Molecular epidemiology of multiresistant Streptococcus pneumoniae with both erm(B)- and mef(A)-mediated macrolide resistance

Of a total of 1043 macrolide-resistant Streptococcus pneumoniae isolates collected from 24 countries as part of PROTEKT 1999-2000, 71 isolates tested positive for both the mef(A) and erm(B) genes. Of 69 isolates subjected to further molecular investigations, all were resistant to tetracycline, 63 (91.3%) were resistant to penicillin, and 57 (82.6%) were resistant to trimethoprim-sulfamethoxazole. One isolate was also fluoroquinolone resistant, and another was resistant to quinupristin-dalfopristin. The ketolide telithromycin retained activity against all of the isolates. Of the 69 of these 71 isolates viable for further testing, 46 were from South Korea, 13 were from the United States, 8 came from Japan, and 1 each came from Mexico and Hungary. One major clonal complex (59 [85.5%] of 69 isolates) was identified by serotyping (with 85.5% of the isolates being 19A or 19F), pulsed-field gel electrophoresis, and multilocus sequence typing. The remaining isolates were less clonal in nature. Representative isolates were shown to carry the mobile genetic elements Tn1545 and mega, were negative for Tn1207.1, had tetracycline resistance mediated by tet(M), and contained the mef(E) variant of mef(A). All isolates were positive for mel, a homologue of the msr(A) efflux gene. These clones are obviously very efficient at global dissemination, and hence it will be very important to monitor their progress through continued surveillance. Telithromycin demonstrated high levels of activity (MIC for 90% of the strains tested, 0.5 micro g/ml; MIC range, 0.06 to 1 micro g/ml) against all isolates.

Activity of gemifloxacin against Streptococcus pneumoniae and Haemophilus influenzae

This review focuses on the activity of gemifloxacin, a new respiratory fluoroquinolone, against the two most important bacterial pathogens associated with lower respiratory tract infections, namely Streptococcus pneumoniae and Haemophilus influenzae.

Worldwide incidence, molecular epidemiology and mutations implicated in fluoroquinolone-resistant Streptococcus pneumoniae: data from the global PROTEKT surveillance programme

OBJECTIVES

To analyse the mutations and epidemiology associated with fluoroquinolone-resistant pneumococci collected as part of the PROTEKT global surveillance programme during 1999-2000.

METHODS

Sixty-nine centres in 25 countries submitted a total of 3362 Streptococcus pneumoniae isolates, for which the MICs of antimicrobial agents were determined using NCCLS methodology.

RESULTS

Levofloxacin resistance was low overall (1% worldwide), with higher rates in: Hong Kong (14.3%), South Korea (2.9%), USA (1.8%), Mexico (1.5%), Canada (1.4%) and Japan (1.3%). Levofloxacin resistance was very low or absent in European countries, and absent in Australia. Worldwide, there was a total of 35 levofloxacin-resistant isolates, of which 22 (63%) were resistant and 10 (29%) were intermediate to moxifloxacin. All levofloxacin-resistant isolates were susceptible to telithromycin (< or =0.5 mg/L), linezolid (< or =2 mg/L) and quinupristin/dalfopristin (< or =1 mg/L). One or more mutations in the topoisomerase genes were identified in all levofloxacin-resistant isolates; most of these isolates (33/35) had a mutation in one of the DNA gyrase encoding genes (gyrA, gyrB) and one of the topoisomerase IV encoding genes (parC, parE). Eighteen (51%) isolates carried the same combination of amino acid substitutions: Ser-81-->Phe in GyrA and Ser-79-->Phe in ParC. Isolates displaying a levofloxacin MIC of 2-4 mg/L generally had no mutation or one mutation in either a DNA gyrase or a topoisomerase IV gene, although most mutations were in parC.

CONCLUSIONS

Most levofloxacin-resistant isolates possess two mutations (one in DNA gyrase and one in topoisomerase IV). Although multilocus sequence typing demonstrated that most of these isolates were unrelated, 12 (34%) were the Spain23F-1 clone: 10 from Hong Kong and one each from Saskatchewan, Canada and Sao Paulo, Brazil.

