Therapeutic Options and Outcomes for the Treatment of Children with Gram-Positive Bacteria with Resistances of Concern: A Systematic Review

Background: Methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant coagulase-negative Staphylococci (MR-CoNS), and vancomycin-resistant Enterococci (VRE) are increasing worldwide and represent a threat for the limited treatment options in pediatric patients and neonates compared to adults. Recommendations in pediatrics are mainly extrapolated from adults’ studies. Methods: A literature search for the treatment of these pathogens in children (<18 years old) was conducted in Embase, MEDLINE, and Cochrane Library. Studies reporting data on single-patient-level outcomes related to a specific antibiotic treatment for multidrug resistant (MDR) Gram-positive bacterial infection in children were included. Studies reporting data from adults and children were included if single-pediatric-level information could be identified (PROSPERO registration: CRD42022383867). Results: The search identified 11,740 studies (since January 2000), of which 48 fulfilled both the inclusion and the exclusion criteria and were included in the analysis: 29 for MRSA, 20 for VRE, and seven for MR-CoNS. Most studies were retrospective studies. Vancomycin was mainly used as a comparator, while linezolid and daptomycin were the most studied antimicrobials showing good efficacy. Conclusions: Linezolid showed a safety and efficacy profile in a neonatal setting; daptomycin is increasingly used for MRSA, but the evidence is scarce for VRE.


Introduction
Gram-positive multidrug-resistant organisms (GP-MDROs) significantly cause hospitalrelated infections in neonatal and pediatric populations. The increased life expectancy for chronically ill individuals is accompanied by the increased use of invasive devices and access to medical services, raising the risk of colonization and infection with MDR organisms.
Due to their clinical and public health impact, the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) have listed both methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) as high-priority pathogens in urgent need of drug research and development [1,2]. In Europe, in 2019, 18.6% of Staphylococcus aureus (SA) isolates showed single or combined resistances with significant geographical variability; methicillin resistance (MR) can be isolated (2.7%) or combined with quinolones (9.6%) or quinolones and rifampicin (0.6%) [3]. The burden of MRSA, overall, increased from 2007 and was higher in infants younger than 12 months, compared to other age groups [3,4]. This study was registered with the International Prospective Register of Systematic Reviews (PROSPERO) under record number CRD42022383867.
We included studies with any method of diagnosing infections with pathogens of interest in children, neonates and preterms. All sites of infection were included. The search results were exported to Rayyan software for further manuscript assessment and handling.

Study Selection
Assessments of the titles, abstracts, and full texts were conducted independently by three investigators (L.C., C.L., and L.R.). Discussion with a fourth senior reviewer (D.D.) resolved any disagreement regarding study selection.

Eligibility Criteria
Eligible study designs included randomized clinical trials, observational studies, prospective or retrospective designs, concomitant or historical control studies, case series, and case reports. Meta-analyses, systematic reviews, and narrative reviews were not included. Studies investigating any antimicrobial treatment for infections caused by the following bacteria were included: MRSA, VRE, and MR-CoNS.
The populations of interest were children, as well as term and preterm newborns, with confirmed GP-MDROs infections receiving antimicrobial treatment and presenting clinical and/or microbiological outcomes.
The outcomes of interest we collected from the selected studies were infection-related mortality from the initiation of treatment until discharge, clinical success (defined as complete resolution or a substantial improvement in signs and symptoms of the index infection), and microbiological success (measured by the suppression, eradication, or relapses of bacterial growth).

