ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Feb. 2010, p. 583–589
Vol. 54, No. 2
0066-4804/10/$12.00 doi:10.1128/AAC.01095-09
Copyright © 2010, American Society for Microbiology. All Rights Reserved.
MINIREVIEW
Global Challenge of Antibiotic-Resistant
Treponema pallidum
Lola V. Stamm*
Program in Infectious Diseases, Department of Epidemiology, Gillings School of Global Public Health, University of
North Carolina, Chapel Hill, North Carolina
Syphilis is a multistage infectious disease that is usually transmitted through contact with active lesions of
a sexual partner or from an infected pregnant woman to her fetus. Despite elimination efforts, syphilis remains
endemic in many developing countries and has reemerged in several developed countries, including China,
where a widespread epidemic recently occurred. In the absence of a vaccine, syphilis control is largely
dependent upon identification of infected individuals and treatment of these individuals and their contacts with
antibiotics. Although penicillin is still effective, clinically significant resistance to macrolides, a second-line
alternative to penicillin, has emerged. Macrolide-resistant strains of Treponema pallidum are now prevalent in
several developed countries. An understanding of the genetic basis of T. pallidum antibiotic resistance is
essential to enable molecular surveillance. This review discusses the genetic basis of T. pallidum macrolide
resistance and the potential of this spirochete to develop additional antibiotic resistance that could seriously
compromise syphilis treatment and control.
Spirochetes are motile, spiral-shaped bacteria that are di-
vided into the families
Spirochaetaceae,
Brachyspiraceae, and
Leptospiraceae (54).
Treponema species, which are members of
the family
Spirochaetaceae, are fastidious anaerobic or mi-
croaerophilic host-associated spirochetes. While the majority
of
Treponema species are found in the ffora of humans and
animals, a few species are pathogenic for humans.
Treponema
pallidum subspp.
pallidum,
endemicum, and
pertenue, the
agents of venereal syphilis, endemic syphilis, and yaws, respec-
tively, together with
Treponema carateum, the agent of pinta,
are primary pathogens of humans that have eluded in vitro
cultivation (70).
Treponema denticola and certain other oral
Treponema species that are associated with human periodontal
disease are cultivable, opportunistic pathogens (22). The pur-
pose of this minireview is to provide an overview of current
antibiotic resistance in
T. pallidum subsp.
pallidum (
T. palli-
dum), the most significant pathogen of the genus globally, and
to discuss the potential of this spirochete to develop additional
antibiotic resistance that could seriously compromise syphilis
treatment and control.
EPIDEMIOLOGY OF SYPHILIS
Syphilis is a multistage disease that is usually transmitted
through contact with active lesions of a sexual partner or from
an infected pregnant woman to her fetus (70). Efforts to elim-
inate syphilis have met with only modest success (24). The
World Health Organization (WHO) estimated that there were
12 million new cases of syphilis in 1999, with more than 90% of
the cases occurring in developing countries (www.who.int/hiv
/pub/sti/who_hiv_aids_2001.02.pdf). Congenital syphilis is a
leading cause of stillbirth and perinatal mortality in many of
these countries (66). Despite the availability of new diagnostic
tests and antibiotic therapy, syphilis has reemerged in several
developed countries. While the widespread epidemics of syphilis
that occurred in Russia in the 1990s and more recently in
China mostly involved heterosexuals, smaller outbreaks in
the United States, Canada, and England predominately
involved men who have sex with men (MSM) (5, 10, 43, 67,
77). However, recent increases in syphilis rates for U.S.
women and infants suggest that heterosexually transmitted
syphilis may be an emerging problem in the United States (5).
A major concern associated with increased rates of syphilis is
that active, early syphilis (i.e., primary and secondary stages)
enhances transmission of human immunodeficiency virus (HIV)
by 2- to 5-fold, thus promoting the spread of HIV (14, 70).
