Antimicrobial susceptibility changes in Enterococcus faecalis following various penicillin exposure regimens

The two-component system CroRS acts as a master regulator of cell envelope homeostasis to confer antimicrobial tolerance in the bacterial pathogen Enterococcus faecalis

Antimicrobial tolerance is the ability of a microbial population to survive, but not proliferate, during antimicrobial exposure. Significantly, it has been shown to precede the development of bona fide antimicrobial resistance. We have previously identified the two-component system CroRS as a critical regulator of tolerance to antimicrobials like teixobactin in the bacterial pathogen Enterococcus faecalis. To understand the molecular mechanism of this tolerance, we have carried out RNA-seq analyses in the E. faecalis wild-type and isogenic∆ croRS mutant to determine the teixobactin-induced CroRS regulon. We identified a 132 gene CroRS regulon and demonstrate that CroRS upregulates biosynthesis of all major components of the enterococcal cell envelope in response to teixobactin. This suggests a coordinating role of this regulatory system in maintaining integrity of the multiple layers of the enterococcal envelope during antimicrobial stress, likely contributing to bacterial survival. Using experimental evolution, we observed that truncation of HppS, a key enzyme in the synthesis of the quinone electron carrier demethylmenaquinone, was sufficient to rescue tolerance in the croRS deletion strain. This highlights a key role for isoprenoid biosynthesis in antimicrobial tolerance in E. faecalis. Here, we propose a model of CroRS acting as a master regulator of cell envelope biogenesis and a gate-keeper between isoprenoid biosynthesis and respiration to ensure tolerance against antimicrobial challenge.

Pheromone Activity after Stimulation with Ampicillin in a Plasmid-Free Enterococcus faecalis Strain

Enterococci exhibit clumping under the selective pressure of antibiotics. The aim of this study was to analyze the effect of supernatants from a plasmid-free clone (C29) of Enterococcus faecalis subjected to 0.25×, 0.5×, and 0.75× of the minimal inhibitory concentration (MIC) of ampicillin on the expression of an aggregation substance (AS) by a donor plasmid clone (1390R). A clumping assay was performed. The relative expression of prgB (gene that encodes AS) was determined and semiquantified in 1390R, and iad1 expression was determined and semiquantified in C29. AS expression was analyzed in the stimulated 1390R cells by confocal microscopy, flow cytometry, and ELISA. Adherence was also measured. Maximal clumping was observed with the pheromone medium 0.25×. Only the 1390R strain stimulated with the C29 supernatant without ampicillin and with 0.25× was able to express prgB. No expression of prgB was observed at 0.5× and 0.75×. The difference in relative expression (RE) of 1390R without ampicillin and with 0.25× was 0.5-fold. AS expression in 1390R showed the greatest increase upon stimulation with 0.25×. When 1390R was stimulated with 0.5× and 0.75×, AS expression was also observed but was significantly lower. Ampicillin stimulated C29 switch-off pheromone expression in recipient cells, which in turn switched off AS expression in donor cells. We observed that although prgB was switched off after 0.5× stimulation in C29, the supernatants induced expression in certain 1390R strains. In conclusion, ampicillin was able to modulate pheromone expression in free plasmid clones which, in turn, modulated AS expression in plasmid donor cells. The fact that PrgB gene expression was switched off after the ampicillin stimulus at 0.5× MIC, whereas AS proteins were present on the surface of the bacteria, suggested that a mechanism of rescue associated with mechanism pheromone sensing may be involved.

Antimicrobial tolerance and its role in the development of resistance: Lessons from enterococci

Bacteria have developed resistance against every antimicrobial in clinical use at an alarming rate. There is a critical need for more effective use of antimicrobials to both extend their shelf life and prevent resistance from arising. Significantly, antimicrobial tolerance, i.e., the ability to survive but not proliferate during antimicrobial exposure, has been shown to precede the development of bona fide antimicrobial resistance (AMR), sparking a renewed and rapidly increasing interest in this field. As a consequence, problematic infections for the first time are now being investigated for antimicrobial tolerance, with increasing reports demonstrating in-host evolution of antimicrobial tolerance. Tolerance has been identified in a wide array of bacterial species to all bactericidal antimicrobials. Of particular interest are enterococci, which contain the opportunistic bacterial pathogens Enterococcus faecalis and Enterococcus faecium. Enterococci are one of the leading causes of hospital-acquired infection and possess intrinsic tolerance to a number of antimicrobial classes. Persistence of these infections in the clinic is of growing concern, particularly for the immunocompromised. Here, we review current known mechanisms of antimicrobial tolerance, and include an in-depth analysis of those identified in enterococci with implications for both the development and prevention of AMR.

Low-Molecular-Weight Polylysines with Excellent Antibacterial Properties and Low Hemolysis

The steady development of bacterial resistance has become a global public health issue, and new antibacterial agents that are active against drug-resistant bacteria and less susceptible to bacterial resistance are urgently needed. Here, a series of low-molecular-weight cationic polylysines (C_x-PLL_n) with different hydrophobic end groups (C_x) and degrees of polymerization (PLL_n) was synthesized and used in antibacterial applications. All the obtained C_x-PLL_n have antibacterial activity. Among them, C₆-PLL₁₃ displays the best antibacterial effect for Gram-positive bacteria, that is, Staphylococcus aureus (S. aureus) and methicillin-resistant Staphylococcus aureus (MRSA), and highest selectivity against Gram-positive bacteria. A mechanistic study revealed that the C₆-PLL₁₃ destroys the integrity of the bacterial cell membrane and causes effective bacterial death. Owing to this membrane-disrupting property, C₆-PLL₁₃ showed rapid bacterial killing kinetics and was not likely to develop resistance after repeat treatment (up to 13 generations). Moreover, C₆-PLL₁₃ demonstrated a significant therapeutic effect on an MRSA infection mouse model, which further proved that this synthetic polymer could be used as an effective weapon against bacterial infections.

