In Vivo Effects of Cefazolin, Daptomycin, and Nafcillin in Experimental Endocarditis with a Methicillin-Susceptible Staphylococcus aureus Strain Showing an Inoculum Effect against Cefazolin

β-Lactam Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus Infective Endocarditis

Importance

Infective endocarditis (IE) caused by Staphylococcus aureus is associated with high mortality, approximately 20% to 30%, mostly in the first month, with no improvement in recent decades. Current opinion is that antistaphylococcal penicillin and cefazolin are equally effective in treating methicillin-susceptible S aureus (MSSA) IE, and both are recommended as possible first-line treatments. Most MSSA strains carry the β-lactamase blaZ gene, and some blaZ-positive strains exhibit an inoculum effect, meaning increased minimum inhibitory concentrations at high inoculum. This reduced susceptibility to an antibiotic at high bacterial inoculum may be particularly relevant in IE, where vegetations have very high bacterial densities.

Objective

To evaluate the association between phenotypic characteristics of S aureus isolates, β-lactam used, and outcome in patients with MSSA IE.

Design, Settings, and Participants

This retrospective case series included MSSA cases treated at 3 French university hospitals between February 2016 and February 2022. The study included patients who had clinical isolates available and had definite or possible S aureus IE that involved native or prosthetic valves. Data were analyzed from July 2023 to June 2024.

Main Outcomes and Measures

MSSA isolates were tested for the presence of blaZ and for inoculum effects to cefazolin and oxacillin. The association between first-month mortality and the β-lactam used, the presence of blaZ, and the presence of an inoculum effect to the treatment received was evaluated.

Results

This study included 216 patients with MSSA IE (median [IQR] age, 65 [49-73] years; 152 [70.4%] male) who were treated with antistaphylococcal penicillin (139 [64.4%]) or cefazolin (77 [35.6%]). One-month mortality of left-sided IE was 44 of 180 patients (24.4%), with no overall difference between patients treated with antistaphylococcal penicillin or cefazolin. However, 1-month mortality was higher in patients infected with blaZ-positive strains than with blaZ-negative strains (38 of 129 [29.5%] vs 6 of 51 [11.8%]; P = .01), and with strains with an inoculum effect to the β-lactam received than with strains without an inoculum effect (25 of 62 [40.3%] vs 13 of 67 [19.4%]; P = .005). On multivariable analysis, the presence of an inoculum effect was independently associated with first-month mortality (HR, 2.84; 95% CI, 1.28-6.30; P = .01).

Conclusions and Relevance

In this case series of MSSA IE, the presence of an inoculum effect to the β-lactam received was a risk factor for death in the first month. Phenotyping MSSA isolates for inoculum effect may guide β-lactam choice and improve outcomes.

Efficacy and safety of antistaphylococcal penicillin or cephazolin-based combinations versus monotherapy for methicillin-susceptible Staphylococcus aureus infective endocarditis: A propensity score analysis of nationwide prospective cohort

OBJECTIVES

We aimed to evaluate the usefulness of antistaphylococcal penicillin (ASP) or cephazolin-based combinations versus monotherapy in patients with native-valve infective endocarditis (IE) caused by methicillin-susceptible Staphylococcus aureus (MSSA).

METHODS

Post-hoc analysis of a multicentre prospective cohort. We include patients from 2008 to 2022 with definite native-valve, left-side IE due to MSSA treated primarily with ASP/cephazolin. Patients were categorized according to whether they initially received monotherapy or combination therapy for more than 72 h. A propensity score-matched cohort was planned.

RESULTS

Out of 420 included cases, 94 (22.4%) received monotherapy and 326 (77.6%) combination. Median combination duration was 14 days (interquartile range 10-20). Sixty-eight combination cases were matched with 68 monotherapy controls. Baseline characteristics were well balanced. There were no differences in in-hospital or one-year mortality between groups (OR 0.85, 95%CI 0.33-2.18 and HR 0.68, 95%CI 0.35-1.31, respectively). Endocarditis relapses and persistent bacteraemia rates were similar (0% vs 1.5%, p = 1.000; and 19.1% vs 13.2%, p = 0.352, respectively). Drug-related adverse events were more frequent in the combination group (15.0% vs 1.1%, p < 0.001).

CONCLUSIONS

Antibiotic combinations for patients with native valve left-sided MSSA endocarditis did not improve patient's outcomes. Drug-related adverse events were more frequent in combination patients.

Effectiveness and safety of cefazolin versus cloxacillin in endocarditis due to methicillin-susceptible Staphylococcus spp.: a multicenter propensity weighted cohort study

PURPOSE

To compare the effectiveness and safety of cefazolin versus cloxacillin for the treatment of infective endocarditis (IE) due to methicillin-sensitive Staphylococci (MSS).

METHODS

Data were retrospectively collected on patients treated for a definite MSS endocarditis who received cefazolin or cloxacillin for at least 10 consecutive days in six French hospitals between January-1 2014 and December-31 2020. The primary endpoint was treatment failure defined as a composite of death within 90 days of starting antibiotherapy, or embolic event during antibiotherapy, or relapse of IE within 90 days of stopping antibiotherapy. We used Cox regression adjusted for the inverse probability of treatment weighting of receiving cefazolin.

RESULTS

192 patients were included (median age 67.8 years). IE was caused by S.aureus in 175 (91.1%) and by coagulase-negative staphylococci in 17 (8.9%). Ninety-four patients (48.9%) received cefazolin, and 98 (51%) received cloxacillin. 34 patients (34.7%) with cefazolin and 26 (27.7%) with cloxacillin met the composite primary endpoint, with no significant differences between groups (adjusted HR = 1.13, 95% CI 0.63 to 2.03). There were no significant differences in secondary efficacy endpoints or biological safety events.

CONCLUSION

The effectiveness of cefazolin did not significantly differ from cloxacillin for the treatment of MSS endocarditis.

The Application of Rat Models in Staphylococcus aureus Infections

Staphylococcus aureus (S. aureus) is a major human pathogen and can cause a wide range of diseases, including pneumonia, osteomyelitis, skin and soft tissue infections (SSTIs), endocarditis, mastitis, bacteremia, and so forth. Rats have been widely used in the field of infectious diseases due to their unique advantages, and the models of S. aureus infections have played a pivotal role in elucidating their pathogenic mechanisms and the effectiveness of therapeutic agents. This review outlined the current application of rat models in S. aureus infections and future prospects for rat models in infectious diseases caused by S. aureus.

Emerging challenges in antimicrobial resistance: implications for pathogenic microorganisms, novel antibiotics, and their impact on sustainability

Overuse of antibiotics is accelerating the antimicrobial resistance among pathogenic microbes which is a growing public health challenge at the global level. Higher resistance causes severe infections, high complications, longer stays at hospitals and even increased mortality rates. Antimicrobial resistance (AMR) has a significant impact on national economies and their health systems, as it affects the productivity of patients or caregivers due to prolonged hospital stays with high economic costs. The main factor of AMR includes improper and excessive use of antimicrobials; lack of access to clean water, sanitation, and hygiene for humans and animals; poor infection prevention and control measures in hospitals; poor access to medicines and vaccines; lack of awareness and knowledge; and irregularities with legislation. AMR represents a global public health problem, for which epidemiological surveillance systems have been established, aiming to promote collaborations directed at the well-being of human and animal health and the balance of the ecosystem. MDR bacteria such as E. coli, Staphylococcus aureus, Pseudomonas aeruginosa, Enterococcus spp., Acinetobacter spp., and Klebsiella pneumonia can even cause death. These microorganisms use a variety of antibiotic resistance mechanisms, such as the development of drug-deactivating targets, alterations in antibiotic targets, or a decrease in intracellular antibiotic concentration, to render themselves resistant to numerous antibiotics. In context, the United Nations issued the Sustainable Development Goals (SDGs) in 2015 to serve as a worldwide blueprint for a better, more equal, and more sustainable existence on our planet. The SDGs place antimicrobial resistance (AMR) in the context of global public health and socioeconomic issues; also, the continued growth of AMR may hinder the achievement of numerous SDGs. In this review, we discuss the role of environmental pollution in the rise of AMR, different mechanisms underlying the antibiotic resistance, the threats posed by pathogenic microbes, novel antibiotics, strategies such as One Health to combat AMR, and the impact of resistance on sustainability and sustainable development goals.

