Culture-negative endocarditis diagnosed using 16S DNA polymerase chain reaction

16S DNA polymerase chain reaction (PCR) is a molecular amplification technique that can be used to identify bacterial pathogens in culture-negative endocarditis. Bacterial DNA can be isolated from surgically excised valve tissue or from blood collected in EDTA vials. Use of this technique is particularly helpful in identifying the bacterial pathogen in cases of culture-negative endocarditis. A case involving a 48-year-old man who presented with severe aortic regurgitation and a four-month prodrome of low-grade fever is reported. Blood and valve tissue cultures following valve replacement were negative. A valve tissue sample was sent for investigation with 16S DNA PCR, which successfully identified Streptococcus salivarius and was interpreted as the true diagnosis. A review of the literature suggests that 16S DNA PCR from valve tissue is a more sensitive diagnostic test than culture. It is also extremely specific, based on a sequence match of at least 500 base pairs.     

Phenotypic methods of greater accuracy to detect the mecA gene product for the recognition of MRSA in resource constraint settings

ObjectiveTo evaluate the detection of methicillin resistant Staphylococcus aureus (MRSA) and analyze the performance of Mastalex MRSA (Mast, UK).MethodsTwo hundred and ten Staphylococcus aureus (S. aureus) strains were isolated from different clinical samples and were tested for methicillin resistance by Oxacillin (1 μg) and Cefoxitin (30 μg) disc diffusion, oxacillin agar screen, and minimum inhibitory concentration of oxacillin and cefoxitin. S. aureus isolates were grown on the blood agar and mannitol salt agar with (2 mg/L) and without oxacillin for the analysis of Mastalex MRSA.ResultsOut of 210 S. aureus strains tested, 103 strains were detected as methicillin resistant by Cefoxitin disk diffusion, Cefoxitin minimal inhibitory concentration (MIC) and Mastalex MRSA test. Whereas oxacillin disc diffusion and oxacillin agar screen detected 91 and 97 MRSA respectively. The Cefoxitin MIC test performance was equivalent to Cefoxitin disc diffusion. 103 (100%) strains grown on blood agar without and with oxacillin, and 76 (74%) and 93 (91%) strains grown on mannitol salt agar without and with oxacillin shown positive agglutination with Mastalex MRSA test respectively.ConclusionsThe cefoxitin disk diffusion/Mastalex MRSA is very suitable for detection of MRSA and the tests can be an alternative to PCR for detection of MRSA in resource constraint settings. Mastalex test would be particularly useful when confirmation of resistance is urgently required.     

Ecology of blood stream infection and antibiotic resistance in intensive care unit at a tertiary care hospital in North India

ObjectiveTo analyse the prevalent microorganisms and their antimicrobial resistance among intensive care unit patients in a tertiary care centre in New Delhi.MethodsA retrospective study of all consecutive blood cultures from various intensive care unit patients in the hospital during four years (January 2008 to December 2011). Antibiotic consumption data in the intensive care units were also analysed during the same period.ResultsOut of the total 22,491 blood cultures processed, 2846 samples were positive and 3771 microorganisms were isolated. The blood culture positivity was estimated as 12.7% of which 67.5% were monomicrobial and 32.5% polymicrobial infections. Gram negative bacilli, Gram positive cocci, and fungi were isolated in 49%, 33%, and 18% cases, respectively. Coagulase negative staphylococcus was the commonest single isolate followed by Candida spp. A drastic shift in the distribution of Candida spp. towards nonalbicans along with high resistance to azole group of antifungals suggest echinocandins for the empiric therapy of candidemia. High penicillin resistance in Gram positive isolates suggest vancomycin, linezolid and tigecycline as the options for empiric therapy, whereas tigecycline and colistin are the only options remaining for highly resistant Gram negative isolates. Aminoglycosides were observed to have better sensitivity and reduced usage when compared with cephalosporins and β-lactam + β-lactam inhibitor combinations.ConclusionsHigh frequencies of multidrug resistant organisms were observed in intensive care units which is a warning as to use the only few effective antimicrobials wisely to reduce selective pressure on sensitive strains.     

