Penicillin Minimum Inhibitory Concentration Research Articles

Role of tcaA, a potential target as a ceftobiprole resistance breaker, in MRSA β-lactam resistance

ObjectivesUsing a random forest algorithm, we previously found that teicoplanin-associated gene A (tcaA) might play a role in resistance of methicillin-resistant Staphylococcus aureus (MRSA) to β-lactams, which we have investigated further here. MethodsRepresentative MRSA strains of prevalent clones were selected to identify the role of tcaA in the MRSA response to β-lactams. tcaA genes were deleted by homologous recombination in the selected MRSA strains, and antibiotic susceptibility tests were applied to evaluate the effect of tcaA on the minimum inhibitory concentrations (MICs) of glycopeptides and β-lactams. Scanning electron microscopy, RNA sequencing, and quantitative reverse transcription-polymerase chain reaction were performed to explore the mechanism of tcaA in MRSA resistance to β-lactams. ResultsThe MIC of penicillin plus clavulanate decreased from 3 mg/L to 0.064 mg/L and that of oxacillin decreased from 16 to 0.5 mg/L when tcaA was knocked out in the LAC strain. Compared with wild-type MRSA isolates, when tcaA was deleted, all selected strains were more susceptible to β-lactams. Susceptibility to ceftobiprole was restored in the ceftobiprole-resistant strain when tcaA was deleted. tcaA knockout caused “log-like” abnormal division of MRSA, and tcaA deficiency mediated low expression of mecA, ponA, and murA2. ConclusionsMachine learning is a reliable tool for identifying drug resistance-related genes. tcaA may be involved in S. aureus cell division and may affect mecA, ponA, and murA2 expression. Furthermore, tcaA is a potential resistance breaker target for β-lactams, including ceftobiprole, in MRSA.

Synergistic Activity of 3-Hydrazinoquinoxaline-2-Thiol in Combination with Penicillin Against MRSA.

The growing resistance seen in various antibiotics, including those considered as last-resort options, underscores the pressing need for novel approaches and new substances to address MRSA infections. Combining antibiotics as a treatment approach can enhance effectiveness, expand the range of targeted bacteria, and minimize the likelihood of resistance emergence. This approach holds promise in addressing the escalating issue of antibiotic resistance. This study seeks to investigate the potential synergy between 3-hydrazinoquinoxaline-2-thiol and penicillin against a diverse array of MRSA isolates, thereby providing insights into their combined antimicrobial action. Twenty-two clinical MRSA isolates subjected to broth microdilution to determine the Minimum Inhibitory Concentrations (MICs) of 3-hydrazinoquinoxaline-2-thiol and penicillin. Subsequently, a checkerboard assay was employed to evaluate the interaction between 3-hydrazinoquinoxaline-2-thiol and penicillin, focusing on the Fractional Inhibitory Concentration Index (FICI). The MICs of penicillin and 3-hydrazinoquinoxaline-2-thiol were determined for 22 clinical MRSA strains. Penicillin exhibited MICs within a range of 1024 to 128 µg/mL, while 3-hydrazinoquinoxaline-2-thiol displayed MICs varying from 64 to 8 µg/mL. Remarkably, the combination of 3-hydrazinoquinoxaline-2-thiol and penicillin yielded a synergistic effect, resulting in a significant reduction of MICs by up to 64-fold. The potential of 3-hydrazinoquinoxaline-2-thiol in combination with penicillin as a viable solution against MRSA appears promising. However, to establish its practical utility, further extensive testing and experiments are essential.

