A tale of two more studies: topical antibiotics applied to tick bites to prevent Lyme disease

Feeding Ixodes ticks harboring Borrelia burgdorferi deposit the Lyme disease spirochete in the skin of the victim.  The spirochetes remain in the skin for a few days before entering the bloodstream to spread throughout the host.  The delay in dissemination provides a window of opportunity to stop the infection by simply applying antibiotics to the skin where the tick was feeding.  Topical application of antibiotics would allow patients to avoid experiencing side effects associated with ingesting antibiotics.

I recently posted a critique of a study by Knauer and colleagues, who tested the ability of a topical antibiotic to prevent B. burgdorferi infection in lab mice bitten by infected ticks.  As I explained in the post, the antibiotic appeared to prevent infection, but the investigators had used a weakened B. burgdorferi strain to inoculate the mice.  Consequently it wasn't possible to draw any conclusions about the effectiveness of their antibiotic formulation in preventing infection.

Now let's look at two more studies that tested the ability of topical antibiotics to prevent infection in the mouse model of Lyme disease.  These studies were conducted properly with highly infectious B. burgdorferi strains.  One study was published almost 20 years ago.  The other appeared online just last month.  Both studies were published in The Journal of Infectious Diseases.

In their 1993 study Shih and Spielman were able to prevent B. burgdorferi infection by applying at least 1 milligram of tetracycline starting up to two days following the tick bite.  The antibiotic had to be applied twice a day for at least three consecutive days (see table below).  The presence of infection four weeks after tick feeding was assessed by xenodiagnosis, which tests whether the spirochetes could be recovered by ticks placed on the skin at a location distant from the original tick bite.  The investigators also found that penicillin, amoxicillin, ceftriaxone, doxycycline, and erythromycin applied for three days beginning one day after tick feeding prevented infection.

Although this study demonstrated the promise of topical antibiotics in preventing Lyme disease in those who discover a tick feeding on them, a limitation of the study was that most of the antibiotics were dissolved in DMSO, which is not approved for use on humans.  The only antibiotic dissolved in a solvent suitable for humans was erythromycin, which was added into 70% ethanol.

For their 2011 study, Wormser and colleagues decided to dissolve the antibiotics in something else that would be acceptable to apply to human skin.  They rubbed a 2% erythromycin ointment or 3% tetracycline gel over the tick bite 1-2 days after the infected ticks finished feeding on the mice.  The antibiotics were applied twice daily for three days.  Four weeks later urinary bladder and ear tissue were cultured to see whether the mice had a disseminated infection.  The authors found that their antibiotic formulations failed to prevent systemic infection, although erythromycin was able to prevent a persistent infection at the tick bite site in some of the mice (see table below).

Erythromycin and tetracycline were tested in both studies.  Why the stark difference in the effectiveness of the same antibiotics in the two studies?   In the Discussion of their paper, Wormser and colleagues highlighted the major methodological differences between the studies:

• Different antibiotic concentrations. A much higher concentration of erythromycin was applied to the tick bites in the 1993 study.
• Different solvents.  DMSO, the solvent used for the 1993 study, may have promoted better penetration of tetracycline into the skin than the ointment and gel formulations selected for the Wormser study.
• Different placement of infected ticks.  In the 1993 study the infected ticks were placed on the ear for feeding.  In the Wormser study the ticks were placed on the back, where the skin may be thicker and hence more resistant to antibiotic penetration.
• Different B. burgdorferi strains.  Wormser and colleagues used ticks infected with the highly invasive BL206 strain to inoculate the mice, whereas Shih and Spielman used ticks infected with the less invasive JD1 strain.
• Different mouse strains.  The C3H mouse strain used in the Wormser study is highly susceptible to dissemination by B. burgdorferi.

For this simple treatment approach to effective, higher concentrations of the antibiotic in a penetrating solvent such as ethanol may be necessary.  Different B. burgdorferi and mouse strains should also be tested in future studies.