The Enterococcus: a Model of Adaptability to Its Environment

The genus Enterococcus comprises a ubiquitous group of Gram-positive bacteria that are of great relevance to human health for their role as major causative agents of health care-associated infections. The enterococci are resilient and versatile species able to survive under harsh conditions, making them well adapted to the health care environment. Two species cause the majority of enterococcal infections: Enterococcus faecalis and Enterococcus faecium Both species demonstrate intrinsic resistance to common antibiotics, such as virtually all cephalosporins, aminoglycosides, clindamycin, and trimethoprim-sulfamethoxazole. Additionally, a remarkably plastic genome allows these two species to readily acquire resistance to further antibiotics, such as high-level aminoglycoside resistance, high-level ampicillin resistance, and vancomycin resistance, either through mutation or by horizontal transfer of genetic elements conferring resistance determinants.

In vitro/vivo Mechanism of Action of MP1102 With Low/Nonresistance Against Streptococcus suis Type 2 Strain CVCC 3928

Streptococcosis is recognized as a leading infectious disease in the swine industry. Streptococcus suis serotype 2 is regarded as the most virulent species, which threatens human and pig health and causes serious economic losses. In this study, multiple in vitro and in vivo effects of MP1102 on multidrug resistant S. suis was studied for the first time. MP1102 exhibited significant antibacterial activity against S. suis (minimum inhibitory concentration, MIC = 0.028–0.228 μM), rapid bacteriocidal action, a longer postantibiotic effect than ceftriaxone, and a synergistic or additive effect with lincomycin, penicillin, and ceftriaxone (FICI = 0.29–0.96). No resistant mutants appeared after 30 serial passages of S. suis in the presence of MP1102. Flow cytometric analysis and electron microscopy observations showed that MP1102 destroyed S. suis cell membrane integrity and affected S. suis cell ultrastructure and membrane morphology. Specifically, a significantly wrinkled surface, intracellular content leakage, and cell lysis were noted, establishing a cyto-basis of nonresistance to this pathogen. DNA gel retardation and circular dichroism analysis indicated that MP1102 interacted with DNA by binding to DNA and changing the DNA conformation, even leading to the disappearance of the helical structure. This result further supported the mechanistic basis of nonresistance via interaction with an intracellular target, which could serve as a means of secondary injury after MP1102 is transported across the membrane. Upon treatment with 2.5–5.0 mg/kg MP1102, the survival of mice challenged with S. suis was 83.3–100%. MP1102 decreased bacterial translocation in liver, lung, spleen, and blood; inhibited the release of interleukin-1β and tumor necrosis factor-α; and relieved the lung, liver, and spleen from acute injury induced by S. suis. These results suggest that MP1102 is a potent novel antibacterial agent for the treatment of porcine streptococcal disease.

Potential effect of selective pressure with different β-lactam molecules on the emergence of reduced susceptibility to β-lactams in group B Streptococci

In this study, the selective potential of group B Streptococcus isolates with reduced penicillin susceptibility (PRGBS) in a neonate-hypervirulent sequence type (ST)17 lineage was investigated by in vitro exposure to β-lactams. After 19 passages of stepwise penicillin exposure, PRGBS with a high penicillin minimum inhibitory concentration MIC (0.5 mg/L), greatly augmented ceftibuten MIC (>512 mg/L), and acquisition of G406D predicted to provide destabilizing effect (ΔΔG 0.099 kcal/mol) on PBP2X structure were identified. In early passages of stepwise cefotaxime exposure, PRGBS possessing G398E predicted to stabilize PBP2X (ΔΔG -0.038 kcal/mol) emerged with high MICs for cefotaxime (0.5 mg/L), ceftibuten (>512 mg/L) and penicillin (0.25 mg/L). Additionally, G398E + G329V + H438Y predicted to provide more stabilizing effect (ΔΔG -0.415 kcal/mol) were detected in mutants with higher MICs to cefotaxime (1 mg/L) and penicillin (0.5 mg/L). PRGBS mutants selected by penicillin and cefotaxime had a marked growth disadvantage compared with the parent strain. After two passages of stepwise ceftibuten exposure, the mutants exhibited increased MICs toward ceftibuten and acquisition of T555S predicted to provide stabilizing effect (ΔΔG -0.111 kcal/mol) in PBP 2X. In subsequent passages, gradual increases in ceftibuten MICs from 128 mg/L to 512 mg/L were found among selected mutants with accompanying stabilizing T555S+A354V (ΔΔG -0.257 kcal/mol) followed by stabilizing T555S + A354V + A536V (ΔΔG -0.322 kcal/mol), resulting in selection of a penicillin-susceptible group B Streptococcus lineage with reduced ceftibuten susceptibility (CTB^r PSGBS). Notably, growth ability of CTB^r PSGBS mutants was comparable to that of the parent strain. These findings may predict future failure of treatment for neonatal invasive infections caused by the neonate-hypervirulent PRGBS ST17 lineage.

