign cells - endocytose pathogens

Question 8

What do phagocytes include?

Answer 8

• macrophages
• neutrophils
• monocytes
• dendritic cells
• mast cells

Question 9

How does a infection occur?

Answer 9

a micro-organism causes ill-health in two ways

• invading host tissues
• exerting effects from mucosal surfaces

Question 10

Commensal definition?

Answer 10

micro-organism which forms part of the normal host microbiota

Question 11

Pathogen definition?

Answer 11

micro-organism capable of causing an infection

Question 12

Pathogenicity definition?

Answer 12

the capacity to cause disease

Question 13

Virulence definition?

Answer 13

measure of the capacity to cause disease

Question 14

What are obligate pathogens?

Answer 14

almost always cause disease

eg. HIV

Question 15

What are conditional pathogens?

Answer 15

may cause disease if certain conditions are met

eg. bacteroides fragilis, staphylococcus aureus

Question 16

What are opportunistic pathogens?

Answer 16

usually only infects immunocompromised host

eg. pneumocystis jiroveci

Question 17

What are the 5/6 steps of infection?

Answer 17

• recognition
• attachment and entry
• multiplication
• evasion of host defences
• shedding - expulsion and release of virus
  (- damage - doesn’t always happen as not too much damage wants to be caused)

Question 18

How can infection enter a host?

Answer 18

microbes with

• specific mechanisms for attachment and penetration of host’s body surfaces
• introduced into host by biting arthropods (spiders…)
• introduced into host via skin wounds or animal bites
• able to infect only when hosts defences are impaired

Question 19

What is tissue tropism,

Answer 19

defines the cells and tissues of a host which support the growth of a particular microbe

• broad tissue tropism - infecting many cells and tissues
• or infect primarily a single tissue

Question 20

What are the influencing factors of tissue tropism?

Answer 20

• presence of cell receptors
• transcription factors
• local temp
• physical barriers
• pH

Question 21

List virulence factors?

Answer 21

• toxin secretion
• antibiotic resistance
• pilus formation
• capsule
• iron transport systems
• adhesion factors
• enzymes

Question 22

What are the features of endotoxins?

Answer 22

• low toxicity
• part of cell wall of gram negative bacteria
• lipopolysacharide
• low specify

Question 23

What are the features of exotoxins?

Answer 23

• highly toxic
• secreted from bacterial cells
• produced by gram negative and positive bacteria
• can be converted into toxoids for vaccine use - only toxin part causes disease
• eg. tetanus toxin, cholera toxin ..

Question 24

What does transmission depend on?

Answer 24

• number of microorganisms shed
• number of microorganisms required to infect a fresh host
• microorganisms stability in environment

Question 25

What Is horizontal transmission?

Answer 25

passed amount same generation

Question 26

What is vertical transmission?

Answer 26

passed down generations

Question 27

What is format transmission?

Answer 27

transmission via inanimate objects

doot handles, cutlery…

Question 28

What are nosocomial infections?

Answer 28

infections acquired during a hospital stay

- antibiotic resistance - clostridium difficile, staphylococcus aureus

Question 29

What causes antibiotic resistance?

Answer 29

• resistant genes on plasmids
• production of ensymes
• impermeability
• efflux mechanisms
• alteration of target sites