References

Shih, C.-M., & Spielman, A. (1993). Topical prophylaxis for Lyme disease after tick bite in a rodent model Journal of Infectious Diseases, 168 (4), 1042-1045 DOI: 10.1093/infdis/168.4.1042

Wormser, G.P., Daniels, T.J., Bittker, S., Cooper, D., Wang, G., & Pavia, C.S. (2011). Failure of topical antibiotics to prevent disseminated Borrelia burgdorferi infection following a tick bite in C3H/HeJ mice Journal of Infectious Diseases DOI: 10.1093/infdis/jir382
Knauer, J., Krupka, I., Fueldner, C., Lehmann, J., & Straubinger, R.K. (2011). Evaluation of the preventive capacities of a topically applied azithromycin formulation against Lyme borreliosis in a murine model Journal of Antimicrobial Chemotherapy DOI: 10.1093/jac/dkr371


Related post

• A flawed study claiming prevention of Lyme spirochete infection with topical antibiotics

Life after leptospirosis, a pilot study

The signs and symptoms associated with acute human leptospirosis are well known.  Less is known about how well patients manage after they begin recovering from the illness.  Most who survive the illness, even those who had severe illness, appear to regain full health eventually.  However careful examination may reveal subtle deficiencies in organ function.²  Moreover the spectrum of health problems that could crop up later is not known.  The only late complication of leptospirosis that has been well documented is uveitis, which can lead to eye pain and vision problems.³  Old case studies have also reported recurring headaches, general malaise, and even dementia, aggression, depression, and psychosis in those surviving leptospirosis,^4,5 although these observations would need to be confirmed in well-designed clinical studies.

To gain a better understanding of the long-term outcomes of leptospirosis, a large systematic study is needed to monitor the health of leptospirosis patients as they heal from the illness.  Such a study was attempted by Spichler and colleagues.¹  Their pilot study targeted patients hospitalized with leptospirosis at any time from December 2008 to May 2009 in São Paulo, Brazil, where leptospirosis is endemic.  There were 180 patients hospitalized with leptospirosis during this period, but for a variety of reasons only 47 of the 180 could be enrolled in the study.  All 47 had been treated for at least seven days with antibiotics (penicillin or ceftriaxone) during their hospitalization, and all had their diagnosis confirmed by serologic testing.  17 of the 47 had severe leptospirosis, defined as those who had experienced jaundice, kidney failure, bleeding (hemorrhage), pulmonary (lung) involvement, or shock while hospitalized.  Fortunately no patient died.

The first outpatient visit was conducted an average of 22 ± 12 days after the patients were discharged from the hospital.  23 of the 47 (49%) continued to experience one or more of the following health problems:  general malaise, headache, muscle pain, dizziness, bronchitis, and abdominal pain.  Three patients still had jaundice.

Only 22 of the 47 patients came back for a second visit, which took place a mean of 40 ± 21 days after they were discharged from the hospital.  Two of the 22 patients continued to suffer from medical problems.

One individual was experiencing general malaise, which wasn't a problem for him before being stricken with leptospirosis.  He did always have high blood pressure, which may or may not have been a contributing factor to his malaise. An ECG showed some abnormalities with his heartbeat.  Leptospira is known to cause myocarditis, an inflammation of the heart muscle.  This patient continued to suffer from profound general malaise when examined one year later.

The second patient started to experience panic attacks between the two clinic visits.  Were the panic attacks related to his earlier bout with leptospirosis?  It's difficult to conclude anything from this single patient.  Although the patient had severe leptospirosis, did not suffer from any of the known neurologic features of leptospirosis during his acute illness⁶ and had not been diagnosed with any neurologic or psychiatric condition before contracting leptospirosis.  Panic disorder has never been described in leptospirosis patients in the scientific literature.

Since this was a pilot study, the investigators probably lacked the resources to address the problems that cropped up during the study.  As pointed out by the authors, the major limitations of the study were that few of the eligible patients were enrolled in the study and that many dropped out between visits, resulting in a potentially biased sampling of leptospirosis patients. Additionally the follow-up visits did not include any laboratory testing to detect lingering functional deficiencies in the kidney, liver, and other organs.  For these reason it's impossible to make any definitive conclusions about the recovery of these subjects.  Despite the preliminary nature of this pilot study, the possible outcomes of acute leptospirosis identified in this study and earlier case studies beg for a future prospective study with a larger number of individuals living in an area where leptospirosis is endemic.  A longer time frame for follow up is also necessary since uveitis can first appear up to four years after recovery from leptospirosis.³

Any future study should also follow those with mild or asymptomatic Leptospira infections.  The reason is that the long-term outcome of mild disease is unknown.  Since those with mild or asymptomatic infections are unlikely to seek medical attention, identifying such individuals will require investigators to monitor healthy high-risk individuals by serologic testing so that the newly infected could be identified as those with increasing anti-Leptospira antibody titers.

