Penicillin-streptomycin treatment of enterococcal endocarditis. A re-evaluation

Treatment of Enterococcus faecalis Infective Endocarditis: A Continuing Challenge

Today, Enterococcus faecalis is one of the main causes of infective endocarditis in the world, generally affecting an elderly and fragile population, with a high mortality rate. Enterococci are partially resistant to many commonly used antimicrobial agents such as penicillin and ampicillin, as well as high-level resistance to most cephalosporins and sometimes carbapenems, because of low-affinity penicillin-binding proteins, that lead to an unacceptable number of therapeutic failures with monotherapy. For many years, the synergistic combination of penicillins and aminoglycosides has been the cornerstone of treatment, but the emergence of strains with high resistance to aminoglycosides led to the search for new alternatives, like dual beta-lactam therapy. The development of multi-drug resistant strains of Enterococcus faecium is a matter of considerable concern due to its probable spread to E. faecalis and have necessitated the search of new guidelines with the combination of daptomycin, fosfomycin or tigecycline. Some of them have scarce clinical experience and others are still under investigation and will be analyzed in this review. In addition, the need for prolonged treatment (6–8 weeks) to avoid relapses has forced to the consideration of other viable options as outpatient parenteral strategies, long-acting administrations with the new lipoglycopeptides (dalbavancin or oritavancin), and sequential oral treatments, which will also be discussed.

Penicillin-Binding Proteins and Alternative Dual-Beta-Lactam Combinations for Serious Enterococcus faecalis Infections with Elevated Penicillin MICs

Ampicillin-ceftriaxone has become a first-line therapy for Enterococcus faecalis endocarditis. We characterized the penicillin-binding protein (PBP) profiles of various E. faecalis strains and tested for synergy to better inform beta-lactam options for the treatment of E. faecalis infections. We assessed the affinity of PBP2B from elevated-MIC strain E. faecalis LS4828 compared to type strain JH2-2 using the fluorescent beta-lactam Bocillin FL. We also characterized pbp4 and pbpA structures and PBP4 and PBP2B expression and used deletion and complementation studies to assess the impact of PBP2B on the levels of resistance. We tested penicillin-susceptible and -resistant E. faecalis isolates against ceftriaxone or ceftaroline combinations with other beta-lactams in 24-h time-kill studies. Two penicillin-susceptible strains (JH2-2 and L2052) had identical pbp sequences and similar PBP expression levels. One reduced-penicillin-susceptibility strain (L2068) had pbp sequences identical to those of the susceptible strains but expressed more PBP4. The second decreased-penicillin-susceptibility strain (LS4828) had amino acid substitutions in both PBP4 and PBP2B and expressed increased quantities of both proteins. PBP2B did not appear to contribute significantly to the elevated beta-lactam MICs. No synergy was demonstrable against the strains with both mutated PBPs and increased expression (L2068 and LS4828). Meropenem plus ceftriaxone or ertapenem plus ceftriaxone demonstrated the most consistent synergistic activity. PBP2B of strain LS4828 does not contribute significantly to reduced penicillin susceptibility. Neither the MIC nor the level of PBP expression correlated directly with the identified synergistic combinations when tested at static subinhibitory concentrations.

Effect of Bacterial and Fungal Microbiota Removal on the Survival and Development of Bryophagous Beetles

Insect microbiota may play a wide range of roles in host physiology. Among others, microbiota can be involved in diet processing or protection against pathogens, both of which are potentially important in bryophagous (moss-feeding) insects, which survive on extreme diets and live in the stable environment of moss clumps suitable for the growth of fungi and bacteria. We treated Cytilus sericeus (Forster, 1771) (Coleoptera: Byrrhidae) as a model organism with bactericides and fungicides to test the effect of bacterial and fungal removal on egg hatching and larval development. Furthermore, we supplied larvae with adult feces to determine whether feces is a source of beneficial microbiota or pathogens. Bactericides had a positive effect, but fungicides had a negative effect on beetle fitness, both of which manifested during egg hatching. The feces did not play a positive role. Our conclusions indicate the presence of beneficial fungal microbiota associated with eggs but not transmitted through feces. Based on preliminary cultivation and fungicide tests, Fusarium or Penicillium may be important for suppressing pathogens, but their exact role needs to be further studied.

