WEEK 11: 11.2 Bacterial Pathogenicity, Growth & Antibiotics

Question 1

What are commensal bacteria?

Answer 1

Bacteria that colonise all surfaces of a host i.e. skin, mucosal epithelium of gut, lungs

Question 2

What benefits do commensals provide to the host?

Answer 2

metabolic benefits
protection against pathogens
protection via competition for colonization sites

Question 3

Can pathogens be commensal? how so?

Answer 3

Yes, for eg, S. pneumonia
they can become an opportunistic pathogen in response to immunological insult. for eg. influenza can cause secondary bacterial infection. trauma/tissue damage allows gut commensals to become opportunistic pathogens.

Question 4

What are primary pathogens?

Answer 4

they do not provide any advantage to the host & not part of the normal microbiome. they also damage the host if they evade normal immune responses.

Question 5

What is a pathogen?

Answer 5

an organism whose growth inflicts damage upon the host

Question 6

describe indirect damage a pathogen can cause

Answer 6

the bacteria disturbs metabolic balance/nutrient acquisition

Question 7

Describe direct damage a pathogen can cause

Answer 7

bacteria secretes toxins

Question 8

What is pathogenesis

Answer 8

the mechanism of disease

Question 9

What is virulence

Answer 9

capacity to cause disease/severity of symptoms

Question 10

Describe the pathogenic cycle

Answer 10

1. transmission from host/reservoir (this is where humans can have the greatest impact by prevention factors like hand-washing)
2. colonisation (motility, adherence, invasion)
3. proliferation within host (host provides nutrients for growth, may result in damage via toxins)
4. evasion of host immune response
5. can spread further to other hosts

Question 11

Name some common transmission routes

Answer 11

• Direct contact via bites/wounds/sex
• Indirect contact via contaminated objects
• Airborne
• Facecal
• Food-borne
• Zoonotic

Question 12

What is infectious dose (ID50)

Answer 12

the number of bacteria needed to affect 50% of individuals

Question 13

What is inoculum size?

Answer 13

the amount of pathogen an individual is actually exposed to

Question 14

What are virulence factors?

Answer 14

they determine the ID50 of a strain
presence/absence of flagella, fimbriae, toxins influence this

Question 15

What extraceullaar enzymes are secreted to play a role in pathogenesis?

Answer 15

Beta haemolysin, a phospholipase

Question 16

What is an exotoxin

Answer 16

a bacterial toxin in which proteins are made/secreted during bacterial growth

Question 17

What are the two groupings of exotoxins, describe their function briefly

Answer 17

cytolysins - disrupt membrane homeostasis
A-B toxins - disrupt cellular processes

Question 18

What is an endotoxin?

Answer 18

bacterial structural components that have toxic activity
lipid A component causes endotoxic shock
released on death of bacteria

Question 19

What are 3 outcomes of infection?

Answer 19

1. clearance (without apparent disease)
2. asymptomatic carriage - host is a reservoir and can spread disease
3. symptomatic disease - high severity and can be followed by clearance, carriage or death

Question 20

What factors can affect the outcome of infection

Answer 20

age, immune status, prior exposure, diet, microbiome

Question 21

Describe disease patterns based on timing

Answer 21

acute- develops & resolves rapidly
chronic- develops slowly
latent- initially asymptomatic, becomes asymptomatic

Question 22

Describe disease patterns in terms of location

Answer 22

local
systemic

Question 23

How do bacterial populations grow?

Answer 23

Via binary fission

Question 24

How can optimal binary fission occur?

Answer 24

with access to nutrients and appropriate environmental conditions eg. temperature, pH

Question 25

What are the 4 distinct phases of growth , describe them

Answer 25

Lag phase: cells adapt to new conditions, enzymes and new metabolites accumulate- minutes/hours Exponential phase: Maximum constant growth rate. Doubling time may be as short as 10 min Stationary phase: Population reaches very high. Oxygen/nutrient demands cannot be met, growth slows. Slow death of cells is balanced by slow multiplication Death phase: no of viable cells drops sharply. rate of cell death > cell division

Question 26

What are the factors that affect growth

Answer 26

temperature, osmolarity pH

Question 27

How can oxygen requirements differ between bacterial cultures

Answer 27

Some like obligate aerobes require O2 to meet energy demands Some like aerotolerant anaerobe do not use O2 and are unaffected by its presence

Question 28

What are chemotherapeutic agents?

Answer 28

chemical agents used to treat disease, includes killing or inhibiting growth of pathogenic microbes within host.

Question 29

What is the primary mechanisms of action of antimicrobial drugs

Answer 29

they are inhibitors of cell wall synthesis they are protein synthesis inhibitors they are metabolic antagonists they inhibit nucleic acid synthesis, which is essential for bacterial survival

Question 30

Give an example of antibiotics that target cell wall synthesis

Answer 30

cycloserine, penicilin

Question 31

What is the mode of action of penicillin

Answer 31

they bind to proteins involved in peptidoglycan assembly inhibit the last step in bacterial wall synthesis prevent cross linkage of peptidoglycan strands hence synthesis of complete cell walls leading to lysis of cell act only on growing bacteria that are synthesizing new peptidoglycan

Question 32

What do protein synthesis inhbitors do?

Answer 32

they either bind specifically to prokaryotic ribosome & interfere with function, or inhibit a stage of protein synthesis

Question 33

What are metabolic antagonists

Answer 33

sulfonamides, trimethoprim they interfere with enzymes involved in folic acid synthesis

Question 34

How does nucleic acid synthesis inhibition work?

Answer 34

It either blocks DNA replication via inhibition of DNA polymerase/gyrase or blocks transcription via inhibition of RNA polymerase

Question 35

Where can resistance genes be found on

Answer 35

bacterial chromosomes plasmids transposons integrons

Question 36

Resistance genes can be freely exchanged between bacteria on mobile genetic elements via

Answer 36

transformation of chromosomal/plasmid DNA conjugation

Question 37

Explain transformation

Answer 37

transfer of plasmid DNA or a fragment of free DNA from a lysed to a living cell

Question 38

Explain conjugation

Answer 38

transfer of chromosomal/plasmid DNA by direct cell to cell contact

Question 39

Explain transduction

Answer 39

transfer of genes between bacteria by bacterial viruses (bacteriophages)

Question 40

What do resistance plasmids do

Answer 40

they can carry multiple antibiotic resistance genes, are usually transferred to other cells by conjugation & transformation often carry transposons that encode antibiotic resistance genes

Question 41

What do composite transposons do

Answer 41

they contain genes for antibiotic resistance within a cell, they move rapidly between the chromosome & plasmids between cells, they move through a bacterial population

Question 42

What are gene cassettes

Answer 42

sets of resistance genes that can exist as seperate genetic elements and can be part of transposons, integrons or located on bacterial chromosome