Pharmacodynamics of Doripenem Alone and in Combination with Relebactam in an In Vitro Hollow-Fiber Dynamic Model: Emergence of Resistance of Carbapenemase-Producing Klebsiella pneumoniae and the Inoculum Effect

The emergence of bacteria resistant to beta-lactam/beta-lactamase inhibitor combinations is insufficiently studied, wherein the role of the inoculum effect (IE) in decreased efficacy is unclear. To address these issues, 5-day treatments with doripenem and doripenem/relebactam combination at different ratios of the agents were simulated in a hollow-fiber dynamic model against carbapenemase-producing K. pneumoniae at standard and high inocula. Minimal inhibitory concentrations (MICs) of doripenem alone and in the presence of relebactam at two inocula were determined. Combination MICs were tested using traditional (fixed relebactam concentration) and pharmacokinetic-based approach (fixed doripenem-to-relebactam concentration ratio equal to the therapeutic 24-h area under the concentration-time curve (AUC) ratio). In all experiments, resistant subpopulations were noted, but combined simulations reduced their numbers. With doripenem, the IE was apparent for both K. pneumoniae isolates in combined treatments for one strain. The pharmacokinetic-based approach to combination MIC estimation compared to traditional showed stronger correlation between DOSE/MIC and emergence of resistance. These results support (1) the constraint of relebactam combined with doripenem against the emergence of resistance and IE; (2) the applicability of a pharmacokinetic-based approach to estimate carbapenem MICs in the presence of an inhibitor to predict the IE and to describe the patterns of resistance occurrence.

Therapeutic Phage Monitoring: A Review

With the global rise in antimicrobial resistance, there has been a renewed interest in the application of therapeutic phages to treat bacterial infections. Therapeutic phage monitoring (TPM) is proposed as an essential element of phage therapy (PT) protocols to generate data and fill knowledge gaps regarding the in vivo efficacy of therapeutic phages, patients' immune responses to PT, and the wider ecological effects of PT. By monitoring phage concentrations in blood and tissues, together with immune responses and possible ecological changes during PT, TPM may enable the optimization of dosing and the implementation of precision medicine approaches. Furthermore, TPM can validate diagnostic surrogates of efficacy, direct research efforts, and establish quality assurance indicators for therapeutic phage products. Thus, TPM holds great potential for enhancing our understanding of the multidirectional phage-bacteria-host interactions and advancing "best practice" PT, ultimately improving patient care.

Therapeutic Options for Adult Patients With Persistent Methicillin-Susceptible Staphylococcus aureus Bacteremia: A Narrative Review

OBJECTIVE

To compare the efficacy of antimicrobial therapies used in the management of persistent methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia.

DATA SOURCES

A literature search using the PubMed database (inception to December 2022) was conducted using the search terms "Staphylococcus aureus bacteremia," "methicillin-susceptible Staphylococcus aureus bacteremia," "persistent methicillin-susceptible Staphylococcus aureus bacteremia," and "refractory methicillin-susceptible Staphylococcus aureus bacteremia ." In addition, therapeutic agents which could be used as treatment for MSSA including "nafcillin," "oxacillin," "cefazolin," "ceftaroline," "gentamicin," "rifampin," and "daptomycin" were also combined with the aforementioned search terms to capture data using these agents.

STUDY SELECTION/DATA EXTRACTION

Clinical data were limited to those published in the English language. Articles and abstracts were considered for inclusion in addition to ongoing trials identified through ClinicalTrials.gov.

DATA SYNTHESIS

A total of 78 articles were reviewed including 17 in vitro or animal model studies and 39 studies including patient data. The remaining 22 articles included guidelines, review articles, and editorials. Recent data evaluating use of dual β-lactam regimens for persistent MSSA bacteremia were limited to 8 case reports or case series.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

At present, there is little guidance on how to best manage patients with persistent MSSA bacteremia. This narrative review collates the available data to assist clinicians in selecting the best possible antimicrobial regimen when facing this clinical conundrum.

CONCLUSIONS

Modification of antimicrobial therapy, in conjunction with source control and infectious diseases consultation, may all be necessary to sterilize blood cultures in patients with persistent MSSA bacteremia.

Disc Test for Detecting Staphylococcus aureus Strains Producing Type A and Type C β-Lactamases

Staphylococcus aureus can produce β-lactamases capable of hydrolyzing penicillins and first-generation cephalosporins. The propensity of type A and type C β-lactamase-producing S. aureus (TAPSA and TCPSA) to hydrolyze cefazolin at a high inoculum is termed the cefazolin inoculum effect (CIE). Strains with a CIE have a theoretical risk of causing treatment failure and are unable to be detected routinely by most laboratories. We developed a high-performing yet straightforward β-lactamase disc test that identifies and differentiates both TAPSA and TCPSA and is suitable for routine diagnostic laboratory workflows. Clinical isolates of S. aureus resistant to penicillin were identified, and their blaZ genes were sequenced. MICs were determined at low and high inocula (5 × 105 CFU/mL and 5 × 107 CFU/mL), and isolates demonstrating a CIE were characterized. A semimechanistic model was established to describe differential hydrolysis patterns, and candidate models were iteratively assessed using area-under-the-curve analysis from competitor receiver operating characteristic (ROC) curves. Biomarker thresholds were derived from Youdon index-derived optimal cutoff values. Genetic analysis of 99 isolates identified 26 TAPSA isolates and 45 TCPSA isolates. The model best differentiating TAPSA from non-TAPSA utilized cefazolin-to-cephalothin ratio analysis (sensitivity, 96.2%; specificity, 98.6%). The model best differentiating TCPSA from non-TCPSA incorporated cefazolin, cephalothin, and oxacillin (sensitivity, 88.6%; specificity, 96.6%). TAPSA and TCPSA can be differentiated using three antibiotic discs on a single agar plate. The test has potential value in typing the β-lactamase type from isolates from patients that are candidates for or have failed cefazolin therapy. IMPORTANCE The key significance of this article is that it details a straightforward method of performing a disc test that can differentiate Staphylococcus aureus isolates that are likely to be associated with a cefazolin inoculum effect and theoretical risk of cefazolin treatment failure from isolates that are less likely to be associated with a cefazolin inoculum effect.

Application of antimicrobial peptides in plant protection: making use of the overlooked merits

Pathogen infection is one of the major causes of yield loss in the crop field. The rapid increase of antimicrobial resistance in plant pathogens has urged researchers to develop both new pesticides and management strategies for plant protection. The antimicrobial peptides (AMPs) showed potential on eliminating plant pathogenic fungi and bacteria. Here, we first summarize several overlooked advantages and merits of AMPs, which includes the steep dose-response relations, fast killing ability, broad synergism, slow resistance selection. We then discuss the possible application of AMPs for plant protection with above merits, and highlight how AMPs can be incorporated into a more efficient integrated management system that both increases the crop yield and reduce resistance evolution of pathogens.

Meropenem MICs at Standard and High Inocula and Mutant Prevention Concentration Inter-Relations: Comparative Study with Non-Carbapenemase-Producing and OXA-48-, KPC- and NDM-Producing Klebsiella pneumoniae

The minimal inhibitory concentration (MIC) is conventionally used to define in vitro levels of susceptibility or resistance of a specific bacterial strain to an antibiotic and to predict its clinical efficacy. Along with MIC, other measures of bacteria resistance exist: the MIC determined at high bacterial inocula (MIC_HI) that allow the estimation of the occurrence of inoculum effect (IE) and the mutant prevention concentration, MPC. Together, MIC, MIC_HI and MPC represent the bacterial "resistance profile". In this paper, we provide a comprehensive analysis of such profiles of K. pneumoniae strains that differ by meropenem susceptibility, ability to produce carbapenemases and specific carbapenemase types. In addition, we have analyzed inter-relations between the MIC, MIC_HI and MPC for each tested K. pneumoniae strain. Low IE probability was detected with carbapenemase-non-producing K. pneumoniae, and high IE probability was detected with those that were carbapenemase-producing. MICs did not correlate with the MPCs; significant correlation was observed between the MIC_HIs and the MPCs, indicating that these bacteria/antibiotic characteristics display similar resistance properties of a given bacterial strain. To determine the possible resistance-related risk due to a given K. pneumoniae strain, we propose determining the MIC_HI. This can more or less predict the MPC value of the particular strain.