Effect of delay of blood cultures on positive detection by automated blood culture system

The effect of entry delayed blood culture bottles until the start of incubation for mechanical detection of organism were compared using 2 major blood culture systems; BACTEC 9240 system and BacT/ALERT 3D system. Total of 13 bacterial strains; 5 gram-positive cocci, 7 gram-negative bacilli and Candida parapsilosis which were isolated mainly from blood cultures were used as the test strains. BACTEC 92F, 93F and BacT/ALERT FA, FN bottles were used as the blood culture bottles. All the bottles inoculated with the test strains were incubated and evaluated immediately after standing at room temperature for 24, 42, 48, 54 or 72 hours, using the respective automated blood culture systems. All the bottles were subcultured. The effect of entry delay the blood culture bottles for the mechanical detection was observed in many gram-negative organisms in BACTEC 9240 system. The blood cultures were evaluated not to be positive in 4 of the 10 samples on delaying for 24 hours or in any of the samples on delaying for 42 hours in the BACTEC 92F bottles inoculated with Escherichia coli. In Serratia marcescens, the blood cultures were evaluated not to be positive in 5 of the 10 samples on delaying for 24 hours or in any of the samples on delaying for 42 hours in the BACTEC 92F bottles. In Klebsiella pneumoniae, the blood cultures were evaluated not to be positive in 9 of the 10 samples on delaying for 42 hours. In Enterococcus faecalis, Pseudomonas aeruginosa and Proteus mirabilis, the blood cultures were evaluated not to be positive in 5-6 of the 10 samples on delaying for 42 hours. On the other hand, the blood cultures were evaluated to be positive in most of the samples of Acinetobacter calcoaceticus (except 3 of the 10 samples which were evaluated not to be positive) on delaying for 42 hours in BacT/ALERT 3 D system. The samples except part of Streptococcus spp. were detected by subculture in both the bottles. These results indicate that the delayed time of blood culture bottles before inoculation with the test bacterial samples affects the positive detection of blood cultures markedly in the blood culture system. Therefore, the immediate incubation was considered to be necessary.     

Two nosocomial outbreaks of sepsis caused by Serratia marcescens, which occurred in July 1999 and January 2002--Tokyo

Two nosocomial outbreaks of sepsis caused by Serratia marcescens, which occurred in Tokyo were the following cases. CASE A: In July 1999, 10 inpatients admitted to the third floor ward of the General Hospital A, developed sudden onset of high fever, coagulation disorders (disseminated intravascular coagulation), and acute renal failure, of which 5 died. Twenty-one strains of Serratia marcescens were isolated from the inpatient's blood and urine, nurse fingers and environmental samples from floor and cooling tower. Serratia infection was strongly suspected as the cause of sepsis. These cases were defined as "inpatients who developed fever 38 degrees C or more during July 26 to 29 and from whom S. marcescens was isolated by blood culture". Ten isolates were detected from the blood. In order to investigate the background of S. marcescens isolation in the hospital and to compare molecular and biochemical characteristics of S. marcescens, cultures were attempted from samples of other inpatients and staffs and hospital environment. Those were classified into 9 groups by various different typings: biotyping with Api Rapid 20; susceptibility typing of antimicrobial agents tested; pulsed-field gel electrophoresis (PFGE) typing of SpeI- or Xba I-restricted chromosome. All 10 isolates causing sepsis were found to be in the same group. CASE B: In January 2002, 24 inpatients, admitted to Neurosurgical Hospital B, developed sudden onset of high fever, of which 7 died. S. marcescens was isolated from a towel, environmental samples and inpatients. These cases were defined as "inpatients who developed fever of 38.5 degrees C and S. marcescens isolated by blood culture". Twelve strains were isolated from the blood samples in 12 cases. In order to investigate the background of S. marcescens isolation in the hospital, cultures were attempted from other inpatient's urine and environmental samples from medical tape, Tshake and a towel. These isolates were classified into 3 groups by the previous typings; biotyping with Api Rapid 20; susceptibility typing of antimicrobial agents tested; and PFGE typing. All 12 isolates in 12 cases were found to be in the same group. These cases of 2 nosocomial outbreaks of sepsis were defined as "in-patient who developed high fever and S. marcescens isolated by blood culture". However in both cases transmission routes of Serratia infection remain unknown by field investigation.     

Detection of the mecA gene and identification of Staphylococcus directly from blood culture bottles by multiplex polymerase chain reaction

Introduction

Staphylococcus spp. – both S. aureus, including methicillin-resistant strains (MRSA) and coagulase negative staphylococci (CoNS) – are relevant agents of healthcare-associated infections. Therefore, the rapid recognition of MRSA and methicillin-resistant CoNS from blood stream infections is critically important for patient management. It is worth noting that inappropriate empiric therapy has been associated with higher in-hospital mortality.