High Prevalence of Sterile Pyuria in the Setting of Sexually Transmitted Infection in Women Presenting to an Emergency Department

Introduction

The clinical presentations for sexually transmitted infections (STI) and urinary tract infections (UTI) often overlap, and symptoms of dysuria and urinary frequency/urgency occur with both STIs and UTIs. Abnormal urinalysis (UA) findings and pyuria are common in both UTIs and STIs, and confirmatory urine cultures are not available to emergency clinicians to aid in decision-making regarding prescribing antibiotics for UTIs. The objective of this study was to determine the frequency of sterile pyuria in women with confirmed STIs, as well as whether the absolute number of leukocytes on microscopy or nitrite on urine dipstick correlated with positive urine cultures in patients with confirmed STIs. We also sought to determine how many patients with STIs were inappropriately prescribed a UTI antibiotic.

Methods

We performed a retrospective chart review of patients aged 18–50 who had a urinalysis and pelvic examination in the emergency department (including cervical cultures), and tested positive for Neisseria gonorrhoeae, Chlamydia trachomatis, and/or Trichomonas vaginalis. Descriptive statistics were obtained for all variables, and associations between various findings were sought using the Fisher’s exact test for categorical variables. We calculated comparison of proportions using the N-1 chi-squared analysis.

Results

A total of 1,052 female patients tested positive for Neisseria gonorrhoeae, Chlamydia trachomatis, and/or Trichomonas vaginalis and were entered into the database. The prevalence of pyuria in all cases was 394/1,052, 37% (95% confidence interval [CI] [0.34–0.40]). Of the cases with pyuria, 293/394, 74% (95% CI [0.70–0.78]) had sterile pyuria with negative urine cultures. The prevalence of positive urine cultures in our study population was 101/1,052, 9.6% (95% CI [0.08–0.11]). Culture positive urines had a mean of 34 leukocytes per high-power field, and culture negative urines had a mean of 24 leukocytes per high-power field, with a difference of 10, (95% CI [3.46–16.15]), which was statistically significant (p=0.003). Only 123 cases tested positive for nitrite on the urinalysis dipstick; 50/123, 41% (95% CI [0.32–0.49]) had positive urine cultures, and 73/123, 59% (95% CI [0.51–0.68]) had negative urine cultures. Nitrite-positive urines were actually 18% more likely to be associated with negative urine cultures in the setting of positive STI cases, (95% CI [4.95–30.42], p=0.0048). Antibiotics were prescribed for 295 patients with suspected UTI. Of these, 195/295, 66% (95% CI [0.61–0.71]) had negative urine cultures, and 100/295, 34% (0.33, 95% CI [0.28–0.39]) had positive urine cultures. Chi-square analysis yielded a difference of these proportions of 32% (95% CI [23.92–39.62], p<0.0001).

Conclusion

This study demonstrated that in female patients with STIs who have pyuria, there is a high prevalence of sterile pyuria. Our results suggest that reliance on pyuria or positive nitrite for the decision to add antimicrobial therapy empirically for a presumed urinary tract infection in cases in which an STI is confirmed or highly suspected is likely to result in substantial over-treatment.

Microbiome Heterogeneity Characterizing Intestinal Tissue and Inflammatory Bowel Disease Phenotype

Inflammatory bowel disease has been associated with differential abundance of numerous organisms when compared to healthy controls (HCs); however, few studies have investigated variability in the microbiome across intestinal locations and how this variability might be related to disease location and phenotype. In this study, we have analyzed the microbiome of a large cohort of individuals recruited at Mount Sinai Hospital in Toronto, Canada. Biopsies were taken from subjects with Crohn's disease, ulcerative colitis, and HC, and also individuals having undergone ileal pouch-anal anastomosis for treatment of ulcerative colitis or familial adenomatous polyposis. Microbial 16S rRNA was sequenced using the Illumina MiSeq platform. We observed a great deal of variability in the microbiome characterizing different sampling locations. Samples from pouch and afferent limb were comparable in microbial composition. When comparing sigmoid and terminal ileum samples, more differences were observed. The greatest number of differentially abundant microbes was observed when comparing either pouch or afferent limb samples to sigmoid or terminal ileum. Despite these differences, we were able to observe modest microbial variability between inflammatory bowel disease phenotypes and HCs, even when controlling for sampling location and additional experimental factors. Most detected associations were observed between HCs and Crohn's disease, with decreases in specific genera in the families Ruminococcaceae and Lachnospiraceae characterizing tissue samples from individuals with Crohn's disease. This study highlights important considerations when analyzing the composition of the microbiome and also provides useful insight into differences in the microbiome characterizing these seemingly related phenotypes.