Eligibility Criteria
Eligible study designs included randomized clinical trials, observational studies, prospective or retrospective designs, concomitant or historical control studies, case series, and case reports. Meta-analyses, systematic reviews, and narrative reviews were not included. Studies investigating any antimicrobial treatment for infections caused by the following bacteria were included: MRSA, VRE, and MR-CoNS.
The populations of interest were children, as well as term and preterm newborns, with confirmed GP-MDROs infections receiving antimicrobial treatment and presenting clinical and/or microbiological outcomes.
The outcomes of interest we collected from the selected studies were infection-related mortality from the initiation of treatment until discharge, clinical success (defined as complete resolution or a substantial improvement in signs and symptoms of the index infection), and microbiological success (measured by the suppression, eradication, or relapses of bacterial growth).
Studies published between 1 January 2000 and 30 October 2022 were included. Further details are reported in the PICOS format (P: problem/patient/population; I: intervention; C: comparison/control; O: outcome; S: study design). P (Participants/population): Children, as well as term and preterm newborns, with confirmed GP-MDROs infections that were receiving antimicrobial treatment and presenting clinical and/or microbiological outcomes clearly specified.
I (Intervention/Exposure): Any antimicrobial treatment clearly defined. C: (Comparator/Control): Standard of care at the time and place where the study was conducted. Not applicable.
O: (outcomes): The primary outcome was mortality-related GP-MDRO infection. Secondary outcomes were clinical success, defined as complete resolution or substantial improvement of signs and symptoms of the index infection, microbiological success measured by suppression, eradication, or relapses of bacterial growth, and treatment-related adverse effects.

Data Extraction and Assessment of Study Quality
The following data were extracted using a standardized data collection form:
Patient characteristics (age, care setting, and inclusion and exclusion criteria); 3.
Main results with accuracy measures; 6.
Main results.
Standardized predetermined study criteria were applied to all full-text documents. The selection process is presented in Figure 1.
The quality and risk of bias in individual studies were jointly assessed at the study and outcome level by all reviewing authors using the Study Quality Assessment Tool from The National Heart, Lung, and Blood Institute [19]. The quality assessment results are presented in Tables 1-3. Each study was quality-rated according to one of the following categories on the basis of the proportion of yes answers to all relevant questions: poor quality, 0-40%; fair quality, 41-80%; good quality, 81-100% [19].

Summary Measures
The following measures of treatment success were included: absolute values, absolute risk differences, hazard ratio (HR), relative risk, and odds ratio. Unadjusted and adjusted measures were included if available.

Study Selection
A total of 11,740 records were found, and 2225 duplicate records were removed. Then, 9515 records were screened and excluded by title or abstract, followed by 138 exclusions after reading the full text as they did not meet the eligibility criteria. Forty-two articles regarding Gram-negative bacteria were also excluded. Therefore, 48 articles were included in the systematic qualitative review (Figure 1). We report the characteristics of the studies in Tables 1-3. Figure 2 shows the studies with a relevant number of patients.

Methicillin-Resistant Staphylococcus aureus (MRSA)
We included 29 studies regarding MRSA: 11 randomized controlled trials, two prospective studies, six retrospective studies, two case series, and nine case reports .
Clinical and microbiological success is reported in Table 1.
Most relevant studies were conducted in high-income countries (19/30), and three were multicenter international studies. The selected studies reported the following MRSA clinical infections: skin and skin structure infections (SSSIs), bloodstream infections (BSIs), central line-associated bloodstream infections (CLABSIs), endocarditis, community and hospital pneumonia, bone infections, and others.

Methicillin-Resistant Staphylococcus aureus (MRSA)
We included 29 studies regarding MRSA: 11 randomized controlled trials, two prospective studies, six retrospective studies, two case series, and nine case reports .
Clinical and microbiological success is reported in Table 1.
Most relevant studies were conducted in high-income countries (19/30), and three were multicenter international studies. The selected studies reported the following MRSA clinical infections: skin and skin structure infections (SSSIs), bloodstream infections (BSIs), central line-associated bloodstream infections (CLABSIs), endocarditis, community and hospital pneumonia, bone infections, and others.
Clinical and microbiological success and antimicrobial safety are reported in Table 2. The settings were different, but most studies (11/20) were conducted in HICs. The selected studies reported the following infections: BSIs, central nervous system (CNS), pneumonia, and other sites.
Clinical and microbiological success and antimicrobial safety are reported in Table 2. The settings were different, but most studies (11/20) were conducted in HICs. The selected studies reported the following infections: BSIs, central nervous system (CNS), pneumonia, and other sites.
Clinical and microbiological success and antimicrobial safety are reported in Table 3. Only two out of six studies were conducted in LMICs. The selected studies reported the following infections: BSIs, CNS, pneumonia, and other sites.