184. Implications of Enterococcus faecalis penicillin susceptibility in patients with bacteremia

Abstract Background Enterococcus faecalis isolates with an elevated penicillin minimum inhibitory concentration (MIC) ≥ 4 mg/L have decreased in vitro ampicillin plus ceftriaxone bactericidal and synergistic activity, despite ampicillin susceptibility. This phenotype’s implication on patient outcomes and prevalence in the US is unknown due to limited penicillin susceptibility reporting. Methods This was a retrospective cohort study of adult patients with E. faecalis bacteremia between August 20, 2018 and May 13, 2021 from the Mount Sinai Health System in New York City. Patients were excluded if they were discharged or deceased within 48 hours of index blood culture, did not receive active definitive therapy, or the blood culture was determined to be contamination. The primary outcome was differences in 30-day and 90-day mortality based on the penicillin MIC and the secondary outcome was microbiological relapse. Multivariable logistic regression was used to identify risk factors for 90-day mortality. Corresponding blood isolates were tested for penicillin susceptibility by broth microdilution in accordance with CLSI. Results A total of 124 patients were included, with 27% having an elevated penicillin MIC ≥ 4 mg/L. Differences in baseline characteristics (Table 1) as well as source of infection and treatments received (Table 2) were similar between groups. Endocarditis infected 18.5% of patients and was less common in cases with an elevated penicillin MIC. Thirty-day and 90-day mortality, and microbiological relapse were higher in cases with an elevated penicillin MIC (14.4% vs. 20.6%, p=0.58; 20% vs. 26.5%, p=0.59; and 4.4% vs. 8.8%, p=0.39, respectively). Pitt bacteremia score and prior ceftriaxone usage had a higher odds of 90-day mortality (OR 1.2 [95% CI 1.03-1.4], p=0.02; OR 2.8 [95% CI 1.02-7.9], p=0.04, respectively), whereas definitive treatment with an aminopenicillin compared to piperacillin/tazobactam was protective of 90-day mortality (OR 0.21 [95% CI 0.05-0.93], p=0.04). Conclusion E. faecalis with an elevated penicillin MIC ≥ 4 mg/L is present in over a quarter of patients across New York City. Larger studies are warranted to determine the impact on patient outcomes. E. faecalis bacteremia treatment with piperacillin/tazobactam may be associated higher mortality. Disclosures Jaclyn A. Cusumano, PharmD, BCIDP, Shionogi: Advisor/Consultant

Impact of Intermediate Susceptibility to Penicillin on Antimicrobial Treatment and Outcomes of Endocarditis Caused by Viridans and Gallolyticus Group Streptococci.

Evidence supporting combination treatment with a beta-lactam plus an aminoglycoside (C-BA) for endocarditis caused by viridans and gallolyticus group streptococci (VGS-GGS) with intermediate susceptibility to penicillin (PENI-I) is lacking. We assessed the clinical characteristics and outcomes of PEN-I VGS-GGS endocarditis and compared the effectiveness and safety of C-BA with third-generation cephalosporin monotherapy. Retrospective analysis of prospectively collected data of a cohort of definite endocarditis caused by penicillin-susceptible and PENI-I VGS-GGS (penicillin minimum inhibitory concentration ranging from 0.25 to 2 mg/L) between 2008 and 2018 in 40 Spanish hospitals. We compared cases treated with monotherapy or with C-BA and performed multivariable analyses of risk factors for in-hospital and 1-year mortality. A total of 914 consecutive cases of definite endocarditis caused by VGS-GGS with complete or intermediate susceptibility to penicillin were included. A total of 688 (75.3%) were susceptible to penicillin and 226 (24.7%) were PENI-I. Monotherapy was used in 415 (45.4%) cases (cephalosporin in 331 cases) and 499 (54.6%) cases received C-BA. In-hospital mortality was 11.9%, and 190 (20.9%) patients developed acute kidney injury. Heart failure (odds ratio [OR]: 6.06; 95% confidence interval [CI]: 1.37-26.87; P = .018), central nervous system emboli (OR: 9.83; 95% CI: 2.17-44.49; P = .003) and intracardiac abscess (OR: 13.47; 95% CI: 2.24-81.08; P = .004) were independently associated with in-hospital mortality among PEN-I VGS-GGS cases, while monotherapy was not (OR: 1.01; 95% CI: .26-3.96; P = .982). Our findings support the use of cephalosporin monotherapy in PEN-I VGS-GGS endocarditis in order to avoid nephrotoxicity without adversely affecting patient outcomes.

Molecular Epidemiology of Penicillinase-Producing Neisseria gonorrhoeae Isolates and Their blaTEM-135 Gene Variant in Bangkok, Thailand, 2015-2017.

Penicillinase-producing Neisseria gonorrhoeae (PPNG) possessing blaTEM-135 is a serious public health threat. With only a single change in the amino acid sequence, blaTEM-135 could evolve into a TEM-type extended-spectrum beta-lactamase (ESBL), which hydrolyzes extended-spectrum cephalosporins, including ceftriaxone and cefixime. We investigated the molecular epidemiological characteristics, types of plasmids in PPNG isolates, and prevalence of PPNG clinical isolates producing TEM-135 beta-lactamases. N. gonorrhoeae multi-antigen sequence typing (NG-MAST) was used to determine the molecular epidemiological characteristics of 99 PPNG isolates collected from 2015 to 2017. A mismatch amplification mutation assay was used to examine the blaTEM-135 gene prevalence. Of the 89 identified NG-MAST sequence types, 65 (73.0%) were novel. Only 17.7% (43/243) of PPNG isolates belonged to 16 genogroups. The most frequent plasmid was African, followed by Rio/Toronto, and Asian. The blaTEM-135 allele was found in Rio/Toronto plasmids. The blaTEM-135 allele was present in 23.2% (23/99) of the PPNG isolates. PPNG isolates expressing TEM-135 beta-lactamase exhibited significantly higher penicillin MIC (minimum inhibitory concentration) values than TEM-1 PPNG isolates. The PPNG isolates showed high genetic diversity and a high proportion of blaTEM-135 alleles. Mutation of the blaTEM-135 allele is worrisome as only one mutation could cause TEM-1 to evolve into an ESBL variant that degrades ceftriaxone. Ongoing surveillance of blaTEM-135 and new PPNG isolates is imperative.