In light of the recent discovery of chronically infected individuals in the Peruvian Amazon,⁷ it would also be prudent to test the urine of healthy individuals for Leptospira being shed from the kidney tubules so that the findings of the Peruvian study could be confirmed.  The long-term effects of chronic infection, if any, could also be identified.

Where could a future study examining the long-term outcomes of Leptospira infection be conducted?  Brazil and India may be best suited for this type of study.  Leptospirosis is highly endemic in those countries, and multiple research teams are already investigating the epidemiology of leptospirosis in those countries.

10-30-2011, edits for clarity.

Featured paper

1.  Spichler A., Athanazio D, Seguro AC, and Vinetz JM (July 2011).  Outpatient follow-up of patients hospitalized for acute leptospirosis.  International Journal of Infectious Diseases 15(7):e486-e490.  DOI:  10.1016/j.ijid.2011.03.020

References

2.  de Francesco Daher E, Zanetta DMT, and Abdulkader RCRM (September 2004).  Pattern of renal function recovery after leptospirosis acute renal failure.  Nephron, Clinical Practice 98(1):c8-c14.  DOI: 10.1159/000079922

3.  Shukla D, Rathinam SR, and Cunningham ET (Spring 2010).  Leptospiral uveitis in the developing world.  International Ophthalmology Clinics 50(2):113-124.  DOI:

4.  Shpilberg O, Shaked Y, Maier MK, Samra D, and Samra Y (April 1990).  Long-term follow-up after leptospirosis.  Southern Medical Journal 83(4):405-407.  Link

5.  Avery TL (July 27, 1983).  Leptospirosis and mental illness.  New Zealand Medical Journal 96(736):589 (Letter).

6.  Panicker JN, Mammachan R, and Jayakumar RV (September 2001).  Primary neuroleptospirosis.  Postgraduate Medical Journal 77(911):589-590. DOI: 10.1136/pmj.77.911.589

7. Ganoza CA, Matthias MA, Collins-Richards D, Brouwer KC, Cunningham CB, Segura ER, Gilman RH, Gotuzzo E, and Vinetz JM (2006). Determining risk for severe leptospirosis by molecular analysis of environmental surface waters for pathogenic Leptospira. PLoS Medicine 3(8):e308.  DOI: 10.1371/journal.pmed.0030308

Related post

• Healthy human carriers of the spirochete Leptospira in the Peruvian Amazon

The Lyme disease spirochete feasts on tick antifreeze

In the northeastern United States the Lyme disease spirochete Borrelia burgdorferi spreads from one white-footed mouse to another by hitching a ride in the tick Ixodes scapularis. Transmission between tick and mouse occurs during the tick's rare blood meals.  The larval tick acquires B. burgdorferi from an infected mouse during a blood meal late in the summer, and the spirochetes take up shelter in the tick's midgut.  Later the larva molts into a nymph, which then completes the transmission cycle by feeding on an uninfected mouse during the next spring or early summer.

Although blood is potentially a rich source of nutrients for both tick and spirochete, the cells lining the tick's gut rapidly engulf the nutrients, including glucose, an energy-rich sugar favored by B. burgdorferi.  The spirochetes must therefore rely on other energy sources if it is to survive the many months between tick feedings.  How does B. burgdorferi fuel its survival during this period?

A study in the July issue of PLoS Pathogens has shown that B. burgdorferi metabolizes the tick's antifreeze while living in its midgut.¹  Many arthropods and insects produce large amounts of antifreeze to protect themselves from freezing temperatures.  The Ixodes tick's antifreeze is glycerol, the same stuff that's often added to enzymes to keep them from freezing in laboratory freezers.  The amount of glycerol found in other organisms is too low to serve as antifreeze.  Instead glycerol is metabolized to extract the chemical energy stored in its bonds and to make cell membrane components.  I describe below how B. burgdorferi handles glycerol, but the same enzymes are found in most organisms that metabolize glycerol, including humans.

