Garenoxacin treatment of experimental endocarditis caused by viridans group streptococci

The activity of garenoxacin was compared to that of levofloxacin or penicillin in a rabbit model of Streptococcus mitis group (penicillin MIC, 0.125 microg/ml) and Streptococcus sanguinis group (penicillin MIC, 0.25 microg/ml) endocarditis. Garenoxacin and levofloxacin had MICs of 0.125 and 0.5 microg/ml, respectively, for both study isolates. Rabbits with catheter-induced aortic valve endocarditis were given no treatment, penicillin at 1.2x10(6) IU/8 h intramuscularly, garenoxacin at 20 mg/kg of body weight/12 h intravenously, or levofloxacin at 40 mg/kg/12 h intravenously. For both isolates tested, garenoxacin area under the curve (AUC)/MIC and maximum concentration of drug in serum (Cmax)/MIC ratios were 368 and 91, respectively. Rabbits were sacrificed after 3 days of treatment; cardiac valve vegetations were aseptically removed and quantitatively cultured. For S. mitis group experimental endocarditis, all studied antimicrobial agents were more active than no treatment (P<0.001), whereas for S. sanguinis group endocarditis, no studied antimicrobial agents were more active than no treatment. We conclude that AUC/MIC and Cmax/MIC ratios may not predict activity of some quinolones in experimental viridans group endocarditis and that garenoxacin and levofloxacin may not be ideal choices for serious infections caused by some quinolone-susceptible viridans group streptococci.     

Antimicrobial therapy of experimental endocarditis caused by nutritionally variant viridans group streptococci.

Rabbits with nutritionally variant viridans group streptococcal experimental endocarditis were treated three times daily for 3 days with procaine penicillin (1.2 X 10(6) U) alone or together with low-dose streptomycin (2 mg/kg), high-dose streptomycin (8 mg/kg), low-dose gentamicin (0.32 mg/kg), or high-dose gentamicin (1.05 mg/kg). The mean 0.5-h serum concentrations of streptomycin were 5.3 and 22.5 micrograms/ml in the low- and high-dose group, respectively, and the concentrations of gentamicin were 0.7 and 2.5 micrograms in the low- and high-dose groups, respectively. The combination of procaine penicillin with each dose of aminoglycoside was significantly more effective (P less than 0.001) than was procaine penicillin alone. In combination with procaine penicillin, the higher dose of streptomycin was significantly more effective (P less than 0.02) than the lower dose of streptomycin. The higher dose of streptomycin was not significantly more effective than either dose of gentamicin. The results of treatment with the high or low dose of gentamicin were virtually identical.     

Efficacy of levofloxacin in the treatment of experimental endocarditis caused by viridans group streptococci

Levofloxacin was investigated against viridans group streptococci in vitro and in rats with experimental aortic endocarditis. The MIC(90)s of levofloxacin and ciprofloxacin for 20 independent isolates of such bacteria were 1 and 8 mg/L, respectively. Rats were infected with two types of organism: either fully susceptible to levofloxacin MIC < or = 0.5 mg/L) or borderline susceptible (MIC 1-2 mg/L). Fully levofloxacin-susceptible bacteria comprised one penicillin-susceptible (MIC 0.004 mg/L) Streptococcus gordonii, and one penicillin-tolerant as well as one intermediate penicillin-resistant (MIC 0.125 mg/L) isogenic strains. Borderline levofloxacin-susceptible bacteria comprised one penicillin-susceptible Streptococcus sanguis and one highly penicillin-resistant Streptococcus mitis (MIC 2 mg/L). Rats were treated for 5 days with drug dosages simulating the following treatments in humans: (i) levofloxacin 500 mg orally once a day (q24 h), (ii) levofloxacin 500 mg orally twice a day (q12 h), (iii) levofloxacin 1 g orally q24 h, (iv) ciprofloxacin 750 mg orally q12 h, and (v) ceftriaxone 2 g iv q24 h. Levofloxacin was equivalent or superior to ceftriaxone, and was successful in treating experimental endocarditis irrespective of penicillin resistance. Nevertheless, standard levofloxacin treatment equivalent to 500 mg q24 h in human was less effective than twice daily 500 mg or once daily 1 g doses against borderline-susceptible organisms. Ciprofloxacin, used as a negative control, was ineffective and selected for resistant isolates. This underlines the importance of MIC determinations when treating severe streptococcal infection with quinolones. In the case of borderline-susceptible pathogens, total daily doses of 1 g of levofloxacin should be considered.     