Evaluation of Neisseria gonorrhoeae Isolates Susceptibility to Antibiotics in Zhejiang Province Since 2007.

This study aimed to assess the drug susceptibility of clinical isolates of Neisseria gonorrhoeae to spectinomycin, ceftriaxone and azithromycin. Moreover, the temporal trends in the minimum inhibitory concentration (MIC) of five antibiotics from Zhejiang, China, are also in the scope of this study. A total of 1710 gonococcal clinical strains were collected between 2007 and 2021 from health-care institutions in Zhejiang. The MICs of ceftriaxone, azithromycin, spectinomycin, penicillin and ciprofloxacin were assessed by agar dilution method on 1710 Neisseria gonorrhoeae isolates. Count data were expressed as strains and rates, and MICs distribution was elucidated using descriptive statistics. The total resistance rates of gonococci to azithromycin, spectinomycin, penicillin and ciprofloxacin in this study were 19.3%, 0.3%, 75.4% and 99.7%, respectively. The in vitro results showed a high prevalence of resistance to ciprofloxacin and penicillin. Azithromycin resistance rate has exceeded 5%, suggested a high prevalence of resistance. Ceftriaxone and spectinomycin are suggested based on this study for the treatment of Neisseria gonorrhoeae in Zhejiang.

The Formation and Drug Resistance Mechanism of Biofilm for Streptococcus pneumoniae Infection in Severe Respiratory Patients.

This study was to explore whether Streptococcus pneumoniae would form biofilms and the formative factors of biofilms, as well as the drug resistance mechanism of S. pneumoniae. In this study, a total of 150 strains of S. pneumoniae were collected from 5 local hospitals in the past two years, and the minimum inhibitory concentrations (MIC) of levofloxacin, moxifloxacin and penicillin were determined by agar double dilution method to select the drug-resistant strains. The polymerase chain reaction (PCR) amplification and sequencing were performed on specific genes of drug-resistant strains. In addition, 5 strains of S. pneumoniae with penicillin MIC ≤ 0.065 μg/mL, 0.5 μg/mL, 2 μg/mL, ≥ 4μg/mL were randomly selected, and the biofilms were cultured on two kinds of well plates for 24 hours. Finally, whether the biofilms were formed was observed. Experimental results revealed that the resistance rate of S. pneumoniae to erythromycin in this area was as high as 90.3%, and the strains that were resistant to penicillin account for only 1.5%. The amplification and sequencing experiment revealed that one (strain 1) of the strains, which was resistant to both drugs, had a GyrA mutation and ParE mutation, and strain 2 had a parC mutation. All strains generated biofilms, and the optical density (OD) value of penicillin MIC ≤ 0.065 μg/mL group (0.235 ± 0.053) was higher than that of 0.5 μg/mL group (0.192 ± 0.073) (P< 0.05) and higher than the OD value of the 4 μg/mL group (0.200 ± 0.041) (P< 0.05), showing statistically great differences. It was confirmed that the resistance rate of S. pneumoniae to erythromycin remained high, the rate of sensitivity to penicillin was relatively high, and the moxifloxacin and levofloxacin-resistant strains had appeared; S. pneumoniae mainly showed QRDR mutations in gyrA, parE, and parC; and it was confirmed that S. pneumoniae can generate biofilms in vitro.