Cardiac Microlesions Form During Severe Bacteremic Enterococcus faecalis Infection

Enterococcus  faecalis is a significant cause of hospital-acquired bacteremia. Herein, the discovery is reported that cardiac microlesions form during severe bacteremic E. faecalis infection in mice. The cardiac microlesions were identical in appearance to those formed by Streptococcus pneumoniae during invasive pneumococcal disease. However, E. faecalis does not encode the virulence determinants implicated in pneumococcal microlesion formation. Rather, disulfide bond forming protein A (DsbA) was found to be required for E. faecalis virulence in a Caenorhabditis elegans model and was necessary for efficient cardiac microlesion formation. Furthermore, E. faecalis promoted cardiomyocyte apoptotic and necroptotic cell death at sites of microlesion formation. Additionally, loss of DsbA caused an increase in proinflammatory cytokines, unlike the wild-type strain, which suppressed the immune response. In conclusion, we establish that E. faecalis is capable of forming cardiac microlesions and identify features of both the bacterium and the host response that are mechanistically involved.

Adaptation to Adversity: the Intermingling of Stress Tolerance and Pathogenesis in Enterococci

Enterococcus is a diverse and rugged genus colonizing the gastrointestinal tract of humans and numerous hosts across the animal kingdom. Enterococci are also a leading cause of multidrug-resistant hospital-acquired infections. In each of these settings, enterococci must contend with changing biophysical landscapes and innate immune responses in order to successfully colonize and transit between hosts. Therefore, it appears that the intrinsic durability that evolved to make enterococci optimally competitive in the host gastrointestinal tract also ideally positioned them to persist in hospitals, despite disinfection protocols, and acquire new antibiotic resistances from other microbes. Here, we discuss the molecular mechanisms and regulation employed by enterococci to tolerate diverse stressors and highlight the role of stress tolerance in the biology of this medically relevant genus.

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.

The structures of penicillin-binding protein 4 (PBP4) and PBP5 from Enterococci provide structural insights into β-lactam resistance

The final steps of cell-wall biosynthesis in bacteria are carried out by penicillin-binding proteins (PBPs), whose transpeptidase domains form the cross-links in peptidoglycan chains that define the bacterial cell wall. These enzymes are the targets of β-lactam antibiotics, as their inhibition reduces the structural integrity of the cell wall. Bacterial resistance to antibiotics is a rapidly growing concern; however, the structural underpinnings of PBP-derived antibiotic resistance are poorly understood. PBP4 and PBP5 are low-affinity, class B transpeptidases that confer antibiotic resistance to Enterococcus faecalis and Enterococcus faecium, respectively. Here, we report the crystal structures of PBP4 (1.8 Å) and PBP5 (2.7 Å) in their apo and acyl-enzyme complexes with the β-lactams benzylpenicillin, imipenem, and ceftaroline. We found that, although these three β-lactams adopt geometries similar to those observed in other class B PBP structures, there are small, but significant, differences that likely decrease antibiotic efficacy. Further, we also discovered that the N-terminal domain extensions in this class of PBPs undergo large rigid-body rotations without impacting the structure of the catalytic transpeptidase domain. Together, our findings are defining the subtle functional and structural differences in the Enterococcus PBPs that allow them to support transpeptidase activity while also conferring bacterial resistance to antibiotics that function as substrate mimics.

