Prevalence and significance of coagulase-negative staphylococci isolated from blood cultures in a tertiary hospital

Clinical Impact of Vancomycin MIC on Outcomes in Patients With Coagulase-negative Staphylococcal Bacteremia

PURPOSE

Coagulase-negative staphylococci (CoNS) are Gram-positive organisms that are a known component of normal skin flora and the most common cause of nosocomial bacteremia. For CoNS species, the vancomycin MIC breakpoint for susceptibility set by the Clinical and Laboratory Standards Institute is ≤4 µg/mL. There has been published reports of vancomycin heteroresistance in CoNS with vancomycin MICs of 2 to 4 µg/mL. The aim of this retrospective cohort analysis was to assess the clinical impact of vancomycin MICs <2 µg/mL versus ≥2 µg/mL in adult patients with CoNS bloodstream infections.

METHODS

Adult patients admitted to University Medical Center New Orleans with a blood culture positive for CoNS were assessed. The primary outcome was difference in 30-day mortality. Secondary outcomes were in-hospital, all-cause mortality; duration of bacteremia; hospital length of stay; and percentage of oxacillin-resistant CoNS.

FINDINGS

There was no difference in mortality in the vancomycin MIC <2 µg/mL group versus the vancomycin MIC ≥2 µg/mL group at 30 days (15.4% vs 17.4%; P = 1). In-hospital, all-cause mortality was also not different between groups (11.5% vs 13%; P = 1). Hospital length of stay between groups was 28.2 days versus 21 days (P = 0.692). Median duration of bacteremia was 1 day in both groups (P = 0.975), and median scheduled duration of antibiotic therapy was 14.9 days and 19.5 days (P = 0.385). The source and mode of acquisition of CoNS were similar between groups. Of all CoNS isolates, 58.7% (44 of 75) were oxacillin resistant. Staphylococcus epidermidis was the most common CoNS species at 66.7% (50 of 75). Of all isolates, 30.7% (23 of 75) had a vancomycin MIC ≥2 µg/mL, and 87% (20 of 23) of these were S. epidermidis. There was a higher percentage of S. epidermidis in the vancomycin MIC ≥2 µg/mL group than in the MIC <2 µg/mL group (87% vs 57.7%; P = 0.012). CoNS with a vancomycin MIC ≥2 µg/mL were also more likely to be oxacillin resistant (78.3% vs 50%; P = 0.005).

IMPLICATIONS

There was no difference in clinical outcomes in adult patients with a CoNS bloodstream infection with a vancomycin MIC <2 µg/mL versus ≥2 µg/mL. At present, vancomycin remains appropriate empiric therapy for CoNS bloodstream infection. Further research is needed to determine if there is a true clinical impact of a vancomycin MIC ≥2 µg/mL in CoNS infections.

Lesson from the COVID-19 pandemic lockdown: A major change of hospital-diagnosed bacteremia epidemiology

OBJECTIVES

When the COVID-19 pandemic reached France early in 2020, the enforced nationwide lockdown deeply altered lifestyle as well as hospital processes and modalities of care. The aim of the study was to evaluate the impact during the lockdown of the first epidemic wave on the epidemiology of bacteremia in one French University Hospital.

PATIENTS AND METHODS

Retrospective cohort study including adult patients with positive blood culture between 23^rd March to 24^th May 2020. The clinical-microbiological characteristics were compared with those of the period from 25^th March to 26^th May 2019. The data were adjusted to the number of hospitalizations (h).

RESULTS

In 2020, 189 bacteremia were diagnosed from 1939 vials (9658 hospitalizations, 10911 emergency room consultations) compared to 143 from 1976 vials (14797 hospitalizations, 16493 emergency room consultations) recorded in 2019. The incidence of bacteremia increased up to 19.7 per 1000h in 2020 vs 9.7 in 2019 (p<0.001). The main differences (2020 vs 2019) were: Staphylococcus aureus bacteremia (2.4 vs 1.0/1000h, p=0.012), polymicrobial bacteremia (2.2 vs 0.9/1000h p=0.013) and Gram-negative bacteremia (8.9 vs 4.3/1000h, p<0.01). Conversely, Streptococcus pneumoniae incidence decreased (0 vs 0.47/1000h, p=0.047). The standardized incidence ratio calculation confirmed these results.

CONCLUSION

The lockdown and the impact of the first wave of the Covid-19 pandemic on the health system resulted in increased hospital-diagnosed bacteremia and decreased pneumococcal bacteremia. Disruption and overload of ICUs, lockdown with preventive control measures, and decrease in human-to-human interaction may have been the main reasons.