A novel method for early detection of MIC value – Broth dilution using indicator solution versus agar dilution: an original article

Introduction: The adequate protocol for treatment of an infection is often determined on the basis of the minimum inhibitory concentration (MIC) of the causative organism. Traditional methods (agar dilution, microbroth dilution, and gradient diffusion) are labour intensive and time consuming (they are usually take over 48 hours to report the results). On the other hand, automated systems (VITEK™, Phoenix™, MicroScan WalkAway™) and rapid methods of MIC detection (using dielectrophoresis (DEP), magnetic bead rotation sensors and microfluidic incubation) require expensive instruments. This study is aimed to develop a rapid MIC detection method with the ability to applied to a resource limited setting. Methods: Agar dilution method and a novel broth dilution method (containing indicator solution) were simultaneously performed using amikacin, ceftriaxone, piperacillin-tazobactam, imipenem, cefoxitin and azithromycin. Results: Isolates of Escherichia coli, Enterobacter spp, Klebsiella spp, Staphylococcus aureus, Proteus mirabilis, Acinetobacter spp and Pseudomonas spp were used. The MIC values for Enterobacteriaceae and S. aureus isolates for each antibiotic were obtained within 4 to 5 hours by a novel broth dilution method. The obtained MIC values were corresponded with the MIC shown on the following day by agar dilution method. Conclusion: Broth dilution method with indicator solution is effective in rapid determination of the MIC for cephalosporins, penicillin, carbapenems, cephamycin, aminoglycosides and macrolides for most isolates of Enterobacteriaceae and S. aureus. Unfortunately this method did not work for the non-fermenter group of organisms like Pseudomonas spp and Acinetobacter spp, as their results could not be obtained before 24 hours. The method is time saving, relatively inexpensive and is applicable to resource limited settings.

1522. Penicillin plus ceftaroline against Enterococcus faecalis blood isolates lacking ampicillin plus ceftriaxone activity

Abstract Background Enterococcus faecalis infective endocarditis standard of care treatment, ampicillin plus ceftriaxone, unfortunately has been unable to decrease mortality rates and additionally increases patient risk for vancomycin-resistant enterococci (VRE). Recent data also demonstrates decreased ampicillin plus ceftriaxone activity in E. faecalis isolates with an elevated penicillin MIC of 4 mcg/mL. Penicillin plus ceftaroline is a promising alternative but limited data is available to support. Methods Seven E. faecalis clinical blood isolates and one wild-type E. faecalis isolate (i.e., JH2-2) were tested against penicillin plus ceftaroline utilizing 24-hour in vitro time-kill assays. All isolates were penicillin susceptible with minimum inhibitory concentrations (MIC) of either 2 (n=4) or 4 (n=4) mcg/mL, and ceftaroline MICs ranged from 1 to 8 mcg/mL. Penicillin and ceftaroline were tested at subinhibitory concentrations (0.25x and 0.5xMIC) as monotherapy and in combination. Antibacterial activity was described as bactericidal or bacteriostatic, defined as a ≥ 3-log10 colony-forming units (CFU)/mL or &amp;lt; 3-log10 CFU/mL decrease from initial inoculum, respectively. Synergy was defined as a ≥ 2-log10 decrease in CFU/mL at 24 hours from the most active single agent. Results Penicillin and ceftaroline monotherapy did not demonstrate any antibacterial activity, except for ceftaroline 0.5xMIC where four isolates demonstrated bacteriostatic activity (Table 1). Penicillin plus ceftaroline was synergistic in all isolates except JH2-2 (penicillin MIC of 2 mcg/mL) due to the bacteriostatic activity of ceftaroline alone. Synergistic and bactericidal activity was also more frequently observed in isolates with a penicillin MIC of 4 mcg/mL with penicillin plus ceftaroline than ampicillin plus ceftriaxone. Conclusion Penicillin plus ceftaroline demonstrates synergistic activity against clinical E. faecalis blood isolates including isolates with a penicillin MIC of 4 mcg/mL, where ampicillin plus ceftriaxone less frequently demonstrated synergy. Further research is warranted to support utilization in patients with E. faecalis infective endocarditis. Disclosures Jaclyn A. Cusumano, PharmD, BCIDP, Shionogi: Advisor/Consultant.

Outcomes of neutropenic hemato-oncological patients with viridans group streptococci (VGS) bloodstream infection based on penicillin susceptibility.