In Vivo and In Vitro Effects of a ClpP-Activating Antibiotic against Vancomycin-Resistant Enterococci

Antibiotics with novel bactericidal mechanisms of action are urgently needed. The antibiotic acyldepsipeptide 4 (ADEP4) activates the ClpP protease and causes cells to self-digest. The effects of ADEP4 and ClpP activation have not been characterized sufficiently for the enterococci, which are important pathogens known for high levels of acquired and intrinsic antibiotic resistance. In the present study, ADEP4 was found to be potently active against both Enterococcus faecalis and Enterococcus faecium, with MIC₉₀s of 0.016 μg/ml and 0.031 μg/ml, respectively. ClpP purified from E. faecium was found to bind ADEP4 in a surface plasmon resonance analysis, and ClpP activation by ADEP4 was demonstrated biochemically with a β-casein digestion assay. In addition, E. faecium ClpP was crystallized in the presence of ADEP4, revealing ADEP4 binding to ClpP in the activated state. These results confirm that the anti-enterococcal activity of ADEP4 occurs through ClpP activation. In killing curve assays, ADEP4 was found to be bactericidal against stationary-phase vancomycin-resistant E. faecalis (VRE) strain V583, and resistance development was prevented when ADEP4 was combined with multiple classes of approved antibiotics. ADEP4 in combination with partnering antibiotics also eradicated mature VRE biofilms within 72 h of treatment. Biofilm killing with ADEP4 antibiotic combinations was superior to that with the clinically used combinations ampicillin-gentamicin and ampicillin-daptomycin. In a murine peritoneal septicemia model, ADEP4 alone was as effective as ampicillin. ADEP4 coadministered with ampicillin was significantly more effective than either drug alone. These data suggest that ClpP-activating antibiotics may be useful for treating enterococcal infections.

Enterococci and Their Interactions with the Intestinal Microbiome

The Enterococcus genus comprises over 50 species that live as commensal bacteria in the gastrointestinal (GI) tracts of insects, birds, reptiles, and mammals. Named "entero" to emphasize their intestinal habitat, Enterococcus faecalis and Enterococcus faecium were first isolated in the early 1900s and are the most abundant species of this genus found in the human fecal microbiota. In the past 3 decades, enterococci have developed increased resistance to several classes of antibiotics and emerged as a prevalent causative agent of health care-related infections. In U.S. hospitals, antibiotic use has increased the transmission of multidrug-resistant enterococci. Antibiotic treatment depletes broad communities of commensal microbes from the GI tract, allowing resistant enterococci to densely colonize the gut. The reestablishment of a diverse intestinal microbiota is an emerging approach to combat infections caused by antibiotic-resistant bacteria in the GI tract. Because enterococci exist as commensals, modifying the intestinal microbiome to eliminate enterococcal clinical pathogens poses a challenge. To better understand how enterococci exist as both commensals and pathogens, in this article we discuss their clinical importance, antibiotic resistance, diversity in genomic composition and habitats, and interaction with the intestinal microbiome that may be used to prevent clinical infection.

Enterococci and Their Interactions with the Intestinal Microbiome

The Enterococcus genus comprises over 50 species that live as commensal bacteria in the gastrointestinal (GI) tracts of insects, birds, reptiles, and mammals. Named "entero" to emphasize their intestinal habitat, Enterococcus faecalis and Enterococcus faecium were first isolated in the early 1900s and are the most abundant species of this genus found in the human fecal microbiota. In the past 3 decades, enterococci have developed increased resistance to several classes of antibiotics and emerged as a prevalent causative agent of health care-related infections. In U.S. hospitals, antibiotic use has increased the transmission of multidrug-resistant enterococci. Antibiotic treatment depletes broad communities of commensal microbes from the GI tract, allowing resistant enterococci to densely colonize the gut. The reestablishment of a diverse intestinal microbiota is an emerging approach to combat infections caused by antibiotic-resistant bacteria in the GI tract. Because enterococci exist as commensals, modifying the intestinal microbiome to eliminate enterococcal clinical pathogens poses a challenge. To better understand how enterococci exist as both commensals and pathogens, in this article we discuss their clinical importance, antibiotic resistance, diversity in genomic composition and habitats, and interaction with the intestinal microbiome that may be used to prevent clinical infection.