Antimicrobial susceptibility patterns and macrolide resistance genes of viridans group streptococci from normal flora

OBJECTIVES: Our aim was to study the antimicrobial susceptibilities and macrolide resistance mechanisms of viridans group streptococci isolated from the normal flora. METHODS: In vitro susceptibilities of 16 antimicrobials were studied for 161 viridans streptococci (on average 5.8 isolates per person) from the normal flora of 28 elderly persons. Resistance mechanisms of erythromycin-resistant isolates were studied by the double disc test and PCR. RESULTS: In all, 16.8% of the isolates were non-susceptible (MIC > or =0.25 mg/L) to penicillin, but none showed high-level resistance (MIC > or =4 mg/L). Resistance to erythromycin, tetracycline, quinupristin/dalfopristin, levofloxacin and moxifloxacin was found in 22.4, 27.3, 13.0, 1.9 and 1.9% of the isolates, respectively. Combined resistance to erythromycin and tetracycline was found in 13.0% of the isolates. Erythromycin-resistant isolates were isolated from 57% of the study persons. Of the erythromycin-resistant isolates 80.6% were of the M phenotype and 19.4% were of the macrolide-lincosamide-streptogramin B (MLSB) phenotype (one isolate with constitutive and six with inducible expression). Isolates with the M phenotype were the least susceptible to telithromycin, a new ketolide. The mef(A) gene was found in the isolates with the M phenotype and the erm(B) gene in the isolates with the MLSB phenotype. CONCLUSIONS: The distribution of phenotypes among the viridans streptococci resembles that found in Streptococcus pyogenes, with predominance of the M phenotype. However, the coding gene for the MLSB phenotype, erm(B), is the same in viridans streptococci as in Streptococcus pneumoniae. Viridans group streptococci carrying different resistance traits provide a pool of resistant bacteria that may transfer resistance determinants to more pathogenic organisms.     

Discrepancies between MBC and actual killing of viridans group streptococci by cell-wall-active antibiotics.

We determined the MBC of amoxicillin and vancomycin, two antibiotics advocated for treatment and prophylaxis of bacterial endocarditis, for 24 strains of viridans group streptococci isolated from patients with endocarditis. We found that the MIC of amoxicillin for all strains was less than or equal to 0.25 micrograms/ml and the MBC was either low (less than 0.5 micrograms/ml) in 6 nontolerant strains or high (greater than 128 micrograms/ml) in 18 tolerant strains. The MIC of vancomycin for the 24 strains was less than or equal to 1 microgram/ml, and the MBC was either low (less than 1 microgram/ml) for 3 nontolerant strains or high (greater than 128 micrograms/ml) for 21 tolerant strains. In addition to the MBC, we determined the actual reduction of the viable bacterial counts in each tube dilution after 24 h of incubation. This determination was made by subtracting the number of colonies observed on the subculture plate from the number of bacteria contained in the initial inoculum. For both antibiotics we found that the maximal reduction in viable counts was achieved at or very close to the MIC and did not increase with increasing antibiotic concentrations (up to 128 micrograms/ml). As expected, the six strains for which the amoxicillin MBC was less than 0.5 micrograms/ml and the three strains for which the vancomycin MBC was less than 1 microgram/ml had a reduction of viable counts of more than 3 log10 (greater than 99.9% killing). In contrast, among the strains defined as tolerant to amoxicillin and vancomycin, there were wide variations in the actual reduction of bacterial counts, ranging from 3 log10 to less than 1 log10. Therefore our observations suggest that the reduction of viable streptococcal counts reflects more accurately the bactericidal effect of amoxicillin and vancomycin than does the MBC, which artificially divides the strains into sensitive or tolerant strains.     