ANTIBIOTIC TREATMENT AND RESISTANCE
Effective antibiotic treatment is a key component of syphilis
control programs (4). According to the U.S. Centers for Dis-
ease Control and Prevention (CDC) 2006 guidelines, the rec-
ommended treatment for uncomplicated, early syphilis in
adults is penicillin G benzathine administered intramuscularly
(i.m.) as a single dose of 2.4 million units (MU) (4). This form
of the drug provides weeks of treponemicidal levels of penicil-
lin in the blood, though it does not efficiently cross the blood-
brain barrier (39, 48; for further details on the form and dose
of penicillin for treatment of syphilis, see reference 4.) Because
there are no proven alternatives to penicillin for treatment of
infected pregnant women, those who are penicillin allergic
should be desensitized and then treated with penicillin G ben-
zathine. Despite over 65 years of extensive clinical experience
with penicillin, the need to administer this antibiotic parenter-
ally has led to the use of second-line oral antibiotics, including
macrolides (e.g., erythromycin and azithromycin) and tetracy-
* Mailing address: 2107 McGavran Hall, Department of Epidemi-
ology, Gillings School of Global Public Health, University of North
Carolina at Chapel Hill, Chapel Hill, NC 27599-7435. Phone: (919)
966-3809. Fax: (919) 966-0584. E-mail: lstamm@e-mail.unc.edu.
Published ahead of print on 5 October 2009.
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clines (e.g., tetracycline and doxycycline), as first-line drugs for
treatment of syphilis. This use of alternative antibiotics, which
is inconsistent with current CDC guidelines, occurs more
frequently outside the United States (76). The resistance of
T. pallidum to macrolides as well as two additional classes of
antibiotics is discussed below.
Macrolide resistance. Macrolides are bacteriostatic antibi-
otics that inhibit protein synthesis by binding reversibly to 23S
rRNA of the 50S ribosomal subunit (80). Shortly after intro-
duction of erythromycin, the first macrolide, in the 1950s, re-
sistance to this antibiotic was observed in several bacterial
pathogens (61). Failure of erythromycin treatment for syphilis
was reported by South et al. (69) in 1964 and Fenton and Light
(15) in 1976 in pregnant women who delivered infants with
congenital syphilis. However, since erythromycin may not ef-
ficiently cross the placental barrier, it is unknown if these
treatment failures were actually due to erythromycin-resistant
T. pallidum (55). In 1977,
T. pallidum Street strain 14 was
isolated from a U.S. patient with active lesions of secondary
syphilis who failed long-term erythromycin therapy (74, 75).
Studies by Stamm et al. (73, 74), using an in vitro assay to
assess the effect of antibiotics on treponemal protein synthesis,
showed that Street strain 14 is resistant to high levels of eryth-
romycin and cross resistant to azithromycin, a newer macrolide
that was approved by the U.S. Food and Drug Administration
(FDA) in the early 1990s. In vivo studies with the rabbit model
of syphilis confirmed that Street strain 14 is resistant to eryth-
romycin and azithromycin (37). Interestingly, the macrolide-
resistant phenotype of Street strain 14 is highly stable, despite
multiple passages in laboratory rabbits in the absence of anti-
biotic pressure (41; L. V. Stamm, unpublished data).
Macrolide resistance is often associated with alteration of
the target site (i.e., the peptidyl transferase region in domain V
of 23S rRNA) via mutation or methylation (61, 80). For over
two decades after its isolation, the genetic basis of Street strain
14 macrolide resistance remained a mystery. However, the
complete sequence of the 1.14-Mb genome of the
T. pallidum
Nichols strain, published in 1998, provided some helpful clues
(17). Analysis of the genome sequence revealed that
T. palli-
dum lacks genetic elements (e.g., plasmids, bacteriophage, and
transposons) commonly associated with horizontal gene trans-
fer mechanisms (i.e., transformation, transduction, and conju-
gation) that are a major means for acquiring antibiotic resis-
tance (60, 61). Based on this observation, it seemed plausible
that Street strain 14 macrolide resistance originated endog-
enously via a spontaneous, low-frequency, chromosomal mu-
tation in the 23S rRNA gene that conferred a survival advan-
tage to treponemes exposed to macrolides. Consistent with this
hypothesis, Stamm and Bergen (71) demonstrated that an ad-
enine (A)-to-guanine (G) transition, at the position cognate to
A2058 in the
Escherichia coli 23S rRNA gene, is present in
both copies of the Street strain 14 23S rRNA gene (Table 1)
(7). This mutation was not present in the
T. pallidum Nichols
strain, the type strain, which was isolated in 1912 and is sen-
sitive to macrolides. Sequencing of the Street strain 14 genome
to high accuracy with oligonucleotide arrays recently con-
firmed the A-to-G transition in both 23S rRNA genes (45).