Antistaphylococcal penicillins vs. cefazolin in the treatment of methicillin-susceptible Staphylococcus aureus infective endocarditis: a quasi-experimental monocentre study

Whether cefazolin is as effective and safer than antistaphylococcal penicillins (ASPs) for the treatment of methicillin-susceptible Staphylococcus aureus (MSSA) infective endocarditis (IE) is still debated in the absence of a randomized controlled trial. In this quasi-experimental study, we aimed to assess the effectiveness and safety of these two treatments in MSSA-IE, using the ASPs nationwide shortage in April 2016 as a unique opportunity to overcome the indication bias associated with observational studies. In this single-centre study, we compared patients with Duke-Li definite MSSA-IE treated with ASPs from January 2015 to March 2016 versus those treated with cefazolin from April 2016 to December 2018, when ASPs were not available. Effectiveness outcome was 90-day all-cause mortality. Safety outcomes included significant decrease in GFR and significant increase in serum liver enzymes. Logrank test was used to compare survival rates. Of 73 patients with MSSA-IE, 35 and 38 were treated with ASPs and cefazolin, respectively. Baseline patients' characteristics (demography, native or prosthetic valve IE, clinical characteristics, cardiac and septic complications) were similar between groups. Ninety-day all-cause mortality was 28.6% and 21.1%, in patients treated with ASPs and cefazolin, respectively (logrank p = 0.5727). There was no difference between groups for incident renal or liver toxicity events: acute kidney injury 45.7% vs. 44.7% (p = 0.933), increased ALT 5.7% vs. 13.2% (p = 0.432), bilirubin increase 5.7% vs. 10.5% (p = 0.676), in ASPs vs. cefazolin groups, respectively. In this quasi-experimental, effectiveness and safety did not statistically differ between ASPs and cefazolin for MSSA-IE treatment.

Effectiveness of vancomycin plus cloxacillin compared with vancomycin, cloxacillin and daptomycin single therapies in the treatment of methicillin-resistant and methicillin-susceptible Staphylococcus aureus in a rabbit model of experimental endocarditis

OBJECTIVES

To investigate if the addition of cloxacillin to vancomycin enhances the activity of both monotherapies for treating MSSA and MRSA experimental endocarditis (EE) in rabbits.

METHODS

Vancomycin plus cloxacillin was compared with the respective monotherapies and daptomycin. In vitro time-kill studies were performed using standard (105 cfu) and high (108 cfu) inocula of five MRSA, one glycopeptide-intermediate (GISA) and five MSSA strains. One MSSA (MSSA-678) and one MRSA (MRSA-277) strain were selected to be used in the in vivo model. A human-like pharmacokinetics model was applied and the equivalents of cloxacillin 2 g/4 h IV and daptomycin 6 mg/kg/day IV were administered. To optimize vancomycin activity, dosage was adjusted to achieve an AUC/MIC ≥400.

RESULTS

Daptomycin sterilized significantly more vegetations than cloxacillin (13/13, 100% versus 9/15, 60%; P = 0.02) and showed a trend of better activity than vancomycin (10/14, 71%; P = 0.09) and vancomycin plus cloxacillin (10/14, 71%; P = 0.09) against MSSA-678. Addition of cloxacillin to vancomycin (13/15, 87%) was significantly more effective than vancomycin (8/16, 50%; P = 0.05) and showed similar activity to daptomycin (13/18, 72%; P = 0.6) against MRSA-277. In all treatment arms, the bacterial isolates recovered from vegetations were re-tested and showed the same daptomycin susceptibility as the original strains.

CONCLUSIONS

Vancomycin plus cloxacillin proved synergistic and bactericidal activity against MRSA. Daptomycin was the most efficacious option against MSSA and similar to vancomycin plus cloxacillin against MRSA. In settings with high MRSA prevalence, vancomycin plus cloxacillin might be a good alternative for empirical therapy of S. aureus IE.

Inoculum effect of antimicrobial peptides

The activity of many antibiotics depends on the initial density of cells used in bacterial growth inhibition assays. This phenomenon, termed the inoculum effect, can have important consequences for the therapeutic efficacy of the drugs, because bacterial loads vary by several orders of magnitude in clinically relevant infections. Antimicrobial peptides are a promising class of molecules in the fight against drug-resistant bacteria because they act mainly by perturbing the cell membranes rather than by inhibiting intracellular targets. Here, we report a systematic characterization of the inoculum effect for this class of antibacterial compounds. Minimum inhibitory concentration values were measured for 13 peptides (including all-D enantiomers) and peptidomimetics, covering more than seven orders of magnitude in inoculated cell density. In most cases, the inoculum effect was significant for cell densities above the standard inoculum of 5 × 105 cells/mL, while for lower densities the active concentrations remained essentially constant, with values in the micromolar range. In the case of membrane-active peptides, these data can be rationalized by considering a simple model, taking into account peptide-cell association, and hypothesizing that a threshold number of cell-bound peptide molecules is required in order to cause bacterial killing. The observed effect questions the clinical utility of activity and selectivity determinations performed at a fixed, standardized cell density. A routine evaluation of the dependence of the activity of antimicrobial peptides and peptidomimetics on the inoculum should be considered.

Effects of cefazolin-containing niosome nanoparticles against methicillin-resistant Staphylococcus aureus biofilm formed on chronic wounds

The ability of biofilm formation in methicillin-resistant Staphylococcus aureus (MRSA) causes significant mortality and morbidity in wound infections. Nanoparticles because of the drug concentration increment at the point of contact of nanoparticles and bacteria, and slower release of the drug at the desired location are considered as proper tools to overcome the therapeutic problem of antimicrobial-resistant infections. This study was aimed to evaluate the anti-biofilm activity of cefazolin-loaded nanoparticles against MRSA isolates. The 27 clinical isolates of MRSA were collected from patients with pressure sores and diabetic ulcers referred to Loghman Hospital in Tehran-Iran. MRSA isolates were detected by polymerase chain reaction (PCR) and biochemical tests. Cefazolin-loaded niosome was synthesized using the thin-film hydration method and were characterized by zeta potential measurement and transmission electron microscopy (TEM). The round-shaped cefazolin-loaded niosomes had a diameter of 100 nm and a -63 mV zeta potential. The cefazolin-containing niosomes removed 1, 3, and 5 d old biofilms at the concentration of 128 µg ml-1, 128 µg ml-1, and 256 µg ml-1, respectively. Histological results indicated that BALB/c mice receiving cefazolin-loaded niosomes were treated effectively faster than those treated by cefazolin or untreated group. In conclusion, the cefazolin-loaded niosome could be considered as a promising candidate for the treatment of biofilm-mediated infections of MRSA.

Bistable Bacterial Growth Dynamics in the Presence of Antimicrobial Agents

The outcome of an antibiotic treatment on the growth capacity of bacteria is largely dependent on the initial population size (Inoculum Effect). We characterized and built a model of this effect in E. coli cultures using a large variety of antimicrobials, including conventional antibiotics, and for the first time, cationic antimicrobial peptides (CAMPs). Our results show that all classes of antimicrobial drugs induce an inoculum effect, which, as we explain, implies that the dynamic is bistable: For a range of anti-microbial densities, a very small inoculum decays whereas a larger inoculum grows, and the threshold inoculum depends on the drug concentration. We characterized three distinct classes of drug-induced bistable growth dynamics and demonstrate that in rich medium, CAMPs correspond to the simplest class, bacteriostatic antibiotics to the second class, and all other traditional antibiotics to the third, more complex class. These findings provide a unifying universal framework for describing the dynamics of the inoculum effect induced by antimicrobials with inherently different killing mechanisms.

In-cell Solid-State NMR Studies of Antimicrobial Peptides

Antimicrobial peptides (AMPs) have attracted attention as alternatives to classic antibiotics due to their expected limited pressure on bacterial resistance mechanisms. Yet, their modes of action, in particular in vivo, remain to be elucidated. In situ atomistic-scale details of complex biomolecular assemblies is a challenging requirement for deciphering the complex modes of action of AMPs. The large diversity of molecules that modulate complex interactions limits the resolution achievable using imaging methodology. Herein, the latest advances in in-cell solid-state NMR (ssNMR) are discussed, which demonstrate the power of this non-invasive technique to provide atomic details of molecular structure and dynamics. Practical requirements for investigations of intact bacteria are discussed. An overview of recent in situ NMR investigations of the architecture and metabolism of bacteria and the effect of AMPs on various bacterial structures is presented. In-cell ssNMR revealed that the studied AMPs have a disruptive action on the molecular packing of bacterial membranes and DNA. Despite the limited number of studies, in-cell ssNMR is emerging as a powerful technique to monitor in situ the interplay between bacteria and AMPs.