Assessing the performance of the Cepheid Xpert in identifying and differentiating methicillin-susceptible Staphylococcus aureus and methicillin-resistant Staphylococcus aureus from blood culture bottles

Staphylococcus aureus bacteraemia is associated with a high morbidity and mortality. Time to effective antibiotics is key to reducing mortality. Current practices yield preliminary susceptibilities approximately 16-18 h after blood culture positivity. Molecular diagnostics could reduce the time from blood culture positivity to organism identification as MRSA or MSSA. The objective was to assess the performance of the GeneXpert in identifying MRSA/MSSA from blood culture bottles in the BACT/ALERT VIRTUO system. Eighty-eight blood culture bottles with Gram-positive cocci resembling staphylococci were analysed at Pathology Queensland using the Cepheid Xpert MRSA/SA BC system. The identification and susceptibilities from standard operating procedures were compared with the results from the Xpert MRSA/SA Blood Culture assay and routine laboratory practice. The overall positive percent agreement between the GeneXpert and standard laboratory practice was 94.1% (95% CI 85.6-98.37%) and the negative percent agreement was 100% (95% CI 83.16-100%). The Cepheid Xpert accurately identifies MRSA, MSSA and coagulase-negative staphylococci. The discordant results were from rarely occurring clinical isolates and were expected limitations of the assay. This kit has the potential to reduce the time to effective antibiotics and minimise the use of unnecessary antibiotics and associated costs.

Clinical and economical improvements after introducing rapid identification of bacteria and early antibiotic susceptibility testing in sepsis and bloodstream infections. Results of the PHENOMENON study

Background: Sepsis and bloodstream infections pose severe challenges in intensive care. Early reliable diagnosis is the key to successful therapy. The objective of the study presented here was to investigate the clinical and economical effects of the new Pheno^TM BC test, which allows bacteria identification (ID) and antimicrobial susceptibility testing (AST) in approximately 7 hours after a blood culture becomes positive (BC+). Methods: Historically controlled interventional study. Population: patients with BC+ and ICU admission. Inadequate initial antimicrobial therapy (IAT) is need of therapy change based on result. Prospectively the new test was used in addition. Primary endpoint: time-to-result in hours. Contribution margin (CM) i.e. revenue - costs was computed. All patients formed the intention-to-treat population (ITT). Patients with complete cost data formed the modified ITT group (mITT). CM results were calculated for mITT and PP. Further analyses: length-of-stay (LOS) and mortality. Results: 223 historical and 200 prospective patients were included. Time to result (ITT) was shortened by 51.1 hours (83 vs. 31.9; p<0.001). Overall savings (mITT) were 257,100 € (-301,264 € vs. -44,164 €). 143 of 181 (79%) patients had a test performed, 126 of 143 (88%) having a clinically useable result. 40 (32%) had IAT vs. 65 (29%) in the historic cohort. Median time to AST in PP was shortened by 61.7 hours (89.5 vs. 27.8; p<0.001). LOS was shortened 7 days (28 vs. 19; p=0.226) and mortality was 8% (40.5% vs. 32.5%; p=0.440) lower. Median CM +3,074.80 € per case (-2,350.50 € vs. +724.70 €; p=0.040). Conclusion: The new Pheno^TM ID+AST test leads to faster and clinically meaningful results and saves money by shortening LOS on the ICU.

A novel approach to eliminate detection of contaminating Staphylococcal species introduced during clinical testing

We describe here a strategy that can distinguish between Staphylococcus species truly present in a clinical sample from contaminating Staphylococcus species introduced during the testing process. Contaminating Staphylococcus species are present at low levels in PCR reagents and colonize lab personnel. To eliminate detection of contaminants, we describe an approach that utilizes addition of sufficient quantities of either non-target Staphylococcal cells (Staphylococcus succinus or Staphylococcus muscae) or synthetic oligonucleotide templates to helicase dependent isothermal amplification reactions to consume Staphylococcus-specific tuf and mecA gene primers such that contaminating Staphylococcus amplification is suppressed to below assay limits of detection. The suppressor template DNA is designed with perfect homology to the primers used in the assay but an internal sequence that is unrelated to the Staphylococcal species targeted for detection. Input amount of the suppressor is determined by a mathematical model described herein and is demonstrated to completely suppress contaminating levels of Staphylococcus while not negatively impacting the appropriate clinical assay limit of detection. We have applied this approach to improve the specificity of detection of Staphylococcus species present in positive blood cultures using a chip-based array that produces results visible to the unaided eye.

Comparison of coagulase-negative staphylococci isolated from blood cultures as a true bacteremia agent and contaminant in terms of slime production and methicillin resistance

OBJECTIVE

The aim of this study is to determine the species distribution, slime activity, and methicillin resistance of coagulase-negative staphylococci (CoNS) isolated from blood cultures as either contaminants or true bacteremia agents.

MATERIALS AND METHODS

In this study, 13.268 blood culture samples sent to our laboratory from various clinics during a two-year period were examined in terms of the presence of CoNS to clarify whether the isolates are true bacteremia agents, as defined by Centers for Disease Control and Prevention (CDC) criteria. The slime activities of true bacteremia agents (58 CoNS strains) and contaminants (50 randomly selected CoNS strains) were investigated by the Christensen method. The methicillin susceptibilities of the strains were determined by the disk diffusion method.