Point mutations at position A2058 have been identified in
many other macrolide-resistant bacteria, including
Brachyspira
hyodysenteriae and
Brachyspira pilosicoli, agents of swine dys-
entery and spirochetal diarrhea, respectively (Table 1) (28, 29,
56, 61, 65, 80). Additionally, Lee et al. (35) reported that an
A2058G mutation, present in one or both copies of the
T.
denticola 23S rRNA gene, conferred equivalent, high-level re-
sistance to erythromycin (Table 1). Macrolide-resistant
T. den-
ticola isolates were obtained after in vitro exposure to eryth-
romycin, suggesting that antibiotic pressure is sufficient to
select mutants with point mutations that confer high-level re-
sistance.
In hindsight, it is remarkable that the first documented re-
port of clinically relevant macrolide resistance in
T. pallidum
Street strain 14 was not greeted with greater apprehension (21,
75). Indeed, after the introduction of azithromycin, there was
considerable enthusiasm for the use of this macrolide for treat-
ment of syphilis, since unlike erythromycin, it can be adminis-
tered orally as a single 2-g dose and has a long tissue half-life
(30). The clinical efficacy of azithromycin for treatment of
syphilis was demonstrated in nonrandomized studies (30, 31,
79) and randomized controlled trials in the United States (23,
26), Africa (59), and China (2) that compared the cure rates
for azithromycin and penicillin. Azithromycin was used for
syphilis treatment in Uganda in the mid-1990s (31), in the
United States (San Francisco, CA) in 1999 and 2000 (47), and
in the United States (Los Angeles, CA) and Canada (Vancou-
ver, BC) in 2000 (9, 58). However, treatment failures were
observed in eight patients in San Francisco from 2002 to 2003
(6, 30). Molecular analysis of
T. pallidum in clinical specimens
from two of these patients revealed the presence of a 23S
rRNA gene mutation identical to the Street strain 14 A2058G
mutation (37). Retrospective analysis of clinical specimens from
Baltimore, San Francisco, Seattle, and Vancouver showed that
while the prevalence of the A2058G mutation varied among
these sites, it increased significantly over time within the sites
that were analyzed temporally (30, 37, 50). For example, the
percentage of San Francisco clinical specimens containing
T.
pallidum with the A2058G mutation increased from 4% in
2002 to 41% in 2003, 56% in 2004, and 76.5% in 2005 (30, 47).
TABLE 1. 23S rRNA gene mutations associated with spirochete
macrolide resistance
Spirochete
Mutation
position
a
Resistance(s)
b
Clinical/
induced
c
Reference
d
T. pallidum
A2058G
Ery, Azi
Clinical
71
A2059G
Spi
Clinical
44
T. denticola
A2058G
Ery
Induced
35
B. hyodysenteriae
A2058G
Ery, Cli, Tyl Induced
28
A2058T
Ery, Cli, Tyl Clinical
28
B. pilosicoli
A2058G
Tyl
Induced
56
A2058T
Ery, Cli, Tyl Clinical
29
A2059C
Ery, Cli, Tyl Clinical
29
A2059G
Ery, Cli, Tyl Clinical
29
Brachyspira spp.
A2062C
Tyl
Induced
56
a Based on the numbering of the
E. coli 23S rRNA gene nucleotide sequence.
b Based on the antibiotics tested. Azi, azithromycin; Cli, clindamycin; Ery,
erythromycin; Spi, spiramycin; Tyl, tylosin.
c Macrolide resistance present in clinical/field isolate versus experimentally
induced resistance.
d Initial description of mutation.
