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Journal of Clinical Microbiology, February 2000, p. 521-525, Vol. 38, No. 2
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Antimicrobial Resistance of Neisseria
gonorrhoeae and High Prevalence of Ciprofloxacin-Resistant
Isolates in Japan, 1993 to 1998
Masatoshi
Tanaka,1,*
Hiroshi
Nakayama,2
Masashi
Haraoka,1
Takeshi
Saika,3
Intetsu
Kobayashi,3 and
Seiji
Naito1
Department of Urology, Faculty of Medicine,
Kyushu University,1 and Nakayama
Urologic Clinic,2 Fukuoka, and
Chemotherapy Division, Mitsubishi-Kagaku BCL,
Tokyo,3 Japan
Received 19 July 1999/Returned for modification 28 September
1999/Accepted 28 October 1999
 |
ABSTRACT |
To assess the antimicrobial resistance of Neisseria
gonorrhoeae isolated from 1993 through 1998 in Japan,
susceptibility testing was conducted on 502 isolates. Selected isolates
were characterized by auxotype and analysis for mutations within the
quinolone resistance-determining region (QRDR) in the gyrA
and parC genes, which confer fluoroquinolone resistance on
the organism. Plasmid-mediated penicillin resistance (penicillinase-producing N. gonorrhoeae) decreased
significantly from 1993-1994 (7.9%) to 1997-1998 (2.0%).
Chromosomally mediated penicillin resistance decreased from 1993-1994
(12.6%) to 1995-1996 (1.9%) and then increased in 1997-1998
(10.7%). Chromosomally mediated tetracycline resistance decreased from
1993-1994 (3.3%) to 1997-1998 (2.0%), and no plasmid-mediated
high-level tetracycline resistance was found. Isolates with
ciprofloxacin resistance (MIC
1 µg/ml) increased
significantly from 1993-1994 (6.6%) to 1997-1998 (24.4%). The
proline-requiring isolates were less susceptible to ciprofloxacin than
the prototrophic or arginine-requiring isolates. Ciprofloxacin-resistant isolates contained three or four amino acid
substitutions within the QRDR in the GyrA and ParC proteins.
 |
INTRODUCTION |
Gonococcal resistance to
antimicrobial agents is an increasing problem in the treatment of
gonorrhea. A high prevalence of plasmid-mediated high-level or
chromosomally mediated low-level resistance to penicillin or
tetracycline has been recognized in southeast Asia and in African
countries (5, 13, 19, 20, 21). Fluoroquinolones, such as
ciprofloxacin or ofloxacin, are highly effective as oral single-dose
treatments for uncomplicated gonococcal infections caused by most
gonococcal strains, including those with previously documented types of
resistance (2, 7). Recently, fluoroquinolone regimens have
been recommended for the treatment of gonorrhea (4, 12).
However, the emergence of gonococcal isolates with reduced
susceptibility or resistance to fluoroquinolones is a significant
concern in several countries, including Japan (12, 14, 17, 25,
29). The failure of treatment of gonococcal infections with
fluoroquinolones has also been reported sporadically (6, 30, 32,
33, 35). The treatment of gonorrhea has now become more
complicated due to resistance to a variety of antimicrobial agents.
This study was performed to characterize the current antimicrobial
susceptibility patterns of Neisseria gonorrhoeae, and in particular, to examine the possibility of an increasing prevalence of
high-level fluoroquinolone resistance in Japan. In addition, we
examined the correlation between auxotype and fluoroquinolone susceptibility and also the relationship between alteration patterns in
DNA gyrase subunit A (GyrA) and topoisomerase IV
parC-encoded subunit (ParC) proteins, which confer quinolone
resistance on the organisms, and the ciprofloxacin resistance level.