Viridans group streptococci (VGS) bloodstream infection (BSI) in neutropenic patients can be a severe complication. A higher prevalence of vancomycin use has been reported due to reduced susceptibility to penicillin. We aimed to assess the impact on mortality of both penicillin minimal inhibitory concentration (MIC) and the use of vancomycin. We conducted a retrospective multicenter study including consecutive neutropenic patients with VGS BSI between 2007 and 2019. Univariable and multivariable analyses were conducted to evaluate risk factors for mortality, including penicillin susceptibility as an independent variable. Non-susceptibility to penicillin was defined as MIC ≥ 0.25. We included 125 neutropenic patients with VGS BSI. Mean age was 53years and ~ 50% were women. Overall, 30-day mortality rate was 25/125 (20%), and 41 patients (33%) had a VGS isolate non-susceptible to penicillin. In univariable analysis, no significant association was demonstrated between penicillin non-susceptibility and mortality (9/25, 26% vs. 32/100, 32%, p = 0.81). Among patients with a non-susceptible strain, the use of vancomycin was not significantly associated with mortality (empirical, p = 0.103, or definitive therapy, p = 0.491). Factors significantly associated with increased mortality in multivariable analysis included functional status (ECOG > 1, adjusted odds ratio [aOR] 12.53, 95% CI 3.64-43.14; p < 0.0001); allogeneic transplantation (aOR 6.33, 95% CI 1.96-20.46; p = 0.002); and co-pathogen in blood cultures (aOR 3.99, 95% CI 1.34-11.89; p = 0.013). Among neutropenic hemato-oncological patients with VGS BSI, penicillin non-susceptibility and the use of vancomycin were not associated with mortality. Thus, vancomycin should not be used routinely as empirical therapy in neutropenic patients with suspected VGS BSI.

Ascorbic Acid-Mediated Modulation of Antibiotic Susceptibility of Major Bovine Mastitis Pathogens.

This study aimed to investigate the effects of ascorbic acid on antibiotic susceptibility of major bovine mastitis pathogens, including Staphylococcus aureus, Streptococcus dysgalactiae, Streptococcus uberis, Streptococcus agalactiae, and Escherichia coli. Minimum inhibitory concentrations (MICs) were determined by E-test method. The presence of 10 mM ascorbic acid decreased the MICs of penicillin and ampicillin while increased the MICs of erythromycin, kanamycin, streptomycin, and ciprofloxacin for all tested strains. Besides, ascorbic acid specifically reduced the MICs of tetracycline for gram-positive bacteria and chloramphenicol for gram-negative bacteria. This study highlights that ascorbic acid is a potential modulator of antibiotic activity against the major bovine mastitis pathogens.

Resistance profiles of Neisseria gonorrhoeae isolates in Vienna, Austria: a phenotypic and genetic characterization from 2013 to 2020

International surveillance data show a constant rise in the number of Neisseria gonorrhoeae infections and an increase in drug resistance of N. gonorrhoeae. As recent N. gonorrhoeae surveillance data in Austria are scarce, this study investigated phenotypic and genotypic antimicrobial resistance in N. gonorrhoeae isolates. In total, 440 N. gonorrhoeae samples were collected at the Medical University of Vienna, and the minimal inhibitory concentrations (MICs) for a range of different antibiotics were determined. Sampling sites and treatments were recorded, and whole-genome sequencing of N. gonorrhoeae isolates was performed using allele libraries to determine genotypic resistance. The median MICs for ceftriaxone, cefixime, azithromycin, ciprofloxacin, tetracycline and penicillin were <0.002 µg/mL, <0.016 µg/mL, 0.25 µg/mL, 2.0 µg/mL, 1.5 µg/mL and 0.25 µg/mL, respectively. Annual comparison showed that MICs were generally stable for all antimicrobial agents except azithromycin, for which an increase in median MIC was observed from 2017 (0.25 µg/mL). There was no genetic resistance to ceftriaxone; 8% of samples displayed resistance mutations against cefixime, primarily located in the penA gene. Resistance to azithromycin increased from 2% in 2013 to 12% in 2020. MtrD mosaic had the highest impact on azithromycin susceptibility; 47% of the resistant isolates showed this mutation. The majority of cases of gonorrhoea were treated successfully with either ceftriaxone or a ceftriaxone/azithromycin regime. Two treatment failures occurred under monotherapy with doxycycline. Overall, genotypic resistance corresponded significantly to all respective MICs. The resistance rate of N. gonorrhoeae to antibiotics has remained stable in Vienna over the last decade, except for azithromycin. The strong correlation found between genetic and phenotypic patterns in this study holds promise for future diagnostics of N. gonorrhoeae resistance based on genotypes.

Influence of Penicillin on Biofilm Formation by Streptococcus agalactiae Serotype Ia/CC23.