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The percentage of Seattle clinical specimens containing
T.
pallidum with the A2058G mutation increased from 9% in
2002 to 23% in 2003, 50% in 2004, and 56% in 2005 (41).
Although only a limited number of clinical specimens were
analyzed in these studies, the rapidly increasing frequency of
detection of the A2058G mutation is nevertheless disturbing.
Macrolide-resistant
T. pallidum with the A2058G mutation
is now present in several areas of the United States, Canada,
Europe, and China (37, 42, 43, 44, 47, 50). Although some
outbreaks are due to multiple macrolide-resistant strains that
emerged independently, there is also evidence for the limited
spread of a macrolide-resistant clone of
T. pallidum (41, 42).
The findings of Marra et al. (41) support the hypothesis that
recent macrolide use for unrelated infections (e.g., oral, skin,
respiratory, and genital infections) contributes to the increased
prevalence of macrolide-resistant
T. pallidum by providing a
selective pressure. Interestingly, macrolide-resistant
T. palli-
dum appears to be uncommon in some developing countries
(e.g., Madagascar, Tanzania, and Uganda) (30, 31, 59). How-
ever, because macrolide use is increasing in developing coun-
tries, it is likely that macrolide-resistant
T. pallidum will even-
tually emerge or will be introduced via travel and/or tourism.
Although potentially problematic in resource-limited settings,
availability of a recently developed real-time PCR assay for
detection of the A2058G mutation and, with modification, a
new mutation (A2059G; see below) would enable molecular
surveillance efforts for rapid identification of macrolide-resis-
tant
T. pallidum in populations where macrolide use has been
limited and therapy might still be effective (53). Regardless of
the setting, the use of macrolides for treatment of syphilis must
be undertaken with the utmost caution, particularly in areas
where the prevalence of macrolide resistance is unknown. All
syphilis patients who receive macrolides must be closely
monitored both clinically and serologically. Clinical speci-
mens (i.e., lesion exudate, blood, and cerebrospinal ffuid
[CSF]) from patients who fail treatment should be subjected
to molecular analysis for detection of mutations in the
T.
pallidum 23S rRNA genes. Monitoring for resistance is impor-
tant not only to identify individual treatment failures but also
to more efficiently target control programs that are based on
the prevalence and geographical distribution of resistance.
Until recently, the A2058G mutation was the only docu-
mented mutation associated with macrolide resistance in
T.
pallidum. In vitro studies by Stamm et al. (73, 74) suggested
that this mutation confers resistance to 14-member (e.g., eryth-
romycin and roxithromycin) and 15-member (e.g., azithromy-
cin) lactone ring macrolides but not to 16-member lactone ring
macrolides (e.g., spiramycin, midecamycin, and tylosin), possi-
bly due to the differential binding of these antibiotics to the
23S rRNA of Street strain 14. In 2009, Matejkova et al. (44)
reported the identification of a new mutation (i.e., A2059G) in
the 23S rRNA gene of
T. pallidum in clinical specimens from
a Czech Republic patient with secondary syphilis who did not
respond to spiramycin therapy. The A2059G mutation is asso-
ciated with resistance to 14-, 15-, and 16-member lactone ring
macrolides in a number of bacteria, including
B. pilosicoli, and
is likely responsible for spiramycin resistance in
T. pallidum
(Table 1) (29, 80). The prevalence of the A2059G or the
A2058G mutation among clinical specimens collected from
syphilis patients in the Czech Republic is
18% for either
mutation (44). There was no direct epidemiological relation-
ship among syphilis patients with the A2059G mutation, sug-
gesting that these individuals had not been infected with a
single macrolide-resistant strain. Currently, there are no re-
ports outside of the Czech Republic of infections due to
T.
pallidum harboring the A2059G mutation. However, recent
experience with the A2058G mutation suggests that emergence
of
T. pallidum with the A2059G mutation is unlikely to be
unique to a specific geographical region, particularly if this
mutation extends the spectrum of macrolide resistance to in-
clude resistance to erythromycin and azithromycin (80).
