Experimental Escherichia coli endocarditis in rats: roles of serum bactericidal activity and duration of catheter placement

Current Challenges in the Management of Infective Endocarditis

Infective endocarditis is a relatively rare, but deadly cause of sepsis, with an overall mortality ranging from 20 to 25% in most series. Although the classic clinical classification into syndromes of acute or subacute endocarditis have not completely lost their usefulness, current clinical forms have changed according to the profound epidemiological changes observed in developed countries. In this review, we aim to address the changing epidemiology of endocarditis, several recent advances in the understanding of the pathophysiology of endocarditis and endocarditis-triggered sepsis, new useful diagnostic tools as well as current concepts in the medical and surgical management of this disease. Given its complexity, the management of infective endocarditis requires the close collaboration of multidisciplinary endocarditis teams that must decide on the diagnostic approach; the appropriate initial treatment in the critical phase; the detection of patients needing surgery and the timing of this intervention; and finally the accurate selection of patients for out-of-hospital treatment, either at home hospitalization or with oral antibiotic treatment.

Right atrial pressure alterations during echocardiography-guided-catheterization predict tricuspid valvular impairment: a novel method for the creation of a rabbit model of Staphylococcus aureus endocarditis

Background

We previously reported the use of a catheter system to damage the tricuspid valve and create infectious endocarditis (IE) in an animal model. The current study aims to create a faint IE model suitable for antibiotic prophylaxis using a low bacterial inoculum. We also aim to explore a way to quantitatively assess valvular impairment and to predict the success of the IE models during catheterization.

Methods

Ninety rabbits were assigned to two groups according to the density of bacteria inoculated (1 × 10⁵ CFU for Group A and 1 × 10⁴ CFU for Group B). A catheter system consisting of a polyethylene catheter and a guide wire were used to damage the valve. The catheter system was passed through the rabbits’ tricuspid valves under echocardiographic guidance. A pressure transducer was used to assess right atrial pressure (P_RA) before and just after valvular damage to calculate the pressure alterations (ΔP_RA). The animals in group A and B were divided into 3 subgroups according to the ΔP_RA (0–5 mmHg for Groups A₁ and B₁; 5–10 mmHg for Groups A₂ and B₂; 10–15 mmHg for Groups A₃ and B₃). Staphylococcus aureus (ATCC 29213) inoculation was performed 24 hr after cardiac catheterization.

Results

Faint IE was confirmed in 20%, 93.3%, 26.7%, 6.7%, 20%, and 33.3% of the rabbits in Groups A₁, A₂, A₃, B₁, B₂, and B₃, respectively. There was no difference in the LV/RV ratio and V_TR of the No-IE, faint-IE, and severe IE animals. Faint IE rabbits had a larger ΔP_RA than No-IE rabbits (7.81 ± 1.21 vs. 2.48 ± 1.0, P < 0.01, for Group A; 7.60 ± 1.32 vs. 2.98 ± 1.08, P < 0.01, for Group B). The ΔP_RA of severe IE and faint IE rabbits was significantly different (13.11 ± 1.31 vs. 7.81 ± 1.21, P < 0.01, for Group A; 12.73 ± 1.44 vs.7.60 ± 1.32, P < 0.01, for Group B).

Conclusion

ΔP_RA could be used to assess valvular impairment. Controlling the value of ΔP_RA during catheterization and inoculating of an appropriate dose of bacteria was associated with a successful IE model.

A rabbit model of right-sided Staphylococcus aureus endocarditis created with echocardiographic guidance

Background

The most widely used experimental models of infective endocarditis (IE) are the rabbit and rat models, in which cardiac valve lesions are induced by a polyethylene catheter introduced into the left ventricle through the aortic valve. Our study was designed to create a rabbit model of IE right-sided with echocardiographic guidance.

Methods

Thirty rabbits underwent both catheterization and inoculation (group A). These were divided into three subgroups of ten based on the time of catheter removal (immediately, after 24 h, and after death or moribundity for groups, A₁, A₂, and A₃, respectively). Ten inoculated-only and ten catheterized-only rabbits served as controls. A catheter system consisted of a polyethylene catheter and a guide wire inside it. This system was passed through the rabbits’ tricuspid valves under echocardiographic guidance to damage them. The ratio of left ventricle to right ventricle (LV/RV) was measured in a four-chamber view before catheterization and at the time of death or moribundity. The peak velocity of tricuspid regurgitation (V_TR) was measured in a four-chamber view at the time of catheterization and at the time of death or moribundity. Staphylococcus aureus (ATCC 29213) inoculation was performed 24 h after right heart catheterization to produce IE.

