Syphilis

Question 1

Describe the spirochetes family

Answer 1

 4 sub-species cause clinical disease:
o Pallidum (syphilis)
o Endemicum (Bejel) - found in hot and dry areas
o Carateum (Pinta) – central/southern America
o Pertenue (Yaws) – Castellini – hot and wet areas
The last 3 are mainly childhood infections found in third world countries, they are associated with poor hygiene.

All 4 are morphologically identical.
Unicellular and helically shaped. 
They are all fragile heat-sensitive (only <37 degrees) bacteria so cannot be grown in vitro. 
Very slow growth and doubling time 
Reproduces via transverse fission.

Question 2

Describe structure of Treponema pallidum

Answer 2

Gram-negative bacteria .
Thin and tightly coiled bacterium.
Have axial filaments which allows it to move through liquids.
The outermembrane has limited proteins and no LPS.
Has an endoflagella found in the periplasmic space between its two membranes. These organelles give T. pallidum its distinctive corkscrew motility.
Chemoheterotroph and Microaerophilic - it requires a very low concentration of oxygen.

Question 3

Describe genome of T. pallidum and explain why it’s difficult to grow in vitro

Answer 3

It has a very small genome and doesn’t code for many enzymes
therefore has limited metabolic capacity:- glycolysis occurs, but no krebs/electron transport chain meaning that it can’t synthesise fatty acids, cofactors and most amino acids.

T. pallidum’s small coding capacity means that it must rely heavily on its host for most of its essential compounds, making it extremely hard to culture in the lab. Usually grown in rabbits.

Question 4

Describe the mechanism of T.pallidum invasion

Answer 4

Syphilis is transmitted venereally via direct contact with primary or secondary sores
Enters the body through a breach in mucous membranes Highly invasive bacterium which rapidly enters the bloodstream after infection in rabbit models.
30% of contacts will develop disease on contact with primary or secondary lesions.

It can attach to a wide variety of cells such as epithelial, endothelial, and fibroblastlike cells.

Then it disseminates widely: CNS, skin, organs, bone -> can easily cross junctions between cells and swim through tissue fluids – propels along by its long axis.

Attachment to cells is mediated by two T. pallidum fibronectin binding proteins

T. pallidum has homologues of both Methyl-accepting chemotaxis and cytoplasmic chemotaxis systems common to gram –ve bacteria -Most likely move along nutrient gradients

Question 5

Outline mechanisms of syphilis immune evasion

Answer 5

Low rate of division
Avoids immune privileged sites
Not dependent on Iron.
Low immunogenicity of bacterial surface:
o No LPS
o Proteins anchored to inner membrane extend into the periplasm (not the outer membrane)

o Antigenic variability of TPR proteins – an antigen present in outer membrane

• TPR proteins can induce a strong antibody and T cell response.
• Sequences differ substantially both within and between strains of T. pallidum, and sequence diversity accumulates in a single infected rabbit over time.
• — New variants arise by segmental gene conversion: Variant DNA segments in the tprD gene non-reciprocally recombine with the variable regions (V1-7) of the tprK gene in the expression site to generate new mosaic TPRK proteins and thus help to evade the immune system.

Question 6

Describe stages of syphilis infection

Answer 6

Primary (9-90 days after infection)

• single, hard-based lesion which appears at site of infection.
• painless but contains infectious fluid.
• In women it can appear on cervix, so that they won’t be aware.
• During this stage the bacteria enters the bloodstream and disseminates around the body.
• lesion disappears after a couple of weeks.

Secondary (6/52 to 6/12)

• multiple lesions/skin rashes on skin and mucosal membranes – especially on palms and soles.
• The tissue damage is caused by an inflammatory response to circulating immune complexes.
• Other symptoms include loss of patches of hair, malaise and mild fever.
• The lesions are highly infectious but at least half of patients will not notice any lesions.

Latent
- Can last for lifetime – most cases do not progress beyond latent stage.

Tertiary
- up to 25% of untreated cases, disease progresses to tertiary stage.
- Several types differentiated on type of tissue affected
 Cardiovascular – Infection occurs when bacterium manifests in blood vessels – usually the ascending aorta. causes weakening of aorta.
 Neurological – damage of parts of the CNS causing symptoms such as seizures, dementia, loss of coordination, pathogens.
 Gummatous – characterised by gummas, form of progressive inflammation that appears as rubbery mass of tissue in various organs – damages these organs.

Question 7

Outline methods of syphilis diagnosis

Answer 7

Microscopic tests - important in detecting primary stage as Abs yet to form. The bacterium can be seen in fluid from lesions. Darkfield microscopy is used since they stain poorly.

Direct fluorescent antibody test using monoclonal antibodies - important at secondary.

Non-treponemal serological tests - non-specific, they detect regain-type antibodies which are produced in response to lipid materials the body forms as an indirect response to spirochetes. E.g. VDRL.

ELISA test - uses IgM in 2nd week and IgG in 4th and 5 weeks.

Question 8

Outline methods of syphilis treatment

Answer 8

 The usual antibiotic treatment is benzathine penicillin which is a long acting formulation that remains active in body for 2 weeks.

 For penicillin sensitive people, azithromycin is used. It is a semi-synthetic macrolide - azalide, but what that means is that is can resist acid degradation, it penetrates tissues better (including the CNS), has greater activity against gram –ve organisms and has a very long half-life. It’s also been used in prophylaxis of syphilis.

Question 9

Describe resistance to azithromycin

Answer 9

o A lot of resistant mutants emerged in those co-infected with HIV at the time and in MSM.
o The origin of azithromycin resistance is thought to be a single mutant strain that was introduced into a specific single sexual network and became endemic.
 All mutant strains in San Francisco and London were from MSM
 Similar types in epidemics: Shanghai data showed 100% mutants and 79% were the same type (5 types in total)

o Selection pressure for the development of mutant strains?
 No clinical failures seen with azithromycin in African trials and no mutants in Madagascar or Sri Lanka
 Marra et al showed Increased risk of resistance with previous macrolide exposure

 The future use of azithromycin
o Resistance seems likely to develop in the hard to reach populations in Africa.
o Surveillance is needed in the UK and Intl to look at resistant mutants.