Activity of quinolones against viridans group streptococci isolated from blood cultures of patients with haematological malignancy

Use of fluoroquinolones for antimicrobial prophylaxis during neutropenia is often cited as a significant predisposing factor for viridans group streptococcus (VGS) bacteraemia. Newer compounds in this class are reputed to have enhanced activity against Gram-positive bacteria, and we determined the minimal inhibitory concentrations (MICs) for ciprofloxacin and three of the newer compounds: trovafloxacin, fleroxacin and clinafloxacin, against 44 isolates of VGS. On a gravimetric basis, clinafloxacin was most active (MIC₉₀ 0.19 mg/l), whereas ciprofloxacin and fleroxacin were the least active (both MIC₉₀ 16 mg/l). Clinafloxacin warrants further study as an agent of prophylaxis against bacterial infection in neutropenic patients.     

Importance of minimizing carry-over effect at subculture in the detection of penicillin-tolerant viridans group streptococci.

Penicillin susceptibility determinations made for 14 strains of Streptococcus mitior by two different broth dilution tests revealed small numbers of tolerant strains regardless of the volumes (0.01 and 0.1 ml) of subcultured broth. The addition of penicillinase to the subculture medium increased the number of tolerant strains significantly in 0.1 ml of subcultured broth, demonstrating that the detection of penicillin-tolerant viridans group streptococci might be hampered by the carry-over of penicillin.     

Emergence of high rates of antimicrobial resistance among viridans group streptococci in the United States.

Three hundred fifty-two blood culture isolates of viridans group streptococci obtained from 43 U.S. medical centers during 1993 and 1994 were characterized. Included were 48 isolates of "Streptococcus milleri," 219 S. mitis isolates, 29 S. salivarius isolates, and 56 S. sanguis isolates. High-level penicillin resistance (MIC, > or = 4.0 micrograms/ml) was noted among 13.4% of the strains; for 42.9% of the strains, penicillin MICs were 0.25 to 2.0 micrograms/ml (i.e., intermediate resistance). In general, amoxicillin was slightly more active than penicillin. The rank order of activity for five cephalosporins versus viridans group streptococci was cefpodoxime = ceftriaxone > cefprozil = cefuroxime >> cephalexin. The percentages of isolates resistant (MIC, > or = 2 micrograms/ml) to these agents were 15, 17, 18, 20, and 96, respectively. The rates of resistance to erythromycin, tetracycline, and trimethoprim-sulfamethoxazole were 12 to 38%. Resistance to either chloramphenicol or ofloxacin was uncommon (i.e., < 1%). In general, among the four species, S. mitis was the most resistant and "S. milleri" was the most susceptible.     

Efficacy of garenoxacin in treatment of experimental endocarditis due to Staphylococcus aureus or viridans group streptococci