Quinupristin-dalfopristin resistance in Streptococcus pneumoniae: novel L22 ribosomal protein mutation in two clinical isolates from the SENTRY antimicrobial surveillance program

Resistance to quinupristin-dalfopristin (Q/D) among gram-positive cocci has been very uncommon. Two clinical isolates among 8,837 (0.02%) Streptococcus pneumoniae isolates were discovered in 2001 to 2002 with Q/D MICs of 4 micro g/ml. Each had a 5-amino-acid tandem duplication (RTAHI) in the L22 ribosomal protein gene (rplV) preventing synergistic ribosomal binding of the streptogramin combination. Similar gene duplication has been reported in Q/D-resistant Staphylococcus aureus.

Macrolide resistance by ribosomal mutation in clinical isolates of Streptococcus pneumoniae from the PROTEKT 1999-2000 study

Sixteen (1.5%) of the 1,043 clinical macrolide-resistant Streptococcus pneumoniae isolates collected and analyzed in the 1999-2000 PROTEKT (Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin) study have resistance mechanisms other than rRNA methylation or efflux. We have determined the macrolide resistance mechanisms in all 16 isolates by sequencing the L4 and L22 riboprotein genes, plus relevant segments of the four genes for 23S rRNA, and the expression of mutant rRNAs was analyzed by primer extension. Isolates from Canada (n = 4), Japan (n = 3), and Australia (n = 1) were found to have an A2059G mutation in all four 23S rRNA alleles. The Japanese isolates additionally had a G95D mutation in riboprotein L22; all of these originated from the same collection center and were clonal. Three of the Canadian isolates were also clonal; the rest were not genetically related. Four German isolates had A2059G in one, two, and three 23S rRNA alleles and A2058G in two 23S rRNA alleles, respectively. An isolate from the United States had C2611G in three 23S rRNA alleles, one isolate from Poland had A2058G in three 23S rRNA alleles, one isolate from Turkey had A2058G in four 23S rRNA alleles, and one isolate from Canada had A2059G in two 23S rRNA alleles. Erythromycin and clindamycin resistance gradually increased with the number of A2059G alleles, whereas going from one to two mutant alleles caused sharp rises in the azithromycin, roxithromycin, and rokitamycin MICs. Comparisons of mutation dosage with rRNA expression indicates that not all alleles are equally expressed. Despite their high levels of macrolide resistance, all 16 isolates remained susceptible to the ketolide telithromycin (MICs, 0.015 to 0.25 microg/ml).

Molecular characterization and antimicrobial susceptibility of fluoroquinolone-resistant or -susceptible Streptococcus pneumoniae from Hong Kong

Fluoroquinolone resistance in Streptococcus pneumoniae isolated from Hong Kong as part of Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin 1999/2000 was found to be due to the spread of the Spain(23F)-1 clone (mainly a Spain(23F)-1-14 variant). All the isolates were multidrug resistant but were susceptible to quinupristin-dalfopristin, linezolid, and telithromycin. The Spain(23F)-1 clone also occurred among antimicrobial-susceptible isolates, which suggests that the primary source of this clone may have been Asia rather than Iberia.

A UK multicentre study of the antimicrobial susceptibility of bacterial pathogens causing urinary tract infection

OBJECTIVES

To determine the prevalence of resistance amongst urinary tract pathogens against antimicrobials used to treat urinary tract infections (UTIs) in the UK to provide data to help direct empirical therapy.

METHOD

During 1999-2000, a total of 1291 bacterial isolates causing UTI were collected from 8 centres in the UK. Isolates were cultured from patients with (1). community-acquired UTI in those less than 65 years old (397), (2). hospital-acquired UTI other than those admitted with pyelonephritis (394), (3). pyelonephritis (108) and (4). community-acquired UTI in those greater than 65 years old (392). After re-identification, MICs for a range of antimicrobials were determined and interpreted using NCCLS procedures and interpretive guidelines.

