Staphylococcal endocarditis in rabbits treated with a low dose of cloxacillin

A systematic review of optimal pharmacokinetic/pharmacodynamic parameters for beta-lactam therapy in infective endocarditis

BACKGROUND

Beta-lactam antibiotics are the mainstay of therapy for most bacterial causes of infective endocarditis (IE). Traditionally considered as agents with a broad therapeutic index, there is increasing recognition that standard doses may be subtherapeutic or toxic in critically ill patients. Optimizing therapy for efficacy requires a defined pharmacokinetic (PK)/pharmacodynamic (PD) target associated with clinical and microbiological cure.

OBJECTIVES

To elucidate the factors that influence beta-lactam PK and PD variability in IE and to examine optimal PK/PD target parameters for therapy.

METHODS

The review was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Clinical and laboratory in vivo animal or human studies examining PK and/or PD of beta-lactam antibiotics in IE were eligible. Ovid MEDLINE, Embase and Cochrane Central Registry were searched using defined terms. The Office of Health Assessment and Translation (OHAT) tool was used for assessing risk of bias.

RESULTS

From 2677 abstracts, 62 articles were selected for review and synthesis, comprising: 45 animal studies investigating the broad categories of beta-lactam diffusion into vegetations, PK/PD determinants of outcome, mode of antibiotic delivery and synergistic impact of agents; and 17 human studies totalling 347 participants. Findings supported the importance of time-dependent killing for beta-lactams but heterogeneous data limited the determination of an optimal PK/PD target for IE treatment.

CONCLUSION

Beta-lactam PK and PD in endocarditis are variable and specific to the particular antibiotic-organism combination. Time-dependent killing is important, consistent with non-endocarditis studies, but there is little agreement on optimal drug exposure. Clinical studies examining PK/PD targets in endocarditis are required to further inform drug selection and dosing.

Post-treatment bacterial endocarditis mimicking fungal organisms: a morphologic comparison and tips for avoiding this diagnostic pitfall

BACKGROUND

Histopathologic differentiation of bacterial endocarditis from yeast-like fungal endocarditis is usually straightforward; however, an underappreciated phenomenon is the effect of antimicrobial therapy on bacterial size, shape and septa (cross-wall) formation resulting in bacterial forms that mimic yeast-like fungi. In this article we illustrate the alterations that occur in antibiotic-treated Staphylococcus aureus endocarditis and compare these changes to histopathologic findings in unaltered S. aureus and Histoplasma endocarditis, respectively.

METHODS

Resected valves from three cases of endocarditis were compared based on the type ofinflammatory reaction, organism morphology and culture results. Case 1 was S. aureus endocarditis initially misclassified as Histoplasma due to its atypical morphologic and histopathologic features. The two cases included for comparison were an S. aureus endocarditis with more classic features and an Histoplasma capsulatum endocarditis. Hematoxylin and eosin (H&E), Gram, periodic acid Schiff (PAS), Gomori-Grocott methenamine silver stains (GMS), and culture results were compared in all cases. Molecular and immunohistochemistry tests were used for confirmation of first case. High power oil-immersion was used to visualize organisms' characteristics in all three cases.

RESULTS

Case 1 and Case 3 (Histoplasma-infected valves) had fibrinous exudates with scattered macrophages. The microorganisms observed in the first case of methicillin-sensitive S. aureus (MSSA) were ∼ 2-3 μm by GMS stain and had prominent septations. Histoplasma yeast were round to oval, ∼ 3-4 μm in size and demonstrated budding. S. aureus without alterations were round, ∼ 1 μm in size, and lacked prominent septations. Necrotizing purulent inflammation was present in the unaltered case of MSSA. The MSSA case with alterations from antibiotic treatment did not stain well with the Gram stain and organisms were best visualized with the PAS and GMS stains.

CONCLUSIONS

Antibiotic therapy for bacterial endocarditis can alter the inflammatory reaction to infection, bacterial size, septa formation, and staining characteristics. Knowledge of these therapy-related effects and use of high-power magnification helps to avoid misclassification as yeast-like fungi.