Antimicrobials: a global alliance for optimizing their rational use in intra-abdominal infections (AGORA)

Intra-abdominal infections (IAI) are an important cause of morbidity and are frequently associated with poor prognosis, particularly in high-risk patients. The cornerstones in the management of complicated IAIs are timely effective source control with appropriate antimicrobial therapy. Empiric antimicrobial therapy is important in the management of intra-abdominal infections and must be broad enough to cover all likely organisms because inappropriate initial antimicrobial therapy is associated with poor patient outcomes and the development of bacterial resistance. The overuse of antimicrobials is widely accepted as a major driver of some emerging infections (such as C. difficile), the selection of resistant pathogens in individual patients, and for the continued development of antimicrobial resistance globally. The growing emergence of multi-drug resistant organisms and the limited development of new agents available to counteract them have caused an impending crisis with alarming implications, especially with regards to Gram-negative bacteria. An international task force from 79 different countries has joined this project by sharing a document on the rational use of antimicrobials for patients with IAIs. The project has been termed AGORA (Antimicrobials: A Global Alliance for Optimizing their Rational Use in Intra-Abdominal Infections). The authors hope that AGORA, involving many of the world's leading experts, can actively raise awareness in health workers and can improve prescribing behavior in treating IAIs.

Enterococcal endocarditis revisited

The Enterococcus species is the third main cause of infective endocarditis (IE) worldwide, and it is gaining relevance, especially among healthcare-associated cases. Patients with enterococcal IE are older and have more comorbidities than other types of IE. Classical treatment options are limited due to the emergence of high-level aminoglycosides resistance (HLAR), vancomycin resistance and multidrug resistance in some cases. Besides, few new antimicrobial alternatives have shown real efficacy, despite some of them being recommended by major guidelines (including linezolid and daptomycin). Ampicillin plus ceftriaxone 2 g iv./12 h is a good option for Enterococcus faecalis IE caused by HLAR strains, but randomized clinical trials are essential to demonstrate its efficacy for non-HLAR EFIE and to compare it with ampicillin plus short-course gentamicin. The main mechanisms of resistance and treatment options are also reviewed for other enterococcal species.

Vancomycin-resistant enterococci: Troublemaker of the 21st century

The emergence of multidrug-resistant and vancomycin-resistant enterococci during the last decade has made it difficult to treat nosocomial infections. Although various enterococcal species have been identified, only two (Enterococcus faecalis and Enterococcus faecium) are responsible for the majority of human infections. Vancomycin is an important therapeutic alternative against multidrug-resistant enterococci but is associated with a poor prognosis. Resistance to vancomycin dramatically reduces the therapeutic options for enterococcal infections. The bacterium develops resistance by modifying the C-terminal d-alanine of peptidoglycan to d-lactate, creating a d-Ala-d-Lac sequence that effectively reduces the affinity of vancomycin for the peptidoglycan by 1000-fold. Moreover, the resistance genes can be transferred from enterococci to Staphylococcus aureus, thereby posing a threat to patient safety and also a challenge for treating physicians. Judicious use of vancomycin and broad-spectrum antibiotics must be implemented, but strict infection control measures must also be followed to prevent nosocomial transmission of these organisms. Furthermore, improvements in clinical practice, rotation of antibiotics, herbal drugs, nanoantibiotics and the development of newer antibiotics based on a pharmacogenomic approach may prove helpful to overcome dreadful vancomycin-resistant enterococcal infections.

Intrinsic and acquired resistance mechanisms in enterococcus

Enterococci have the potential for resistance to virtually all clinically useful antibiotics. Their emergence as important nosocomial pathogens has coincided with increased expression of antimicrobial resistance by members of the genus. The mechanisms underlying antibiotic resistance in enterococci may be intrinsic to the species or acquired through mutation of intrinsic genes or horizontal exchange of genetic material encoding resistance determinants. This paper reviews the antibiotic resistance mechanisms in Enterococcus faecium and Enterococcus faecalis and discusses treatment options.

Treatment of resistant enterococcal urinary tract infections

Enterococcus spp have emerged as important pathogens in urinary tract infection (UTI), especially in hospitalized patients. Resistance to multiple antibiotics, including vancomycin, has become common, particularly in infections involving Enterococcus faecium. The management of UTIs caused by Enterococcus spp has become challenging given the presence of underlying comorbidities in these patients and the limited therapeutic options available to treat multidrug-resistant (MDR) Enterococcus. Routine therapy for asymptomatic bacteriuria with MDR-Enterococcus is not recommended. Removal of indwelling urinary catheters should be considered. Appropriate antibiotic therapy selection should be guided by urine culture and susceptibility results. Data are limited on the treatment of UTIs caused by MDR-Enterococcus. Potential oral agents active against MDR-Enterococcus that may be considered for acute uncomplicated UTI include nitrofurantoin, fosfomycin, and fluoroquinolones. Potential parenteral agents for the treatment of pyelonephritis and complicated UTIs caused by MDR-Enterococcus include daptomycin, linezolid, and quinipristin-dalfopristin. Aminoglycosides or rifampin may be considered as adjunctive therapy in serious infections.

Contribution of the autolysin AtlA to the bactericidal activity of amoxicillin against Enterococcus faecalis JH2-2

The bactericidal activity of amoxicillin was investigated against Enterococcus faecalis JH2-2 and against an isogenic mutant deficient in the production of the N-acetylglucosaminidase AtlA. Comparison of the two strains indicated that this autolysin contributes to killing by amoxicillin both in vitro and in a rabbit model of experimental endocarditis.

Characterization of the inhibitory effect of voriconazole on the fungicidal activity of amphotericin B against Candida albicans in an in vitro kinetic model

OBJECTIVES

The aim of the present investigation was to study and characterize the effect of voriconazole on the fungicidal activity of amphotericin B.