Clindamycin resistance. Clindamycin, a semisynthetic deriv-
ative of lincomycin, was introduced in the 1960s for treatment
of bacterial infections (13). Although chemically unrelated to
the macrolides, clindamycin is grouped with these antibiotics
since its binding site on 23S rRNA overlaps with those of
macrolides. Clindamycin resistance can result from modifica-
tion of the 23S rRNA target site by mutation, methylation, or
through efffux or inactivation of the antibiotic (33). In 1976,
Brause et al. (3) reported the use of a rabbit model of intra-
dermal infection to determine the relative efficacies of clinda-
mycin, erythromycin, and penicillin for treatment of infection
with the
T. pallidum Nichols strain. Single i.m. injection of two
different doses of clindamycin (15 mg/kg and 40 mg/kg) did not
significantly affect treponemal cell counts. Multiple i.m. injec-
tions of clindamycin reduced treponemal cell counts by 5- to
7-fold, whereas multiple i.m. doses of erythromycin and peni-
cillin reduced treponemal cell counts by 300-fold, suggesting
that
T. pallidum has a level of intrinsic resistance to clindamy-
cin. In vitro studies by Stamm et al. (74) with
T. pallidum
Nichols strain and Street strain 14 showed that while clinda-
mycin partially inhibited protein synthesis in both strains, the
effect on the Nichols strain was somewhat stronger. Failure of
clindamycin treatment to cure syphilis in humans was reported
by Meljanac et al. in 1999 (46) and by Woznicova in 2007 (84).
In the latter report, an infected pregnant woman treated with
clindamycin, which has been shown to cross the placenta (55),
gave birth to an infant with congenital syphilis. DNA se-
quences of
T. pallidum present in the infant’s lesions matched
those of Street strain 14. Based on limited clinical data, it
appears that clindamycin is unlikely to be effective for treat-
ment of syphilis, presumably due to the intrinsic resistance of
T. pallidum to clindamycin. However, it should be noted that
the A2058G mutation present in
T. pallidum Street strain 14 is
associated with high-level clindamycin resistance in some iso-
lates of
Helicobacter pylori,
B. hyodysenteriae, and certain other
bacterial species (Table 1) (28, 29, 81). Thus, the higher level
of clindamycin resistance observed by Stamm et al. (74) for
Street strain 14 may be due to an additive effect of the A2058G
mutation on the intrinsic level of clindamycin resistance in this
spirochete. The genetic basis of the latter is unknown.
Rifampin resistance. Rifampin binds to the -subunit of
DNA-dependent RNA polymerase (RpoB) encoded by the
rpoB gene, thus preventing RNA synthesis. All spirochetes
tested thus far, including
T. pallidum, are intrinsically resistant
to rifampin (36, 72). Several investigators have reported the
absence of a treponemicidal effect of rifampin on humans with
syphilis (27). Rifampin has been used as a selective agent for
the isolation of cultivable
Treponema from human or animal
specimens in which bacterial contaminates are present (e.g.,
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oral cavity, genital and intestinal tracts, and bovine foot and
mammary lesions, etc.). If continuous in vitro cultivation of
T.
pallidum is eventually achieved, rifampin resistance, though
not clinically relevant, would enable the use of this antibiotic
for culturing
T. pallidum from syphilis patients, thus benefiting
molecular and epidemiologic studies.