The B. burgdorferi genome encodes homologs of a glycerol transporter (GlpF), glycerol kinase (GlpK), and glycerol-3-phosphate dehydrogenase (GlpD), which are used by the bacteria to take up and metabolize glycerol (see figure below).  The figure also shows that B. burgdorferi can break down glucose by the glycolytic pathway (glycolysis) to supply its carbon and energy needs.

Modified from Figure 1 of Pappas et al., 2011.  The BB numbers are the gene ID numbers assigned when the B. burgdorferi genome was sequenced.  The individual steps of glycolysis are not shown.  Source

After the glycerol transporter brings glycerol into the cytoplasm, glycerol kinase (GlpK) quickly phosphorylates glycerol at the expense of ATP to generate glycerol-3-phosphate.

Glycerol-3-phosphate is located at a branch point in glycerol metabolism.  This key metabolite can be shunted to one of two pathways.  One pathway leads to assembly of lipids, and the other leads to the glycolytic pathway, which generates ATP for B. burgdorferi.

To make more lipids, additional molecules are attached to glycerol-3-phosphate by other enzymes to generate phospholipids, glycolipids, and lipoproteins.  For example, one of the two major phospholipids in B. burgdorferi membranes is phosphotidylcholine (the other is phosphotidylglycerol).  Note in the figure below that glycerol-3-phosphate (in black and blue) makes up the core of phosphotidylcholine.  (R₁ and R₂ denote fatty acid chains.)  The glycerol or glycerol-3-phosphate base also forms the core of other phospholipids, glycolipids, and lipoproteins needed to assemble the bacterial cell membrane.

ATP provides the energy to build lipids and other components of B. burgdorferi.  To generate ATP, glycerol-3-phosphate is converted by glycerol-3-phophate dehydrogenase (GlpD or G3PDH) into dihydroxyacetone phosphate, which feeds into the middle of the glycolytic pathway.

Glycerol is not a great energy source.  For each molecule of glycerol, one ATP is consumed to make glycerol-3-phosphate, and two molecules of ATP are made via glycolysis, netting B. burgdorferi one molecule of ATP.  On the other hand, each molecule of glucose, which is plentiful in blood, nets two molecule of ATP, twice the amount of energy extracted from glycerol.

To enlarge the glycolytic pathway, click on the image above

B. burgdorferi lacks the TCA cycle enzymes and electron transport chain, which could unleash the chemical energy stored in the bonds of pyruvate, the end product of glycolysis, to generate even more ATP.  Instead pyruvate is converted into the fermentation end product lactate by lactate dehydrogenase.

Modified from Figure 2c of Harper and Harris 2005

For their study the investigators knocked out the B. burgdorferi glpD gene encoding glycerol-3-phosphate dehydrogenase so that the spirochete couldn't use glycerol as an energy source to make ATP.  As expected, the glpD mutant was unable to grow to a high cell density when glycerol was the major carbon and energy source in the culture medium.  Nevertheless the mutant was still able to infect laboratory mice and spread throughout their bodies almost as well as the wild-type (unmutated) strain.  This makes sense since energy sources other than glycerol (such as glucose) are readily available in mammals.

To see how well the glpD mutant survived in ticks, larval Ixodes scapularis ticks were allowed to feed to satiation on groups of mice infected with the mutant and wild-type strains.  Similar numbers of the mutant (632 ± 343 spirochetes/tick) and wildtype (737 ± 369 spirochetes/tick) ended up in the larva (P = 0.5646).  The infected larva were maintained in the lab and allowed to molt into nymphs.  7-8 weeks after larval feeding, the number of mutant spirochetes in the nymphs (254 ± 137 spirochetes/tick) was much lower than the number of wildtype (1173 ± 637 spirochetes/tick; P = 2.76 x 10^-8).  This result suggests that to thrive in the tick's midgut, B. burgdorferi has to break down glycerol, the tick's antifreeze, to generate ATP.

The glpD mutation also slowed the rapid increase in spirochete numbers seen when the infected nymph starts to feed on a mouse (see below).  It's unclear how much of the blood nutrients are available to B. burgdorferi early during feeding.  Blood consumption by the tick is slow initially, and a membrane called a peritrophic matrix forms in the tick midgut to encase the blood.  The spirochetes in the midgut may therefore rely primarily on glycerol to power its rapid multiplication even as the nymph is feeding.  Within a few days a small number of spirochetes eventually break through the midgut lining and make their way to the salivary glands, where they end up as passengers in the saliva flowing into the mouse's skin.