Fourier transform infrared microspectroscopy of endocarditis vegetation

The objectives of this work were to compare the infrared spectra of bacterial endocarditis vegetation with those of native valvular tissue and the infrared spectra of vegetation bacterial masses with those of surrounding vegetation tissue. Streptococcal aortic endocarditis was induced in three rabbits. Vegetation slices were cryo-sectioned for study by Fourier transform infrared (FT-IR) microspectroscopy. Valvular apparatus, vegetation, and bacterial masses within the vegetation were localized on hematoxylin and eosin (H&E) stained contiguous slices. Infrared images of whole vegetations and images of bacterial masses were acquired with apertures set to 80 x 80 and 20 x 20 microm, respectively. Valvular apparatus and vegetation showed different infrared spectra, mainly in the amide I and amide II bands (1674-1518 cm(-1)), and at about 1450, 1400, 1340, 1280, 1240, 1200, 1080, and 1030 cm(-1). Valvular collagen, elastin, and proteoglycans may explain these differences. Bacterial masses and surrounding vegetation showed different infrared patterns, mainly in the amide I and amide II bands and in the 1142-991 cm(-1) carbohydrate spectral range. Bacterial nucleic acids and polysaccharides may partly explain these differences. Study of experimental endocarditis vegetation using FT-IR microspectroscopy distinguishes (1) the vegetation from the valvular tissue, and (2) the bacterial masses from the surrounding tissue. This study demonstrates for the first time that FT-IR microspectroscopy is able to detect bacterial growth in infected tissue. FT-IR microspectroscopy appears to be a useful tool for investigation of the biochemical structure of endocarditis vegetation.

Animal models of endocarditis

Bacterial endocarditis is a difficult infection to cure, due to poor penetration of antibiotics into infected vegetations, altered metabolic state of bacteria within the lesion, and absence of adequate host-defense cellular response which could cooperate with antibiotic action. The contribution of animal models to a better understanding of the pathophysiology of the infection and to definition and improvement of therapeutic regimens of endocarditis in humans remains of great importance due to the difficulties encountered in clinical trials. The advantage of the experimental model is that besides the fact that it closely simulates the characteristics of the infection in humans, it provides clear endpoints which allow statistical comparisons among different therapeutic regimens: number of bacteria per gram of tissue, frequency of emergence of resistance, positivity of blood cultures, death vs. survival rates, and percentage of relapses after treatment has been stopped. All these parameters are more sensitive and more easy to study than in humans. The animal model has definitively established that bactericidal therapy is warranted and that in vitro susceptibility tests, especially those evaluating the killing rate, have a good predictive value on the therapeutic outcome. Two main aspects are discussed for their relevance to human therapy: (i) the kinetics of antibiotic diffusion into vegetations, with special reference to data obtained with autoradiography; and (ii) the specificity of some pharmacodynamic aspects of antibiotics in endocarditis, including the clinical consequences of these two parameters with respect to antibiotic dosing regimens and length of therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance and efficacy of parenterally administered penicillin-streptomycin and orally administered amoxicillin or penicillin V for prophylaxis of experimentally induced streptococcal endocarditis

A regimen of a single intramuscular dose of penicillin G-streptomycin was compared with regimens of three oral doses of amoxicillin and two oral doses of penicillin V to prevent Streptococcus sanguis endocarditis in rabbits with experimentally induced valvular heart lesions. Challenge doses of 10(4), 10(6), and 10(8) CFU of a strain of S. sanguis highly tolerant to penicillin and amoxicillin were used. The combination of penicillin and streptomycin was the only regimen tested that provided full protection even against the highest inoculum concentration. A single oral dose of penicillin V (36 mg/kg) or amoxicillin (50 mg/kg), two oral doses of penicillin V (36 and 18 mg/kg with a 7-h interval between doses), or six oral doses of amoxicillin (50 mg/kg followed by 8.5 mg/kg at 8-h intervals) protected recipients of the lowest inoculum concentration; protection diminished with increasing inocula. In contrast, administration of two high oral doses of amoxicillin (50 mg/kg) with a 10-h interval between doses provided full protection against challenge doses of 10(4) and 10(6) CFU, preventing endocarditis in 10 (66%) of 15 recipients of 10(8) CFU. All regimens evaluated were highly effective in preventing endocarditis when rabbits were challenged with 10(4) CFU. The combination of penicillin and streptomycin was the best regimen tested. Administration of two high oral doses of amoxicillin (50 mg/kg) with a 10-h interval between doses led to significantly fewer infections when compared with the other oral regimens when rabbits were challenged with 10(6) and 10(8) CFU.

Evaluation of cilofungin (LY121019) for treatment of experimental Candida albicans endocarditis in rabbits

The efficacy of cilofungin (LY121019) for aortic valve endocarditis caused by Candida albicans in rabbits was studied. Vegetation titers were similar for cilofungin-treated and untreated rabbits. No rabbit survived beyond 5 days in either group. All rabbits given amphotericin B survived, and titers were reduced. Cilofungin was ineffective in this model.