Comparative outcomes of cefazolin versus antistaphylococcal penicillins in methicillin-susceptible Staphylococcus aureus infective endocarditis: a post hoc analysis of a prospective multicentre French cohort study

OBJECTIVES

Current guidelines recommend cefazolin as an alternative to antistaphylococcal penicillins (ASPs) in methicillin-susceptible Staphylococcus aureus (MSSA) infective endocarditis despite the lack of comparative study. The objective of this study was to evaluate the comparative outcomes of cefazolin vs. ASPs in MSSA infective endocarditis.

METHODS

This was a retrospective analysis of an observational multicentre cohort study using prospectively collected data from patients with MSSA endocarditis confirmed by endocarditis team and treated either with cefazolin or ASPs between July 2013 and December 2018. Patients were excluded if they received both treatments. The primary outcome was 90-day all-cause mortality.

RESULTS

Of 210 patients included, 53 patients (25.2%) received cefazolin and 157 (74.8%) received ASPs. The overall 90-day mortality rate was 27.6% (58/210 patients), 24.5% (13/53) in the cefazolin group vs. 28.7% (45/157) in the ASP group (p 0.561). Premature antimicrobial discontinuation due to adverse events occurred less frequently with cefazolin than with ASPs (0/53 vs. 13/157 patients; p 0.042). In multivariate analysis, there was no difference in 90-day mortality between cefazolin and ASPs (adjusted odds ratio (aOR), 1.2; 95% confidence interval (CI), 0.49-2.91; p 0.681), while age (aOR, 1.06; 95% CI, 1.03-1.09; p < 0.001), Charlson comorbidity index (aOR, 1.18; 95% CI, 1.02-1.36 p 0.023), cerebral embolism (aOR, 2.83; 95% CI, 1.33-6.14; p 0.007) and intensive care unit admission (aOR, 4.16; 95% CI, 1.89-9.59; p 0.001) were factors significantly associated with higher mortality.

CONCLUSIONS

Cefazolin seems to be a possible alternative to ASPs in MSSA endocarditis. More studies are needed to confirm these results and determine which treatment should be recommended as first-line therapy.

Cefazolin and Ertapenem Salvage Therapy Rapidly Clears Persistent Methicillin-Susceptible Staphylococcus aureus Bacteremia

Cefazolin and ertapenem combination therapy was used successfully to salvage 11 cases (6 endocarditis) of persistent methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia, including immediate clearance (≤24 hours) in 8 cases. While in vitro synergy was modest, cefazolin plus ertapenem exhibited synergistic action in a rat model of MSSA endocarditis. The combination of cefazolin and ertapenem provides potent in vivo activity against MSSA beyond what is predicted in vitro and warrants further clinical study in the treatment of refractory MSSA bacteremia and endocarditis.

Cefazolin Inoculum Effect and Methicillin-Susceptible Staphylococcus aureus Osteoarticular Infections in Children

Select methicillin-susceptible Staphylococcus aureus (MSSA) strains may produce β-lactamases with affinity for first-generation cephalosporins (1GCs). In the setting of a high inoculum, these β-lactamases may promote the cleavage of 1GCs, a phenomenon known as the cefazolin inoculum effect (CzIE). We evaluated the prevalence and impact of CzIE on clinical outcomes among MSSA acute hematogenous osteomyelitis (AHO) cases. MSSA AHO isolates obtained from two children's hospitals between January 2011 and December 2018 were procured through ongoing surveillance studies. Isolates were tested for CzIE via a broth macrodilution assay using an inoculum of 10⁷ CFU/ml; CzIE was defined as a cefazolin MIC of ≥16 μg/ml. Isolates were characterized by accessory gene regulator group (agr). The progression from acute to chronic osteomyelitis was considered an important outcome. A total of 250 cases with viable isolates were included. Notably, 14.4% of isolates exhibited CzIE with no observed temporal trend; and 4% and 76% of patients received a 1GC as an empirical and definitive therapy, respectively. CzIE isolates were more often resistant to clindamycin, belonged to agrIII, and associated with the development of chronic osteomyelitis. In multivariable analyses, agrIII, multiple surgical debridements, delayed source control, and CzIE were independently associated with progression to chronic osteomyelitis. A higher rate of chronic osteomyelitis was observed with CzIE isolates regardless of definitive antibiotic choice. CzIE is exhibited by 14.4% of MSSA AHO isolates in children. CzIE is independently associated with progression to chronic osteomyelitis in cases of AHO irrespective of final antibiotic choice. These data suggest that negative outcomes reported with CzIE may more accurately reflect strain-dependent virulence factors rather than true antibiotic failure.

Inoculum effect of β-lactam antibiotics

The phenomenon of attenuated antibacterial activity at inocula above those utilized for susceptibility testing is referred to as the inoculum effect. Although the inoculum effect has been reported for several decades, it is currently debatable whether the inoculum effect is clinically significant. The aim of the present review was to consolidate currently available evidence to summarize which β-lactam drug classes demonstrate an inoculum effect against specific bacterial pathogens. Review of the literature showed that the majority of studies that evaluated the inoculum effect of β-lactams were in vitro investigations of Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Across all five pathogens, cephalosporins consistently displayed observable inoculum effects in vitro, whereas carbapenems were less susceptible to an inoculum effect. A handful of animal studies were available that validated that the in vitro inoculum effect translates into attenuated pharmacodynamics of β-lactams in vivo. Only a few clinical investigations were available and suggested that an in vitro inoculum effect of cefazolin against MSSA may correspond to an increased likeliness of adverse clinical outcomes in patients receiving cefazolin for bacteraemia. The presence of β-lactamase enzymes was the primary mechanism responsible for an inoculum effect, but the observation of an inoculum effect in multiple pathogens lacking β-lactamase enzymes indicates that there are likely multiple mechanisms that may result in an inoculum effect. Further clinical studies are needed to better define whether interventions made in the clinic in response to organisms displaying an in vitro inoculum effect will optimize clinical outcomes.

Genome Sequence Comparison of Staphylococcus aureus TX0117 and a Beta-Lactamase-Cured Derivative Shows Increased Cationic Peptide Resistance Accompanying Mutations in relA and mnaA

Staphylococcus aureus strain TX0117 is a methicillin-susceptible bacterium with type A beta-lactamase exhibiting a high cefazolin inoculum effect. TX0117 was cured of blaZ, yielding TX0117c with increased antimicrobial peptide resistance. The sequencing and genome assembly of TX0117 elucidate six mutations between TX0117 and TX0117c, including relA truncation and mnA_1 substitution.

Tedizolid as Step-Down Therapy following Daptomycin versus Continuation of Daptomycin against Enterococci and Methicillin- and Vancomycin-Resistant Staphylococcus aureus in a Rat Endocarditis Model

Tedizolid (TZD) and daptomycin (DAP) were assessed in a rat endocarditis model against Enterococcus faecalis, Enterococcus faecium (resistant to vancomycin and ampicillin), and Staphylococcus aureus As a monotherapy, TZD for 5 days was not effective in a comparison with no-treatment controls, while DAP for 5 days was significantly effective against these bacteria. Step-down therapy (DAP for 3 days followed by TZD for 2 days) was as effective as DAP for 5 days and was comparable to 3 days of DAP plus ceftriaxone against all bacteria and to 3 days of DAP plus gentamicin against E. faecalis OG1RF.

Antibiotic resistance: bioinformatics-based understanding as a functional strategy for drug design

The use of antibiotics to manage infectious diseases dates back to ancient civilization, but the lack of a clear distinction between the therapeutic and toxic dose has been a major challenge. This precipitates the notion that antibiotic resistance was from time immemorial, principally because of a lack of adequate knowledge of therapeutic doses and continuous exposure of these bacteria to suboptimal plasma concentration of antibiotics. With the discovery of penicillin by Alexander Fleming in 1924, a milestone in bacterial infections' treatment was achieved. This forms the foundation for the modern era of antibiotic drugs. Antibiotics such as penicillins, cephalosporins, quinolones, tetracycline, macrolides, sulphonamides, aminoglycosides and glycopeptides are the mainstay in managing severe bacterial infections, but resistant strains of bacteria have emerged and hampered the progress of research in this field. Recently, new approaches to research involving bacteria resistance to antibiotics have appeared; these involve combining the molecular understanding of bacteria systems with the knowledge of bioinformatics. Consequently, many molecules have been developed to curb resistance associated with different bacterial infections. However, because of increased emphasis on the clinical relevance of antibiotics, the synergy between in silico study and in vivo study is well cemented and this facilitates the discovery of potent antibiotics. In this review, we seek to give an overview of earlier reviews and molecular and structural understanding of bacteria resistance to antibiotics, while focusing on the recent bioinformatics approach to antibacterial drug discovery.