METHODS

Four strains of Candida albicans susceptible to voriconazole were exposed to voriconazole and amphotericin B, either alone, simultaneously or sequentially in an in vitro kinetic model. Bolus doses resulting in voriconazole and amphotericin B concentrations of 0.005-5 and 2.5 mg/L, respectively, were administered. Antifungal-containing RPMI 1640 was eliminated and replaced by a fresh medium using a peristaltic pump, with a flow rate adjusted to obtain the desired half-lives. With two drugs tested, a computer-controlled dosing pump compensated for differences in the elimination rates. Using static time-kill methodology, one C. albicans strain was exposed to 5 mg/L voriconazole for varying durations followed by 2.5 mg/L amphotericin B after three repeated washes of voriconazole.

RESULTS

Voriconazole and amphotericin B treatment alone resulted in fungistatic and fungicidal activities, respectively. Simultaneous administration of voriconazole and amphotericin B resulted in fungicidal activity, whereas only fungistatic activity was observed when repeated doses of amphotericin B were administered sequentially after voriconazole at 24-96 h. The inhibition of the fungicidal activity of amphotericin B was voriconazole dose-dependent, but seemed to be recovered once the voriconazole concentration fell below the MIC. The fungicidal activity was quickly regained after the removal of voriconazole, irrespective of the duration of voriconazole pre-exposure.

CONCLUSIONS

Voriconazole inhibited the fungicidal effect of sequentially administered amphotericin B in a concentration- and time-dependent manner; the clinical significance of this needs further investigation.

Persistence of Borrelia burgdorferi following antibiotic treatment in mice

The effectiveness of antibiotic treatment was examined in a mouse model of Lyme borreliosis. Mice were treated with ceftriaxone or saline solution for 1 month, commencing during the early (3 weeks) or chronic (4 months) stages of infection with Borrelia burgdorferi. Tissues from mice were tested for infection by culture, PCR, xenodiagnosis, and transplantation of allografts at 1 and 3 months after completion of treatment. In addition, tissues were examined for the presence of spirochetes by immunohistochemistry. In contrast to saline solution-treated mice, mice treated with antibiotic were consistently culture negative, but tissues from some of the mice remained PCR positive, and spirochetes could be visualized in collagen-rich tissues. Furthermore, when some of the antibiotic-treated mice were fed on by Ixodes scapularis ticks (xenodiagnosis), spirochetes were acquired by the ticks, as determined based upon PCR results, and ticks from those cohorts transmitted spirochetes to naïve SCID mice, which became PCR positive but culture negative. Results indicated that following antibiotic treatment, mice remained infected with nondividing but infectious spirochetes, particularly when antibiotic treatment was commenced during the chronic stage of infection.

Infective endocarditis: a review of the best treatment options

Despite significant advances in antimicrobial therapy and an enhanced ability to diagnose and treat complications, infective endocarditis is still associated with substantial morbidity and mortality today, and its incidence has not decreased over the past decades. This apparent paradox may be explained by a progressive change in risk factors, leading to an evolution in its epidemiological and clinical features. In fact, new risk factors for endocarditis have emerged, such as intravenous drug abuse, diffusion of heart surgery procedures and prosthetic valve implantation, atherosclerotic valve disease in elderly patients, and nosocomial disease. Recently identified microorganisms (including Bartonella spp., Abiotrophia defectiva, and the HACEK group of bacteria [including Haemophilus spp., Actinobacillus spp., Cardiobacterium hominis, Eikenella corrodens and Kingella kingae]) are sometimes the cause of culture-negative endocarditis, and emerging resistant bacteria (such as methicillin- or vancomycin-resistant Staphylococci and vancomycin-resistant Enterococci) are becoming a new challenge for conventional antibiotic therapy. New therapeutic approaches need to be developed for the treatment of infective endocarditis caused by drug-resistant Gram-positive cocci, and some antimicrobial compounds recently introduced in clinical practice (such as streptogramins and oxazolidinones) may be an effective alternative, but further clinical studies are needed in order to confirm their effectiveness and safety. This review should help redefine the best therapeutic and preventive strategies against infective endocarditis.

Vancomycin-resistant enterococci: a road map on how to prevent the emergence and transmission of antimicrobial resistance

Nosocomial acquisition of microorganisms resistant to multiple antibiotics represents a threat to patient safety. Here we review the mechanisms that have allowed highly resistant strains belonging to the Enterococcus genus to proliferate within our health-care institutions. These mechanisms indicate that decreasing the prevalence of resistant organisms requires active surveillance, adherence to vigorous isolation, hand hygiene and environmental decontamination measures, and effective antibiotic stewardship. We suggest how to tailor such a complex, multidisciplinary program to the needs of a particular health-care setting so as to maximize cost-effectiveness.