Enterococcus faecalis Infective Endocarditis

Background—

Because of the nephrotoxic effects of aminoglycosides, the Danish guidelines on infective endocarditis were changed in January 2007, reducing gentamicin treatment in enterococcal infective endocarditis from 4 to 6 weeks to only 2 weeks. In this pilot study, we compare outcomes in patients with Enterococcus faecalis infective endocarditis treated in the years before and after endorsement of these new recommendations.

Methods and Results—

A total of 84 consecutive patients admitted with definite left-sided E faecalis endocarditis in the period of 2002 to 2011 were enrolled. Forty-one patients were treated before and 43 patients were treated after January 1, 2007. There were no significant differences in baseline characteristics. At hospitalization, the 2 groups had similar estimated glomerular filtration rates of 66 and 75 mL/min ( P =0.22). Patients treated before January 2007 received gentamicin for a significantly longer period (28 versus 14 days; P <0.001). The primary outcome, 1-year event-free survival, did not differ: 66% versus 69%, respectively ( P =0.75). At discharge, the patients treated before 2007 had a lower estimated glomerular filtration rate (45 versus 66 mL/min; P =0.008) and a significantly greater decrease in estimated glomerular filtration rate (median, 11 versus 1 mL/min; P =0.009) compared with those treated after 2007.

Conclusions—

Our present pilot study suggests that the recommended 2-week treatment with gentamicin seems adequate and preferable in treating non–high-level aminoglycoside-resistant E faecalis infective endocarditis. The longer duration of gentamicin treatment is associated with worse renal function. Although the certainty of the clinical outcomes is limited by the sample size, outcomes appear to be no worse with the shorter treatment duration. Randomized, controlled studies are warranted to substantiate these results.

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.

Enterococcal endocarditis on native and prosthetic valves: a review of clinical and prognostic factors with emphasis on hospital-acquired infections as a major determinant of outcome

Enterococci are the third leading cause of infectious endocarditis, and despite advances in diagnosis and treatment, the mortality of enterococcal endocarditis has not changed in recent decades. Although variables such as advanced age, cardiac failure, and brain emboli have been recognized as risk factors for mortality, cooperative multi-institutional studies have not assessed the role of other variables, such as nosocomial acquisition of infection, the presence of comorbidities, or the changing antimicrobial susceptibility of enterococci, as factors determining prognosis.We conducted the current study to determine the risk factors for mortality in patients with enterococcal endocarditis in a single institution. We reviewed 47 consecutive episodes of enterococcal endocarditis in 44 patients diagnosed according to the modified Duke criteria from a retrospective cohort study of cases of infectious endocarditis. The main outcome measure was inhospital mortality. We applied stepwise logistic regression analysis to identify risk factors for mortality.Predisposing heart conditions were observed in 86.3% of patients, and 17 had prosthetic valve endocarditis. A portal of entry was suspected or determined in 38.2%; the genitourinary tract was the most common source of the infection (29.7%). Comorbidities were present in 52.2% of cases. Twelve episodes (25.5%) were acquired during hospitalization. Only 3 isolates of Enterococcus faecalis were highly resistant to gentamicin. Eighteen patients (40.9%) needed valve replacement due to cardiac failure or relapse. Comparing cases of native valve and prosthetic valve endocarditis, we found no differences regarding complications, the need for surgical treatment, or mortality. Eight of 44 (18%) episodes were fatal. Age over 70 years (p = 0.05), heart failure (odds ratio [OR], 1.61; 95% confidence interval [CI], 1.15-2.25; p = 0.001), presence of 1 or more comorbidities (OR, 3.2; 95% CI, 1.11-9.39; p = 0.02), and nosocomial acquisition (OR, 8.05; 95% CI, 1.50-43.2; p = 0.01) were associated with mortality. In the multivariate analysis, only nosocomial acquisition increased the risk of mortality. In patients with enterococcal endocarditis, nosocomial acquisition of infection is an important factor determining outcome. As the incidence of bacteremia and the population of elderly people at risk continue to grow, the hazard of acquiring nosocomial enterococcal endocarditis may increase; hence, major emphasis must be put on prevention.