Understanding the evolution of antibiotic resistance at the molecular level as a functional tool for bioinformatic-based drug design.

Retrospective Analysis of Adverse Drug Events Between Nafcillin Versus Cefazolin for Treatment of Methicillin-Susceptible Staphylococcus aureus Infections

Background: Nafcillin or cefazolin are drugs of choice for methicillin-susceptible Staphylococcus aureus (MSSA) infections. Prior studies indicate a higher incidence of acute kidney injury (AKI) with nafcillin, although AKI classification and time to occurrence is not well described. Objective: To characterize the incidence and time to adverse drug events for nafcillin versus cefazolin in the inpatient setting. Methods: A retrospective cohort study evaluated hospitalized, adult patients receiving intravenous nafcillin or cefazolin for treatment of MSSA infection. Incidence and time to AKI based on RIFLE criteria were measured. Secondary end points included antibiotic discontinuation and incidence of neutropenia, thrombocytopenia, elevated transaminases, and Clostridioides difficile infection (CDI). Results: Of 324 patients who received nafcillin (n = 119) or cefazolin (n = 205), higher rates of AKI were found for nafcillin versus cefazolin (19% vs 2%, respectively; P < 0.0001). Median time to AKI with nafcillin was 6.5 days (range, 3-14 days). The majority of patients were classified as RIFLE "Risk" stratum. Nafcillin treatment discontinuations were more frequent than for cefazolin (17.6% vs 0.9%, respectively; P < 0.0001). Nafcillin was an independent predictor of AKI (odds ratio = 12.4; 95% CI = 4.14-47.60, P < 0.0001). No differences in neutropenia, thrombocytopenia, elevated transaminases, or CDI were observed. Conclusion and Relevance: Nafcillin displayed higher rates of AKI at a median of 1 week of therapy, which provides a framework for clinician monitoring and consideration of antibiotic modification. Most patients developed "Risk" class AKI (RIFLE classification), which may be reversible with prompt intervention.

The Cefazolin Inoculum Effect and the Presence of type A blaZ Gene according to agr Genotype in Methicillin-Susceptible Staphylococcus aureus Bacteremia

BACKGROUND

Recent data suggests the inoculum effect of methicillin-susceptible Staphylococcus aureus (MSSA) against beta-lactam antibiotics and their association with functionality or genotypic variation of agr locus.

METHODS

MSSA blood isolates were collected at a tertiary care hospital in Korea from June 2014 to December 2017. The functionality of the agr operon was measured by δ-hemolysin assays. Multiplex PCR was performed to determine the agr genotype. The cefazolin minimum inhibitory concentrations (MICs) at a high inoculum concentration (~5 × 10⁷ CFU/ml) were compared to the MICs at a standard inoculum concentration (~5 × 10⁵ CFU/ml) to identify strains with the cefazolin inoculum effect (CIE). The DNA sequencing of blaZ gene was performed to classify the blaZ genotype.

RESULTS

Among the 195 MSSA blood isolates, agr genotype I was most common (68.2%), followed by type III (16.4%), type IV (9.2%), and type II (6.2%). Sixty-seven (34.3%) MSSA isolates had dysfunctional agr, but neither CIE nor blaZ genotype was associated with dysfunctional agr. The MSSA with agr type III genotype exhibited significantly higher CIE positivity (agr III 43.8% vs. non-agr III 5.5%, P <0.01) and erythromycin/clindamycin resistance. In the subgroup analysis of type A blaZ possessing MSSA, almost all of the agr III MSSA isolates exhibited CIE, while only 20% of non-agr III isolates had CIE (P <0.01).

CONCLUSION

In MSSA blood isolates, CIE might be associated with agr genotype rather than with dysfunctional agr.

Adjunctive Clavulanic Acid Abolishes the Cefazolin Inoculum Effect in an Experimental Rat Model of Methicillin-Sensitive Staphylococcus aureus Endocarditis

We tested the ability of clavulanic acid to restore the efficacy of cefazolin against Staphylococcus aureus TX0117, which exhibits the cefazolin inoculum effect (CzIE). In the rat infective endocarditis model, the coadministration of cefazolin plus clavulanic acid resulted in a significant reduction of bacterial counts (7.1 ± 0.5 log₁₀ CFU/g) compared to that with cefazolin alone (2 ± 0.6 log₁₀ CFU/g; P < 0.0001). The addition of a β-lactamase inhibitor may be a viable strategy for overcoming the CzIE.

A comparison of safety and outcomes with cefazolin versus nafcillin for methicillin-susceptible Staphylococcus aureus bloodstream infections

BACKGROUND

Methicillin-susceptible Staphylococcus aureus (MSSA) is a frequent cause of bloodstream infections (BSI). Treatment with nafcillin (NAF) has been preferred to cefazolin (CFZ). However, comparable outcomes have been found with CFZ with possibly lower risk for side-effects. This study compared safety and effectiveness of NAF versus CFZ for MSSA BSI.

METHODS

This single center retrospective study evaluated adults admitted with MSSA BSI who received NAF or CFZ. Patients receiving ≥24 h of antibiotics were included for safety analyses. Patients receiving NAF or CFZ for ≥75% of a 14 day minimum treatment course were assessed for clinical effectiveness. The primary safety outcome was incidence of renal toxicity with multiple secondary safety endpoints. Clinical success was defined as symptom resolution, repeat negative cultures, lack of additional therapy for presumed failure, and lack of recurrence within 30 days.

RESULTS

A total of 130 patients receiving NAF (n = 79) or CFZ (n = 51) were included for safety analysis. Of those, 90 met criteria for effectiveness assessment (NAF n = 40, CFZ n = 50). Baseline characteristics were well matched. NAF was associated with a higher incidence of nephrotoxicity compared to CFZ (25% vs. 2%, RR 1.31, 95% CI 1.15-1.5, p < 0.001), allergic reactions (p = 0.01) and a trend for hepatotoxicity (p = 0.08). Clinical success was achieved in 82% NAF and 94% CFZ treated patients (p = 0.1).

CONCLUSION

CFZ was associated with less nephrotoxicity and no difference in clinical success compared to NAF for MSSA BSI. A prospective study comparing NAF to CFZ for MSSA BSI should be conducted to elucidate differences in therapies.

The Cefazolin Inoculum Effect Is Associated With Increased Mortality in Methicillin-Susceptible Staphylococcus aureus Bacteremia

Background

Recent studies have favored the use of cefazolin over nafcillin for the treatment of methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia. The clinical influence of the cefazolin inoculum effect (CzIE) in the effectiveness of cephalosporins for severe MSSA infections has not been evaluated.

Methods

We prospectively included patients from 3 Argentinian hospitals with S. aureus bacteremia. Cefazolin minimum inhibitory concentrations (MICs) were determined at standard (10⁵ colony-forming units [CFU]/mL) and high (10⁷ CFU/mL) inoculum. The CzIE was defined as an increase of MIC to ≥16 µg/mL when tested at high inoculum. Whole-genome sequencing was performed in all isolates.

Results

A total of 77 patients, contributing 89 MSSA isolates, were included in the study; 42 patients (54.5%) had isolates with the CzIE. In univariate analysis, patients with MSSA exhibiting the CzIE had increased 30-day mortality (P = .034) and were more likely to have catheter-associated or unknown source of bacteremia (P = .033) compared with patients infected with MSSA isolates without the CzIE. No statistically significant difference between the groups was observed in age, clinical illness severity, place of acquisition (community vs hospital), or presence of endocarditis. The CzIE remained associated with increased 30-day mortality in multivariate analysis (risk ratio, 2.65; 95% confidence interval, 1.10–6.42; P = .03). MSSA genomes displayed a high degree of heterogeneity, and the CzIE was not associated with a specific lineage.