 |
MATERIALS AND METHODS |
N. gonorrhoeae strains.
From January 1993 through December 1998, a total of 502 isolates of N. gonorrhoeae were collected from consecutive male patients with
urethritis attending a sexually transmitted diseases clinic in Fukuoka
City. However, posttreatment isolates or repeat isolates from the same
patients were excluded in this study. Specimens from each patient were
inoculated directly onto Thayer-Martin selective agar (Becton
Dickinson, Cockeysville, Md.), transported to the Mitsubishi Kagaku
Laboratory by a commercially available transport system (Bio-Bag
environmental chamber type C; Becton Dickinson), and incubated for 24 to 48 h at 35°C in a 5% CO2 atmosphere. N. gonorrhoeae was identified as a gram-negative diplococcus and by
oxidase reaction and sugar utilization patterns. The isolates were
suspended in cryoprotective medium as described by Obara et al.
(24) and stored at
80°C until they were tested.
Antimicrobial susceptibility testing.
MICs for all isolates
were determined by an agar dilution technique with a GC agar base
(Becton Dickinson) containing 1% IsoVitaleX (Becton Dickinson) and
twofold dilutions of antibiotic (22). Plates were inoculated
with 5 µl of 106 CFU of each isolate/ml with a multipoint
inoculator (Microplanter; Sakuma Seisakusho, Tokyo, Japan). Five World
Health Organization N. gonorrhoeae reference strains (A, B,
C, D, and E) were included as quality control strains. The plates were
incubated for 24 h at 35°C in a 5% CO2 atmosphere.
MICs were defined as the lowest antibiotic concentrations that
inhibited bacterial growth.
-Lactamase production was assayed by the
chromogenic cephalosporin test. The antimicrobial agents tested were
penicillin G (Sigma Chemical Co., St. Louis, Mo.), tetracycline
(Lederle Japan, Tokyo, Japan), ceftriaxone (Nippon Roche, Tokyo,
Japan), cefixime (Fujisawa Pharmaceutical, Osaka, Japan), ciprofloxacin
(Bayer Yakuhin, Osaka, Japan), azithromycin (Pfizer Pharmaceuticals,
Tokyo, Japan), and spectinomycin (Sigma Chemical). All of the
antibiotics were obtained as powders of stated potency from their
manufacturers. The ranges of concentrations of the antibiotics tested
were as follows: penicillin G, 0.004 to 64 µg/ml; tetracycline, 0.004 to 64 µg/ml; ceftriaxone, 0.001 to 4 µg/ml; cefixime, 0.001 to 4 µg/ml; ciprofloxacin, 0.001 to 64 µg/ml; and spectinomycin, 1 to
128 µg/ml. The isolates were sequentially grouped into mutually
exclusive categories according to National Committee for Clinical
Laboratory Standards guidelines as follows (22):
penicillinase-producing N. gonorrhoeae (PPNG) (
-lactamase
positive; tetracycline MIC, <16 µg/ml); plasmid-mediated tetracycline-resistant N. gonorrhoeae (TRNG) (
-lactamase
negative; tetracycline MIC,
16 µg/ml); PPNG-TRNG (
-lactamase
positive; tetracycline MIC,
16 µg/ml); chromosomally mediated
penicillin-resistant N. gonorrhoeae (non-PPNG and non-TRNG;
penicillin MIC,
2 µg/ml; tetracycline MIC, <2 µg/ml);
chromosomally mediated tetracycline-resistant N. gonorrhoeae
(non-PPNG and non-TRNG; penicillin MIC, <2 µg/ml; tetracycline MIC,
2 µg/ml); chromosomally mediated penicillin- and
tetracycline-resistant N. gonorrhoeae (CMRNG) (non-PPNG and non-TRNG; penicillin MIC,
2 µg/ml; tetracycline MIC,
2 µg/ml). Proposed criteria were used to interpret the ciprofloxacin MIC (18); isolates for which the MICs were
1 µg/ml were
classified as resistant to ciprofloxacin, and isolates for which the
MICs were 0.125 to 0.5 µg/ml were interpreted as exhibiting reduced susceptibility to ciprofloxacin. Furthermore, isolates for which the
ceftriaxone or cefixime MICs were
0.5 µg/ml were categorized as
having decreased susceptibility to the respective agent
(22).