Results

IE was confirmed in 28 of 30 rabbits by macroscopic and histologic examination of tricuspid valves, blood cultures, and bacterial count in cardiac vegetations. Cardiac vegetations were confirmed in 25 of 28 IE rabbits by echocardiography. Enlargement of right ventricle dimension with a significantly decreased LV/RV ratio was confirmed in all IE rabbits at the time of death or moribundity than at the initial state (1.11 ± 0.35 vs. 1.95 ± 0.39, P < 0.01; 1.21 ± 0.34 vs. 1.98 ± 0.35, P < 0.01; 1.04 ± 0.31 vs. 2.00 ± 0.41, P < 0.01 for groups A₁, A₂, and A₃, respectively). V_TR was significantly higher in all the IE rabbits at the time of death or moribundity than at the time of catheterization (1.89 ± 0.46 vs 0.76 ± 0.45, P < 0.01; 2.04 ± 0.73 vs 0.68 ± 0.66, P < 0.01; 2.24 ± 0.51 vs 0.87 ± 0.55, P < 0.01 for group A₁, A₂ and A₃, respectively).

Conclusion

The models described herein closely reproduced the pathogenesis and pathophysiology of right heart catheter-induced endocarditis in humans. Echocardiographic guidance is helpful in the process of right heart catheterization. Some echocardiographic parameters, such as V_TR and the LV/RV ratio could be used to assess the success or failure of the IE models.

In vitro assays of Staphylococcus epidermidis characteristics and outcome in an endocarditis model

OBJECTIVE

Staphylococcus epidermidis adherence to indwelling polymers is important in prosthetic valve endocarditis. Earlier studies have related streptococcal endocarditis to isolates with high levels of cell-associated hexoses. The objective of the present study was to determine if a relationship exists between an S epidermidis isolate assay score and production/severity of experimental endocarditis.

DESIGN

Groups of patient S epidermidis isolates were screened for surface hexoses and an animal model of endocarditis with isolates testing highest and lowest on the screen was produced. Disease severity produced by 'high hexose' versus 'low hexose' organisms was evaluated. Endocarditis responding variables were bacterial vegetation weight and log(10) colony forming units (cfu) and in survival tests, comparative time to death with different isolates. Bacterial characteristics were not measured. Baseline data showed a vegetation weight difference so that with a β error of 0.20 and a two-tailed α error of 0.05, a significant difference would be noted using 30 animals. A total of 64 animals was used. POPULATION STUDDED: Bacterial isolates from two patient groups (n=42 and n=68) on which in vitro assays were run. An animal model of endocarditis (n=64) was used to evaluate four selected isolates for vegetation size, log(10) cfu/g, and survival time.

MAIN RESULTS

In a group of S epidermidis endocarditis animals evaluated for time of death, a significantly more rapid death time resulted in the group dosed with the high hexose-scoring organism (P<0.025). Vegetations and log(10) cfu produced by test high hexose isolates averaged larger but were not significantly different.

CONCLUSIONS

A significantly more rapid death rate occurs in untreated endocarditis using a high hexose isolate than with S epidermidis with low surface hexoses. Using bacterial vegetation and cfu as endpoints, however, experimental endocarditis using patient isolates of S epidermidis does not show the same strong correlation to bacterial surface hexoses as does streptococcal endocarditis.

Infective endocarditis

Infective endocarditis (IE) is lethal if not aggressively treated with antibiotics alone or in combination with surgery. The epidemiology of this condition has substantially changed over the past four decades, especially in industrialized countries. Once a disease that predominantly affected young adults with previously well-identified valve disease--mostly chronic rheumatic heart disease--IE now tends to affect older patients and new at-risk groups, including intravenous-drug users, patients with intracardiac devices, and patients exposed to healthcare-associated bacteremia. As a result, skin organisms (for example, Staphylococcus spp.) are now reported as the pathogen in these populations more often than oral streptococci, which still prevail in the community and in native-valve IE. Moreover, progress in molecular diagnostics has helped to improve the diagnosis of poorly cultivable pathogens, such as Bartonella spp. and Tropheryma whipplei, which are responsible for blood-culture-negative IE more often than expected. Epidemiological data indicate that IE mostly occurs independently of medico-surgical procedures, and that circumstantial antibiotic prophylaxis is likely to protect only a minute proportion of individuals at risk. Therefore, new strategies to prevent IE--including improvement of dental hygiene, decontamination of carriers of Staphylococcus aureus, vaccination, and, possibly, antiplatelet therapy--must be explored.

