Penicillin therapy of experimental endocarditis induced by tolerant Streptococcus sanguis and nontolerant Streptococcus mitis

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.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

Combining diagnostic methods for antimicrobial susceptibility testing - A comparative approach

BACKGROUND

The minimum inhibitory concentration (MIC) is a measure of antimicrobial susceptibility testing (AST) of a given antibiotic but provides insufficient information when bacterial killing is crucial, e.g., when treating immunocompromised patients. In these cases, the minimum bactericidal concentration (MBC) is a more reliable measure of antibiotic activity. Here, we aim to demonstrate and recommend combinations of methods for MIC and MBC measurements. We also aim to emphasize the importance of uniform protocols for these procedures including the time point for reading MIC results, which the authors suggest to be 20h.

METHODS

To address the challenges with obtaining fast and reliable readouts on MIC as well as the kinetic and end-point effects of antibiotics, the broth micro dilution method, a calorimetric method and a microscopy-based screening system (MBSS) were evaluated in this study. For MBC determination, fluorophore staining with SYTO9 and propidium iodide was compared to the broth regrowth method.

RESULTS

Three scenarios for combining the MIC and MBC methods depending on the investigators' primary concern (time, cost or sensitivity) are presented. Further, as the MBSS and the isothermal microcalorimetry method detected delayed bacterial growth up to 18h after initiation of experiments, the importance of reading MIC testing after a full 20h is emphasized. A one-fold change in MIC values can be observed when comparing data obtained at 16h and 20h of incubation.

CONCLUSION

The authors suggest that combining MIC and MBC determinations will provide more detailed understanding of the bacteria susceptibility to antibiotic drugs and result in more clinically relevant data and optimized therapies. Furthermore, establishing 20h as a time point for reading MIC results will provide more uniform data across laboratories.

Efficacy of Antibiotic Prophylactic Regimens for the Prevention of Bacterial Endocarditis of Oral Origin

Despite the controversy about the risk of individuals developing bacterial endocarditis of oral origin, numerous Expert Committees in different countries continue to publish prophylactic regimens for the prevention of bacterial endocarditis secondary to dental procedures. In this paper, we analyze the efficacy of antibiotic prophylaxis in the prevention of bacteremia following dental manipulations and in the prevention of bacterial endocarditis (in both animal models and human studies). Antibiotic prophylaxis guidelines remain consensus-based, and there is scientific evidence of the efficacy of amoxicillin in the prevention of bacteremia following dental procedures, although the results reported do not confirm the efficacy of other recommended antibiotics. The majority of studies on experimental models of bacterial endocarditis have verified the efficacy of antibiotics administered after the induction of bacteremia, confirming the efficacy of antibiotic prophylaxis in later stages in the development of bacterial endocarditis. There is no scientific evidence that prophylaxis with penicillin is effective in reducing bacterial endocarditis secondary to dental procedures in patients considered to be "at risk". It has been suggested that there is a high risk of severe allergic reactions secondary to prophylactically administered penicillins, but, in reality, very few cases have been reported in the literature. It has been demonstrated that antibiotic prophylaxis could contribute to the development of bacterial resistance, but only after the administration of several consecutive doses. Future research on bacterial endocarditis prophylactic protocols should involve the re-evaluation of the time and route of administration of antibiotic prophylaxis, and a search for alternative antimicrobials.

Streptococcus mitis: walking the line between commensalism and pathogenesis

Streptococcus mitis is a viridans streptococcus and a normal commensal of the human oropharynx. However, S. mitis can escape from this niche and cause a variety of infectious complications including infective endocarditis, bacteraemia and septicaemia. It uses a variety of strategies to effectively colonize the human oropharynx. These include expression of adhesins, immunoglobulin A proteases and toxins, and modulation of the host immune system. These various colonization factors allow S. mitis to compete for space and nutrients in the face of its more pathogenic oropharyngeal microbial neighbours. However, it is likely that in vulnerable immune-compromised patients S. mitis will use the same colonization and immune modulation factors as virulence factors promoting its opportunistic pathogenesis. The recent publication of a complete genome sequence for S. mitis strain B6 will allow researchers to thoroughly investigate which genes are involved in S. mitis host colonization and pathogenesis. Moreover, it will help to give insight into where S. mitis fits in the complicated oral microbiome. This review will discuss the current knowledge of S. mitis factors involved in host colonization, their potential role in virulence and what needs to be done to fully understand how a an oral commensal successfully transitions to a virulent pathogen.

