Synergism with aminoglycosides of penicillin, ampicillin and vancomycin against non-enterococcal group-D streptococci and viridans streptococci

The antibiotic susceptibility of 10 strains of non-enterococcal group-D streptococci was compared with that of 20 strains of viridans streptococci. The minimal inhibitory concentrations of penicillin, ampicillin, oxacillin, nafcillin, cephalothin, vancomycin, erythromycin and clindamycin for the two groups of streptococci were very similar in range and median values. Both groups of streptococci were resistant to the aminoglycosides. The effect of the combination of penicillin, ampicillin or vancomycin with streptomycin, kanamycin, gentamicin or tobramycin on the in-vitro killing of the two groups of streptococci was compared. For all the antibiotic combinations tested, synergism was demonstrated against all strains of non-enterococcal group-D streptococci after one or more of the time-intervals 6, 24 and 48 h. Some or all of the antibiotic combinations were synergistic against all strains of viridans streptococci after one or more of the same time-intervals. The other aminoglycosides (kanamycin, gentamicin and tobramycin) offered no advantage over streptomycin in synergism with penicillin, ampicillin or vancomycin against nonenterococcal group-D streptococci or viridans streptococci. These results suggest that non-enterococcal group-D streptococcal endocarditis may be treated by the same regimen as endocarditis caused by the viridans streptococci.     

Phylogenetic analysis of viridans group streptococci causing endocarditis

Identification of viridans group streptococci (VGS) to the species level is difficult because VGS exchange genetic material. We performed multilocus DNA target sequencing to assess phylogenetic concordance of VGS for a well-defined clinical syndrome. The hierarchy of sequence data was often discordant, underscoring the importance of establishing biological relevance for finer phylogenetic distinctions.     

Association of cell-adherent glycocalyx and endocarditis production by viridans group streptococci.

To assess the role of glycocalyx production in the pathogenesis of endocarditis caused by viridans group streptococci in adult patients, glycocalyx production was examined for 49 blood culture isolates. The tryptophan assay, a quantitative spectrophotometric test, was used to measure cell-adherent glycocalyx production. Absorbance values of the isolates that produced endocarditis were significantly higher (means, 0.166 versus 0.060 [P less than 0.001]). At a breakpoint of absorbance of 0.120, the sensitivity of the test was 0.83, the specificity was 0.96, and the predictive value was 0.95. These data suggest that the in vitro tryptophan assay of glycocalyx production by viridans group streptococci has potential value as a predictor of clinical pathogenicity.     

Glucosyltransferases of viridans streptococci are modulins of interleukin-6 induction in infective endocarditis

The glucosyltransferases (GTFs) of viridans streptococci, common pathogens of infective endocarditis, are extracellular proteins that convert sucrose into exopolysaccharides and glucans. GTFs B, C, and D of Streptococcus mutans are modulins that induce, in vitro and in vivo, the production of cytokines, in particular interleukin-6 (IL-6), from monocytes. The roles of S. mutans GTFs in infectivity and inflammation in situ were tested in a rat experimental model of endocarditis. No significant differences in infectivity, in terms of 95% infective dose and densities of bacteria inside vegetations, were observed between laboratory strain GS-5 and two clinical isolates or isogenic mutant NHS1DD, defective in the expression of GTFs. In aortic valves and surrounding tissues, IL-6 was detected by Western blots and immunostaining 24 h after GS-5 infection, was maintained over 72 h, and was followed by production of tumor necrosis factor alpha but not IL-1beta. Animals infected with NHS1DD showed markedly lower levels of IL-6 (less than 5% of that of parental GS-5-infected rats), while tumor necrosis factor alpha was unaffected. In contrast, animals infected with NHR1DD, another isogenic mutant expressing only GtfB, showed a much smaller reduction (down to 56%). These results suggest that GTFs are specific modulins that act during acute inflammation, inducing IL-6 from endothelial cells surrounding the infected valves without affecting bacterial colonization in vegetations, and that IL-6 might persist in chronic inflammation in endocarditis.     

Quantitative assay of glycocalyx produced by viridans group streptococci that cause endocarditis.