Streptococcus agalactiae (Group B Streptococcus , GBS) is a major agent of perinatal infections. Biofilms have been associated with GBS colonization and disease, as well as with infection persistence and recurrence. Although GBS remains susceptible to beta-lactams, it is still unknown how sessile cells respond to these antibiotics. Here, we evaluated the effect of different concentrations of penicillin (3-48 mg/L) on in vitro biofilm formation by four GBS strains belonging to serotype Ia/clonal complexes23 that were recovered from the oropharynx or urine of pregnant women and were previously characterized as strong biofilm producers. All four GBS strains were fully susceptible to penicillin (minimum inhibitory concentration = 0.023 mg/L), but penicillin was not able to fully prevent biofilm formation by these GBS strains. Biofilms formed in the presence of penicillin had reduced biomasses and thickness, but they were still classified as strong. Penicillin significantly reduced the density of live cells, but higher penicillin concentrations did not lead to improved prevention of biofilm formation. Biofilms formed in the presence of penicillin had no channels or long cocci chains observed in penicillin-free biofilms. Overall, results highlight the concerning possible impacts of biofilm formation in penicillin-based treatment and preventive strategies of GBS infections, even when the bacterial strain involved is fully antibiotic-susceptible.

Genome-wide association, prediction and heritability in bacteria with application to Streptococcus pneumoniae.

Whole-genome sequencing has facilitated genome-wide analyses of association, prediction and heritability in many organisms. However, such analyses in bacteria are still in their infancy, being limited by difficulties including genome plasticity and strong population structure. Here we propose a suite of methods including linear mixed models, elastic net and LD-score regression, adapted to bacterial traits using innovations such as frequency-based allele coding, both insertion/deletion and nucleotide testing and heritability partitioning. We compare and validate our methods against the current state-of-art using simulations, and analyse three phenotypes of the major human pathogen Streptococcus pneumoniae, including the first analyses of minimum inhibitory concentrations (MIC) for penicillin and ceftriaxone. We show that the MIC traits are highly heritable with high prediction accuracy, explained by many genetic associations under good population structure control. In ceftriaxone MIC, this is surprising because none of the isolates are resistant as per the inhibition zone criteria. We estimate that half of the heritability of penicillin MIC is explained by a known drug-resistance region, which also contributes a quarter of the ceftriaxone MIC heritability. For the within-host carriage duration phenotype, no associations were observed, but the moderate heritability and prediction accuracy indicate a moderately polygenic trait.

1132. Evaluating an Amoxicillin Dosing Regimen for Community Acquired Pneumonia: A Quality Improvement Initiative Using Clinical and Laboratory Data

BackgroundAmoxicillin 90 mg/kg/day divided twice daily is recommended for children with mild community acquired pneumonia (CAP). While adequate for fully susceptible Streptococcus pneumoniae isolates, three times daily dosing allows achievement of greater amoxicillin exposure, which may be necessary for isolates with penicillin minimum inhibitory concentrations (MIC) of ≥ 2 μg/mL. We evaluated our current twice daily amoxicillin dosing strategy by characterizing 1) the MIC distribution among S. pneumoniae isolates and 2) the frequency of clinical amoxicillin treatment failures.MethodsWe performed a retrospective cohort study of all S. pneumoniae isolates from sterile and non-sterile sites between 2017-2020. Breakpoints established by the CLSI were used for both meningitis and non-meningitis isolates. Only the first isolate per patient was included. We also evaluated the frequency of amoxicillin treatment failure in patients diagnosed with CAP who were discharged from the ED in 2019. CAP was defined as a discharge diagnosis code for pneumonia and an antibiotic prescription. Treatment failure was defined as an ED or primary care revisit, or admission, within 14 days during which an antibiotic change was made. Results28 S. pneumoniae isolates were identified from sterile sites between 2017-2020 and 171 isolates were identified overall. All isolates from sterile sites had penicillin MICs of ≤ 2 μg/mL and 165 (96%) of isolates overall had penicillin MICs of ≤ 2 μg/mL (Table 1). Of these, 10 isolates had MICs of 2 μg/mL, all from non-sterile sites. In 2019, 589 patients were treated for CAP in the ED; 447 (76%) received amoxicillin and 142 (24%) were treated with alternative antibiotics. Treatment failures occurred in 15 amoxicillin-treated patients (3.3%, 95% confidence interval 1.9-5.5%) and in 5 patients (3.5%, 95% confidence interval 1.2-8.0%) treated with alternative antibiotics. Conclusion In vitro penicillin resistance was rare at our institution. Further, given that S. pneumoniae is rarely identified by culture, we also demonstrated that clinical amoxicillin treatment failures were infrequent using twice daily amoxicillin dosing. Coupled with provider and family preference, these data supported continuing our current practice of twice daily amoxicillin dosing. Disclosures All Authors: No reported disclosures

