Bacterial strategies for host colonisation

Question 1

What are key abilities of bacterial pathogens?

Answer 1

1. Transmit between hosts
2. Colonise hosts
3. Cause host damage

Question 2

What are pathogenic routes of entry and host removal strategies?

Answer 2

Routes of entry include sites of vulnerability such as the Respiratory tract, Intestinal tract, Urogenital tract, Conjunctiva. Physical removal strategies include Coughing and sneezing, Vomiting and diarrhea, Urination and Tear production.

Question 3

What are attributes involved in host colonisation?

Answer 3

Adhere to host cells and resist physical removal

Invade host cells.

Compete for iron and other nutrients.

Evade the immune system

Question 4

Fimbriae and Pili

Answer 4

ound in virtually all Gram-negative bacteria but not in many Gram-positive bacteria

Made up of a protein called pilin

Bind to sugar (e.g. mannose) receptors on the surface of eukaryotic cells

e.g. pathogenic Escherichia coli

Can colonise gut and urinary tract – diarrhea and UTI

Adhere to host cells and resist physical removal

Question 5

Gram +ve attachment

Answer 5

Adhesins enable attachment and resistance to physical removal
Bind to specific receptors to allow intimate attachment

e.g. Streptococcus pyogenes

Protein F that binds to fibronectin

Lipoteichoic acid binds to fibronectin on epithelial cells.

M-protein also functions as an adhesin

Question 6

Gram -ve attachment

Answer 6

Colonises gut – stomach ulcers and cancer

Flagella help move bacteria through the mucus attach to gut epithelial cell

Urease produced in bacterial cytosol produces ammonia from urea

Ammonia passes into periplasmic space – buffer against acidic pH of the gut

Resist physical removal

Question 7

Invasion of host cells

Answer 7

Invasins are molecules that activate the host cells cytoskeleton and promote cell entry by phagocytosis, facilitating the growth and spread of the pathogen

Bacteria within host cells provide the bacterium with a ready supply of nutrients and protect the bacteria from complement, antibodies, and other body defence molecules

Extra-cellular proteases also help with invasion, e.g. metalloproteases

Question 8

How do bacteria invade epithelial cells?

Answer 8

Bacterial secretion systems co-opt the functions of the host cell (Gram +ve and Gram –ve)

Type 3 secretion system most common (Gram –ve) - an injectosome filled with effector molecules that interfere with cytoskeleton, encouraging phagocytosis

Eventual cell to cell spread

e.g. Escherichia coli, Shigella dysenteriae.

Question 9

How do bacteria grow and survive in the host?

Answer 9

Host is a source of nutrients for growth of bacteria, such as Carbon, Nitrogen, Iron

Have the ability to compete for iron and other nutrients. Iron is insoluble and carried in the host in complexes with glycoproteins: Transferrin – serum, Haemoglobin – red blood cells, Lactoferrin – tears, sweat, saliva, mucus

Question 10

What are the key cells of the innate immune system?

Answer 10

Macrophages, DC, neutrophils, engulf and break down bacteria - enzymes and nutrient starvation

In s.aureus:
Coagulase also causes fibrinogen to fibrin which causes clotting

Fibrin coats the surface of the bacteria which allows evasion of phagocytosis

Coagulase activity is almost always associated with pathogenic strains and almost never associated with non-pathogenic strains of S. aureus

Question 11

What are other methods of bacterial defence against the immune system?

Answer 11

N-deacetylation protects bacteria from lyzozymes; gram +ve are particularly vulnerable

Altered epitopes on antigen interfere with the function of antibodies as it can no longer bind

Surface proteins on bacteria can also prevent correct binding of IgG