Auxotyping.
Auxotyping of the selected gonococcal isolates
was undertaken as described by Catlin (3). Isolates were
tested on chemically defined media for their nutritional requirements
for proline, arginine, hypoxanthine, uracil, methionine, and histidine.
Strains with no requirements for the substances used were designated
prototrophic (Proto).
Molecular study.
PCR and direct DNA sequencing were
performed to identify mutations in the gyrA and
parC genes of the gonococcal isolates. Chromosomal DNA was
extracted by standard methods and was subjected to PCR. The
oligonucleotide primers for the PCR amplification were as follows: for
the gyrA gene, forward primer,
5'-160CGGCGCGTACTGTACGCGATGCA182-3',
and reverse primer,
5'-438AATGTCTGCCAGCATTTCATGTGAGA413-3';
for the parC gene, forward primer,
5'-166ATGCGCGATATGGGTTTGAC185-3',
and reverse primer,
5'-420GGACAACAGCAATTCCGCAA401-3'.
These primers were produced with a DNA synthesizer, according to
sequences previously reported by Belland et al. (1). The gyrA gene sequence was determined from nucleotides 160 to
438, which correspond to amino acids 54 to 146 of the GyrA protein. This includes the quinolone resistance-determining region (QRDR) (amino
acids 55 to 110 of the gonococcal GyrA protein) (1). The
parC gene sequence was also determined from nucleotides 166 to 420, which correspond to amino acids 56 to 140 of the gonococcal ParC protein. This includes the QRDR of the ParC protein (amino acids
66 to 119 of the gonococcal ParC protein) (1).
The PCR amplification was performed in 25 µl of a reaction mixture
which contained 2.5 µl of 10× Taq polymerase buffer (500 mM KCl, 100 mM Tris-HCl [pH 8.3], 15 mM MgCl2, 0.1%
gelatin), 0.25 µl of each of the two primers (25 pmol/µl), 0.5 µl
of each of the four deoxynucleoside triphosphates (10 mM), 0.2 µl of
Taq DNA polymerase (5 U/µl) (Takara Biomedicals, Tokyo,
Japan), 2.5 µl of Triton X-100 (2 mg/ml), and 1.0 µl of template
DNA (100 ng/µl). Thirty-five cycles were performed for each reaction.
Each cycle consisted of 30 s at 93°C, 1 min at 52°C, and 1 min
at 72°C. The PCR amplification products were directly sequenced with
a Taq DyeDeoxy terminator cycle-sequencing kit and a Model
373A autosequencer (Applied Biosystems, Foster City, Calif.).
Statistical analysis.
The data were analyzed with StatView
J-4.5 software (Abacus Concepts, Berkeley, Calif.). Median MIC values
were compared by the Mann-Whitney U test or by the
Kruskal-Wallis test. Proportions were compared by the chi-square test.
Statistical significance for all P values was set at 0.05.
 |
RESULTS |
Ciprofloxacin resistance.
The proportion of isolates resistant
to ciprofloxacin (MIC
1 µg/ml) increased remarkably from
6.6% in 1993-1994 to 24.4% in 1997-1998 (Fig.