681. Penicillin Plus Ceftriaxone Against Enterococcus faecalis In Vitro Time-Kill Studies

BackgroundSynergistic ampicillin plus ceftriaxone (AC) for Enterococcus faecalis infective endocarditis outpatient use is precluded by ampicillin’s poor room temperature stability. Penicillin has superior stability and has been combined with ceftriaxone (PC), however there is a lack of studies to demonstrate synergy. MethodsAC and PC were evaluated, in duplicate, for synergy utilizing 24-hour in vitro time-kill assays with a starting inoculum of 106 colony forming units (CFU)/mL. Six clinical E. faecalis blood isolates and one wild-type E. faecalis isolate (JH2-2) were included. All isolates were susceptible to ampicillin and penicillin, with minimum inhibitory concentrations (MICs) ranging from 0.5-1 µg/mL and 2-4 µg/mL, respectively. Ampicillin and penicillin were tested at subinhibitory concentrations (0.25x and 0.5xMIC) as monotherapy and in combination with ceftriaxone average steady state concentrations for a dose of 2g IV q12hr (CPss 17.2 µg/mL), as all ceftriaxone MICs were high due to intrinsic resistance (MICs 128-2048 µg/mL). Synergy was defined as a ≥ 2 log10 decrease in CFU/mL at 24 hours from the most active single agent. ResultsAn average increase in bacterial density from the starting inoculum was observed for all isolates against ampicillin 0.25xMIC alone, penicillin 0.25x and 0.5xMIC alone, and ceftriaxone alone (+1.60 ± 0.62, +1.91 ± 0.37, +1.48 ± 0.42, and +1.84 ± 0.46 log10 CFU/mL, respectively) [Table 1]. Ampicillin 0.5xMIC alone average increase in bacterial density from starting inoculum for all but two isolates (e2008 and e2009) was +1.21 ± 0.59 log10 CFU/mL. Isolates e2008 and e2009 were the only isolates with a higher penicillin MIC of 4 µg/mL, and did not display synergy for all AC and all PC combinations. AC synergy was observed for all other isolates, with only one isolate (e2012) displaying synergy at 0.5xMIC. PC synergy was observed for four isolates at 0.5xMIC (-3.47 ± 0.94 log10 CFU/mL) and for only one isolate (e2014) at 0.25xMIC but the change in bacterial density was -0.38 ± 0.24 log10 CFU/mL. ConclusionPC synergy against E. faecalis was observed with higher penicillin concentrations. AC and PC did not demonstrate synergy against isolates with a higher penicillin MIC of 4 µg/mL. Further research is warranted to better understand PC synergy against E. faecalis. Disclosures All Authors: No reported disclosures

Susceptibilities of invasive Neisseria meningitidis strains to agents used for prophylaxis and to penicillin G

The antimicrobial susceptibility of isolates of Neisseria meningitidis to agents used for chemoprophylaxis and to penicillin G was determined. Fifty N. meningitidis strains detected in Nova Scotia between 2004 and 2018 were included. The isolates were originally isolated from sites that might prompt chemoprophylaxis (27 blood, 18 cerebrospinal fluid [CSF], 3 CSF-blood, and 2 conjunctiva). Minimal inhibitory concentrations (MICs) were determined to azithromycin, ciprofloxacin, minocycline, rifampin, trimethoprim-sulfamethoxazole, and penicillin G, using a diffusion gradient strip on Mueller-Hinton agar with 5% sheep blood in 5% CO2 for 20-24 hours. All isolates remained susceptible to azithromycin, ciprofloxacin, minocycline, and rifampin, but there was 26% resistance to trimethoprim-sulfamethoxazole. There was a rise in penicillin MIC of the isolates over the study period. In our province, N. meningitidis isolates causing invasive infection remain susceptible to commonly used antimicrobial agents used for chemoprophylaxis. Significant resistance to trimethoprim-sulfamethoxazole is present and the degree of susceptibility to penicillin G is lessening.

Ambient NO2 is associated with Streptococcus pneumoniae-induced pneumonia in children and increases the minimum inhibitory concentration of penicillin.

The present study aimed to assess the correlation between ambient air pollutants and Streptococcus pneumoniae (S. pneumoniae)-induced pneumonia in children and retrospectively reviewed the daily data regarding S. pneumoniae from children with pneumonia in a tertiary hospital of Hangzhou City, between January 1st, 2018, and December 31st, 2018. The excess risk (ER) of NO2 with regard to the daily number of S. pneumoniae isolates obtained from the respiratory tract specimens of children with pneumonia was 13.31% (95% confidence interval [CI]: 3.12-24.51%, P = 0.010) in the single-pollutant model. An increase of 10μg/m3 in NO2 exposure was associated with a 23.30% increased risk for the acquisition of S. pneumoniae-induced pneumonia in children (95% CI: 2.02-49.02%; P = 0.031) according to the multi-pollutant model. The ER of NO2 with regard to the daily number of S. pneumoniae isolates (1μg/ml ≤ minimum inhibitory concentration (MIC) to penicillin ≤ 2μg/ml) obtained from the respiratory tract specimens of children with pneumonia was 15.80% (95% CI: 2.02-31.45%; P = 0.024) in the single-pollutant model. According to the multi-pollutant model, the ER of NO2 with regard to the daily number of S. pneumoniae isolates (1μg/ml ≤ MIC to penicillin ≤ 2μg/ml) obtained from the respiratory tract specimens of children with pneumonia was 37.09% (95% CI: 5.70-77.81%; P = 0.018). In conclusion, ambient NO2 is associated with S. pneumoniae-induced pneumonia in children. More importantly, NO2 exposure is associated with the increased MICs of penicillin against S. pneumoniae from children with pneumonia.