A quantitative method to determine glycocalyx production by strains of viridans group streptococci from patients with endocarditis is presented. There is good correlation between this new tryptophan quantitative assay and qualitative assays employing polysaccharide stains (ruthenium red, periodic acid-Schiff, and Cellufluor) or the Molisch test. The quantification of the glycocalyx production in glucose substrate in vitro by viridans group streptococci correlates with the size of cardiac vegetation and ease of antimicrobial sterilization in experimental endocarditis. The relationship of in vitro quantification of glycocalyx to maintenance of infection, morbidity of infection, and antimicrobial treatment is discussed.     

Identification of viridans streptococci isolated from clinical specimens

Of 532 strains of viridans streptococci isolated from clinical specimens, 517 were identified by using a scheme based on the work of Facklam (R. R. Facklam, J. Clin. Microbiol. 5:184-201, 1977). The strains were distributed among nine of the species described by Facklam and an additional group of physiologically homogeneous streptococci not recognized in Facklam's scheme. The method for identification involves a battery of 10 tests that employ both conventional media and commercially prepared disks. The identification of most of the isolates was accomplished within 48 h.     

Involvement of bactericidal factors from thrombin-stimulated platelets in clearance of adherent viridans streptococci in experimental infective endocarditis.

Platelets activated with thrombin release bactericidal factors. We studied the role of the susceptibility of viridans streptococci to these bactericidal factors in the development of infective endocarditis (IE). By using the experimental endocarditis rabbit model, the initial adherence and the development of IE were assessed for 10 viridans streptococcal strains differing in their susceptibilities to releasate (material released) from thrombin-activated platelets. Six strains were susceptible and four strains were resistant to these releasates. The numbers of vegetations (VGs) colonized at 5 min and 48 h after intravenous challenge with 10(4) CFU were determined. At 5 min after challenge, significantly more VGs were colonized with bacteria of the six platelet releasate-susceptible strains than with bacteria of the four releasate-resistant strains (P < 0.005). In the releasate-susceptible group of strains, the number of colonized VGs decreased significantly between 5 min and 48 h after intravenous inoculation (P < 0.001). Such a decrease was not observed with the releasate-resistant strains. As a result, the final developments of IE due to releasate-susceptible and -resistant strains were not significantly different. The releasate-susceptible strain 1 and the releasate-resistant strain 2 were selected for more detailed experiments. Rabbits were killed at 5 and 30 min and 2, 4, and 48 h after inoculation. The number of culture-positive VGs as well as the number of adherent bacteria on the individual VGs were determined. The 90% infective dose for each strain was 10(5) CFU. At low inoculum concentrations (10(3) and 10(4) CFU) a larger proportion of the inoculated bacteria of both strains was found to be adherent on VGs than at higher challenge doses. The number of culture-positive VGs as well as the number of adherent bacteria per VG decreased rapidly in the first 30 min after challenge with strain 1 but not after challenge with strain 2. Additional experiments with the platelet releasate-susceptible strain S224 and the platelet releasate-resistant stain S182 confirmed the data obtained with strains 1 and 2 and indicated that releasate-susceptible strains disappeared from the VGs with time, whereas releasate-susceptible strains persisted. In vitro studies with VGs excised 5 min after challenge with stain 1 or 2 showed that clearance of the releasate-susceptible strain 1 was not caused by complement bactericidal activity or surface phagocytosis by polymorphonuclear cells. Bacterial cells of strain 1 adherent on excised VTGs were rapidly cleared by exposure to fresh clotting blood or to releasates from thrombin-stimulated platelet suspension.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparative bactericidal activities of daptomycin, glycopeptides, linezolid and tigecycline against blood isolates of Gram-positive bacteria in Taiwan.