The activity of garenoxacin was investigated in rats with experimental endocarditis due to staphylococci and viridans group streptococci (VGS). The staphylococci tested comprised one ciprofloxacin-susceptible and methicillin-susceptible Staphylococcus aureus (MSSA) isolate (isolate 1112), one ciprofloxacin-susceptible but methicillin-resistant S. aureus (MRSA) isolate (isolate P8), and one ciprofloxacin-resistant mutant (grlA) of P8 (isolate P8-4). The VGS tested comprised one penicillin-susceptible isolate and one penicillin-resistant isolate (Streptococcus oralis 226 and Streptococcus mitis 531, respectively). To simulate the kinetics of drugs in humans, rats were infused intravenously with garenoxacin every 24 h (peak and trough levels in serum, 6.1 and 1.0 mg/liter, respectively; area under the concentration-time curve [AUC], 63.4 mg. h/liter) or levofloxacin every 12 h (peak and trough levels in serum, 7.3 and 1.5 mg/liter, respectively; AUC, 55.6 mg. h/liter) for 3 or 5 days. Flucloxacillin, vancomycin, and ceftriaxone were used as control drugs. Garenoxacin, levofloxacin, flucloxacillin, and vancomycin sterilized >/=70% of the vegetations infected with both ciprofloxacin-susceptible staphylococcal isolates (P < 0.05 versus the results for the controls). Garenoxacin and vancomycin also sterilized 70% of the vegetations infected with ciprofloxacin-resistant MRSA isolate P8-4, whereas treatment with levofloxacin failed against this organism (cure rate, 0%; P < 0.05 versus the results obtained with the comparator drugs). Garenoxacin did not select for resistant derivatives in vivo. In contrast, levofloxacin selected for resistant variants in four of six rats infected with MRSA isolate P8-4. Garenoxacin sterilized 90% of the vegetations infected with both penicillin-susceptible and penicillin-resistant isolates of VGS. Levofloxacin sterilized only 22 and 40% of the vegetations infected with penicillin-susceptible S. oralis 226 and penicillin-resistant S. mitis 531, respectively. Ceftriaxone sterilized only 40% of those infected with penicillin-resistant S. mitis 531 (P < 0.05 versus the results obtained with garenoxacin). No quinolone-resistant VGS were detected. In all the experiments successful quinolone treatment was predicted by specific pharmacodynamic criteria (D. R. Andes and W. A. Craig, Clin. Infect. Dis. 27:47-50, 1998). The fact that the activity of garenoxacin was equal or superior to those of the standard comparators against staphylococci and VGS indicates that it is a potential alternative for the treatment of infections caused by such bacteria.     

Enhanced activity of the combination of penicillin G and gentamicin against penicillin-resistant viridans group streptococci.

We evaluated the effects of the combination of penicillin G and gentamicin against 10 penicillin-resistant bacteremic isolates of viridans group streptococci for which the MICs of penicillin were 4 to 64 micrograms/ml. In time-kill studies, the combination resulted in more killing of eight isolates for which the MICs of penicillin were from 8 to 64 micrograms/ml than any of the antimicrobial agents tested alone. In general, clearly enhanced antimicrobial activity was observed with the combination.     

Oropharyngeal carriage of macrolide-resistant viridans group streptococci: a prevalence study among healthy adults in Belgium

OBJECTIVES: Viridans group streptococci (VGS) are gaining significance as reservoirs of resistance determinants for respiratory tract pathogens. Our aim was to investigate healthy adults for oropharyngeal carriage of VGS that are resistant to macrolides, as well as to other common antibiotics. METHODS: Macrolide-resistant VGS were isolated from throat samples of 154 healthy Belgian adults, and phenotyped and genotyped for erm(A), erm(B) and mef(A). In vitro susceptibilities to 10 antimicrobials and the presence of tetracycline resistance genes were also determined. RESULTS: Carriage was detected in 71% of the population screened, from whom 157 unique, macrolide-resistant VGS were isolated. A constitutive (cMLS) phenotype was present in 105 isolates, of which 102 isolates carried either erm(B) or erm(B) + mef(A). The remaining three isolates did not present with any of the genes studied. All 45 isolates showing the M phenotype carried mef(A). The least abundant inducible (iMLS) isolates (n = 7) carried either erm(B) or erm(B) + mef(A). The most abundant macrolide-resistant VGS species was Streptococcus mitis (51%). Co-resistance to tetracycline was identified in 114 isolates, of which tet(M) was present in 105, tet(O) in two and both tet(M) and tet(O) in one, while the remaining six isolates did not present with either gene. tet(M) was also present in four tetracycline susceptible and two intermediately resistant isolates. Fluoroquinolone resistance (ciprofloxacin MIC > or = 4 mg/L) was detected in 16 isolates. Resistance to telithromycin, penicillin and chloramphenicol was appreciably low. CONCLUSIONS: This study highlights a high oropharyngeal carriage of macrolide-resistant VGS and its co-resistance to tetracycline and fluoroquinolones among healthy Belgian adults.     