RESULTS

Escherichia coli was the predominant pathogen in all categories but the total percentage for each category varied (56.3-77.3%). The next three pathogens of importance were Enterococcus faecalis, Klebsiella pneumoniae and Proteus mirabilis which varied in prevalence slightly from category to category. The activity of amoxycillin against E. coli (51.3% susceptible) was greatly reduced as a result of beta-lactamase production and only partially restored by the addition of clavulanic acid (78.8% susceptible). Cefuroxime was very active against E. coli using parenteral form breakpoints (97.1% susceptible) but less so using oral form breakpoints (68.6% susceptible). Cefuroxime was inactive against Enterococcus spp. and Pseudomonas spp. Nitrofurantoin was very active against isolates of E. coli (96.3% susceptible) and E. faecalis but not against K. pneumoniae, P. mirabilis or Pseudomonas aeruginosa. Overall susceptibility to trimethoprim ranged from 58.1% to 84.5% for the most prevalent pathogens. Ciprofloxacin was highly active against the UTI pathogens examined in this study with susceptibilities of between 88.6% and 97.7% for the most prevalent pathogens (E. coli, n=864, 97.7% susceptible) and was the only oral agent tested with activity against Pseudomonas spp.

CONCLUSION

These data provide much needed information on the prevalence of antimicrobial resistance amongst pathogens currently causing UTI in the UK.

Molecular characterization of macrolide resistance mechanisms among Streptococcus pneumoniae and Streptococcus pyogenes isolated from the PROTEKT 1999-2000 study

In this study, the distribution of macrolide resistance mechanisms was determined for isolates of Streptococcus pneumoniae and Streptococcus pyogenes obtained from the PROTEKT 1999-2000 study (a global, longitudinal study of the antibacterial susceptibility of bacterial pathogens associated with community-acquired lower respiratory tract infections). The global macrolide resistance mechanism distribution results for 1043 macrolide-resistant S. pneumoniae isolates collected from 25 countries were as follows: 35.3% mef(A), 56.2% erm(B), 6.8% both mef(A) and erm(B), 0.2% erm(A) subclass erm(TR) and 1.5% negative for mechanisms tested. Mechanisms of macrolide resistance were found to vary widely between countries and different geographical regions with mef(A) predominating in North America and erm(B) in Europe. Approximation of genotype from macrolide MIC without molecular determination of the mechanism of resistance resulted in an error of 10.2% (106 isolates). Overall, for 143 macrolide-resistant S. pyogenes isolates, 46.1% of the isolates tested were mef(A), 30.8% were erm(B), 23.1% were erm(A) subclass erm(TR) and no isolates were negative for all the genetic markers tested. Again, the distribution varied widely between countries and geographical regions. This study provides valuable baseline data for the continued monitoring of the evolution of macrolide resistance development in these important respiratory tract pathogens. The ketolide telithromycin retained excellent anti-pneumococcal activity irrespective of macrolide resistance mechanism, having a MIC(90) of 0.25, 0.5 and 0.5 mg/L against mef(A), erm(B) and mef(A)+erm(B) macrolide-resistant S. pneumoniae, respectively. It also exhibited potent activity against S. pyogenes that had become resistant to macrolides via either mef(A), (MIC(90 )0.5 mg/L) or erm(TR), (MIC(90) 0.03 mg/L).

Serial passage of Chlamydia spp. in sub-inhibitory fluoroquinolone concentrations

We investigated the in vitro development of fluoroquinolone resistance in Chlamydia trachomatis and Chlamydia (Chlamydophila) pneumoniae grown in McCoy cell monolayers in supplemented Eagle's minimum essential medium. With C. trachomatis, initial passages at sub-inhibitory fluoroquinolone concentrations did not affect fluoroquinolone susceptibility. However, after an initial lag of 10-24 passages (depending upon the fluoroquinolone used), fluoroquinolone resistance developed rapidly. The final fluoroquinolone MIC after a total of 30 passages was >256 times the MIC of the original wild-type strain with ofloxacin or ciprofloxacin passage. Analysis of the quinolone-resistance determining regions of two quinolone-resistant C. trachomatis mutants obtained after 30 passages showed that both isolates had a single serine to isoleucine substitution at amino acid position 83 in GyrA. In stark contrast, with C. pneumoniae no reduced fluoroquinolone susceptibility could be sustained, even after 30 passages with moxifloxacin or ofloxacin. With sparfloxacin passage, some indication of resistance was observed but no viable organisms could be isolated for further investigation. It is possible that fluoroquinolone-resistant C. pneumoniae are less able to survive than wild type, which may explain why resistance does not develop readily.