Enterococcal prosthetic valve infective endocarditis: report of 45 episodes from the International Collaboration on Endocarditis-merged database

Enterococcal prosthetic valve infective endocarditis (PVE) is an incompletely understood disease. In the present study, patients with enterococcal PVE were compared to patients with enterococcal native valve endocarditis (NVE) and other types of PVE to determine differences in basic clinical characteristics and outcomes using a large multicenter, international database of patients with definite endocarditis. Forty-five of 159 (29%) cases of definite enterococcal endocarditis were PVE. Patients with enterococcal PVE were demographically similar to patients with enterococcal NVE but had more intracardiac abscesses (20% vs. 6%; p=0.009), fewer valve vegetations (51% vs. 79%; p<0.001), and fewer cases of new valvular regurgitation (12% vs. 45%; p=0.01). Patients with either enterococcal PVE or NVE were elderly (median age, 73 vs. 69; p=0.06). Rates of in-hospital mortality, surgical intervention, heart failure, peripheral embolization, and stroke were similar in both groups. Patients with enterococcal PVE were also demographically similar to patients with other types of PVE, but mortality may be lower (14% vs. 26%; p=0.08). Notably, 93% of patients with enterococcal PVE came from European centers, as compared with only 79% of patients with enterococcal NVE (p=0.03). Thus, patients with enterococcal PVE have higher rates of myocardial abscess formation and lower rates of new regurgitation compared to patients with enterococcal NVE, but there are no differences between the groups with regard to surgical or mortality rates. In contrast, though patients with enterococcal PVE and patients with other types of PVE share similar characteristics, mortality is higher in the latter group. Importantly, the prevalence of enterococcal PVE was higher in the European centers in this study.

Enterococcal endocarditis: 107 cases from the international collaboration on endocarditis merged database

PURPOSE

To describe clinical features and outcomes of enterococcal left-sided native valve endocarditis and to compare it to endocarditis caused by other pathogens.

SUBJECTS AND METHODS

Patients in the International Collaboration on Endocarditis-Merged Database were included if they had left-sided native valve endocarditis. Demographic characteristics, clinical features, and outcomes were analyzed. Multivariable analysis evaluated enterococcus as a predictor of mortality.

RESULTS

Of 1285 patients with left-sided native valve endocarditis, 107 had enterococcal endocarditis. Enterococcal endocarditis was most frequently seen in elderly men, frequently involved the aortic valve, tended to produce heart failure rather than embolic events, and had relatively low short-term mortality. Compared to patients with non-enterococcal endocarditis, patients with enterococcal endocarditis had similar rates of nosocomial acquisition, heart failure, embolization, surgery, and mortality. Compared to patients with streptococcal endocarditis, patients with enterococcal endocarditis were more likely to be nosocomially acquired (9 of 59 [15%] vs 2 of 400 [1%]; P <.0001) and have heart failure (49 of 107 [46%] vs 234 of 666 [35%]; P = 0.03). Compared to patients with S. aureus endocarditis, patients with enterococcal endocarditis were less likely to embolize (28 of 107 [26%] vs 155 of 314 [49%]; P <.0001) and less likely to die (12 of 107 [11%] vs 83 of 313 [27%]; P = 0.001). Multivariable analysis of all patients with left-sided native valve endocarditis showed that enterococcal endocarditis was associated with lower mortality (odds ratio [OR] 0.49; 95% confidence interval [CI] 0.24 to 0.97).

CONCLUSIONS

Enterococcal native valve endocarditis has a distinctive clinical picture with a good prognosis.