Rifampin resistance is usually due to mutations in the
rpoB
gene that lead to changes in the RpoB amino acid sequence,
resulting in the reduced binding of rifampin by RpoB. Alek-
shun et al. (1) proposed that rifampin resistance in
Borrelia
burgdorferi, and possibly other spirochetes, is due to the sub-
stitution of an asparagine (N) for a serine (S) at the RpoB
residue cognate to
E. coli S531. Lee et al. (34) observed a N531
substitution in the RpoB amino acid sequence of 22
Borrelia
strains. Stamm et al. (72) reported that the N531 substitution
is present in
T. pallidum Street strain 14 and in several other
Treponema species, including
T. denticola. Analysis of the
T.
pallidum Nichols strain genome sequence confirmed the pres-
ence of the N531 substitution in this spirochete. While not
commonly observed in rifampin-resistant bacteria, the N531
substitution is associated with high-level resistance in
Myco-
bacterium celatum, an organism that is intrinsically resistant to
rifampin (32). Thus, the N531 substitution in RpoB is likely
responsible for the intrinsic resistance of
T. pallidum, as well as
other spirochetes, to rifampin.
POTENTIAL MECHANISMS FOR DEVELOPMENT OF
RESISTANCE TO TETRACYCLINES AND PENICILLIN
In view of the emergence of clinically significant macro-
lide resistance in
T. pallidum, it is of paramount importance
to consider how this spirochete could develop resistance to
tetracycline, an alternative antibiotic, and to penicillin, the
recommended first-line antibiotic for syphilis treatment. If
resistance to these drugs emerges, awareness of potential
mechanisms would enable a more focused approach to detect-
ing mutations in
T. pallidum that are known to be associated
with tetracycline or penicillin resistance in other bacteria.
Identification of the genetic basis of resistance is a prerequisite
for developing molecular methods (e.g., real-time PCR assays
and oligonucleotide arrays) for the rapid detection and sur-
veillance of antibiotic-resistant
T. pallidum in clinical speci-
mens. Presumably, the latter information will also aid clinicians
in the choice of antibiotic for syphilis treatment.
Tetracycline resistance. Tetracyclines are bacteriostatic an-
tibiotics that inhibit protein synthesis by binding reversibly to
16S rRNA of the 30S ribosomal subunit (60). Four small-scale
studies demonstrated that doxycycline, a tetracycline derivative
with better bioavailability, with a more convenient dosing
schedule, and with fewer gastrointestinal side effects than tet-
racycline, has a success rate similar to that of penicillin for
treatment of early adult syphilis (18, 20, 52, 83). No treatment
failures were reported for three of the four studies. However,
one patient in the study by Onoda (52) did not show serological
evidence of response to treatment at 4 months after doxycy-
cline therapy. Since this patient was lost to follow-up, it is
unclear as to why the patient was nonresponsive. However,
treatment failure due to doxycycline resistance is a possibility.
Tetracycline resistance, which confers cross-resistance to doxy-
cycline, can be due to efffux systems, ribosomal protection
proteins, mutation of certain ribosomal proteins, or enzymatic
inactivation of the antibiotic (60). Additionally, single point
mutations in the 16S rRNA genes of
H. pylori and
Propionibac-
terium spp. at residues cognate to positions 965 to 967 or 1058,
respectively, in the
E. coli 16S rRNA gene confer tetracycline
or doxycycline resistance in these bacteria (64, 85). Pringle et
al. (57) hypothesized that point mutations in the 16S rRNA
genes of spirochetes, which have one or two copies of this gene,
could result in decreased susceptibility to doxycycline, as ob-
served with
B. hyodysenteriae (i.e., G1058C). Based on this
information and knowledge of
T. pallidum genetics, it appears
that point mutations in one or both copies of the 16S rRNA
gene (7) are the most plausible mechanism for development of
doxycycline resistance. Due to macrolide resistance, doxycy-
cline may become more widely used as an alternative drug for
syphilis. Since treatment requires a 14-day course of 100 mg of
doxycycline taken orally twice daily, compliance is an issue (4).
If treatment failure occurs in patients who have been compli-
ant, the 16S rRNA genes of
T. pallidum in the patients’ clinical
specimens should be high-priority targets for detection of mu-
tations associated with doxycycline resistance.