Evaluation of a mass spectrometry and Vitek 2 combined protocol for rapid identification and susceptibility testing of Enterobacterales directly from positive blood cultures

ObjectiveThe aim was to evaluate a rapid method which would combine identification and susceptibility testing directly from positive blood cultures for Gram-negative bacilli of the Enterobacterales.Material and methodsGram-negative rods from blood cultures were directly identified by MALDI-TOF. Samples with Enterobacterales were selected for direct antimicrobial susceptibility testing by Vitek 2. The results were compared to those obtained with our laboratory's standard method.ResultsMALDI-TOF directly from blood cultures identified correctly 83% of the samples. Enterobacterales (n = 68) were identified at gender and species level in 85% of blood cultures with a score >1.7. In general, MICs were obtained after 7 h. MICs of amoxicillin-clavulanate, amikacin and ciprofloxacin showed in almost 50% of the cases after 5 h.ConclusionsA simple procedure with low cost and reduced working time makes it possible to integrate both identification and susceptibility testing directly from blood cultures. Thus, this protocol could offer advantages when it comes to selection and cost of treatment and patients’ clinical outcomes.     

贵州省首次从山羊分离到布鲁氏菌及其种型鉴定

目的从贵州布鲁氏菌抗体阳性的山羊血液分离布鲁氏菌病原体并对其进行种型鉴定。方法采用血培养法对经试管凝集试验检测为布鲁氏菌抗体阳性的山羊血血液进行细菌分离培养,并应用传统方法和和分子生物学方法对可疑菌落进行种型鉴定。结果11份（GZB 01—11）抗体阳性的山羊血标本中有4份（GZB03、GZB04、GZB09、GZB11）经血培养瓶培养出布鲁氏菌可疑菌落,可疑菌落经传统方法鉴定为羊种生物3型布鲁氏菌,4株菌进一步采用布鲁氏菌属特异性PCR（BCSP31-PCR）鉴定为布鲁氏菌属细菌,GZB03,GZB04,GZB11经种／型特异性PCR（AMOS-PCR）将其定为羊种布鲁氏菌,而GZB09采用该方法不能定种。结论从11份贵州省布鲁氏菌抗体阳性山羊血样中分离到4株羊种布鲁氏菌,首次证实贵州省存在羊种布鲁氏菌。     

In vitro activity of delafloxacin against highly levofloxacin-resistant invasive isolates of Streptococcus pneumoniae

IntroductionWe report the activity of delafloxacin, a new fluoroquinolone with high affinity for both topoisomerase IV and DNA gyrase, against highly-levofloxacin-resistant invasive strains of Streptococcus pneumoniae.MethodsA total of 173 highly-levofloxacin-resistant (MIC >32 mg/L) S. pneumoniae invasive isolates were studied. The strains were isolated from blood (n = 162) and other sterile fluids (n = 11). Serotyping was performed by the Pneumotest-Latex and Quellung reaction.Delafloxacin, levofloxacin, penicillin, cefotaxime, erythromycin and vancomycin MICs were determined by the gradient diffusion method following EUCAST guidelines and breakpoints.ResultsAmong the isolates, 32.9% were penicillin non-susceptible, 19.7% cefotaxime non-susceptible, and 76.9% erythromycin resistant. All were susceptible to vancomycin. Delafloxacin MIC₅₀ and MIC₉₀ (mg/L) values were 0.064 and 0.12, respectively; 60% (15/25) of serotype 9V isolates showed delafloxacin MICs ≥ 0.12 mg/L.ConclusionsDelafloxacin was very active against highly-levofloxacin-resistant invasive isolates of S. pneumoniae. Isolates belonging to serotype 9V showed higher delafloxacin MIC values.     