Detection of macrolide resistance mechanisms in Streptococcus pneumoniae and Streptococcus pyogenes using a multiplex rapid cycle PCR with microwell-format probe hybridization

In this study, a multiplex rapid cycle PCR with microwell-format probe hybridization method was developed to perform high-volume screening for macrolide resistance determinants in isolates of Streptococcus pneumoniae and Streptococcus pyogenes. The method was then utilized to determine the distribution of macrolide resistance mechanisms in recent isolates of S. pneumoniae and S. pyogenes from Great Britain and Ireland. For 83 strains of macrolide resistant S. pneumoniae tested, 51 (61.4%) were positive for mef(A), 29 (34.9%) erm(B), two (2.4%) double mechanisms mef(A) + erm(B), and one (1.2%) negative for all mechanisms tested. For 56 strains of macrolide-resistant S. pyogenes tested, 33 (58.9%) were positive for erm(A) subclass erm(TR), 18 (32.1%) mef(A) and five (8.9%) erm(B).

A comparison of the bactericidal activity of quinolone antibiotics in a Mycobacterium fortuitum model

New agents are urgently needed to meet the threat of multiple drug-resistant tuberculosis and to manage infection with the naturally resistant non-tuberculosis mycobacteria. Earlier fluoroquinolones have been shown to have promising in-vitro activity, although mouse infection and clinical studies suggested that they lack sufficient bactericidal activity. Methods were evaluated to measure the bactericidal activity of fluoroquinolones and to compare the new agent moxifloxacin with other fluoroquinolones with M. fortuitum as a model system. The optimum bactericidal concentrations (OBC) for the fluoroquinolones were: moxifloxacin, 0.5 mg/L; ciprofloxacin and sparfloxacin, 2 mg/L and ofloxacin, 8 mg/L. The bactericidal indices (BI) for moxifloxacin, ciprofloxacin, sparfloxacin and ofloxacin were 1.8, 0.5, 0.2 and 0.2, respectively. Similar ranking was obtained when the time taken to produce one log10 reduction in viable count was calculated. These data indicate that moxifloxacin was the most bactericidal of the fluoroquinolones tested. Such methods provide a simple in-vitro measure that correlates with in-vivo models.

Measurement of the bactericidal activity of fluoroquinolones against Streptococcus pneumoniae using the bactericidal index method

Kill curve analysis alone showed trovafloxacin to be more bactericidal than levofloxacin, grepafloxacin and moxifloxacin against four isolates of Streptococcus pneumoniae. However, using the bactericidal index (BI) method, levofloxacin was the most bactericidal fluoroquinolone using serum or lung biopsy concentration levels against the ofloxacin-susceptible strains and trovafloxacin was the most bactericidal against the ofloxacin-intermediate strain. None of the fluoroquinolones was bactericidal against the ofloxacin-resistant strain. With BIs using epithelial lining fluid or alveolar macrophage concentration levels, trovafloxacin or grepafloxacin was most bactericidal, respectively. These data illustrate that simple analysis of traditional kill curves may not be adequate in the evaluation of fluoroquinolone bactericidal activity. The results of this study suggest a need for further investigation to assess the role of tissue concentration and bactericidal activity in antimicrobial efficacy.

The bactericidal activity of gemifloxacin (SB-265805)

The bactericidal activity and mechanisms of action of the new fluoroquinolone gemifloxacin were investigated against the laboratory strains Escherichia coli KL16, Staphylococcus aureus E3T and Streptococcus pneumoniae C3LN4. Gemifloxacin was found to be highly bactericidal against these bacteria, producing a biphasic dose-response curve typical of the fluoroquinolones. This novel fluoroquinolone was more bactericidal than all other fluoroquinolones so far tested (ciprofloxacin, ofloxacin, enoxacin, lomefloxacin, levofloxacin, clinafloxacin, trovafloxacin, DV-7751 and sitafloxacin) against S. aureus and was more bactericidal than most other fluoroquinolones against E. coli or Str. pneumoniae. These data show gemifloxacin to be an improved member of the fluoroquinolone class of antibacterial agents.