Figure 11 from Pappas et al., 2011.  Infected nymphs were placed on mice at time zero.  Filled circles, wild-type B. burgdorferi; open squares, glpD mutant.  Source

The impaired growth of the glpD mutant in the feeding nymph also delayed their transmission into the mice.  The nymphs fed for 62 hours before the wild-type strain was transmitted to the mice, whereas 72 hours elapsed before transmission of the glpD mutant was detected.

Why does the glpD mutant survive at all in the ticks?  The answer is that there are probably other energy sources available to B. burgdorferi.  The authors proposed that the sugar chitobiose, a component of the tick's cuticle and peritrophic membrane, can be consumed by B. burgdorferi living in the midgut.  The transporter encoded by chbC brings chitobiose into the spirochete, where it is processed by several enzymes before being fed into the glycolytic pathway.  In fact the authors found that B. burgdorferi expressed larger amounts of the chbC mRNA when in the unfed nymph than it did when in the mouse host.  This result would be expected if B. burgdorferi was trying to metabolize the tick's chitobiose, which is not found in the mouse.

So to sum things up, B. burgdorferi appears to use different organic carbon sources to fulfill its energy needs depending on where it's living.  In the mouse host B. burgdorferi most likely breaks down glucose, a sugar rich with potential chemical energy.  Since glucose isn't available in the tick, the spirochete consumes glycerol and possibly chitobiose while living in the tick's midgut.

Reference

Pappas, C.J., Iyer, R., Petzke, M.M., Caimano, M.J., Radolf, J.D., & Schwartz, I. (2011). Borrelia burgdorferi requires glycerol for maximum fitness during the tick phase of the enzootic cycle PLoS Pathogens, 7 (7) DOI: 10.1371/journal.ppat.1002102

Image sources

Unless otherwise stated in the figure legends, the chemical reactions were taken from Biochemistry (5th Edition) by Berg, Tymoczko, and Stryer.

Harper ET and Harris RA (2005).  Glycolytic Pathway, from eLS.  DOI: 10.1038/npg.els.0003883


Related posts

• The Lyme disease spirochete has flagella but doesn't use them to penetrate the gut of the feeding tick

A flawed study claiming prevention of Lyme spirochete infection with topical antibiotics

Two recent papers tested the effectiveness of topical antibiotics in preventing Borrelia burgdorferi infection in mice following a tick bite.  Infection by the Lyme disease spirochete was successfully halted in the Knauer et al. study from Germany¹ but not in the Wormser et al. study conducted in New York.²  However a flaw in the Knauer study may have unfairly tipped the outcome in the antbiotic's favor.  (I'll save the Wormser study for another post.)

The paper by Knauer and colleagues¹ presented two trials, which differed in how the mice were inoculated with B. burgdorferi,  In the first trial the spirochetes were injected into the skin, and azithromycin was applied topically one hour, three days, and five days later at the injection site.  In the second trial infected ticks transmitted the spirochetes to the mice.   Azithromycin was applied topically to the feeding site immediately after the ticks stopped feeding.  In both trials azithromycin was dissolved in ethanol for application to the inoculation site.  Disseminated infection of the mice was assessed by culturing the heart, bladder, ear, and tarsus 56 days after inoculation.

The results from the first trial reveals the problem with the study (Table 1).  Among the ten mice in the placebo group (first row), which received only ethanol, only one (10%) had any culture positive organs 56 days later.  The spirochetes failed to establish a persistent infection in the other nine mice, suggesting that the investigators were working with a weakened strain of B. burgdorferi.  The ethanol could have had a slight killing effect (see the second trial) yet could not have accounted for the poor infection rate in the placebo group.

The table legend claims that the difference between the placebo and treatment groups was significant, but the statistics were done on the numbers in the column labeled "Infection Status."  According to the text of the paper, a positive "Infection Status" refers to those animals that managed to produce antibodies against B. burgdorferi antigens.  Infection status is therefore not the proper metric to assess the infectivity of B. burgdorferi.  When the statistics are performed with the appropriate numbers, located in the column under "Culture," the effect of azithromycin (1/10 culture positive in control group vs. 0/10 culture positive in any treatment group) is not significant (P = 0.9 for control vs. any treatment group).