1). This difference was statistically
significant (P < 0.0001). However, the proportions of
isolates resistant to ciprofloxacin plus isolates with reduced
susceptibility to ciprofloxacin (MIC, 0.125 to 0.5 µg/ml) were
similar for all three study periods (Fig. 1). The ciprofloxacin MICs at
which 90% of the isolates tested were inhibited (MIC90s)
for the isolates in 1995-1996 (1 µg/ml) and in 1997-1998 (8 µg/ml) were 2- and 16-fold, respectively, higher than that for the
isolates in 1993-1994 (0.5 µg/ml) (Table 1). The median MICs of penicillin G
and tetracycline were significantly higher for the isolates with
resistance or reduced susceptibility to ciprofloxacin than for
ciprofloxacin-susceptible isolates (median MIC of penicillin G, 1 versus 0.125 µg/ml; median MIC of tetracycline, 0.5 versus 0.125 µg/ml; P < 0.0001 for both comparisons). The median
MICs of ceftriaxone and cefixime were also significantly higher for the
isolates with resistance or reduced susceptibility to ciprofloxacin
than for ciprofloxacin-susceptible isolates (median MIC of ceftriaxone,
0.063 versus 0.004 µg/ml; median MIC of cefixime, 0.063 versus 0.008 µg/ml; P < 0.0001 for both comparisons). However, there were no significant differences in susceptibility to
spectinomycin and azithromycin between isolates with resistance or
reduced susceptibility to ciprofloxacin and ciprofloxacin-susceptible
isolates (median MIC of spectinomycin, 8 versus 8 µg/ml
[P = 0.7141]; median MIC value of azithromycin, 0.125 versus 0.125 µg/ml [P = 0.2474]).

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|
FIG. 1.
Increase in ciprofloxacin-resistant N. gonorrhoeae. , resistance (MIC, 1 µg/ml); , reduced
susceptibility (MIC, 0.125 to 0.5 µg/ml).
|
|
We then analyzed the relationship between auxotype and ciprofloxacin
susceptibility in 116
N. gonorrhoeae isolates collected
consecutively during the first three quarters of 1995-1996 (Table
2). Three predominant auxotypes were
Proto (39.7%), proline requiring
(Pro) (29.3%), and arginine
requiring (Arg) (25.9%). The MIC at
which 50% of the isolates tested
were inhibited (MIC
50) and MIC
90 of
ciprofloxacin for the Pro isolates were higher than those for
the Proto
or Arg isolates.
We also investigated the genetic alterations within the QRDR in the
gyrA and
parC genes in the isolates susceptible
or resistant
to ciprofloxacin (Table
3).
Interestingly, all the isolates resistant
to ciprofloxacin contained
three or four amino acid substitutions
in the GyrA and ParC proteins,
while the isolates with decreased
susceptibility to ciprofloxacin
contained one to three amino acid
substitutions in GyrA alone or in
GyrA and ParC. Moreover, of
the ciprofloxacin-susceptible isolates
tested (MIC, 0.004 to 0.063
µg/ml), 41.3% (26 of 63) had a single
alteration in GyrA alone.
The frequently identified substitutions were
a serine-to-phenylalanine
substitution at position 91 (serine-91 in
N. gonorrhoeae GyrA
corresponds to Ser-83 in
Escherichia coli [
1]) and an aspartic
acid-to-asparagine substitution at position 95 in GyrA, while
they
were a serine-to-proline substitution at position 88, an
aspartic
acid-to-asparagine substitution at position 86, and a
glutamic
acid-to-glycine or -lysine substitution at position 91
in ParC.
Susceptibilities to penicillin and tetracycline.
The
proportion of isolates with any type of resistance to penicillin or
tetracycline decreased from 23.2% in 1993-1994 to 6.5% in 1994-1995
and then increased to 12.7% in 1997-1998 (Table 4). The prevalence of PPNG strains
significantly decreased from 7.9% in 1993-1994 to 2.0% in 1997-1998
(P = 0.0326). However, the percentage of chromosomally
mediated resistance to penicillin alone decreased from 11.9% in
1993-1994 to 1.9% in 1994-1995 and then increased to 9.6% in
1997-1998. The MIC50 and MIC90 of penicillin for the non-PPNG strains in 1997-1998 were equal to those for the
non-PPNG strains in 1993-1994 (Table 1). No TRNG or PPNG-TRNG isolates
were detected during the study period. The percentage of chromosomally
mediated resistance to tetracycline alone decreased from 2.6% in
1993-1994 to 0% in 1994-1995 and 1997-1998. Only three strains of
CMRNG were identified during the study period. The tetracycline
MIC50 and MIC90 for the isolates in 1997-1998 were only twofold lower and twofold higher, respectively, than those
for the isolates in 1993-1994.