Discussion
Gram-positive bacteria may harbor several types of resistance to one or more antimicrobial class agents, with specific criteria to be fulfilled for being defined as MDR [67]. This research focused on the treatment of the most common drug-resistant Gram-positive pathogen infections.
Only one retrospective study on uncomplicated SSSIs caused by MRSA was included, in which cephalexin was as effective as clindamycin. However, as cephalexin does not exert any activity against MRSA, the authors concluded that wound care and drainage are more important than the antimicrobial choice [31].
For invasive infections, vancomycin, which is usually considered and recommended as first-line treatment for MRSA infections in pediatrics [68,69], was mainly used in our research as a comparator to assess the efficacy of other molecules: daptomycin, ceftaroline, and linezolid. We found no studies (except a case report) on teicoplanin, as it is not approved by the FDA and is used mainly in European countries.
Ceftaroline is currently approved by FDA and EMA for children and newborns to treat cSSSIs and community-acquired pneumonia [70,71]; however, efficacy data for MR organisms in pediatrics remain scarce. In our research, ceftaroline was evaluated for community-acquired MRSA (CA-MRSA) in children with SSSIs compared with vancomycin in 25 patients, resulting in a similar efficacy rate [20]. However, children with complicated infections or bacteremia were excluded from the study. In a case report, ceftaroline was successfully used (in combination therapy with rifampicin) to treat MRSA bloodstream infection and pneumonia in a preterm infant [45]. Ceftaroline showed a favorable efficacy and safety profile in newborns with late-onset sepsis in a case series, but MRSA was not considered, and antibiograms of CoNs were not provided [72]. Ceftaroline showed efficacy similar to vancomycin in children with cystic fibrosis and pulmonary exacerbations, which are often colonized (and infected) with MRSA [73]. Extensive studies and trials evaluating ceftaroline for treating MRSA/MRSE bacteremia and other invasive infections are lacking. In addition, children and newborns with sepsis often suffer from renal failure and are exposed to other nephrotoxic drugs; therefore, a vancomycin-sparing regimen may ease the management avoiding therapeutic drug monitoring and protecting the renal function. In this view, ceftaroline is a promising agent due to its safety and broad activity spectrum [74]. Further studies may explore its applications in special populations such as oncohematological patients and neonates, as well as for device-related infections (endocarditis and CLABSIs).
Daptomycin is a novel lipopeptide approved by FDA and EMA for the treatment of cSSSIs and bacteremia caused by Gram-positive bacteria in children older than 1 year, due to its rapid bactericidal effect [75,76]. It is active against MRSA, VRE, and MR-CoNS. Bradley et al. evaluated daptomycin for complicated SSSI in a randomized trial, including 97 MRSA infections treated with daptomycin and 46 with the standard of care, with a similar safety profile. However, children with bacteremia (or other invasive infections), renal insufficiency, or any clinical or laboratory findings suggestive of potential daptomycin toxicity were excluded from this study. The study was not designed to statistically compare the efficacy of daptomycin with the standard of care; however, overall, the efficacy rate was similar between arms [48]. The same authors described results from a randomized trial including eight children with hematogenous MRSA osteomyelitis treated with daptomycin or vancomycin; however, those cases were insufficient to achieve conclusions [21]. Seven children (neonates and infants excluded) with MRSA cSSSIs and one with bacteremia were successfully treated with daptomycin in a prospective Japanese study [39]. On the other hand, we found no trials or prospective studies evaluating the safety and efficacy of daptomycin for VRE infections in children, with publications limited to case series or case reports [22,46,53,60].