Production of enterolysin A by a raw milk enterococcal isolate exhibiting multiple virulence factors

Even though enterococci are a common cause of human infection they can readily be isolated from a range of food sources, including various meat and dairy products. An enterococcal strain, DPC5280, which exhibits a broad spectrum of inhibition against many Gram-positive bacteria was recently isolated from an Irish raw milk sample. Characterization of the inhibition revealed that the strain exhibits haemolytic activity characteristic of the two-component lantibiotic cytolysin and also produces a heat-labile antimicrobial protein of 34 kDa. The latter protein displayed cell wall hydrolytic activity, as evidenced by zymogram gels containing autoclaved lactococcal cells. N-terminal sequencing of the purified protein yielded the sequence ASNEWS which is 100 % identical to enterolysin A (accession no. AF249740), a protein which shares 28 and 29 % identity to the Gly-Gly endopeptidases, lysostaphin and zoocin A, respectively. Indeed, amplification of entL from DPC5280 and sequencing revealed that the protein is 100 % identical to enterolysin A. The DPC5280 strain also contained the determinants associated with multiple virulence factors, including gelatinase, aggregation substance and multiple antibiotic resistance. The linkage of this cell-wall-degrading enzyme to other virulence factors in enterococci may contribute to the competitiveness of pathogenic enterococci when found in complex microbial environments such as food and the gastrointestinal tract.

Prevalence of the fsr locus in Enterococcus faecalis infections

The fsr locus of Enterococcus faecalis confers virulence in animal models. A retrospective analysis of fsr prevalence in diverse E. faecalis clinical isolates demonstrated fsr in all endocarditis isolates versus 53% of stool isolates (P = 0.005). This supports a role for fsr-mediated virulence in the pathogenesis of enterococcal infections in humans.

Enterococcal endocarditis in Sweden, 1995-1999: can shorter therapy with aminoglycosides be used?

A 5-year nationwide prospective study in Sweden during 1995-1999 identified 881 definite episodes of infective endocarditis. Definite enterococcal endocarditis was diagnosed in 93 episodes (11%), the largest series of enterococcal endocarditis so far presented. Mortality during treatment was 16%, the relapse rate was 3%, and clinical cure was achieved in the remaining 81% of the episodes. Clinical cure was achieved with a median duration of cell wall-active antimicrobial therapy of 42 days combined with an aminoglycoside (median treatment time, 15 days). International guidelines generally recommend a 4-6-week combined synergistic treatment course with a cell wall-active antibiotic and an aminoglycoside. Treatment regimens in Sweden often include a shortened aminoglycoside treatment course in order to minimize adverse effects in older patients. Fatal outcome seemed not to be due to the shortened aminoglycoside therapy course. In many enterococcal endocarditis episodes, duration of aminoglycoside therapy could probably be shortened to 2-3 weeks.

Vancomycin-resistant enterococci: mechanisms and clinical observations

Enterococci are not generally regarded as highly virulent bacterial pathogens. However, resistance to many antimicrobial drugs complicates treatment of enterococcal infections. Acquired resistance to high concentrations of glycopeptide antibiotics, specifically vancomycin, has exacerbated this problem. This article seeks to concisely review the mechanisms of that resistance and its effects on clinical management of enterococcal infections, as well as clinical microbiology and infection control.

Efficacy of ampicillin plus ceftriaxone in treatment of experimental endocarditis due to Enterococcus faecalis strains highly resistant to aminoglycosides

The purpose of this work was to evaluate the in vitro possibilities of ampicillin-ceftriaxone combinations for 10 Enterococcus faecalis strains with high-level resistance to aminoglycosides (HLRAg) and to assess the efficacy of ampicillin plus ceftriaxone, both administered with humanlike pharmacokinetics, for the treatment of experimental endocarditis due to HLRAg E. faecalis. A reduction of 1 to 4 dilutions in MICs of ampicillin was obtained when ampicillin was combined with a fixed subinhibitory ceftriaxone concentration of 4 micrograms/ml. This potentiating effect was also observed by the double disk method with all 10 strains. Time-kill studies performed with 1 and 2 micrograms of ampicillin alone per ml or in combination with 5, 10, 20, 40, and 60 micrograms of ceftriaxone per ml showed a > or = 2 log10 reduction in CFU per milliliter with respect to ampicillin alone and to the initial inoculum for all 10 E. faecalis strains studied. This effect was obtained for seven strains with the combination of 2 micrograms of ampicillin per ml plus 10 micrograms of ceftriaxone per ml and for six strains with 5 micrograms of ceftriaxone per ml. Animals with catheter-induced endocarditis were infected intravenously with 10(8) CFU of E. faecalis V48 or 10(5) CFU of E. faecalis V45 and were treated for 3 days with humanlike pharmacokinetics of 2 g of ampicillin every 4 h, alone or combined with 2 g of ceftriaxone every 12 h. The levels in serum and the pharmacokinetic parameters of the humanlike pharmacokinetics of ampicillin or ceftriaxone in rabbits were similar to those found in humans treated with 2 g of ampicillin or ceftriaxone intravenously. Results of the therapy for experimental endocarditis caused by E. faecalis V48 or V45 showed that the residual bacterial titers in aortic valve vegetations were significantly lower in the animals treated with the combinations of ampicillin plus ceftriaxone than in those treated with ampicillin alone (P < 0.001). The combination of ampicillin and ceftriaxone showed in vitro and in vivo synergism against HLRAg E. faecalis.