In-vitro MICs and minimum bactericidal concentrations (MBCs) of daptomycin, linezolid, tigecycline, vancomycin and teicoplanin against Gram-positive bacteria were determined using the broth microdilution method for ten blood isolates each of methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant S. aureus (MRSA), including two vancomycin-intermediate S. aureus (VISA), vancomycin-resistant Enterococcus faecium and Enterococcus faecalis. One strain of VISA was tested in a time-kill synergism assay of daptomycin combined with oxacillin, imipenem, rifampicin and isepamicin. Daptomycin showed excellent in-vitro bactericidal activity against all the isolates tested, with no tolerance or synergism effects when combined with other agents, except with rifampicin against VISA. Vancomycin had better bactericidal activity against MRSA and MSSA than did teicoplanin. Linezolid had the poorest bactericidal activity against the isolates tested, with 100% tolerance by the MSSA and VRE isolates, and 80% tolerance by the MRSA isolates. Tolerance towards tigecycline was exhibited by 40% of the MRSA isolates, 100% of the MSSA and vancomycin-resistant E. faecalis isolates, and 90% of the vancomycin-resistant E. faecium isolates.     

Identity of streptococcal blood isolates and oral isolates from two patients with infective endocarditis.

The purpose of this study was to isolate streptococcal strains from the oral cavities of streptococcal endocarditis patients and compare these strains biochemically and genetically with the corresponding streptococcal blood isolates. Total identity was observed between the blood and oral cavity isolates from the two patients studied.     

Vancomycin‐resistant Enterococcus spp.: validation of susceptibility testing and in vitro activity of vancomycin,linezolid, tigecycline and daptomycin

Rathe M, Kristensen L, Ellermann‐Eriksen S, Thomsen MK, Schumacher H. Vancomycin‐resistant Enterococcus spp.: validation of susceptibility testing and in vitro activity of vancomycin, linezolid, tigecycline and daptomycin. APMIS 2010; 118: 66–73. Vancomycin‐resistant enterococci (VRE) have emerged to become a significant nosocomial pathogen. However, detection may be challenging and treatment possibilities are limited. Reports of resistance to linezolide, daptomycin and tigecycline underline the need for reliable susceptibility testing with respect to these compounds. We evaluated the in vitro activity of vancomycin, linezolid, daptomycin and tigecycline against a panel of VRE and vancomycin‐susceptible enterococci by broth microdilution (BMD). Etest for determination of minimum inhibitory concentration of these four antibiotics and two disc diffusion assays for detecting VRE and for susceptibility testing against tigecycline and linezolid were evaluated. Before susceptibility testing, all isolates were classified by polymerase chain reaction as vanA or vanB gene positive or vanA/B gene negative. Linezolid, daptomycin and tigecycline had excellent in vitro activity towards all isolates. For daptomycin and tigecycline, the overall agreement between BMD and Etest was suboptimal. For both disc diffusion assays, use of current break points was inadequate to detect vancomycin resistance for isolates carrying the vanB gene. Inspection of the inhibition zone for a diffuse edge, as recommended, accurately predicted presence of the vanB gene.     

Erythromycin susceptibility of viridans streptococci from the normal throat flora of patients treated with azithromycin or clarithromycin

Objective   To study the emergence of macrolide resistance in throat flora following treatment with clarithromycin or azithromycin.
Methods   Throat samples were collected before and after treatment and plated as a lawn on Columbia blood agar with an erythromycin E test strip. Minimum inhibitory concentrations (MICs) of erythromycin, clarithromycin and azithromycin were determined against isolates of distinct morphology with erythromycin E test MIC results equal to or greater than 2 mg/L. Polymerase chain reaction techniques were used to determine the genetic mechanisms of resistance.
Results   There were 749 resistant isolates of which 474 (63%) were streptococci. Only a quarter of the patients had no resistant streptococci before treatment started. There were increases in the numbers of resistant isolates and in the number of patients carrying a resistant flora during and after treatment. The most common genes identified were mef A/E in isolates with low-level resistance and erm A/M in isolates with high-level resistance.
Conclusions   There is a pool of streptococci carrying genes associated with macrolide resistance in the normal respiratory flora of generally healthy adults. Differences between the patients treated with clarithromycin and those treated with azithromycin were difficult to assess because of the large number of patients in each group with macrolide-resistant streptococci before treatment. Although there were some differences these were not statistically significant.     

Clinical significance of viridans streptococci isolated from blood cultures.

The clinical significance of viridans streptococci isolated from the blood cultures of 86 patients was determined. Isolates that were significant or suggestive of infection represented only 21% of the cases. Among 54 isolates for which the species was known, Streptococcus sanguis II was the most common. However, a significant association between species and clinical significance was not found.     