Antimicrobial susceptibility patterns and macrolide resistance genes of viridans group streptococci from blood cultures in Korea

OBJECTIVES: Our aim was to study the antimicrobial susceptibilities and macrolide resistance mechanisms of viridans group streptococci (VGS) in a Korean tertiary hospital. METHODS: MICs of five antimicrobials were determined for 106 VGS isolated from blood cultures. The macrolide resistance mechanisms of erythromycin non-susceptible isolates were studied by the double-disc test and PCR. RESULTS: In all, 42.4% of the isolates were susceptible to penicillin. Nine of 61 penicillin non-susceptible isolates were fully resistant (MIC >/= 4 mg/L). Rates of non-susceptibility to erythromycin, clindamycin and ceftriaxone were 33.9%, 17.9% and 9.4%, respectively. Twenty-two (61.1%) of 36 erythromycin non-susceptible isolates expressed constitutive resistance to macrolide-lincosamide-streptogramin B antibiotics (a constitutive MLS(B) phenotype); 13 isolates (36.1%) expressed an M phenotype; and one isolate, a Streptococcus bovis isolate, had an inducible MLS(B) resistance phenotype. erm(B) was found in isolates with constitutive/inducible MLS(B) phenotypes, and mef(A) in isolates with the M phenotype. In three isolates (two isolates with a constitutive MLS(B) phenotype and in one isolate with the M phenotype), none of erm(A), erm(B), erm(C) or mef(A) was detected by PCR. CONCLUSIONS: Penicillin non-susceptible VGS were more resistant to erythromycin, clindamycin and ceftriaxone than were penicillin-susceptible isolates. A constitutive MLS(B) phenotype associated with erm(B) was the predominant mechanism of macrolide resistance among erythromycin non-susceptible isolates from this Korean hospital.     

Macrolide resistance mechanisms and in vitro susceptibility patterns of viridans group streptococci isolated from blood cultures

OBJECTIVES: Our aim was to study the macrolide resistance mechanisms and antimicrobial susceptibilities of viridans group streptococci (VGS) isolated from blood cultures. METHODS: In vitro susceptibilities to nine antimicrobials were studied for 85 VGS isolated from blood cultures by agar dilution. Pheno- and genotyping of erythromycin-resistant isolates were studied by the double disc test and PCR. RESULTS: Resistance to erythromycin was found in 27% (n = 23) of the isolates. Erythromycin-resistant Streptococcus oralis (n = 13) predominated among the other erythromycin-resistant species isolated. The phenotypes among 23 erythromycin-resistant isolates were as follows: 12 constitutive macrolide-lincosamide-streptogramin (cMLS(B)) resistance phenotype and 11 macrolide (M) resistance phenotype. Of the cMLS(B) isolates 11 had erm(B) genes and 11 of the M phenotype isolates had mef(A) genes. Four of the cMLS(B) isolates had both erm(B) and mef(A) genes. None of the isolates had erm(TR) genes. Combined resistance to erythromycin with penicillin, clindamycin, chloramphenicol, tetracycline and quinupristin/dalfopristin was found in 100, 61, 74, 100 and 100% of the isolates, respectively. No resistance was found for vancomycin, linezolid and levofloxacin. CONCLUSIONS: The macrolide resistance mechanisms of our VGS isolates revealed that the cMLS(B) phenotype associated with erm(B) and the M phenotype associated with mef(A) genes are found with similar frequencies.     

Tigecycline (GAR-936) activity against Streptococcus gallolyticus (bovis) and viridans group streptococci