Vancomycin resistance in the enterococcus. Relevance in pediatrics

Enterococci are nosocomial pathogens intrinsically resistant to a variety of commonly used antimicrobial agents. The frequent use of antimicrobial agents such as cephalosporins has been associated with the increased isolation of enterococci in pediatric hospitals. In addition to their intrinsic resistance traits, the enterococci have rapidly accumulated a variety of acquired resistance determinants. Strains that are resistant to all currently available antibiotics are now being isolated from infected children. The threat of untreatable enterococcal infection and the possibility that vancomycin resistance may spread from the enterococci to the more virulent pneumococci or staphylococci argue for vigilant surveillance for resistant strains, isolation and barrier precautions for infected patients, increased research into the mechanisms of resistance, and a reinvigorated effort to identify new classes of antimicrobial agents.

The epidemiology of enterococci

The enterococci are emerging as a significant cause of nosocomial infections, accounting for approximately 10% of hospital acquired infections. They are found as normal inhabitants of the human gastrointestinal tract, but may also colonize the oropharynx, vagina, perineal region and soft tissue wounds of asymptomatic patients. Until recently, evidence indicated that most enterococcal infections arose from patients' own endogenous flora. Recent studies, however, suggest that exogenous acquisition may occur and that person-to-person spread, probably on the hands of medical personnel, may be a significant mode of transmission of resistant enterococci within the hospital. The use of broad-spectrum antibiotics, especially cephalosporins, is another major factor in the increasing incidence of enterococcal infections. These findings suggest that barrier precautions, as applied with other resistant nosocomial pathogens, along with more judicial use of antibiotics may be beneficial in preventing nosocomial spread of resistant enterococci.

Comparison of the post-antibiotic effect of Streptococcus faecalis and Streptococcus faecium with ampicillin alone or combined with streptomycin: studies on a novel type of antimicrobial interaction

Determination of post-antibiotic effect (PAE) and time-kill studies were made with twelve strains of Streptococcus faecalis and nine strains of Streptococcus faecium, comparing the effect of ampicillin alone with a combination of ampicillin and streptomycin at achievable serum concentrations. Bactericidal synergism (greater than or equal to one log10 decrease in viable counts) and prolongation in PAE (greater than or equal to 0.5 h) were demonstrated in all streptomycin-susceptible strains (Minimum inhibitory Concentration less than 2000 mcg/ml), whereas only one of five highly streptomycin-resistant strains exhibited a synergistic effect. A significant correlation between the magnitude of increased killing and the increase in recovery period by the combinations of ampicillin and streptomycin was demonstrated.

Optimal therapy for enterococcal endocarditis

Enterococcal endocarditis accounts for an increasing proportion of cases of endocarditis in recent years. The combination of a penicillin and an aminoglycoside has become an accepted standard of treatment for this disease. However, the optimal choice of antibiotics, duration of therapy, and timing of surgical intervention remain controversial. This study reviews the presentation, clinical course, treatment, and outcome in 37 patients with 42 separate episodes of enterococcal endocarditis at four Yale University hospitals. Patients treated with aminoglycosides and penicillins or vancomycin had significantly better outcomes than those who did not receive aminoglycosides. However, the duration of aminoglycoside therapy (more than four versus less than four weeks) did not appear to affect outcome significantly. These results suggest that excellent cure rates may be achieved after treatment for less than four weeks with an aminoglycoside in combination with penicillin or vancomycin, thus potentially avoiding significant renal and vestibular toxicity.

Bacterial endocarditis on a prosthetic valve. Oral treatment with amoxicillin

A patient with endocarditis due to Streptococcus faecalis on an aortic valvular prosthesis was successfully treated using large oral doses of amoxicillin concurrently with intramuscular administration of streptomycin. Oral therapy was employed because of a persistent reaction to intravenously administered antibiotics. Oral therapy for bacterial endocarditis occurring on an artificial valve may be attempted as a last resort when all other accepted therapeutic measures have failed.

Recurrence of group B streptococcal infection

It is clear that the group B streptococcus has become a major pathogen of young infants within the comparatively recent past. Further it is clear that, as with other endemic and epidemic pathogens, increasing clinical and laboratory experience brings to light variations not initially evident. In addition, therapy presumably effective in initial cases may not continue to be so. The following papers by coincidence were received over a relatively brief period of time. Hence, for emphasis of some of the problems related to this organism, they are presented as a group.