Conclusions

In patients with MSSA bacteremia where cephalosporins are used as firstline therapy, the CzIE was associated with increased 30-day mortality. Clinicians should be cautious when using cefazolin as firstline therapy for these infections.

Inoculum effect of high concentrations of methicillin-susceptible Staphylococcus aureus on the efficacy of cefazolin and other beta-lactams

The existence of a cefazolin inoculum effect (InE) of methicillin-susceptible Staphylococcus aureus (MSSA), which is speculated to be a reason for cefazolin treatment failure in MSSA infections, is controversial. In Japan, although cefazolin is one of the therapeutic choices for patients with MSSA infection, there are few reports of this effect. Additionally, the association between InE and blaZ type in beta-lactams other than cefazolin has not been well documented. In this study, we confirmed an MSSA InE in several beta-lactams, including cefazolin, and its relationship with blaZ, using 52 MSSA isolates from blood cultures. Three isolates (5.8%) that possessed type A blaZ showed a pronounced cefazolin InE. Five isolates (9.6%) showed pronounced InE with sulbactam/ampicillin; four isolates had type C blaZ and one had type A blaZ. However, we confirmed InE in MSSA isolates with blaZ not only type A and C but also B and D. For cefotaxime, ceftriaxone, imipenem, and meropenem, regardless of the presence of blaZ, we did not observe a significant increase in MICs at a high inoculum of MSSA. Hence, our results suggest that the above four beta-lactams are good alternatives to cefazolin if InE leads to treatment failure in a patient.

Cefazolin pharmacokinetics in cats under surgical conditions

Objectives The aim of this study was to determine the plasma pharmacokinetic profile, tissue concentrations and urine elimination of cefazolin in cats under surgical conditions after a single intravenous dose of 20 mg/kg. Methods Intravenous cefazolin (20 mg/kg) was administered to nine young mixed-breed cats 30 mins before they underwent surgical procedures (ovariectomy or orchiectomy). After antibiotic administration, samples from blood, some tissues and urine were taken. Cefazolin concentrations were determined in all biological matrices and pharmacokinetic parameters were estimated. Results Initial plasma concentrations were high (Cp(0), 134.80 ± 40.54 µg/ml), with fast and moderately wide distribution (distribution half-life [t½(d)] 0.16 ± 0.15 h; volume of distribution at steady state [V(d[ss])] 0.29 ± 0.10 l/kg) and rapid elimination (body clearance [ClB], 0.21 ± 0.06 l/h/kg; elimination half-life [t½], 1.18 ± 0.27 h; mean residence time 1.42 ± 0.36 h). Thirty to 60 mins after intravenous administration, cefazolin tissue concentrations ranged from 9.24 µg/ml (subcutaneous tissue) to 26.44 µg/ml (ovary). The tissue/plasma concentration ratio ranged from 0.18 (muscle) to 0.58 (ovary). Cefazolin urine concentrations were high with 84.2% of the administered dose being eliminated in the first 6 h postadministration. Conclusions and relevance Cefazolin plasma concentrations remained above a minimum inhibitory concentration of ⩽2 µg/ml up to 4 h in all the studied cats. This suggests that a single intravenous dose of 20 mg/kg cefazolin would be adequate for perioperative prophylactic use in cats.

Mechanisms of Antibiotic Resistance

Emergence of resistance among the most important bacterial pathogens is recognized as a major public health threat affecting humans worldwide. Multidrug-resistant organisms have not only emerged in the hospital environment but are now often identified in community settings, suggesting that reservoirs of antibiotic-resistant bacteria are present outside the hospital. The bacterial response to the antibiotic "attack" is the prime example of bacterial adaptation and the pinnacle of evolution. "Survival of the fittest" is a consequence of an immense genetic plasticity of bacterial pathogens that trigger specific responses that result in mutational adaptations, acquisition of genetic material, or alteration of gene expression producing resistance to virtually all antibiotics currently available in clinical practice. Therefore, understanding the biochemical and genetic basis of resistance is of paramount importance to design strategies to curtail the emergence and spread of resistance and to devise innovative therapeutic approaches against multidrug-resistant organisms. In this chapter, we will describe in detail the major mechanisms of antibiotic resistance encountered in clinical practice, providing specific examples in relevant bacterial pathogens.

Cefazolin versus anti-staphylococcal penicillins for treatment of methicillin-susceptible Staphylococcus aureus bacteraemia: a narrative review

BACKGROUND

Anti-staphylococcal penicillins (ASPs) are recommended as first-line agents in methicillin-susceptible Staphylococcus aureus (MSSA) bacteraemia. Concerns about their safety profile have contributed to the increased use of cefazolin. The comparative clinical effectiveness and safety profile of cefazolin versus ASPs for such infections remain unclear. Furthermore, uncertainty persists concerning the use of cefazolin due to controversies over its efficacy in deep MSSA infections and its possible negative ecological impact.

AIMS

The aim of this narrative review was to gather and balance available data on the efficacy and safety of cefazolin versus ASPs in the treatment of MSSA bacteraemia and to discuss the potential negative ecological impact of cefazolin.

SOURCES

PubMed and EMBASE electronic databases were searched up to May 2017 to retrieve available studies on the topic.

CONTENTS

Although described in vitro and in experimental studies, the clinical relevance of the inoculum effect during cefazolin treatment of deep MSSA infections remains unclear. It appears that there is no significant difference in rate of relapse or mortality between ASPs and cefazolin for the treatment of MSSA bacteraemia but these results should be cautiously interpreted because of the several limitations of the available studies. Compared with cefazolin, there is more frequent discontinuation for adverse effects with ASP use, especially because of cutaneous and renal events. No study has evidenced any change in the gut microbiota after the use of cefazolin.

IMPLICATIONS

Based on currently available studies, there are no data that enable a choice to be made of one antibiotic over the other except in patients with allergy or renal impairment. This review points out the need for future prospective studies and randomized controlled trials to better address these questions.

Efficacy of Ceftaroline against Methicillin-Susceptible Staphylococcus aureus Exhibiting the Cefazolin High-Inoculum Effect in a Rat Model of Endocarditis

Certain Staphylococcus aureus strains exhibit an inoculum effect (InE) with cefazolin (CFZ) that has been associated with therapeutic failures in high-inoculum infections. We assessed the in vitro activities of ceftaroline (CPT), CFZ, and nafcillin (NAF) against 17 type A β-lactamase (βla)-producing, methicillin-susceptible S. aureus (MSSA) strains, including the previously reported TX0117, which exhibits the CFZ InE, and its βla-cured derivative, TX0117c. Additionally, we determined the pharmacokinetics of CPT in rats after single intramuscular doses of 20 and 40 mg/kg of body weight and evaluated the activities of CPT (40 mg/kg every 8 h [q8h]), CFZ, and NAF against TX0117 and TX0117c in a rat model of infective endocarditis. No InE was observed for CPT or NAF, whereas a marked InE was detected for CFZ (MIC, 8 to ≥128 μg/ml). CPT and NAF treatment against TX0117 resulted in mean bacterial counts of 2.3 and 2.1 log10 CFU/g in vegetations, respectively, compared to a mean of 5.9 log10 CFU/g in the CFZ-treated group (CPT and NAF versus CFZ, P = 0.001; CPT versus NAF, P = 0.9830). Both CFZ and CPT were efficacious against the βla-cured derivative, TX0117c, compared to time zero (t0) (P = <0.0001 and 0.0015, respectively). Our data reiterate the in vivo consequences of the CFZ InE and show that CPT is not affected by this phenomenon. CPT might be considered for high-inoculum infections caused by MSSA exhibiting the CFZ InE.