Viridans group streptococci including Streptococcus gallolyticus (formerly S. bovis) represent serious invasive pathogens often associated with endocarditis or sepsis among immunocompromised or cancer patients. Tigecycline (GAR-936), the first clinically studied glycylcycline, has a potent gram-positive activity with a potential treatment option for these streptococcal infections. The studied collection (848 strains) included 100 isolates each of Streptococcus anginosus, Streptococcus constellatus, Streptococcus intermedius, Streptococcus mitis, Streptococcus oralis, Streptococcus salivarius, Streptococcus sanguis, and fewer strains of S. gallolyticus (98 strains) and Streptococcus mutans (50 strains). These strains were isolated from patients on 3 continents in the SENTRY Antimicrobial Surveillance Program and tested for susceptibility and interpreted by Clinical and Laboratory Standards Institute broth microdilution methods and criteria (< or = 0.25 microg/mL for tigecycline per US Food and Drug Administration). Penicillin susceptibility rates for the entire collection varied from 61% (S. sanguis) to 98% (S. constellatus), and macrolide susceptibility was also compromised (49-88%; average, 69%). Tigecycline was active against all isolates tested, in contrast to tetracycline resistance rates of 8-66%, and highest for S. gallolyticus. In conclusion tigecycline was quite active against bacteremic isolates of viridans group streptococci species and S. gallolyticus with an overall MIC90 at < or = 0.06 microg/mL; the highest MIC was only 0.25 microg/mL.     

Daptomycin tested against 915 bloodstream isolates of viridans group streptococci (eight species) and Streptococcus bovis

OBJECTIVES: To evaluate the activity of daptomycin tested against numerous species of viridans group streptococci and Streptococcus bovis, which are associated with wound infections, sepsis, cellulitis, endocarditis, abscesses and dental caries. The incidence of penicillin-resistant (non-susceptible) and MLS(B)-resistant strains among viridans group streptococci often varies by species. METHODS: The activity of daptomycin was compared with seven other antimicrobial classes using reference broth microdilution and disc diffusion methods tested against 915 bacteraemic isolates of streptococci (815 viridans group strains; 100 S. bovis). RESULTS: Among all species of viridans group streptococci and S. bovis, 99.9% of isolates were susceptible to daptomycin (MIC values, < or = 0.016-2 mg/L). In contrast, penicillin, erythromycin and tetracycline susceptibility varied widely between species. Erythromycin susceptibility was in the range 48.6-88.7%, penicillin susceptibility in the range 65.5-98.1% and tetracycline in the range 35.0-93.9%. The inter-method agreement between daptomycin and linezolid resistance (comparison agent) disc diffusion and broth microdilution test results was high, each showing near complete susceptibility (99.9%). CONCLUSIONS: Daptomycin is an active antimicrobial agent that has a usable potency against eight species of viridans group streptococci, as well as S. bovis, with all MIC values at < or =2 mg/L.     

Fluoroquinolone resistance associated with target mutations and active efflux in oropharyngeal colonizing isolates of viridans group streptococci

Oropharyngeal samples from 60 hospitalized patients (30 fluoroquinolone [FQ]-treated and 30 non-FQ-treated patients) and 30 untreated nonhospitalized healthy control subjects yielded 20 isolates of viridans group streptococci with reduced susceptibility to FQ, mostly from the hospitalized patients. An efflux phenotype was commonly encountered, expressed either alone or with topoisomerase mutations. Interspecies transfer of the efflux phenotype was demonstrated via transformation of Streptococcus pneumoniae R6 with DNA from S. mitis and S. oralis.     

Penicillin-induced effects on streptomycin uptake and early bactericidal activity differ in viridans group and enterococcal streptococci.

In vitro studies with penicillin and [3H]streptomycin in four strains of streptococci (S. faecalis, S. sanguis, and S. mitis) were performed by simultaneously measuring the rates of bacterial killing and uptake of streptomycin. In S. faecalis, penicillin stimulated streptomycin uptake, as has been shown by Moellering and Weinberg (R. C. Moellering, Jr., and A. N. Weinberg, J. Clin. Invest. 50:2580-2584, 1971). Moreover, the antibiotic combination was associated with an enhanced bactericidal rate which temporally correlated with beta-lactam-induced aminoglycoside uptake. In contrast, in viridans group streptococci (S. sanguis and S. mitis) penicillin had no effect on streptomycin uptake and a minimal effect on bactericidal rate when compared with either drug alone. These data suggested that the stimulation of streptomycin uptake in streptococci by penicillin is strain or species specific and that important differences exist between enterococci and viridans group streptococci regarding the mechanisms of beta-lactam-aminoglycoside potentiation.