Results from the second trial are shown in Table 2.  This trial included an extra control group ("No treatment") that received neither antibiotic nor ethanol.  Four of the seven mice in the untreated group (57%) ended up culture positive.  This is still a low infection rate compared to the rates observed in other studies, in which 90-100% of the control animals end up infected following tick inoculation of B. burgdorferi.  Two of the nine mice treated with ethanol alone (placebo) were culture positive, suggesting that ethanol alone helps prevent infection (57% culture positive in "no treatment" group vs. 22% in placebo group, P = 0.152), although the experiment would need to be repeated with larger groups of animals to make a statistically convincing case.

None of the animals treated with azithromycin were culture positive.  However the number of animals was again too low to conclude that antibiotic treatment was effective (2/9 culture positive in placebo group vs. 0/9, 0/8, and 0/5 in the treatment groups, P > 0.4 for comparison of each treatment group with placebo group).  The authors were able to claim statistical significance by combining the two control groups (no treatment and placebo) and the treatment groups.  However it is inappropriate to combine groups in this manner to attain statistical significance.

Even if the investigators had used a larger number of animals, the problem of their weakened challenge strain remains.  Application of topical antibiotics may turn out to be effective in preventing Lyme disease after a tick bite, but the study presented by Knauer and colleagues was not a fair test of this treatment approach.

References

1. Knauer, J., Krupka, I., Fueldner, C., Lehmann, J., & Straubinger, R. (2011). Evaluation of the preventive capacities of a topically applied azithromycin formulation against Lyme borreliosis in a murine model Journal of Antimicrobial Chemotherapy DOI: 10.1093/jac/dkr371

2. Wormser, G., Daniels, T., Bittker, S., Cooper, D., Wang, G., & Pavia, C. (2011). Failure of topical antibiotics to prevent disseminated Borrelia burgdorferi infection following a tick bite in C3H/HeJ mice Journal of Infectious Diseases DOI: 10.1093/infdis/jir382

BAM! A rare outer membrane protein of the stealth pathogen Treponema pallidum hidden in plain sight

Our immune system is capable of generating antibody against bacterial proteins located inside and outside of the bacterial cell.  However only those antibodies targeting surface proteins (or other surface components) have the potential to eliminate the bacteria.

Unlike typical Gram negative bacteria, the external surface of the Treponema pallidum outer membrane is bare except for a tiny number of proteins protruding from the membrane.  Most of the proteins targeted by the host antibody response are safely tucked away beneath the surface, inaccessible to the antibodies that recognize them.  T. pallidum even lacks LPS, which is a major target found on typical Gram negative bacteria.  The barren surface of T. pallidum is therefore one factor that may allow the "stealth pathogen" to persist in the host despite a strong antibody response.

None of the rare T. pallidum outer membrane proteins (Omps) have been identified with certainty, until now.  Scientists at the University of Connecticut have finally confirmed that TP0326 is one of these rare Omps.¹  Desrosiers and colleagues showed that the protein TP0326 was digested when proteinase K was added to live T. pallidum.  Proteins known to be located in the periplasm were left untouched by the protease, indicating that the fragile outer membrane remained intact while the spirochetes were being harvested for the experiment.  These results indicated that TP0326 was exposed on the surface of T. pallidum.

TP0326 (also called "Tp92") was first identified as a candidate Omp over a decade ago when rabbit antibodies against the protein were shown to promote opsonophagocytosis (engulfment) of T. pallidum.²   Antibodies must physically link bacteria to phagocytes for opsonophagocytosis to proceed.  Opsonophagocytosis therefore occurs only when antibodies against surface-exposed proteins are present.  The amino acid sequence of TP0326 also gave clues to its location.  TP0326 was identified as a homolog of the Omp85 family,² a set of proteins known to reside in the outer membrane of other Gram negative bacteria.  Omp85 was later renamed BamA when it was shown to be the core component of the outer membrane protein complex "BAM" that inserts newly expressed Omps into the outer membrane.³

A number of computer programs predicted that TP0326 spanned the outer membrane as a β-barrel structure, which I described in an earlier post.⁴  The gallery of E. coli transmembrane Omps displayed below shows that the loops on one side of the barrel are exposed on the surface of the bacterium.  The transmembrane portion of BamA is depicted as a box because its structure has yet to be determined experimentally, but it is also likely to have a β-barrel structure.

