Iron metabolism

Question 1

Overview of role of iron in bacterial infections

Answer 1

Environment High Iron Entry

host low iron immune clearance/ enhance iron acquisition

Question 2

how is Iron used in biological systems

Answer 2

• essential nutrient for almost all living cells
• functioning of biological processes
- Fe3+ Fe2+ inter-conversion generates redox potential
- necessary for activity of key metabolic enzymes
• Free iron is limited in the environment
- free ferric iron virtually insoluble
- bound to proteins in body
• Acquisition of iron is major microbial virulence determinant

Question 3

whats the The trouble with iron?

Answer 3

• Ferric iron is a catalyst in Haber-Weiss-Fenton reactions producing free radicals
• Free radicals cause breakage of DNA and disruption of membranes by peroxidation of lipids
• In mammals/birds iron is complexed with proteins: haemoglobin (in red blood cells), haemoglobin-haptoglobin (in serum), haem-haemopexin (in serum), transferrin (in serum, lymph),

Question 4

what are the Consequences of iron-binding capacity

Answer 4

• free ferric iron levels are too low to support bacterial growth
• Injection of animals with iron compounds increases susceptibility to infection
• Upon infection, iron levels are reduced even further - hypoferraemia. IL-1 is the key cytokine in this process increased transferrin levels, release of lactoferrin from PMNs, transfer of iron from transferrin to lactoferrin,
  and transport to liver for storage in ferritin

Question 5

explain Bacterial iron acquisition at low iron

Answer 5

• environmental signal to switch on virulence determinants

Question 6

what are the specialised bacterial mechanisms for scavenging iron form the host at low iron?

Answer 6

• surface ferric reductases (Listeria monocytogenes)
• haemolysins, cytotoxins
• transferrin-specific proteases (Bacteriodes)
• receptor-mediated systems
• haem (Yersinia pestis)
• haemoglobin, haptoglobin (H. influenzae, S. aureus)
• transferrin (Neisseria, H. influenzae, B. pertussis)

Question 7

describe Receptor mediated iron uptake

Answer 7

• Dependent on direct contact between bacterial receptor and host iron binding protein
• Involves specific receptor-protein interaction
• Ferric ion transferred from binding protein to receptor (energy-dependent mechanism)
• Then via a periplasmic binding lipoprotein to an inner membrane permase

Question 8

what is Transferrin species specificity?

Answer 8

– Neisseria - human only
– Haemophilus influenzae - human, bovine, rabbit
– Bordetella pertussis - can use any source

Question 9

Specialised mechanisms for scavenging iron from host

Answer 9

• surface ferric reductases
• haemolysins, cytotoxins
• transferrin-specific proteases
• receptor-mediated systems
• siderophore systems

Question 10

what are Siderophores

Question 11

name the two types of siderophores

Answer 11

catechols and hydroxamates

Question 12

give an example of each type of Siderophore

Answer 12

c- vibriobactin - Vibrio cholera

h- mycobactin - Mycobacterium spp

Question 13

explain Siderophore-mediated iron uptake

Answer 13

• Bacteria that synthesise a particular siderophore will express the cognate transporter
• Many bacteria also express transporters for siderophores produced by other microorganisms
siderophore piracy!

Question 14

how is the Ability to use a wide range of siderophores is highly advantageous

Answer 14

• obtain iron with minimum expenditure of energy

* prevent other bacteria from using the iron

Question 15

name a iron acquisition mechanisms required for virulence

Answer 15

aerobactin in invasive E. coli – UTI

Question 16

name a pathogenicity island that contains iron uptake systems

Answer 16

yersinia enterocolitica

Question 17

name 4 virulence determinants that are iron regulated

Answer 17

• Shiga toxin of Shigella dysenteriae
• Vero-cytotoxin of enterohaemorrhagic E. coli
• Haemolytic toxin of Vibrio cholerae
• Diphtheria toxin

Question 18

how does Iron regulation of genes work?

Answer 18

• Bacterial iron uptake systems are tightly regulated, primarily at transcriptional level
• Global iron regulator – Fur (ferric uptake regulation)
• 17-kDa polypeptide with high histidine content
• Acts as repressor when complexed to ferrous iron
• Binds ‘Fur box’ in promoters of iron-regulated genes

Question 19

explain how Fur homologues work in 3 organisms

Answer 19

• Yersinia enterocolitica haem receptor expression regulated by a Fur protein 84% homologous with E. coli Fur
• Neisseria meningitidis has a Fur protein, regulates the iron-responsiveness of iron-receptor proteins plus some secreted proteins and energy metabolism genes
• Vibrio cholerae has a Fur protein highly homologous with E. coli Fur, controls siderophore production, etc

Question 20

when is Fur repressed

Answer 20

High Iron conditions

Question 21

explain Regulation of Diphtheria Toxin

Answer 21

• Diphtheria Toxin (DT) is major virulence factor of
  Corynebacterium diphtheria
• Lethal dose is 0.1 mg/kg of body weight
• DT production is greatest when bacteria grown under
  iron-limiting conditions
• DT is encoded by tox gene on corynephage
• The regulator DtxR is encoded on bacterial chromosome and represses DT expression when bound to a divalent metal ion

Question 22

explain the Corynebacterium diphtheriae DtxR Regulon at High Iron

Answer 22

DtxR
226 aa
Binds Fe2+, Ni2+, Co2+, and others Functions as a dimer Dimers stabilized by metal ion interaction
DT – no expression

Question 23

what are DtxR Homologs

Answer 23

DtxR homologs are called IdeR
Foundin Mycobacterium,Staphylococci,Treponema
palladium, Brevibacterium and Streptomyces
High conservation – 50-60% identity in amino acid sequence Regulons include iron uptake and storage systems
Some species have both DtxR and Fur homologs

Question 24

summarise Iron Metabolism

Answer 24

•  Multiple pathways exist in humans to restrictiron levels and hence growth of bacteria
•  Bacteria encode multiple pathways to acquire iron from their hosts; including receptor proteins for iron-containing host proteins and siderophores
•  Iron acquisition and other virulence factors are regulated by iron levels (usually through the iron- response regulator, Fur, but also by IdeR homologs)