Purification of pneumococcal type II topoisomerases and inhibition by gemifloxacin and other quinolones

Topoisomerase IV and DNA gyrase were purified from a ciprofloxacin-sensitive Streptococcus pneumoniae strain and from two clinical isolates of S. pneumoniae with high-level resistance to ciprofloxacin by means of a gene cloning method in Escherichia coli. All the quinolones tested (gemifloxacin, trovafloxacin, levofloxacin, ciprofloxacin and grepafloxacin) were able to inhibit topoisomerase IV at lower concentrations than those required for DNA gyrase, suggesting that topoisomerase IV is the primary target in the three pneumococci, in agreement with recently published enzyme data. Gemifloxacin (SB-265805) was found to be the most active agent against topoisomerase IV but, surprisingly, not against DNA gyrase. These findings indicate that the potent in vitro activity of gemifloxacin against S. pneumoniae, including ciprofloxacin-resistant strains, results from a strong affinity for pneumococcal topoisomerase IV.

Bactericidal activity of gemifloxacin and other quinolones against Streptococcus pneumoniae

This study compared the bactericidal activity of gemifloxacin (SB-265805) and a panel of test quinolones against two ciprofloxacin-resistant pneumococcal strains (Streptococcus pneumoniae 502226 and 503244) and one ciprofloxacin-sensitive strain (S. pneumoniae C3LN4). Activities were compared by calculating the bactericidal index of these agents. Gemifloxacin was found to be the most bactericidal quinolone tested against these strains. This finding confirms previous data indicating the superior in vitro activity of gemifloxacin against pneumococci, including ciprofloxacin-resistant strains. Although both ciprofloxacin-resistant strains tested had similar quinolone MICs, they differed considerably in their susceptibility to the bactericidal action of these agents. S. pneumoniae 502226 was more readily killed by quinolones than S. pneumoniae 503244 but, as would be expected, both were less susceptible than the ciprofloxacin-sensitive strain. Of the quinolones tested, trovafloxacin showed disproportionally poor activity against the ciprofloxacin-resistant strains even though potent activity was present against the ciprofloxacin-sensitive strain. These data highlight the importance of assessing quinolone bactericidal activity in addition to the MIC when evaluating new members of this antimicrobial class.

Activities of fluoroquinolones against Streptococcus pneumoniae type II topoisomerases purified as recombinant proteins

Streptococcus pneumoniae topoisomerase IV and DNA gyrase have been purified from a fluoroquinolone-susceptible Streptococcus pneumoniae strain, from first-step mutants showing low-level resistance to ciprofloxacin, sparfloxacin, levofloxacin, and ofloxacin, and from two clinical isolates showing intermediate- and high-level fluoroquinolone resistance by a gene cloning method that produces recombinant proteins from Escherichia coli. The concentrations of ciprofloxacin, sparfloxacin, levofloxacin, or ofloxacin required to inhibit wild-type topoisomerase IV were 8 to 16 times lower than those required to inhibit wild-type DNA gyrase. Furthermore, low-level resistance to these fluoroquinolones was entirely due to the reduced inhibitory activity of fluoroquinolones against topoisomerase IV. For all the laboratory strains, the 50% inhibitory concentration for topoisomerase IV directly correlated with the MIC. We therefore propose that with S. pneumoniae, ciprofloxacin, sparfloxacin, levofloxacin, and ofloxacin target topoisomerase IV in preference to DNA gyrase. Sitafloxacin, on the other hand, was found to be equipotent against either enzyme. This characteristic is unique for a fluoroquinolone. A reduction in the sensitivities of both topoisomerase IV and DNA gyrase are required, however, to achieve intermediate- or high-level fluoroquinolone resistance in S. pneumoniae.