Penicillin resistance. Penicillins and cephalosporins are
bactericidal -lactam antibiotics that interfere with the ac-
tion of transpeptidase enzymes (i.e., penicillin binding pro-
teins [PBPs]) that carry out the cross-linking of the cell wall of
actively growing bacteria (86). Penicillin is the only antibiotic
currently recommended by the CDC for treatment of all stages
of syphilis (4). The form of penicillin (i.e., benzathine, aqueous
procaine, or aqueous crystalline), dose, and duration of treat-
ment are dependent upon the stage and clinical manifestations
of syphilis (4). Although reports of penicillin treatment fail-
ures, particularly for patients with HIV infection, are not un-
common, to date there is no documented penicillin resistance
in
T. pallidum (14, 16, 62, 75). Most serologically defined treat-
ment failures are thought to be due to reinfection or to patient-
to-patient variation in the decline of nontreponemal test titers
after treatment (i.e., 4-fold decrease), rather than to relapse,
which is rare. However, it can be difficult to distinguish be-
tween reinfection and relapse since molecular methods for
epidemiologic typing of
T. pallidum may lack discriminatory
power (51).
It is important to note that
T. pallidum can invade the cen-
tral nervous system (CNS) early in infection and that some
strains may have a greater propensity for neuroinvasion (38,
39). Rolfs et al. (62) demonstrated that
T. pallidum was present
before therapy in at least one-fourth of patients with early
syphilis, regardless of their HIV infection status. This finding is
similar to those of other studies (38). Nonetheless, most pa-
tients with early, uncomplicated syphilis respond well to treat-
ment with penicillin G benzathine i.m., although this therapy
does not result in treponemicidal levels of the antibiotic in
cerebrospinal ffuid (CSF) (48, 62). However, because
T. palli-
dum has been isolated from CSF following i.m. administration
of 2.4 to 10.8 MU of penicillin G benzathine, the CDC recom-
mends that patients with known CNS involvement be treated
with 18 to 24 MU/day of aqueous crystalline penicillin G ad-
ministered at 3 to 4 MU intravenously (i.v.) every 4 h or by
continuous infusion for 10 to 14 days (4, 38, 78).
Myint et al. (51) proposed that treponemes that survive in
the CNS are responsible for the clinical relapse that occurs
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when therapeutic levels of antibiotic wane in body ffuids and
tissues. This appears to be the case, since a more intensive
course of high-dose aqueous penicillin given i.v. can cure re-
lapse (16, 19, 78). Such information provides a basis for spec-
ulating how
T. pallidum might develop penicillin resistance.
Presumably, treponemes that invade the CNS of patients with
early syphilis would encounter subtherapeutic levels of antibi-
otic that act as a selective pressure for mutants with low-level
penicillin resistance when the patients are treated with i.m.
penicillin G benzathine. This resistance would not be readily
detected since it would be easily overcome when the patients
are retreated with higher doses of i.v. penicillin upon treatment
failure. Continued passage of treponemes with low-level pen-
icillin resistance to new hosts, prior to treatment of relapsing
infection, and repeated exposure of these treponemes to in-
creasing levels of penicillin might eventually select mutants
with a clinically significant level of penicillin resistance.
Unlike macrolide or tetracycline resistance, for which a sin-
gle point mutation can confer stable, high-level resistance,
penicillin resistance often involves the acquisition of new ge-
netic information via horizontal gene transfer (86). The latter
is unlikely to occur in
T. pallidum due to the lack of plasmids,
bacteriophage, or transposons (17). Strategies commonly used
by bacteria to resist the effect of penicillin include produc-
tion of -lactamases that inactivate penicillin, acquisition of
novel PBPs with low affinity for penicillin, alterations of
PBPs through homologous recombination, changes in the
structure and number of porins resulting in decreased perme-
ability to penicillin, efffux pumps that decrease the intracellular
concentration of penicillin, or various combinations of these
strategies (86). Analysis of the
T. pallidum genome sequence
predicts three putative PBPs but no typical -lactamases (17).