Effect of disruption of a gene encoding an autolysin of Enterococcus faecalis OG1RF

A mutant (TX5127) of Enterococcus faecalis OG1RF was generated by disruption mutagenesis of a previously described autolysin gene. TX5127 formed longer chains (2 to 10 cells per chain) than wild-type OG1RF (mainly single cells) during growth in broth even though it had a growth rate similar to that of the parental strain as measured by turbidity and cell count. Autolysin activity, as defined by the ability to lyse heat-killed Micrococcus lysodeikticus cells, was absent in TX5127, while this activity was easily detectable in OG1RF. However, disruption of this autolysin gene did not block the ability of TX5127 to hydrolyze E. faecalis cell walls compared to that of OG1RF. The autolysis rate of cells of TX5127 in 10 mM sodium phosphate buffer (pH 6.8) was slower than that of wild-type OG1RF. TX5127 also showed a decreased rate of lysis in the presence of penicillin, as measured by changes in the turbidity of the culture during 24 h of incubation at 37 degrees C and a slightly decreased effect of penicillin as measured by time-kill curves. The virulence of TX5127 was similar to that of OG1RF in the mouse peritonitis model, indicating that the autolysin of E. faecalis is not important for infection in this model.

Diversity among multidrug-resistant enterococci

Enterococci are associated with both community- and hospital-acquired infections. Even though they do not cause severe systemic inflammatory responses, such as septic shock, enterococci present a therapeutic challenge because of their resistance to a vast array of antimicrobial drugs, including cell-wall active agents, all commercially available aminoglycosides, penicillin and ampicillin, and vancomycin. The combination of the latter two occurs disproportionately in strains resistant to many other antimicrobial drugs. The propensity of enterococci to acquire resistance may relate to their ability to participate in various forms of conjugation, which can result in the spread of genes as part of conjugative transposons, pheromone-responsive plasmids, or broad host-range plasmids. Enterococcal hardiness likely adds to resistance by facilitating survival in the environment (and thus enhancing potential spread from person to person) of a multidrug-resistant clone. The combination of these attributes within the genus Enterococcus suggests that these bacteria and their resistance to antimicrobial drugs will continue to pose a challenge.

Characterization of clinical isolates of beta-lactamase-negative, highly ampicillin-resistant Enterococcus faecalis

We analyzed the penicillin-binding protein (PBP) profiles of two clinical isolates of Enterococcus faecalis for which ampicillin MICs were 32 and 64 micrograms/ml. Six PBPs were detected in both isolates, demonstrating an apparently increased amount of PBP 5 and decreased penicillin binding of PBPs 1 and 6. These results suggest that ampicillin resistance in the clinical isolates of E. faecalis described could be associated with alterations in different PBPs.

Synergistic effect of amoxicillin and cefotaxime against Enterococcus faecalis

The antibacterial efficacy of the combination of amoxicillin and cefotaxime was assessed against 50 clinical strains of Enterococcus faecalis. For 48 of 50 strains, the MIC of amoxicillin that inhibited 50% of isolates tested decreased from 0.5 microgram/ml (range, 0.25 to 1 microgram/ml) to 0.06 microgram/ml (range, 0.01 to 0.25 microgram/ml) in the presence of only 4 micrograms of cefotaxime per ml. Alternatively, the MIC of cefotaxime that inhibited 50% of isolates tested decreased from 256 micrograms/ml (range, 8 to 512 micrograms/ml) to 1 micrograms/ml (range, 0.5 to 16 micrograms/ml) in the presence of only 0.06 microgram of amoxicillin per ml. For JH2-2, a reference strain of E. faecalis, the MICs of amoxicillin, cefotaxime, and amoxicillin in the presence of cefotaxime (4 micrograms/ml) were 0.5, 512, and 0.06 microgram/ml, respectively. By using a penicillin-binding protein (PBP) competition assay, it was shown that with cefotaxime, 50% saturation of PBPs 2 and 3 was obtained at very low concentrations (< 1 microgram/ml), while 50% saturation of PBPs 1, 4, and 5 was obtained with > or = 128 micrograms/ml. With amoxicillin, 50% saturation of PBPs 4 and 5 was obtained at 0.12 and 0.5 microgram/ml, respectively. Therefore, the partial saturation of PBPs 4 and 5 by amoxicillin combined with the total saturation of PBPs 2 and 3 by cefotaxime could be responsible for the observed synergy between these two compounds.

Antibiotic selection factors and description of a hospital-based outpatient antibiotic therapy program in the USA

A variety of pharmacodynamic, pharmacokinetic and drug stability factors can influence the choice of drug, the dosing regimen and the method of drug administration for out-patient parenteral antibiotic therapy (OPAT). Beta-lactam antibiotics exhibit little if any concentration-dependent killing and produce short-term or no persistent effects with most bacterial pathogens. Optimal dosing regimens for these agents should provide serum levels that continually exceed the minimal inhibitory concentration (MIC) of the pathogen. Beta-lactam agents with long half-lives (greater than 2 hours) can provide these levels with intermittent dosing once or twice daily. Beta-lactam agents with shorter half-lives can be administered by programmable pumps or by continuous infusion providing the drug is sufficiently stable to degradation in solution. Imipenem and ampicillin are examples of drugs with short half-lives that are unstable in solution and must be dosed intermittently. Intramuscular administration slows absorption and can also prolong the length of time during which serum levels exceed the MIC of infecting bacteria. Aminoglycosides and fluoroquinolones, on the other hand, exhibit concentration-dependent killing and produce prolonged persistent effects. Optimal dosage regimens of these drugs should maximize serum levels. Once-daily dosing regimens for the aminoglycosides meet this goal and also appear to reduce drug-induced nephrotoxicity. Application of these principles to drug selection and administration in a hospital-based OPAT program has provided efficacious therapy and a low incidence of adverse reactions in an elderly population distributed over a wide geographic area.