Efficacy of Telavancin Alone and in Combination with Ampicillin in a Rat Model of Enterococcus faecalis Endocarditis

We first assessed telavancin (TLV) pharmacokinetics in rats after a single subcutaneous dose of 35 mg/kg of body weight. The pharmacokinetic data were used to predict a TLV dose that simulates human exposure, and the efficacy of TLV was then evaluated using a TLV dose of 21 mg/kg every 12 h against Enterococcus faecalis OG1RF (TLV MIC of 0.06 μg/ml) in a rat endocarditis model with an indwelling catheter. Therapy was given for 3 days with TLV, daptomycin (DAP), or ampicillin (AMP) monotherapy and with combinations of TLV plus AMP, AMP plus gentamicin (GEN), and AMP plus ceftriaxone (CRO); rats were sacrificed 24 h after the last dose. Antibiotics were given to simulate clinically relevant concentrations or as used in other studies. TLV treatment resulted in a significant decrease in bacterial burden (CFU per gram) in vegetations from 6.0 log₁₀ at time 0 to 3.1 log₁₀ after 3 days of therapy. Bacterial burdens in vegetations were also significantly lower in the TLV-treated rats than in the AMP (P = 0.0009)- and AMP-plus-GEN (P = 0.035)-treated rats but were not significantly different from that of the AMP-plus-CRO-treated rats. Bacterial burdens from vegetations in TLV monotherapy and TLV-plus-AMP-and-DAP groups were similar to each other (P ≥ 0.05). Our data suggest that further study of TLV as a therapeutic alternative for deep-seated infections caused by vancomycin-susceptible E. faecalis is warranted.

β-Lactam Therapy for Methicillin-Susceptible Staphylococcus aureus Bacteremia: A Comparative Review of Cefazolin versus Antistaphylococcal Penicillins

Methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia is associated with high morbidity and mortality. Traditionally, antistaphylococcal penicillins (ASPs) have been considered the agents of choice for the treatment of MSSA bacteremia. Vancomycin has been demonstrated to have poorer outcomes in several studies and is only recommended for patients with severe penicillin allergies. Although cefazolin is considered as an alternative to the ASPs for patients with nonsevere penicillin allergies, cefazolin offers several pharmacologic advantages over ASPs, such as more convenient dosing regimens, and antimicrobial stewardship programs are increasingly using cefazolin as the preferential agent for MSSA infections as part of cost-saving initiatives. Concerns about susceptibility to hydrolysis by type A β-lactamases, particularly at high inocula seen in deep-seated infections such as endocarditis; selective pressures from unnecessary gram-negative coverage; and lack of comparative clinical data have precluded recommending cefazolin as a first-line therapy for MSSA bacteremia. Recent clinical studies, however, have suggested similar clinical efficacy but better tolerability, with lower rates of discontinuation due to adverse drug reactions, of cefazolin compared with ASPs. Other variables, such as adequate source control (e.g., intravascular catheter removal, debridement, or drainage) and enhanced pharmacodynamics through aggressive cefazolin dosing, may mitigate the role of cefazolin inoculum effect and factor into determining improved clinical outcomes. In this review, we highlight the utility of cefazolin versus ASPs in the treatment of MSSA bacteremia with a focus on clinical efficacy and safety.

Significant publications on infectious diseases pharmacotherapy in 2015

PURPOSE

The most important articles on infectious diseases (ID) pharmacotherapy published in the peer-reviewed literature in 2015, as nominated and selected by panels of pharmacists and others with ID expertise, are summarized.

SUMMARY

Members of the Houston Infectious Diseases Network were asked to nominate articles published in prominent peer-reviewed journals in 2015 that were thought to have a major impact in the field of ID pharmacotherapy. A list of 55 nominated articles on general ID-related topics and 10 articles specifically related to human immunodeficiency virus (HIV) infection or acquired immunodeficiency syndrome (AIDS) was compiled. In a national online survey, members of the Society of Infectious Diseases Pharmacists (SIDP) were asked to select from the list 10 general ID articles believed to have made a significant contribution to the field of ID pharmacotherapy and 1 article contributing to HIV/AIDS pharmacotherapy. Of the 361 SIDP members surveyed, 153 (42%) and 76 (21%) participated in the selection of general ID-related articles and HIV/AIDS-related articles, respectively. The 11 highest-ranked publications (10 general ID-related articles and 1 HIV/AIDS-related article) are summarized here.

CONCLUSION

With the growing number of significant ID-related publications each year, it can be challenging to stay current with the literature. This review of important ID pharmacotherapy publications in 2015 may be helpful in identifying key articles and lessening this burden.

Comparable Effectiveness of First Week Treatment with Anti-Staphylococcal Penicillin versus Cephalosporin in Methicillin-Sensitive Staphylococcus aureus Bacteremia: A Propensity-Score Adjusted Retrospective Study

The objective was to compare the prognostic impact of first week treatment with anti-staphylococcal penicillin (ASP) versus cephalosporin in methicillin-sensitive Staphylococcus aureus bacteremia (MS-SAB). Altogether 580 patients were retrospectively followed and categorized according to first week treatment; 84% (488) received ASP (cloxacillin) and 16% (92) cephalosporin (cefuroxime or ceftriaxone). SAB management was optimized with formal bedside infectious disease specialist consultation in 88%, deep infection foci diagnosed in 77% and adjunctive rifampicin therapy given to 61% of patients. The total case fatality in 580 patients was 12% at 28 days and 18% at 90 days. When comparing effectiveness of first week ASP versus cephalosporin treatment there were no significant differences in 28-days (11% vs. 12%, OR; 1.05, 95% CI, 0.53-2.09) or 90-days (17% vs. 21% OR; 1.25, 95% CI, 0.72-2.19) outcome. In univariate analysis no prognostic impact of either first week ASP or cephalosporin treatment was observed for 28-days (OR; 0.96, 95% CI, 0.48-1.90 and OR; 1.05, 95% CI, 0.53-2.09) or 90-days (OR; 0.80, 95% CI, 0.46-1.39 and OR; 1.25, 95% CI, 0.72-2.19) outcome. Propensity-score adjusted Cox proportional regression analysis for first week treatment with cephalosporin demonstrated no significant prognostic impact at 28-days (HR 1.54, 95% CI 0.72-3.23) or 90-days (HR 1.56, 95% CI 0.88-2.86).

IN CONCLUSION

There is a comparable effectiveness with respect to 28- and 90-days outcome for first week treatment with ASP versus cephalosporin in MS-SAB. The results indicate that the difference in prognostic impact between first week ASP and cephalosporin may be non-significant in patient cohorts with SAB management optimized by infectious disease specialist consultation.

Cefazolin and Ertapenem, a Synergistic Combination Used To Clear Persistent Staphylococcus aureus Bacteremia

Ertapenem and cefazolin were used in combination to successfully clear refractory methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia. In addition, recent work has demonstrated activity of combination therapy with beta-lactams from different classes against methicillin-resistant S. aureus (MRSA). The ertapenem-plus-cefazolin combination was evaluated for synergy in vitro and in vivo in a murine skin infection model using an index MSSA bloodstream isolate from a patient in whom persistent bacteremia was cleared with this combination and against a cadre of well-described research strains and clinical strains of MSSA and MRSA. Against the index MSSA bloodstream isolate, ertapenem and cefazolin showed synergy using both checkerboard (fractional inhibitory concentration [FIC] index = 0.375) and time-kill assays. Using a disk diffusion ertapenem potentiation assay, the MSSA isolate showed a cefazolin disk zone increased from 34 to 40 mm. In vitro pharmacokinetic/pharmacodynamic modeling at clinically relevant drug concentrations demonstrated bactericidal activity (>3 log₁₀-CFU/ml reduction) of the combination but bacteriostatic activity of ether drug alone at 48 h. A disk diffusion potentiation assay showed that ertapenem increased the cefazolin zone of inhibition by >3 mm for 34/35 (97%) MSSA and 10/15 (67%) MRSA strains. A murine skin infection model of MSSA showed enhanced activity of cefazolin plus ertapenem compared to monotherapy with these agents. After successful use in clearance of MSSA bacteremia, the combination of ertapenem and cefazolin showed synergy against MSSA in vitro and in vivo. This combination may warrant consideration for future clinical study in MSSA bacteremia.

Association between Type A blaZ Gene Polymorphism and Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus

Some proportion of type A blaZ gene-positive methicillin-susceptible Staphylococcus aureus strains exhibit the cefazolin inoculum effect (CIE). The type A blaZ gene was divided into two groups by single nucleotide polymorphisms (SNPs) at Ser226Pro and Cys229Tyr. The median cefazolin MICs at a high inoculum concentration were 5.69 μg/ml for the Ser-Cys group and 40.32 μg/ml for the Pro-Tyr group (P = 0.01). The SNPs at codons 226 and 229 in the amino acid sequence encoded by the blaZ gene were closely associated with the CIE.