A morphological study of the effect of treatment with the antibiotic ceftazidime on experimental staphylococcal endocarditis and aortitis

The morphological effects of antibiotic therapy with either single or repeated (8 hourly) injections of ceftazidime were examined in rabbits with experimentally induced staphylococcal endocarditis and aortitis. At 3 h after initiating treatment, many of the bacteria, irrespective of the location of the colony, showed evidence of abnormal ultrastructural changes of the cytoplasm and/or cell wall. By 8 h many degenerate lysed bacteria were present. By 24 h, in rabbits which received a single injection, bacterial colonies contained many normal and dividing bacteria. In comparison, bacterial colonies at 24 h in rabbits receiving repeated injections consisted of large masses of lysed bacteria with only a few viable appearing thick-walled 'persistent' cells. At 48 and 72 h, no viable appearing bacteria were observed although they could be isolated by culture methods. Treatment was associated with an increased inflammatory cell response at the surface of the vegetation and within the vasculature. In the later stages there was evidence of healing with endothelialization of the lesions. It would appear, therefore, that ceftazidime penetrates efficiently into the vegetations with only a short transitory phase at sub-bactericidal concentrations. The few 'persistent' bacteria appear to be protected from the host defences by the surrounding thrombus which prevents their eradication.

Comparative capacity of orally administered amoxicillin and parenterally administered penicillin-streptomycin to protect rabbits against experimentally induced streptococcal endocarditis

A single-intramuscular-dose immunization regimen with a penicillin G-streptomycin combination was compared with three oral-dose amoxicillin regimens for the capacity to prevent Streptococcus sanguis infections of experimentally induced valvular heart lesions in rabbits. Challenge doses of 10(4), 10(6), and 10(8) CFU of a strain of S. sanguis equally susceptible to penicillin and amoxicillin were used in this study. Measured by recovery of test organisms from endocardial lesions, the lowest concentration of these inocula was infective for 60% of the recipients; the two higher-concentration inocula were infective for all recipients. The penicillin G-streptomycin combination provided complete protection against infection with inocula of all sizes. A single-oral-dose amoxicillin regimen (50 mg/kg of body weight) prevented endocarditis when rabbits were challenged with 10(4) CFU, but protection diminished with increasing inoculum concentrations. Similar results were achieved when five oral doses of amoxicillin (8.5 mg/kg of body weight) added at 8-h intervals were included in the single-oral-dose regimen. In contrast, when rabbits received two oral doses of amoxicillin (50 mg/kg of body weight) with a 10-h interval between doses, prophylaxis was fully effective with even the highest inoculum concentration.

Staphylococci, in vitro and in vivo

Strains of Staphylococcus aureus were grown in broth and by the membrane technique; both drug-free media and media containing cloxacillin were used. The staphylococci grown in broth containing cloxacillin showed one thick cross wall and were larger than those grown in drug-free broth: 1.6 micron in diameter as opposed to 0.9 micron. The staphylococci grown on membranes placed on agar containing cloxacillin were 2-3 microns in diameter and contained three or more cross walls. Mice were infected intraperitoneally with staphylococci. After treatment with cloxacillin, the peritoneal fluid and spleens contained staphylococci that were 2-3 microns in diameter with three or more cross walls. A staphylococcal endocarditis was induced in rabbits that were then treated with cloxacillin. The staphylococci in the vegetation of the treated rabbits were 2-3 microns in diameter and contained multiple cross walls. Large staphylococci with multiple cross walls were observed in specimens from patients with respiratory infections treated with beta-lactam antibiotics. It appears, therefore, that the ultrastructure of staphylococci in vivo is comparable to that of staphylococci grown on a solid support medium such as a membrane, and different from that of staphylococci grown in a liquid medium.

Influence of the developmental state of valvular lesions on the antimicrobial activity of cefotaxime in experimental enterococcal infections

Cefotaxime has little antimicrobial activity in vitro against most strains of enterococci, as measured by conventional MICs and MBCs. However, the MICs of cefotaxime against many enterococci are markedly reduced by the addition of serum to the test medium. To assess the relevance of this observation in vivo, we examined the efficacy of cefotaxime in experimental Streptococcus faecalis endocarditis. Since response to antimicrobial agents may vary with the degree of vegetation development, therapeutic efficacy was assessed both in rabbits with newly formed vegetations and in rabbits with well-developed endocardial lesions. Peak serum levels of cefotaxime (50.1 +/- 20.0 micrograms/ml) exceeded the MIC in medium supplemented with serum (4 micrograms/ml), but not in Mueller-Hinton broth alone (greater than 64 micrograms/ml). After 4 days of therapy, animals with newly formed lesions (therapy initiated 1 h after infection, transvalvular catheters removed) had lower mean vegetation bacterial titers than did untreated controls. Among animals with mature vegetations (therapy initiated 12 h after infection, catheters indwelling), the rate of mortality was significantly reduced by cefotaxime therapy. However, no difference in vegetation titers was observed. Thus, cefotaxime demonstrated antienterococcal activity within newly formed vegetations, but did not inhibit bacterial proliferation within well-established vegetations.