However, Cha et al. (8) showed that Tp47, an abundant, mem-
brane-bound lipoprotein that was initially identified and char-
acterized by Norgard and colleagues (12, 82), binds penicillin
and has high -lactamase activity that is subject to strong
product inhibition. Furthermore, Cha et al. (8) hypothesized
that if a mutant variant of Tp47 emerges that overcomes the
product inhibition of its -lactamase activity, this would confer
novel, bona fide resistance to penicillin. Fortunately, the Tp47
-lactamase does not appear to be active against cephalospo-
rins, leaving available the option of using these drugs for syph-
ilis treatment should a Tp47 mutant emerge (25, 40, 49, 68, 87).
It is important to note that the absence of documented peni-
cillin resistance in
T. pallidum after more than 6 decades of its
use for treatment of syphilis suggests that the development of
penicillin resistance will likely require a multistep mutational
process whose probability of occurrence is much rarer than
those of the single point mutations that are responsible for
macrolide resistance. Although this may have forestalled the
emergence of penicillin-resistant
T. pallidum, it provides no
guarantee that such resistance will not emerge.
CONCLUSION
Syphilis has many of the hallmarks of a disease that should
be susceptible to elimination and perhaps ultimately to eradi-
cation, since (i) infected humans are the only natural reservoir;
(ii) diagnostic methods, though not perfect, are relatively
cheap and widely available; and (iii) early infection is usually
treatable with a single dose of penicillin G benzathine (63).
However, the recent, widespread syphilis epidemic in China,
where syphilis had been virtually eliminated, and the resur-
gence of syphilis in many Western countries emphasize the
urgent need for renewed vigilance (10, 11). In the absence of a
vaccine, syphilis control is largely dependent upon identifica-
tion of infected individuals and treatment of these individuals
and their contacts with antibiotics. Although penicillin treat-
ment is still effective, clinically significant resistance to macro-
lides has emerged in
T. pallidum and is prevalent in several
countries, including China. Macrolide resistance has compro-
mised the effectiveness of azithromycin for syphilis treatment,
such that patients who receive this antibiotic must be closely
monitored for treatment failure. This development clearly war-
rants placing tighter restrictions on the use of azithromycin, a
drug that was once deemed the most promising alternative to
penicillin for syphilis treatment, particularly in settings where
injections are problematic or for nonpregnant, penicillin-aller-
gic patients (21, 30).
To date, there is no documented resistance of
T. pallidum to
the tetracyclines, which are the other main class of alternative
antibiotic used for treatment of early syphilis in adults (4).
Decreased use of macrolides could result in increased use of
tetracyclines. Inadequate tetracycline therapy due to poor pa-
tient compliance could provide selective pressure for resistant
mutants. If tetracycline-resistant
T. pallidum were to emerge
and spread, it would undoubtedly have an effect on syphilis
control, particularly if the prevalence and geographical distri-
bution of macrolide-resistant
T. pallidum increase. This prob-
lem would be exacerbated if macrolide-resistant
T. pallidum
were also to develop resistance to the tetracyclines, since there
are few alternatives to penicillin, except for ceftriaxone, which
has not been extensively tested in clinical settings (25, 40, 49,
68, 87).
The global persistence of syphilis and the emergence and
rapid spread of macrolide-resistant
T. pallidum are reminders
that there is no room for complacency. Addressing these issues
requires renewed effort on several fronts, including the follow-
ing: (i) developing new, single-dose oral antibiotics to ensure
patient compliance; (ii) improving availability and reliability of
rapid diagnostic tests to identify those who need treatment;
(iii) supporting molecular surveillance to enable early detec-
tion of antibiotic-resistant
T. pallidum; and (iv) pursuing de-
velopment of a vaccine to prevent infection and thereby limit
the need for antibiotics. We have been warned that the time is
at hand when antibiotics will no longer be useful for the treat-
ment of many infectious diseases due to the emergence and
spread of multidrug-resistant bacteria. We must not allow this
to come to pass for syphilis, the disease once designated by
U.S. Surgeon General Thomas Parran as the “shadow on the
land” (14).
ACKNOWLEDGMENTS
I thank Robert Nicholas and Lee Tuckwiller for helpful conversa-
tions and for critically reading the manuscript.
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