Bacterial properties and disease

Question 1

Meanings of coccus, bacillus, spirillus

Answer 1

Coccus - spherical
Bacillus - Rod-shaped
Spirillus - spiral

Question 2

Gram-negative bacteria features and colour under Gram stain

Answer 2

• Cell wall has two membranes with a thin peptidoglycan layer sandwiched between membranes.
• Lipopolysaccharide in outer membrane which resists the dye.
• Appear violet after stain

Question 3

Examples of Gram-negative bacteria

Answer 3

Escherichia coli
Salmonella
Shigella
Vibrio cholerae
Neisseria

Question 4

Gram-positive bacteria features and colour under Gram stain

Answer 4

• Cell wall has one membrane with a thick peptidoglycan layer and teichoic and lipoteichoic acid.
• Peptidoglycan retains the dye.
• Appear deep purple after stain

Question 5

Examples of Gram-positive bacteria

Answer 5

Staph. aureus
Strep. pneumoniae
Strep. pyogenes

Question 6

Acid-fast bacteria features and colour under Gram stain

Answer 6

• Cell wall has one membrane, a thin peptidoglycan layer, and mycolic acid.
• Mycolic acid helps cell wall resist dye.
• Appear light purple under Gram stain

Question 7

Aerobic bacteria feature

Answer 7

Require oxygen for survival

Question 8

Anaerobic bacteria feature

Answer 8

Do not require oxygen for survival

Question 9

Bacterial life cycle stages

Answer 9

• Colonise in host
• Persist by finding unique niche and avoid host defences
• Replicate using nutrients from the host
• Disseminate throughout tissues
• Cause disease by producing toxins/inducing diarrhoea/deregulate immune responses

Question 10

Extracellular bacteria examples

Answer 10

Staphylococcus
Streptococcus
Yersinia
Neisseria

Question 11

Intracellular bacteria examples

Answer 11

• Phagocytosed by host cell and proliferate inside host cell
• Coxiella - survives in phagolysosome upon uptake
• Salmonella, mycobacteria - survive by preventing fusion of lysosomes with endo-/phagosome
• Listeria, shigella - escape from endo-/phagosome

Question 12

Salmonella’s techniques of motility and invasion

Answer 12

• Uses flagellum for movement

- Possesses an injectisome which transfers virulence proteins into the host cell.

Question 13

Listeria’s techniques of motility and invasion

Answer 13

• Uses actin cytoskeleton of host cell to move intracellularly and spread from cell to cell
• Escapes the endoscope and reorganises the cytoskeleton of the host cell by inducing polymerisation of actin at one end of the bacteria giving it a “rocket tail”

Question 14

Cause of variation in the genomes of pathogens

Answer 14

Bacteria genomes have core genes and accessory genes. These accessory genes are extremely variable and thus produce a very wide gene repertoire.

Question 15

Method of transfer of genetic information between bacteria

Answer 15

Horizontal transmission

Question 16

The 3 types of horizontal transmission

Answer 16

Transformation
Transduction
Conjugation

Question 17

Process of transfer of genetic material between bacteria via transformation

Answer 17

Bacterial cell takes up free exogenous DNA from extracellular fluid and is integrated into chromosome by homologous recombination

Question 18

Process of transfer of genetic material between bacteria via transduction

Answer 18

• Phage replicates its DNA in the bacterium and cuts bacterial DNA into small pieces
• Phages replicate and package bacterial DNA into its head
• Phage infects another bacteria and injects previous bacterial DNA which may be then incorporated into the new host’s genome

Question 19

Process of transfer of genetic material between bacteria via conjugation

Answer 19

Transfer of plasmids through conjugative mating bridge.

• Mating bridge forms between the two
• Plasmid shared between both bacteria
• Each bacteria synthesises a new complementary strand to the plasmid

Question 20

Infectivity definition

Answer 20

The ability of a pathogen to establish infection

Question 21

Virulence definition

Answer 21

The degree of damage an infective pathogen can cause

Question 22

Infectivity is affected by…

Answer 22

• Transmission to host
• Ability to colonise
• Replication
• Tropism
• Immune evasion at site of colonisation or niche

Question 23

Virulence is affected by…

Answer 23

• Toxin production
• Enzymes that degrade host molecules
• Complete immune system evasion
• Interference with host cell function

Question 24

Infective dose definition

Answer 24

The amount of pathogen required to cause an infection in the host

Question 25

Infective dose affected by…

Answer 25

Combined affecting factors of infectivity and virulence

Question 26

Intrinsic sources of infection

Answer 26

• Nasal cavity, sinuses and Upper Respiratory Tract = staph, strep
• Stomach = helicobacter pylori
• Skin = staph. epidermis
• Intestines = E.coli, clostridium species

Question 27

Extrinsic sources of infection

Answer 27

• Secretions from mouth, nasal cavity, and upper respiratory tract = neiisseria meningitidis, strep pneumoniae
• Contaminated food and water = vibrio cholerae
• Sexual transmission = neisseria gonorrhoea, syphilis
• Nosocomial (health care workers) = staph. aureus, clostridium difficile

Question 28

Portals of entry for bacteria

Answer 28

• Upper to lower respiratory tract = bacterial migration to lower respiratory tract, pneumococcus is normal URT flora but when it migrates downwards it becomes dangerous
• Lower GI to urogenital tract = e.coli, enterococci, candida are likely to cause UTI
• Broken skin = surgery, intravenous drug abuse, insects, pre-existing skin breaches, cuts, burns, strep pyogenes can infect wounds in the mouth

Question 29

Transmission route for bacteria

Answer 29

• Mouth droplet transmission = strep pyogenes, meningococcal septicaemia
• Upper to lower respiratory tract = nasal sinuses, strep pneumoniae
• Inoculation through skin = staph. aureus
• Face-oral transmission = vibrio cholerae