[In vivo activity of amoxicillin in a non-toxigenic Corynebacterium diphtheriae rabbit endocarditis experimental model]

The mortality of non-toxigenic Corynebacterium diphtheriae endocarditis is high (27%). One explanation could be tolerance to amoxicillin. The aim of this work was to evaluate in vivo the tolerance phenomenon, in a rabbit endocarditis experimental model. Two strains of non-toxigenic Corynebacterium diphtheriae were compared: a tolerant one and a non-tolerant one. EACH ANIMAL WAS RANDOMLY ASSIGNED TO ONE OF THE FOLLOWING GROUPS: no treatment, continuous infusion of amoxicillin simulating 200 mg/kg/24 h in humans, or fractionated infusion of amoxicillin simulating 66 mg/kg/8 h in humans. Surviving bacteria were counted in vegetations after one or three days of treatment. The 24 h fractionated amoxicillin infusion was more efficacious on the non-tolerant strain than on the tolerant strain. On the tolerant strain, 24 h amoxicillin was more efficacious as a continuous infusion than as a fractionated one.

High-dose cytosine arabinoside and viridans streptococcus sepsis in children with leukemia

We report three cases of fulminant sepsis due to viridans streptococci in leukemic children treated with high-dose cytosine arabinoside (Ara-C). The major predisposing factors to this occurrence are the presence of oropharingeal mucositis, which is the entry of streptococci into the bloodstream, and the use of antibiotic prophylactic regimens against gram-negative bacteria. In order to avoid fatal events during viridans streptococci sepsis, specific measures such as penicillin prophylaxis or early antibiotic treatment are needed. We suggest that the prompt empiric use of a glycopeptide antibiotic in addition to the conventional association of a beta-lactam plus an aminoglycoside may significantly decrease the mortality rate due to fulminant streptococci sepsis while the patient is severely neutropenic. In this regard, our current policy considers the addition of an anti-gram-positive antibiotic to the first-choice fever treatment in neutropenic patients who have received high-dose Ara-C.

In vitro development and stability of tolerance to cloxacillin and vancomycin in Staphylococcus aureus

The stability of tolerance of Staphylococcus aureus during subculturing at 37 degrees C and development of this property after repeated exposure to cloxacillin or vancomycin were investigated in vitro. Four of five tolerant strains lost this property during repeated subculturing at 37 degrees C for 50 days. Conversely, tolerance emerged in two of four nontolerant strains after repeated cycles of exposure to 25 micrograms of cloxacillin/ml or 10 micrograms of vancomycin/ml alternating with growth in antibiotic-free medium. Previous in vivo exposure to cloxacillin did not enhance the development of tolerance in vitro. MICs of both cloxacillin and vancomycin did not change significantly during this procedure. Whether the conversion of nontolerant strains to the tolerant state can also occur during antibiotic exposure in treatment of patients remains to be determined.

Distribution, antibiotic susceptibility and tolerance of bacterial isolates in culture-positive cases of endocarditis in The Netherlands

During a two-year period data were collected nationwide in The Netherlands on 438 episodes of bacterial endocarditis (BE) in 432 patients. Of the strains isolated in these patients 419 were available for analysis. Of these, 326 were isolated in native valve endocarditis (NVE) and 93 in prosthetic valve endocarditis (PVE). Viridans streptococci, staphylococci and enterococci together constituted 87% of the isolates. More than 46% of the viridans streptococci consisted of Streptococcus sanguis. Enterococcus faecalis and Staphylococcus aureus were the predominant species in the late form of PVE. The majority of the viridans streptococci and haemolytic streptococci were highly susceptible to penicillin. Five of 35 strains of coagulase negative staphylococci were resistant to methicillin. Eleven percent of a random sample of the streptococci collected were tolerant to penicillin. After repeated exposure to a concentration gradient of an appropriate beta-lactam antibiotic, this figure increased to 49%. Of the staphylococci, 5-6% of the strains were tolerant before induction and 16-20% after induction. Of the Enterococcus strains (n = 40), 12.5% showed high-level resistance to one or more aminoglycoside.