Cephalosporin-aminoglycoside synergism in experimental enterococcal endocarditis

Despite in vitro demonstrations of synergism against enterococci, combinations of cephalosporin and aminoglycoside antibodies have been ineffective in the therapy of enterococcal endocarditis. Penicillin-gentamicin, cephalothin-gentamicin, and cefazolin-gentamicin were used to treat enterococcal endocarditis in rabbits. A direct relation was observed between the rate of cure and the degree by which the peak serum concentration of penicillin and the cephalosporins exceeded the minimal inhibitory concentration of the enterococcus. Thus, cephalosporin doses which produce serum concentrations which exceed the minimal inhibitory concentration of the enterococcus by several orders of magnitude may, in combination with aminoglycosides, be effective in treating human enterococcal endocarditis.

Comparative synergistic activity of nafcillin, oxacillin, and methicillin in combination with gentamicin against

The effectiveness of three semisynthetic, penicillinase-resistant penicillins alone and in combination with gentamicin was tested against 29 clinical isolates of enterococci. The minimal inhibitory concentrations of nafcillin were considerably lower than those of oxacillin and methicillin but were slightly higher than those of penicillin. At clinically achievable concentrations, the combination of nafcillin plus gentamicin produced enhanced killing against 13 of 14 strains of enterococci and was synergistic (by very rigid criteria) against 10 of 14 strains. In contrast, combinations of oxacillin plus gentamicin were synergistic against only 3 of 14 strains, and methicillin plus gentamicin produced synergistic killing against only 1 of 14 strains.

Studies of cephalothin: aminoglycoside synergism against enterococci

Combinations of cephalothin and aminoglycoside antibiotics are not currently used in the therapy of serious enterococcal infections, because clinical trials of these combinations have been unsuccessful. Studies of 28 enterococci isolated from patients with enterococcal bacteremia suggested three possible mechanisms for this in vivo antibiotic failure: (i) a relatively high level of resistance to cephalothin among all enterococci and especially those characterized as Streptococcus faecium, (ii) a significant incidence of high-level resistance to the aminoglycosides among certain strains of enterococci, and (iii) a failure of synergism to occur when cephalothin concentrations fall below the minimal inhibitory concentration of the enterococcus, as occurs during the in vivo metabolism and excretion of this antibiotic when given in standard doses for endocarditis.

Synergism of vancomycin-gentamicin and vancomycin-streptomycin against enterococci

The in vitro activity of vancomycin and combinations of vancomycin-gentamicin and vancomycin-streptomycin against enterococci was investigated. The minimal inhibitory concentration of vancomycin for 99 of 100 enterococcal strains isolated clinically was 3.12 mug or less/ml. When cultures of eight strains were incubated with vancomycin, regardless of the inoculum size (10(6), 10(5), or 10(4)) and concentration of vancomycin (10 or 20 mug/ml), there was no significant reduction in the number of viable enterococci at 6, 24, and 48 h. Gentamicin and streptomycin in concentrations attainable clinically were not effective against enterococci. Vancomycin combined with gentamicin or streptomycin was tested against 41 enterococcal strains. With the combination of vancomycin at 10 mug/ml and gentamicin at 4 mug/ml or vancomycin at 5 mug/ml and gentamicin at 4 mug/ml, synergism was demonstrated against all 41 strains at 6 h. The combination of vancomycin at 10 mug/ml and streptomycin at 10 mug/ml was only synergistic against 25 of 41 strains at 6 h, and only 22 of 41 strains were affected synergistically at 6 h by vancomycin at 5 mug/ml with streptomycin at 10 mug/ml. With few exceptions, the enhanced killing was more pronounced at 24 and 48 h. The combination of vancomycin and gentamicin or vancomycin and streptomycin (where in vitro studies demonstrate synergism) may be a useful alternate therapy in enterococcal endocarditis.