Beta-lactam antibiotics: is continuous infusion the preferred method of administration?

OBJECTIVE

To examine the pharmacodynamic properties of the beta-lactam class of antibiotics and the rationale for their continuous infusion (CI), and to explore reasons that this mode of administration has not replaced intermittent infusion as the standard of practice.

DATA SOURCES

A Medline search of the English-language literature evaluating CI administration of beta-lactam antibiotics was conducted. Bibliographic searches of these articles also were performed.

STUDY SELECTION

Because there were few human trials, all available trials were considered for review. A cross section of clinical trials, animal studies, and in vitro studies examining the impact of the route of antibiotic administration was selected for each pharmacodynamic variable addressed.

DATA SYNTHESIS

The support for CI as the preferred method of beta-lactam administration comes primarily from in vitro and animal data. Most beta-lactam antibiotics do not demonstrate concentration-dependent killing and have an appreciable postantibiotic effect only against gram-positive cocci. Their efficacy appears to be optimized by maintaining suprainhibitory concentrations throughout the dosing interval. Therefore, CI of beta-lactams could potentially enhance the efficacy of treatment or allow less drug to be used on a daily basis. This has yet to be demonstrated convincingly in human clinical trials. Comparative trials need to continue to explore the impact of the method of administration on patient outcomes such as duration and cost of therapy, as well as morbidity and mortality.

CONCLUSIONS

Results of many animal and in vitro studies suggest that CI may be the optimal method of beta-lactam administration. Clinical trials need to further document the impact of the method of beta-lactam administration on the incidence of adverse effects, emergence of bacterial resistance, and patient outcome. Pharmacodynamic studies defining target beta-lactam concentrations, the practicality of CI in patients requiring multiple intravenous fluids and medications, and the pertinence of this issue when beta-lactam antibiotics are used as sole agents or in combination with other antimicrobials require further exploration.

[Microbiologic criteria of the choice of antibiotic for antibiotic prophylaxis in surgery]

The choice of an antibiotic for antimicrobial prophylaxis is based on microbiological, pharmacokinetic, chemical and pharmacodynamic parameters. The knowledge of the bacterial flora allows the identification of bacteria which could be responsible for postsurgical infections. These floras are complex and their equilibrium can be modified by various factors, such as hospitalization, co-existing disease, medico-surgical procedure, administration of antibiotics, which cause the selection of the so-called hospital-bacteria feared by therapists. The infection will develop according to the quantity of bacteria that have been introduced and to their virulence, which is often altered by local factors, especially the biomaterials. The knowledge of pharmacodynamic parameters such as bactericidal activity, postantibiotic effect, activity on virulence factors (bacterial adhesion), allows the refinement of the choice of the antibiotic and the optimization of its posology.

Enterococcus, an emerging pathogen

OBJECTIVE

To review the bacterial genus Enterococcus with respect to its epidemiology, specific infections in humans, mechanisms of resistance and tolerance, and antimicrobial treatment.

DATA SOURCES

A MEDLINE search of English-language journal articles published from 1977 to 1992 was completed. Articles published prior to 1977 were identified through Index Medicus and from references appearing in the bibliographies of other journal articles. Information also was acquired from abstracts, personal communication with infectious disease specialists with active research in the area of enterococcal infection, and conference proceedings.

STUDY SELECTION

In vitro data; animal models of enterococcal infection; case reports; and case-controlled, cohort, and randomized controlled trials in humans were evaluated for relevant information.

DATA EXTRACTION

Studies were evaluated by their methodologic strength (e.g., randomized controlled trial), reporting of clinically relevant outcomes (e.g., clinical response to antimicrobial therapy), statistical analyses, and accountability of all patients who entered the study.

DATA SYNTHESIS

The incidence of enterococcal infections has increased in recent years and enterococci are now the second most frequently reported nosocomial pathogens. Enterococcus faecalis is the pathogen responsible for most enterococcal infections seen today; it has been implicated as an important cause of endocarditis, bacteremia, urinary tract infections, and intraabdominal infections.

CONCLUSIONS

Enterococcal infection is of particular concern clinically because of its resistance to several antibiotics. Controlled comparative clinical trials of antimicrobial therapy in humans are lacking for several enterococcal infections. Therefore, the recommendations for antimicrobial therapy presented in this review are guidelines that reflect our current understanding of antibiotics used for enterococcal infection.

Influence of high-level gentamicin resistance and beta-hemolysis on susceptibility of enterococci to the bactericidal activities of ampicillin and vancomycin

The bactericidal activities of ampicillin and vancomycin against 40 recent isolates of Enterococcus faecalis were examined by kill-kinetic studies at concentrations of 4 x the MIC and 20 micrograms/ml. Greater killing was seen with ampicillin (3.57 +/- 0.87 and 2.50 +/- 1.09 log10 CFU/ml, respectively; mean +/- standard deviation) than with vancomycin (1.23 +/- 0.65 and 1.05 +/- 0.57 log10 CFU/ml, respectively). Highly gentamicin-resistant strains showed a tendency toward reduced susceptibility to killing; beta-hemolytic strains were more susceptible than nonhemolytic strains when exposed to ampicillin at 20 micrograms/ml. Within each group, individual isolates demonstrated great variability in susceptibility to killing by the drugs.