Figure 1 from Burgess 2008.  The outer membrane is colored gray.  The numbers indicate the number of β strands that cross the membrane.  The periplasm is located below the outer membrane.

The amino terminus of BamA consists of five repeating structures called POTRA, which are thought to guide the β strands of new transmembrane Omps into the outer membrane.³  Note that the POTRA domains protrude into the periplasmic space.  The amino terminus of TP0326 is thought to possess the POTRA domains as well.

Given the prediction that TP0326 structurally resembles BamA, it wasn't too surprising that TP0326 was exposed on the surface of T. pallidum.  What was surprising was how TP0326 was targeted by the immune system in syphilis patients.  Among the six patients examined by Desrosiers et al., three lacked any antibody whatsover against TP0326.  Although the other three syphilis patients managed to generate antibody against TP0326, the antibodies reacted weakly (1 patient) or not at all (2 patients) with the β-barrel domain, which contained the surface-exposed loops.  Instead, the antibodies targeting TP0326 reacted strongly with the subsurface POTRA domains in these three patients.  Assuming that the results with these six patients can be extrapolated to other syphilis patients, humans infected with T. pallidum are incapable of generating a strong antibody response against the surface-exposed loops of TP0326.

Since the exposed regions of TP0326 appear to be an Achilles heel of T. pallidum, TP0326 may have evolved to avoid generation of antibodies targeting its vulnerable segments.  Experimentally infected rabbits, which are not a natural host of T. pallidum, succeeded in generating antibody against the TP0326 β-barrel domain. Unfortunately the authors didn't present any data indicating whether the surface-exposed loops were recognized by the rabbit antibodies.  However, as I mentioned above, earlier work showed that infected rabbits produce opsonic antibody against TP0326.²  The same paper showed that TP0326 was somewhat effective as a subunit vaccine in the rabbit model of syphilis.²  Both of these observations suggest that rabbits are able to produce antibody that reacts with the surface-exposed regions of TP0326.  The rabbit model could be used to figure out conclusively whether the effectiveness of TP0326 as a vaccine relies upon generation of antibodies against the surface-exposed loops of the protein.  If so, an approach for developing a syphilis vaccine would be to coax the human immune system into generating antibodies that target the surface-exposed loops of TP0326.


Featured paper

1. Desrosiers DC, Anand A, Luthra A, Dunham-Ems SM, LeDoyt M, Cummings MAD, Eshghi A, Cameron CE, Cruz AR, Salazar JC, Caimano MJ, and Radolf JD (June 2011).  TP0326, a Treponema pallidum β-barrel assembly machinery A (BamA) orthologue and rare outer membrane protein.  Molecular Microbiology 80(6):1496-1515.  DOI: 10.1111/j.1365-2958.2011.07662.x

Related papers

2. Cameron CE, Lukehart SA, Castro C, Molini, B, Godornes C, and Van Voorhis WC (April 2000).  Opsonic potential, protective capacity, and sequence conservation of the Treponema pallidum subspecies pallidum Tp92.  The Journal of Infectious Diseases 181:1401-1413.  DOI: 10.1086/315399


3. Knowles TJ, Scott-Tucker A, Overduin M, and Henderson IR (March 2009).  Membrane protein architects: the role of the BAM complex in outer membrane protein assembly.  Nature Reviews Microbiology 7(3):206-214.  DOI: 10.1038/nrmicro2069

4. Cox DL, Luthra A, Dunham-Ems S, Desrosiers DC, Salazar JC, Caimano MJ , and Radolf JD (December 2010).  Surface immunolabeling and consensus computational  framework to identify candidate rare outer membrane proteins of Treponema pallidum.  Infection and Immunity 78(12):5178-5194.  DOI: 10.1128/IAI.00834-10

Image source

Burgess NK, Dao TP, Stanley AM, and Fleming KG (September 26, 2008).  β-barrel proteins that reside in the Escherichia coli outer membrane in vivo demonstrate varied folding behavior in vitro.  Journal of Biological Chemistry 283(39):26748-26758.  DOI: 10.1074/jbc.M80275420

Related posts

• The quest for outer membrane proteins of the stealth pathogen Treponema pallidum:  the cliffhanger episode