Susceptibilities to ceftriaxone, cefixime, azithromycin, and
spectinomycin.
There were no significant changes in the gonococcal
susceptibilities to ceftriaxone, cefixime, azithromycin, or
spectinomycin during this period. In general, the MIC50s
and MIC90s of these agents for the 1997-1998 isolates were
equal, only twofold higher or twofold lower than those for 1993-1994
(Table 1). Only three isolates in 1997-1998 showed reduced
susceptibility to cefixime (MIC, 0.5 µg/ml).
 |
DISCUSSION |
This study shows that the incidence of PPNG strains significantly
decreased from 1993-1994 (7.9%) to 1997-1998 (2.0%) and the
incidence of chromosomally or plasmid-mediated tetracycline resistance was very low during the study period. However, the percentage of ciprofloxacin-resistant isolates increased
significantly from 1993-1994 (6.6%) to 1997-1998 (24.4%). Frequent
usage of fluoroquinolones against gonorrhea may lead to the rapid
development of resistance to these agents. We have recently undertaken
a survey of the usage of antimicrobial agents against gonococcal
infections by mailing a questionnaire to sexually transmitted disease
clinics in Fukuoka city. Fluoroquinolones were most frequently used
(46%) as first-line treatment for gonorrhea, followed by penicillins with or without a
-lactamase inhibitor (28%), cephems (16%), and
others (10%). Of various fluoroquinolones available in Japan, a 7-day
course of levofloxacin (200 mg twice a day or 100 mg three times a day)
was most frequently used in the treatment of gonococcal infections (M. Tanaka, unpublished data), because this regimen is effective against
urethritis caused not only by N. gonorrhoeae alone but also
by both N. gonorrhoeae and Chlamydia trachomatis (15). The increase in fluoroquinolone resistance has also
been substantial in some countries: in Hong Kong; it rose from 7.7% in
1995 to 24% in 1996; in Singapore, it rose from 0.3% in 1993 to 3.5%
in 1996; and in Australia, it rose from 0.1% in 1992 to 2.6% in 1996 (12). However, in the United States, where fluoroquinolone regimens are recommended as first-line therapy for gonorrhea, quinolone
resistance is still uncommon, except for certain areas. It has been
reported that in 1994 quinolone resistance in N. gonorrhoeae isolates was only 0.04% (2 of 4,996) in the United States
(10). Standard usage of fluoroquinolones in Japan has
preceded the use of many of these agents in the United States by many
years. Thus, findings in Japan may well indicate that the increase in
fluoroquinolone-resistant N. gonorrhoeae isolates will also
become a significant problem in the United States in the future.
Auxotyping involves characterization of nutritional requirements for
the growth of N. gonorrhoeae isolates. A variety of
gonococcal auxotypes with different geographic distributions have been
reported (26). In our study, the three predominant auxotypes
were Proto (39.7%), Pro (29.3%), and Arg (25.9%) in 1995-1996. Our
results showed an association between the auxotype and ciprofloxacin
susceptibility. The median MIC of ciprofloxacin for the Pro isolates
was significantly higher than that for the Proto or Arg isolates.