Pharmacodynamic comparison of linezolid, teicoplanin and vancomycin against clinical isolates of Staphylococcus aureus and coagulase-negative staphylococci collected from hospitals in Brazil.

Pharmacodynamic exposures, measured as the ratio of steady-state total drug area under the curve to MIC (AUC/MIC), were modelled using a 5000-patient Monte-Carlo simulation against 119 non-duplicate clinical isolates of Staphylococcus aureus and 82 coagulase-negative staphylococci (CNS) collected from hospitals in Brazil between 2003 and 2005. Pharmacodynamic targets included an AUC/MIC >82.9 for linezolid and >345 for teicoplanin and vancomycin, as well as a free drug AUC/MIC >180 for vancomycin. The cumulative fractions of response (CFRs) against all S. aureus isolates were 96.0%, 30.1%, 71.6%, 48.0% and 65.1% for linezolid 600 mg every 12 h, teicoplanin 400 mg every 24 h and 800 mg every 24 h, and vancomycin 1000 mg every 12 h and every 8 h, respectively. Using a free drug target for vancomycin improved the CFR to 94.6% for the high-dose regimen, but did not substantially alter results for the lower dose. CFRs against all CNS isolates were 97.8%, 13.4%, 34.6%, 10.9% and 31.3%, respectively, for the same antibiotic regimens. The CFR was reduced for all compounds among the methicillin-resistant isolates, except for linezolid against methicillin-resistant CNS. Sensitivity analyses did not alter the final order of pharmacodynamic potency against these isolates. Although higher doses of vancomycin and teicoplanin increased the CFR, the likelihood of achieving bactericidal targets was still lower than with linezolid. The results for the high-dose vancomycin regimen were highly dependent on the pharmacodynamic target utilised. These data suggest that linezolid has a greater probability of attaining its requisite pharmacodynamic target than teicoplanin and vancomycin against these staphylococci.     

Prevalence of penicillin-resistant viridans streptococci in healthy children and in patients with malignant haematological disorders

The prevalence of penicillin-resistant viridans streptococci was studied in healthy children and in paediatric and adult patients with leukaemia to determine whether the frequent presence of penicillin-resistant streptococci in the oral cavity of children with leukaemia is the result of antibiotic therapy. Twenty of the oral swabs from 50 healthy children who had not received antibiotics in the three months prior to sampling yielded viridans streptococci that could be cultured on blood agar containing 2 µg/ml benzylpenicillin. In 11 of the 20 cases the streptococci were resistant to penicillin (MIC4µg/ml). This prevalence is significantly higher than that found in adult leukaemia patients (40 % vs. 5 %) but is about the same as that found in paediatric patients with leukaemia. The high prevalence of penicillin-resistant streptococci in the paediatric age group should be considered when selecting therapy and prophylaxis, especially when the risk of infection with one of these cocci is enhanced.     

Effects of vancomycin, teicoplanin, linezolid, quinupristin-dalfopristin, and telithromycin on murine gut colonization by Candida albicans.

Crl:CDI(ICR) BR adult mice were fed chow containing Candida albicans or regular chow. Both groups were subsequently given either antibiotics acting mainly against Gram-positive organisms or normal saline for 10 days. Stool cultures were performed before, at the end, and one week after discontinuation of treatment to determine the effects on the stool yeast concentration. Candida colonized mice treated with vancomycin, teicoplanin, linezolid, quinupristin-dalfopristin or telithromycin had higher colony counts of yeast in their stools than control Candida fed mice treated with saline. This increase was not statistically significant. Mice fed regular chow treated with the study drugs or saline did not have any yeasts in their stools. Dissemination of Candida was not observed in the visceral organs of any mouse.     

Characterization of Aerococcus viridans isolates from swine clinical specimens

We present here the biochemical and genetic characterization and antimicrobial susceptibility of 58 isolates of Aerococcus viridans isolated in pure culture from different clinical specimens of normally sterile body sites of pigs. A. viridans isolates were commonly susceptible to beta-lactam antimicrobials and exhibited a great genetic heterogeneity as determined by pulsed-field gel electrophoresis typing. The results indicate that A. viridans might be included in the list of possible etiological agents causing disease in pigs.