Characteristics of cefazolin inoculum effect-positive methicillin-susceptible staphylococcus aureus infection in a multicentre bacteraemia cohort

Cefazolin treatment failure has been observed in high-inoculum infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) with a cefazolin inoculum effect (CIE). However, data on the characteristics and risk factors for the acquisition of CIE-positive MSSA infection are scarce. CIE positivity was measured as an MIC ≥ 16 μg/ml with a high inoculum (∼5 × 10⁷ CFU/ml). The blaZ gene type was assessed through sequence analysis. The clinical characteristics and risk factors for the acquisition of CIE-positive MSSA infection were assessed. The association between the antimicrobial susceptibility profile and CIE positivity was evaluated. A total of 303 MSSA bacteraemia cases and their corresponding isolates were collected from ten hospitals: 61 (20.1 %) isolates showed a positive CIE; 254 (83.8 %) were positive for the blaZ gene. No significant association was found between CIE positivity and the site of infection. Metastatic cancer (aOR 2.86, 95 % CI, 1.10-7.48) and recent (≤1 month) close contact with a chronically ill patient (aOR 4.69, 95 % CI, 1.76-12.50) were identified as significant risk factors for CIE-positive MSSA infection through multivariate analyses. Resistances to clindamycin (OR 3.55, 95 % CI, 1.62-7.80) and erythromycin (OR 5.00, 95 % CI, 2.50-9.99) were associated with CIE positivity, presenting high specificity (92.9 %) and a negative predictive value (82.3 %). CIE-positive MSSA constituted approximately one-fifth of MSSA bacteraemia cases. Although CIE positivity was not clinically discernible, CIE positivity was associated with clindamycin or erythromycin susceptibility. Therefore, our findings suggest that cefazolin can be used in the treatment of high-inoculum MSSA infection if the isolates are susceptible to clindamycin or erythromycin.

Cefazolin versus Nafcillin for Methicillin-Sensitive Staphylococcus aureus Bloodstream Infection in a California Tertiary Medical Center

Recent observational studies have suggested possible reductions in mortality in patients receiving cefazolin versus antistaphylococcal penicillins. We examined 90-day mortality in patients receiving cefazolin compared to nafcillin for methicillin-susceptible Staphylococcus aureus (MSSA) bloodstream infection (BSI). We identified persons with MSSA BSI admitted to San Francisco General Hospital from January 2008 to July 2013 through a hospital-wide infection surveillance system and confirmed 90-day mortality using U.S. national vital registries. We included persons receiving cefazolin or nafcillin as the predominant intravenous antimicrobial agent; all participants received inpatient Infectious Diseases service consultation. We estimated the association between receipt of cefazolin and 90-day risk of death by multivariate logistic regression, including a propensity score for receiving cefazolin as the second predictor. Of 230 MSSA BSI cases, 30 received nafcillin and 70 received cefazolin as the predominant antimicrobial; 10 died within 90 days, 5 from each group. Unadjusted analysis showed substantial but not statistically significant reduced odds of death in those receiving cefazolin (odds ratio, 0.38; 95% confidence interval [CI], 0.10 to 1.44). Multivariate analysis with propensity scores found a similar adjusted odds ratio (0.40; 95% CI, 0.09 to 1.74; P = 0.22). We found a large reduction in 90-day mortality in those receiving cefazolin compared to nafcillin for MSSA BSI, but this finding was not statistically significant. The magnitude of effect seen in this and other studies justifies further study.

Evolving resistance among Gram-positive pathogens

Antimicrobial therapy is a key component of modern medical practice and a cornerstone for the development of complex clinical interventions in critically ill patients. Unfortunately, the increasing problem of antimicrobial resistance is now recognized as a major public health threat jeopardizing the care of thousands of patients worldwide. Gram-positive pathogens exhibit an immense genetic repertoire to adapt and develop resistance to virtually all antimicrobials clinically available. As more molecules become available to treat resistant gram-positive infections, resistance emerges as an evolutionary response. Thus, antimicrobial resistance has to be envisaged as an evolving phenomenon that demands constant surveillance and continuous efforts to identify emerging mechanisms of resistance to optimize the use of antibiotics and create strategies to circumvent this problem. Here, we will provide a broad perspective on the clinical aspects of antibiotic resistance in relevant gram-positive pathogens with emphasis on the mechanistic strategies used by these organisms to avoid being killed by commonly used antimicrobial agents.

Antistaphylococcal penicillins versus cephalosporins for definitive treatment of meticillin-susceptible Staphylococcus aureus bacteraemia: a systematic review and meta-analysis

The objective of this study was to assess the comparative effectiveness and safety of antistaphylococcal penicillins (ASPs) and cephalosporins for the definitive treatment of patients with meticillin-susceptible Staphylococcus aureus (MSSA) bacteraemia. PubMed and Scopus electronic databases were searched up to December 2013. All-cause mortality was the primary outcome of interest. A meta-analysis of unadjusted and adjusted data was performed. Seven articles (1643 patients) were included; all but one were retrospective studies, and three of them employed propensity score matching. The studies enrolled primarily adults hospitalised in medical wards for primary or secondary community-acquired, healthcare-associated or nosocomial MSSA bacteraemia. Several ASPs and cephalosporins were compared. Unadjusted 30-day mortality was lower in patients treated with ASPs than in those treated with cephalosporins [risk ratio (RR)=0.62, 95% confidence interval (CI) 0.40-0.98]. Propensity score-adjusted 30-day mortality was not different in patients receiving ASPs or cephalosporins (RR=0.75, 95% CI 0.41-1.39). Substantial heterogeneity and publication bias were found in these analyses. Both unadjusted (RR=0.85, 95% CI 0.54-1.32) and adjusted (RR=1.42, 95% CI 0.22-9.06) 90-day mortality did not differ between patients receiving ASPs or cephalosporins. Limited data regarding adverse events, development of resistance and recurrence were available. In conclusion, the limited available published data derive from retrospective studies and show that there appears to be no statistically significant difference in mortality between ASPs and cephalosporins for the treatment of MSSA bacteraemia.

Comparative efficacies of cloxacillin-daptomycin and the standard cloxacillin-rifampin therapies against an experimental foreign-body infection by methicillin-susceptible Staphylococcus aureus

We compared the efficacies of daptomycin (doses equivalent to 8 to 10 mg/kg of body weight/day in humans) and cloxacillin alone with those of cloxacillin-rifampin and cloxacillin-daptomycin combinations, using a tissue cage methicillin-susceptible Staphylococcus aureus (MSSA) infection model. Monotherapies were less effective than combinations (P<0.05), and daptomycin resistance emerged. Cloxacillin-daptomycin proved as effective as cloxacillin-rifampin and prevented the appearance of resistance; this combination may be an alternative anti-MSSA therapy, which may offer greater benefits in the early treatment of prosthetic joint infections (PJI).

Comparison of cefazolin versus oxacillin for treatment of complicated bacteremia caused by methicillin-susceptible Staphylococcus aureus

Contrary to prior case reports that described occasional clinical failures with cefazolin for methicillin-susceptible Staphylococcus aureus (MSSA) infections, recent studies have demonstrated no difference in outcomes between cefazolin and antistaphylococcal penicillins for the treatment of MSSA bacteremia. While promising, these studies described low frequencies of high-inoculum infections, such as endocarditis. This retrospective study compares clinical outcomes of cefazolin versus oxacillin for complicated MSSA bacteremia at two tertiary care hospitals between January 2008 and June 2012. Fifty-nine patients treated with cefazolin and 34 patients treated with oxacillin were included. Osteoarticular (41%) and endovascular (20%) sources were the predominant sites of infection. The rates of clinical cure at the end of therapy were similar between cefazolin and oxacillin (95% versus 88%; P=0.25), but overall failure at 90 days was higher in the oxacillin arm (47% versus 24%; P=0.04). Failures were more likely to have received surgical interventions (63% versus 40%; P=0.05) and to have an osteoarticular source (57% versus 33%; P=0.04). Failures also had a longer duration of bacteremia (7 versus 3 days; P=0.0002), which was the only predictor of failure. Antibiotic selection was not predictive of failure. Rates of adverse drug events were higher in the oxacillin arm (30% versus 3%; P=0.0006), and oxacillin was more frequently discontinued due to adverse drug events (21% versus 3%; P=0.01). Cefazolin appears similar to oxacillin for the treatment of complicated MSSA bacteremia but with significantly improved safety. The higher rates of failure with oxacillin may have been confounded by other patient factors and warrant further investigation.