Moreover, there was a correlation between the increase in the
proportion of Pro isolates and the increase in the percentage of
ciprofloxacin resistance. The incidence of the Pro isolates increased
from 4.4% in 1992 (M. Tanaka, unpublished data) to 29.3% in
1995-1996, while ciprofloxacin resistance also increased from 0% in
1992 (28) to 15.6% in 1995-1996. To investigate whether
these correlations are due to dissemination of the same clone requiring
proline in Japan, we will examine gonococcal serovar or pulsed-field
gel electrophoresis patterns, which are also useful epidemiological
markers for monitoring gonococcal epidemics. Interestingly, in the
United States, all isolates with decreased susceptibility to
ciprofloxacin were non-PPNG and Pro isolates (16). Some of
the isolates with decreased susceptibility to ciprofloxacin in the
United States may have been imported from Japan. Pro isolates of
non-PPNG strains are generally less susceptible to antibiotics than
non-Pro isolates (23, 36). Therefore, Pro gonococcal
isolates may more easily acquire chromosomally mediated resistance not
only to
-lactams but also to structurally unrelated fluoroquinolones
than non-Pro isolates.
Alterations in serine-91 and aspartic acid-95 in GyrA, corresponding to
serine-83 and aspartic acid-87 in the E. coli GyrA (1), respectively, were most frequently found among the
clinical isolates with resistance or reduced susceptibility to
ciprofloxacin. Changes in ParC were clustered within the region between
aspartic acid-86 and alanine-92, which corresponds to the region from
aspartic acid-79 to alanine-85 in the E. coli ParC
(1). These amino acid substitutions were also identified in
our preliminary results (31) and other investigations
(1, 8). Surprisingly, 41.3% of the
ciprofloxacin-susceptible isolates had various single alterations in
GyrA alone. There was a unique isolate containing a serine-to-tyrosine substitution at position 91 in GyrA and an arginine-to-histidine substitution at position 116 in ParC. Recently, gonococcal isolates with a substitution at position 116 in ParC were also detected in the
United States (34).
The MICs of structurally unrelated penicillin, tetracycline, and
cephems for the isolates with reduced susceptibility to ciprofloxacin were higher than those for isolates susceptible to ciprofloxacin. The
median MICs of penicillin G, tetracycline, ceftriaxone, and cefixime
for the isolates with resistance or reduced susceptibility to
ciprofloxacin were 4 to 16 times higher than those for the isolates
susceptible to ciprofloxacin. In this investigation, all the isolates
resistant to ciprofloxacin contained three or four amino acid
substitutions within the QRDR in the GyrA and ParC proteins, and the
isolates with decreased susceptibility to ciprofloxacin contained one
to three amino acid substitutions in GyrA alone or in GyrA and ParC.
These results suggest that the isolates showing resistance or decreased
susceptibility to ciprofloxacin, which have chromosomal mutations in
gyrA with or without parC genes, may be combined
with well-known chromosomal mutations at other loci, such as
penA, penB, and mtr (9).
Isolates containing the gyrA and parC mutations,
combined with chromosomal mutations at other loci, are likely to
acquire resistance not only to fluoroquinolones but also to
penicillins, cephalosporins, and tetracycline. A recent study showed
that mutations in the mtrR coding region or a
single-base-pair deletion in a 13-bp inverted repeat in its promoter
enhances mtrCDE gene expression, leading to elevated levels
of multidrug resistance in gonococci (multidrug efflux pump)
(27). Other studies indicated that a mutation in penB confers resistance to structurally unrelated
-lactams and tetracycline and reduced susceptibility to quinolones
on N. gonorrhoeae (reduced porin permeability)
(11).
In Japan, where a high prevalence of isolates with resistance or
reduced susceptibility to ciprofloxacin has been shown, ceftriaxone, cefixime, or spectinomycin is recommended as the first-line treatment for gonococcal infections. Fluoroquinolones should be avoided, and the
surveillance of antimicrobial resistance of N. gonorrhoeae, and in particular, the evolution of high-level fluoroquinolone resistance, should be continued in Japan.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Urology, Faculty of Medicine, Kyushu University, 3-1-1 Maidashi,
Higashi-ku, Fukuoka 812-8582, Japan. Phone: 81-92-642-5603. Fax:
81-92-642-5618. E-mail:
masatosh{at}uro.med.kyushu-u.ac.jp.
 |
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Journal of Clinical Microbiology, February 2000, p. 521-525, Vol. 38, No. 2
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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