Linezolid is the first agent of oxazolidinones and is approved by FDA for adults and children with SSSI or pneumonia, including MRSA or VRE etiologies [77]. In pediatrics, linezolid was studied in a randomized trial, showing clinical efficacy comparable to vancomycin in 20 MRSA severe infections, and showing microbiological eradication for three cases of VRE [78]. In a prospective series of immunocompromised children, linezolid was effective as a single therapy for MRSA and VRE infections without a concomitant worsening of chemotherapy-induced myelotoxicity [23]. Furthermore, linezolid has been described in many pediatric case reports to treat ventriculitis, bloodstream infections, endocarditis, and others [27,49,51,52,56,59,61].
Despite its bacteriostatic activity, linezolid has a favorable efficacy for bacteremia in neonates, comparable to vancomycin [79,80]. Linezolid has the advantages of tolerability for a prolonged course, possible oral shift, and less concern for resistance, particularly for MRSA, compared to vancomycin.
Two recent meta-analyses showed an increase in the last years of vancomycin-inte rmediate, resistant, and heterogenous intermediate SA (VISA, VRSA, and hVISA), particularly in the USA and Asia [81,82]. The prevalence of VRSA increased from 1.2% before 2010 to 2.4% after 2010, with the highest frequency in the USA (3.6%) [81]. We found only one case of SA in our research with a MIC of 4 mg/dL for vancomycin [42]. VRE resistance to linezolid is rare but possible. Two case reports described a combination of antibiotics to treat severe VRE infections: linezolid with daptomycin [49], and daptomycin with tigecycline [51]. To date, no recommendations of combination therapy for VRE or other Gram-positive MDR infections have been provided. Concerns exist because the in vitro inhibition of the growth induced by a bacteriostatic drug may reduce efficacy when combined with a bactericidal drug [83]. Clinical data in vivo are missing.
Tedizolid is a newer oxazolidinone approved by EMA for treating SSSIs in children older than 12 years [84]. In the pediatric population, the safety and efficacy of tedizolid were evaluated in a phase III trial in adolescents with SSSIs caused by multiple pathogens, comparing tedizolid with the standard of care. The study included two MRSA in the tedizolid group and one in the SOC group, but specific outcomes were not reported, and the study was then excluded from our collection [85].
The use of quinupristin/dalfopristin has been described in a retrospective series of children with concomitant comorbidities and serious Gram-positive MDR infection without other drug possibilities [36]. It resulted in clinical success for 71 out of 101 VRE infections (in combination with other antimicrobials). As limitations, quinupristin/dalfopristin is ineffective against E. faecalis, requires a central line because it is highly irritant for peripheral veins, and has several adverse effects: metabolic interactions, severe myalgias, arthralgias, nausea, and hyperbilirubinemia [86]. In adults, studies comparing quinupristin to linezolid showed similar efficacy with a better versatility for linezolid [86,87]. In the pediatric population, the paucity of data does not allow us to conclude, but safety concerns led to its progressive desertion in favor of linezolid and daptomycin.
Novel antibiotics are currently approved by the FDA and/or EMA to treat Grampositive MDROs in adults [88]. Oritavancin is a new-generation lipoglycopeptide, currently approved by the FDA and EMA for acute bacterial soft skin and skin structures infections due to MSSA, MRSA, Streptococcus spp., and E. faecalis [89]. Despite its potential activity, oritavancin has not been clinically evaluated against VRE. A phase I clinical trial is recruiting pediatric patients with Gram-positive bacterial infections to evaluate the safety and tolerability of oritavancin [90].
Dalbavancin is a semisynthetic lipoglycopeptide with a mechanism of action similar to vancomycin, active against MRSA but not against VRE. It is approved by the EMA and FDA to treat adults with cSSSI [91]. Its long half-life (8.5 days) allows a weekly administration, which would be a massive advantage in pediatrics, as prolonged admissions and vascular catheter management are uncomfortable and expensive. A phase III trial (dalbavancin versus standard of care) recruiting patients aged 3 months to 17 years with SSSI is currently ongoing [92].