RESULTS

Although the frequency of slime production was 39.7% among the true bacteremia CoNS agents, it was 18% in CoNS that were judged to be contaminants (p<0.05). S. epidermidis was the most frequently isolated species for both the true bacteremia agent group (56.9%) and contaminant group (74%). Additionally, S. epidermidis was the bacterium most frequently characterized as slime producing in both groups. The methicillin resistance of slime-producing CoNS was determined to be 82.6% for the true bacteremia agent group and 77.8% for the contaminant group.

CONCLUSION

The presence of slime activity in CoNS isolated from blood culture samples is supportive evidence that they are most likely the agents of true bacteremia cases.

Utility of sepsis biomarkers and the infection probability score to discriminate sepsis and systemic inflammatory response syndrome in standard care patients

Physicians are regularly faced with severely ill patients at risk of developing infections. In literature, standard care wards are often neglected, although their patients frequently suffer from a systemic inflammatory response syndrome (SIRS) of unknown origin. Fast identification of patients with infections is vital, as they immediately require appropriate therapy. Further, tools with a high negative predictive value (NPV) to exclude infection or bacteremia are important to increase the cost effectiveness of microbiological examinations and to avoid inappropriate antibiotic treatment. In this prospective cohort study, 2,384 patients with suspected infections were screened for suffering from two or more SIRS criteria on standard care wards. The infection probability score (IPS) and sepsis biomarkers with discriminatory power were assessed regarding their capacity to identify infection or bacteremia. In this cohort finally consisting of 298 SIRS-patients, the infection prevalence was 72%. Bacteremia was found in 25% of cases. For the prediction of infection, the IPS yielded 0.51 ROC-AUC (30.1% sensitivity, 64.6% specificity). Among sepsis biomarkers, lipopolysaccharide binding protein (LBP) was the best parameter with 0.63 ROC-AUC (57.5% sensitivity, 67.1% specificity). For the prediction of bacteremia, the IPS performed slightly better with a ROC-AUC of 0.58 (21.3% sensitivity, 65% specificity). Procalcitonin was the best discriminator with 0.78 ROC-AUC, 86.3% sensitivity, 59.6% specificity and 92.9% NPV. Furthermore, bilirubin and LBP (ROC-AUC: 0.65, 0.62) might also be considered as useful parameters. In summary, the IPS and widely used infection parameters, including CRP or WBC, yielded a poor diagnostic performance for the detection of infection or bacteremia. Additional sepsis biomarkers do not aid in discriminating inflammation from infection. For the prediction of bacteremia procalcitonin, and bilirubin were the most promising parameters, which might be used as a rule for when to take blood cultures or using nucleic acid amplification tests for microbiological diagnostics.

[Interest of real-time PCR Xpert MRSA/SA on GeneXpert(®) DX System in the investigation of staphylococcal bacteremia]

AIM OF THE STUDY

Recently, a rapid, fully automated real-time PCR test has become available for detection of Staphylococcus aureus in positive blood cultures, Xpert MRSA/SA blood culture. This study was defined to evaluate the use of this product in our hospital setting to assist in optimizing antibiotic treatment.

MATERIALS AND METHODS

Over a period of 18months (from February 2008 to July 2009), 51 positive blood cultures were examined for Staphylococcus using the Xpert MRSA/SA assay on the GeneXpert(®) System. The PCR results were transferred to the clinician as soon as available. The presence of empirical antibiotic therapy was noted and modified if necessary after discussions between the clinician and the infectious disease specialist.

RESULTS

Twenty-three blood bottles were positive for S. aureus, two were resistant to methicillin. Twenty-eight were coagulase negative staphylococci. No discrepancy between identification (S. aureus) and methicillin resistance was observed. Thirty-two samples had clinically significant bacteremia (23 S. aureus and nine coagulase negative staphylococci). Sixteen (50%) of these patients had received inappropriate antibiotic therapy (11 without antibiotic therapy, five with betalactam antibiotics). For these patients, an appropriate antibiotic therapy was prescribed according to these results. Sixteen patients had adequate empirical antibiotic therapy at the time of receiving the PCR result. Among these 16 patients, eight switches were performed from broad-spectrum treatment to a more restrictive antistaphylococcal treatment. Of the 19 patients with a nonclinically relevant coagulase negative staphylococci infection, four were already on antibiotics for other infections and these treatments were not modified. Empirical treatment could be avoided in 13 patients who had a clinical presentation consistent with staphylococcal bacteremia (multiple sores, history of carrying methicllin-resistant or susceptible S. aureus infection, presence of intravascular material or prosthesis).

CONCLUSION

The real-time PCR Cepheid Xpert MRSA/SA on GeneXpert(®) DX System has become an essential tool in our laboratory enhancing the reports of positive blood cultures for staphylococci. This test is fast (50min) and reliable. It allows optimization of antibiotic therapy in hospital.