Resistance of Streptococcus gordonii to polymorphonuclear leukocyte killing is a potential virulence determinant of infective endocarditis

Significant differences in virulence among seven representative Streptococcus gordonii strains were observed by using the rat model of infective endocarditis. Five strains, including S. gordonii DL1, caused severe disease, while the other two strains, including S. gordonii SK12, caused minimal or no disease. The differences in virulence were evident from the visible presence of streptococci in the vegetations present on the aortic valves of catheterized rats that were challenged with individual strains and also from the much greater recovery of rifampin-resistant S. gordonii DLl than of streptomycin-resistant S. gordonii SK12 from the hearts of animals coinfected with both organisms. Each S. gordonii strain aggregated with human platelets and bound to polymorphonuclear leukocytes (PMNs), as shown by the stimulation of PMN superoxide anion production. These interactions were reduced or abolished by pretreatment of the platelets or PMNs with sialidase, indicating that there was bacterial recognition of host sialic acid-containing receptors. Adhesin-mediated binding of each S. gordonii strain to PMNs also triggered phagocytosis. However, the subsequent PMN-dependent killing differed significantly for the seven strains. The five virulent strains included three strains that were not killed and two strains whose numbers were reduced by approximately 50%. In contrast, the level of killing of each avirulent strain under the same conditions was significantly greater and approached 90% of the bacteria added. Parallel studies performed with rat PMNs revealed comparable differences in the resistance or susceptibility of representative virulent and avirulent strains. Thus, the ability of S. gordonii to survive in PMNs following adhesin-mediated phagocytosis may be an important virulence determinant of infective endocarditis.

Interleukin 1alpha increases the susceptibility of rabbits to experimental viridans streptococcal endocarditis

Major predisposing conditions for infective endocarditis (IE) are the presence of a cardiac platelet-fibrin vegetation and of circulating bacteria with relatively low susceptibility to microbicidal activity of blood platelets. The influence of proinflammatory conditions on development of IE is unknown. We studied the effects of the presence of a catheter, inserted to induce platelet-fibrin vegetations, and of the proinflammatory cytokine interleukin-1alpha in rabbit experimental IE. Leaving the catheter in place after challenge with viridans streptococci predisposed for experimental IE. IE susceptibility rapidly decreased between 0 to 6 h after catheter removal. The catheter did not predispose for IE by providing a site for bacterial adherence, as almost all explanted catheters were culture negative. To mimic the proinflammatory influence of the catheter, rabbits were injected with interleukin-1alpha at 24 h after catheter removal and at 0, 1, and 3 h before bacterial challenge. Interleukin-1alpha injected 3 h prior to challenge significantly increased IE incidence due to a platelet releasate-susceptible Streptococcus oralis strain, with rapidly increasing numbers of bacteria within the vegetations. IE due to the Streptococcus sanguis strain less susceptible to platelet releasate was not enhanced. We conclude that proinflammatory stimuli, either a catheter or interleukin-1alpha, enhanced susceptibility to IE due to the platelet releasate-susceptible S. oralis. As with rabbits, temporary intravascular proinflammatory conditions may predispose for IE in humans at risk for this serious infection.

Infective endocarditis

Despite improvements in health care, the incidence of infective endocarditis has not decreased over the past decades. This apparent paradox is explained by a progressive evolution in risk factors; while classic predisposing conditions such as rheumatic heart disease have been all but eradicated, new risk factors for infective endocarditis have emerged. These include intravenous drug use, sclerotic valve disease in elderly patients, use of prosthetic valves, and nosocomial disease. Newly identified pathogens, which are difficult to cultivate--eg, Bartonella spp and Tropheryma whipplei--are present in selected individuals, and resistant organisms are challenging conventional antimicrobial therapy. Keeping up with these changes depends on a comprehensive approach, allying understanding of the pathogenesis of disease with the development of new drugs for infective endocarditis. Infection by staphylococci and streptococci is being dissected at the molecular level. New ideas for antimicrobial agents are being developed. These novel insights should help redefine preventive and therapeutic strategies against infective endocarditis.

Guinea pig model for Staphylococcus aureus native valve endocarditis

We present a new experimental model of Staphylococcus aureus infective endocarditis in guinea pigs. Permanent aortic valve damage was produced by electrocoagulation after catheterization of the right carotid artery, which allowed avoidance of the intracardiac catheter to produce cardiac vegetations. Our model closely mimics pathological mechanisms of native valve endocarditis.