MR-CoNS
Compared to other Gram-positive bacteria, there are fewer studies regarding the treatment of MR-CoNS in pediatrics. However, MR-CoNS are widely diffused and poses a challenge, particularly for the limited possibilities in the neonatal population. In our research, neonates were under-represented compared to children. We included only one randomized trial where linezolid was successfully used in 13 neonatal MRSE infections with an 85% eradication rate, compared with vancomycin (100% eradication on six patients). However, no specific information on clinical outcomes was provided in this study [25]. There is a paucity of data regarding daptomycin's safety and efficacy in neonates, which seems to need higher doses to match the clinically effective exposure [93]. A retrospective case series by Mohzari et al. described 15 preterm infants with MRSE infections treated with daptomycin after vancomycin failure, with a success rate of 11 out of 15 [94].

Neonatal Safety
Safety concerns and the lack of pharmacokinetics data in neonates and preterm neonates explain why antimicrobial research often leaves this population behind in drug approvals. This study was not designed to assess antimicrobial safety in pediatrics; however, due to the paucity of data, we looked at safety outcomes in neonates included in the research. Deville performed a sub-analysis on neonatal patients included in the randomized trial by Kaplan et al., comparing vancomycin and linezolid [79]; the two groups of neonates were homogeneous except for postnatal age (newborns were younger in the linezolid group), and adverse effects were overall similar. Of note, the percentage of patients with abnormal hematology or serum chemistry values was not statistically different in linezolid-treated patients and vancomycin-treated patients. In the case series by Ergaz, three preterm newborns infected with VRE were successfully treated with linezolid without abnormalities in white blood cell count and liver enzymes [62]; the same was observed for the two preterms treated with linezolid presented by Hoehn [52]. A review by Garazzino on the use of linezolid in pediatric and neonates described an excellent safety profile [95]. Daptomycin use was described in the series by Mohzari (median weight 870 g) without reporting muscular or neurologic toxicity in preterms treated for MR-CoNS invasive infections [94]. In the neonatal case reports included in our study, daptomycin was used with good tolerability [64,66].
In the case series by Bradley, 11 neonates treated with ceftaroline for LOS experienced mild adverse effects, except for one serious effect (salmonellosis), but only a case of diarrhea was related to the study treatment [72].

Limitations
This review had some limitations, which were intrinsic to the object of our research. The main limiting factor was the paucity of articles exploring the treatment of Grampositive in children and neonates; in particular, data about MR-CoNS, albeit widely diffused, are very few. Furthermore, the included studies had different study designs, making it difficult to compare results and excluding the capacity for a meta-analysis. We extrapolated information on MDR pathogens from larger studies that were not designed ad hoc to study them but often described the experience with a single antibiotic. In addition, the quality of evidence was low for most studies due to their retrospective nature (or case reports); the definition of the outcomes (clinical and microbiological success) was not detailed in every study, and it could vary, impairing the strength of the results.

Conclusions
In conclusion, robust evidence on the treatment of MRSA, VRE, and MR CoNS is lacking in the pediatric and neonatal populations. However, a trend toward newer and safer molecules is observed (ceftaroline and linezolid) compared to the standard of care (vancomycin) for MRSA. Further studies are needed to investigate their effectiveness in particular settings (immunocompromised children, device-related infections, and critical care settings). Linezolid seems promising in the neonatal population for its activity against MRSA and MR CoNS, safety profile, and optimal bioavailability. Daptomycin and linezolid remain the cornerstones of VRE treatment. Novel molecules (oritavancin and dalbavancin) are currently being studied in pediatrics, with promising applications.
Author Contributions: L.C. and D.D. conceptualized the presented review; L.C., E.B., C.L. and L.R. contributed to the literature search, study selection, and data collection; L.C., C.L. and L.R. discussed the study selection and study details, and decided which studies were to be included in the review; L.C. and C.L. conducted the assessments of the titles, abstracts, and full texts independently; L.C.

Conflicts of Interest:
The authors declare no conflict of interest.