Gram-positive bacteraemia in granulocytopenic cancer patients

In the four EORTC International Antimicrobial Therapy Cooperative Group trials, the frequency of gram-positive isolates has increased significantly from 29% of single-organism bacteraemias in trial I (1973-1976) to 41% in trial IV (1983-1985). In trial IV febrile and neutropenic (less than 1000 polymorphonuclear lymphocytes per microliter) cancer patients were randomized prospectively to receive either azlocillin plus a long course (at least 9 days) of amikacin, or ceftazidime plus a short course (3 days) of amikacin, or ceftazidime plus a long course of amikacin. Without modification of the allocated antibiotics, the overall response rates for gram-positive bacteraemias were similar for all three regimens (19/37 [51%], 8/23 [35%] and 14/30 [47%]), respectively. However, in patients with prolonged and severe neutropenia, treatment with azlocillin plus amikacin was significantly more effective than with ceftazidime plus 3 days' amikacin (7/10 vs. 0/7). The overall response rate for these infections was significantly lower than that observed in trial I (46% vs. 74%), but this was not associated with increased mortality. The response to treatment was significantly influenced by the susceptibility of the infecting strain to the beta-lactam. Multivariate analysis revealed that increasing age, presence of a central venous catheter and resistance to beta-lactam adversely affected outcome. Future studies should be designed to improve the outcome of gram-positive bacteraemia in neutropenic patients with cancer.

Penicillin-induced effects on streptomycin uptake and early bactericidal activity differ in viridans group and enterococcal streptococci

In vitro studies with penicillin and [3H]streptomycin in four strains of streptococci (S. faecalis, S. sanguis, and S. mitis) were performed by simultaneously measuring the rates of bacterial killing and uptake of streptomycin. In S. faecalis, penicillin stimulated streptomycin uptake, as has been shown by Moellering and Weinberg (R. C. Moellering, Jr., and A. N. Weinberg, J. Clin. Invest. 50:2580-2584, 1971). Moreover, the antibiotic combination was associated with an enhanced bactericidal rate which temporally correlated with beta-lactam-induced aminoglycoside uptake. In contrast, in viridans group streptococci (S. sanguis and S. mitis) penicillin had no effect on streptomycin uptake and a minimal effect on bactericidal rate when compared with either drug alone. These data suggested that the stimulation of streptomycin uptake in streptococci by penicillin is strain or species specific and that important differences exist between enterococci and viridans group streptococci regarding the mechanisms of beta-lactam-aminoglycoside potentiation.

Infective endocarditis due to penicillin-tolerant Streptococcus bovis

Two Streptococcus bovis isolates obtained from patients with endocarditis were found to be tolerant to penicillin and other cell wall active agents. By time-kill analysis, penicillin and streptomycin acted synergistically against these strains. The existence of tolerant S. bovis strains should be considered when initially choosing antibiotics for the treatment of serious S. bovis infections.

Discrepancies between MBC and actual killing of viridans group streptococci by cell-wall-active antibiotics

We determined the MBC of amoxicillin and vancomycin, two antibiotics advocated for treatment and prophylaxis of bacterial endocarditis, for 24 strains of viridans group streptococci isolated from patients with endocarditis. We found that the MIC of amoxicillin for all strains was less than or equal to 0.25 micrograms/ml and the MBC was either low (less than 0.5 micrograms/ml) in 6 nontolerant strains or high (greater than 128 micrograms/ml) in 18 tolerant strains. The MIC of vancomycin for the 24 strains was less than or equal to 1 microgram/ml, and the MBC was either low (less than 1 microgram/ml) for 3 nontolerant strains or high (greater than 128 micrograms/ml) for 21 tolerant strains. In addition to the MBC, we determined the actual reduction of the viable bacterial counts in each tube dilution after 24 h of incubation. This determination was made by subtracting the number of colonies observed on the subculture plate from the number of bacteria contained in the initial inoculum. For both antibiotics we found that the maximal reduction in viable counts was achieved at or very close to the MIC and did not increase with increasing antibiotic concentrations (up to 128 micrograms/ml). As expected, the six strains for which the amoxicillin MBC was less than 0.5 micrograms/ml and the three strains for which the vancomycin MBC was less than 1 microgram/ml had a reduction of viable counts of more than 3 log10 (greater than 99.9% killing). In contrast, among the strains defined as tolerant to amoxicillin and vancomycin, there were wide variations in the actual reduction of bacterial counts, ranging from 3 log10 to less than 1 log10. Therefore our observations suggest that the reduction of viable streptococcal counts reflects more accurately the bactericidal effect of amoxicillin and vancomycin than does the MBC, which artificially divides the strains into sensitive or tolerant strains.