The bactericidal activity of levofloxacin compared with ofloxacin, D-ofloxacin, ciprofloxacin, sparfloxacin and cefotaxime against Streptococcus pneumoniae

The bactericidal activity of levofloxacin was compared with that of four other quinolones and one cephalosporin against four strains of Streptococcus pneumoniae with varying degrees of penicillin G susceptibility. Levofloxacin was found to be the most bactericidal quinolone at its optimum bactericidal concentration, followed by ofloxacin, ciprofloxacin, sparfloxacin and D-ofloxacin, in that order. There was no correlation between quinolone-susceptibility and penicillin-susceptibility with any quinolone tested. To allow a comparison between the bactericidal activity of the quinolones and cefotaxime to be made a bactericidal index was used. Against two penicillin-sensitive pneumococci and a penicillin-intermediate pneumococcus, cefotaxime was the most potent antibacterial agent tested, followed by levofloxacin. However, against the penicillin-resistant pneumococcus, levofloxacin was considerably more potent than cefotaxime. The remaining quinolones tested were inferior to levofloxacin. Levofloxacin has enhanced activity against pneumococci compared with clinically available quinolones. In addition, levofloxacin may be the future quinolone of choice in the treatment of penicillin-resistant pneumococci.

Bactericidal index: a new way to assess quinolone bactericidal activity in vitro

A new method of assessing the bactericidal activity of quinolones and other antibacterial agents, known as the bactericidal index or BI, is introduced and discussed. The BI represents total bacterial kill over a drug concentration range limited to levels achievable in vivo. This method allows the bactericidal activity of drugs to be readily compared. Calculation of BI produces a single value per bacterial strain/antibacterial agent combination that represents overall bactericidal activity at clinically relevant concentrations. As an example, the bactericidal activities of quinolones against Enterococcus faecalis are compared.

Bactericidal activity of trovafloxacin (CP-99,219)

Trovafloxacin was found to be a typical quinolone producing a biphasic dose response against Escherichia coli, Staphylococcus aureus or Streptococcus pneumoniae. A reduced rate of kill was seen against Enterococcus faecalis, which is also typical of the quinolones. However, the bactericidal activity of trovafloxacin against S. aureus and S. pneumoniae was greater than most other quinolones previously tested. The enhanced bactericidal activity of trovafloxacin against S. aureus may be explained by the possession of a very strong bactericidal mechanism B. Trovafloxacin may prove to be a quinolone of choice against S. aureus infections.

Mechanism of differential activities of ofloxacin enantiomers

Ofloxacin, a potent quinolone antibacterial agent, has a tricyclic ring structure with a methyl group attached to the asymmetric carbon at the C-3 position on the oxazine ring. The S isomer (DR-3355) of ofloxacin has antibacterial activity up to 2 orders of magnitude greater than that of the R isomer (DR-3354). This differential antibacterial activity was not due to different drug transport mechanisms of the two isomers but was found to be derived from the inhibitory activity against the target enzyme, DNA gyrase. Previous mechanistic studies have suggested that the bactericidal effect of the drug is mediated through the stabilization of a cleavable complex via a cooperative drug binding process to a partially denatured DNA pocket created by DNA gyrase. The drug binds to supercoiled DNA in a manner similar to that to which it binds to the enzyme-DNA complex. In the present studies, we first examined the binding of the two radiolabeled ofloxacin enantiomers to supercoiled pUC9 plasmid DNA. Surprisingly, the two enantiomers possessed similar apparent binding affinities and binding cooperatives. The major difference in binding between the two stereoisomers was the molar binding ratio: 4 for the more active S isomer versus 2 for the less active R isomer. We next examined the relative binding potencies of the stereoisomers to the DNA-DNA gyrase complex. The results of a competition assay showed that (S)-ofloxacin binds 12-fold better to the complex than (R)-ofloxacin. The binding potencies of the two enantiomers and two other quinolones correlated well with their respective concentrations causing 50% inhibition against DNA gyrase. The results are interpreted by a stacking model by using the concept of the cooperative drug-DNA binding mechanism, indicating that the potencies of quinolones cannot be determined solely by the DNA binding affinity and cooperativity but can also be determined by their capability in maximally saturating the binding site. The capability of the drug in saturating the binding pocket manifests itself in an increased efficacy at inhibiting the enzyme through a direct interaction between the drug and the enzyme. The results augment the previous suggestion that the binding pocket in the enzyme-DNA complex involves multiple receptor groups including not only DNA bases but also a gyrase subunit. The higher level of potency of (S)-ofloxacin is proposed to derive from the fact that a greater number of molecules are assembled in the pocket. This greater number of molecules optimizes the interaction between the drug and the enzyme, possibly through a contact between the C-7 substituent and the quinolone pocket on the B subunit of DNA gyrase.