Predicting Oral Beta-lactam susceptibilities against Streptococcus pneumoniae

BackgroundOral beta-lactam antimicrobials are not routinely tested against Streptococcus pneumoniae due to presumed susceptibility based upon penicillin minimum inhibitory concentration (MIC) testing. Currently, Clinical and Laboratory Standards Institute provides comments to use penicillin MIC ≤0.06 to predict oral cephalosporin susceptibility. However, no guidance is provided when cefotaxime MIC is known, leading to uncertainty with interpretation. The purpose of this study was to evaluate cefotaxime and penicillin MICs and their respective correlation to oral beta-lactam categorical susceptibility patterns.Methods249 S. pneumoniae isolates were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-ToF) and then tested by broth microdilution method to penicillin, cefotaxime, amoxicillin, cefdinir, cefpodoxime, and cefuroxime.ResultsUsing Clinical and Laboratory Standards Institute (CLSI) non-meningitis breakpoints for cefotaxime, 240/249 isolates were classified as susceptible. Of the cefotaxime susceptible isolates, 23% of the isolates are misrepresented as cefdinir susceptible. Amoxicillin correlated well with penicillin MIC breakpoints with only 1 discordant isolate out of 249.ConclusionThe correlation between amoxicillin and penicillin creates a very reliable predictor to determine categorical susceptibility. However oral cephalosporins were not well predicted by either penicillin or cefotaxime leading to the possible risk of treatment failures. Caution should be used when transitioning to oral cephalosporins in cefotaxime susceptible isolates, especially with higher cefotaxime MICs.

First Report of Oxacillin Susceptible mecA-Positive Staphylococcus aureus in a Children's Hospital in Kunming, China.

PurposeThe present study investigated the prevalence characteristics of oxacillin susceptible mecA-positive Staphylococcus aureus (OS-MRSA) in a children’s hospital in Kunming from January 2019 to December 2020.MethodsA total of 499 S. aureus strains were included in the study and tested for oxacillin susceptibility using the VITEK 2 Compact automated antimicrobial susceptibility test system. All oxacillin-susceptible strains were detected mecA and mecC by polymerase chain reaction (PCR). E-test was used to compare the minimum inhibitory concentration (MIC) values of methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and OS-MRSA for oxacillin, cefoxitin, penicillin, vancomycin, erythromycin, and clindamycin. Molecular typing of OS-MRSA was performed by MLST and SCCmec typing. Toxin genes were detected by PCR.ResultsForty-five OS-MRSA strains were detected, for an overall rate of 9.02% (45/499). The MICs of MSSA, OS-MRSA, and MRSA against oxacillin were concentrated at 0.38, 0.38, and 12 μg/mL, respectively; the cefoxitin MICs of MSSA and MRSA were concentrated at 2 and 32 μg/mL respectively; and MICs of OS-MRSA were concentrated at 2 and 8 μg/mL; penicillin, vancomycin and erythromycin MICs against MSSA, OS-MRSA, and MRSA showed same centralized points and were 32, 1, and 256 μg/mL, respectively; the MICs of clindamycin against MSSA were 0.5 μg/mL, while that against OS-MRSA and MRSA were concentrated at 256 μg/mL. Molecular typing of OS-MRSA was dominated by ST59-SCCmec IV. The carrier rates of hemolysin genes (hl-a, hl-d) and fibrinogen-binding clumping factor genes (clfA, clfB) were 100% in OS-MRSA, followed by 40% (18/45) for enterotoxin genes (sea, seb).ConclusionOS-MRSA has a high detection rate in children, and main molecular typing is ST59-SCCmecIV in Kunming. The identification ability of automated antibacterial drug sensitivity test detection systems for OS-MRSA is very limited. A combination of phenotypic analysis and molecular detection should be used to improve OS-MRSA identification.