Colonization of the central venous catheter by Stenotrophomonas maltophilia in an ICU setting: An impending outbreak managed in time

BackgroundStenotrophomonas maltophiliacauses opportunistic infections in immunocompromised and patients in intensive care units (ICUs). An outbreak of S. maltophilia in ICU is described which highlights the importance of the risk of infection from contaminated medical devices and suction fluids in ventilated patients.MethodsThe investigation of the outbreak was carried out. Environmental sampling was done. This was followed by MALDI-TOF MS typing and recA gene-based-phylogeny.ResultsIn February, S. maltophilia was reported from the central line blood of six patients from ICU within a span of two weeks. The peripheral line blood cultures were sterile in all patients. Relevant environmental sampling of the high-touch surface and fluids revealed S. maltophilia strains in normal saline used for suction and in the inspiratory circuit of two patients. The isolated strains from patients and environment (inspiratory fluid) showed a minimum of 95.41% recA gene sequence identity between each other. Strict cleaning and disinfection procedures were followed. Continuous surveillance was done and no further case of S. maltophilia was detected. Timely diagnosis and removal of central line prevented development of central-line associated blood stream infection.ConclusionThis outbreak report illustrates that environmental sources like suction fluid and normal saline could be the source of S. maltophilia in ICU patients.     

Surveillance of multidrug resistance of 10 enteropathogens in a teaching hospital and in vitro efficacy of 25 ethnomedicinal plants used by an Indian aborigine

ObjectiveTo have an antibiogram of hospital acquired (HA) and community acquired (CA) enteropathogens against 16 antibiotics to assess the infection dynamics for plausible help to the antimicrobial stewardship. To check extracts of 25 lesser-known plants used by an Indian aborigine, for antimicrobial efficacy in vitro and as complementary and alternate medicines against resistant pathogens.MethodsTen strains of enteric bacteria (Enterobacter aerogenes, Escherichia coli, Klebsiella sp., Salmonella paratyphi, S. typhi, Shigella boydii, S. dysenteriae, S. flexneri, S. sonnei and Vibrio cholerae) were isolated from clinical samples in 6 months and their antibiotic sensitivity was assessed by the disc-diffusion method. Concentrated aqueous and ethanolic extracts of leaves and barks of plants were used for monitoring their antibacterial potencies, by the agar-well diffusion method.ResultsIsolated bacterial strains were invariably multidrug resistant (MDR). E. coli was the most frequently isolated organism from HA and CA samples, followed next by Klebsiella sp. From the surveillance, it was evident that the distribution of MDR strains of each was more in HA than CA isolates. Aqueous and ethanolic extracts of Aegle marmelos, Azadirachta indica, Cassia fistula, Holarrhena antidysenterica, Salvadora persica and Terminalia arjuna were highly effective against the all isolated enteropathogenic strains. From the preliminary phytochemical analysis, it was confirmed that both extracts of A. indica, T. arjuna and T. alata contained all the detected phytochemicals (alkaloids, glycosides, terpenoids, reducing sugars, saponins, tannins, flavonoids and steroids), which plausibly attributed to their significant antibacterial activity.ConclusionsPhytoextracts were highly effective against the all enteropathogenic bacterial isolates, in vitro. These 25 plants could be used further for the isolation of pure compounds for use as complementary medicines.     

Infections Caused by Stenotrophomonas maltophilia– A Prospective Study

Background: Stenotrophomonas maltophilia is an opportunistic microorganism, often highly resistant to routinely tested antibiotics. This microorganism is isolated in specimens from patients with nosocomial infections with increasing frequency. Patients and Methods: During a 1-year period (1998/1999) S. maltophilia was isolated from 137 specimens (0.26% of all investigated specimens) from 80 patients who were treated in a 1,500 bed major tertiary care teaching hospital in Leipzig. The data of 76 patients (133 specimens) could be collected and analyzed completely. Results: The pathogen was most frequently detected in specimens from the respiratory tract (54%). In five patients (six cases) S. maltophilia was isolated from blood cultures (0.3% of all positive blood cultures; 1.4% of all gram-negative isolates from blood cultures). 70 of the infected patients were inpatients and 32 (42%) of them were treated on the internal medicine wards. Of these 32 patients only six (19%) were pretreated with imipenem. The length of stay at the hospital resulted in an independent increased risk of infection with S. maltophilia. In addition, this organism was detected in six infected outpatients. Conclusion: S. maltophilia is not only a nosocomial pathogen. Pretreatment with a carbapemnem is no longer an unequivocal risk factor for an infection with S. maltophilia. Received: April 13, 2000 · Revision accepted: February 27, 2001     

Rapid Ribosequencing – an Effective Diagnostic Tool for Detecting Microbial Infection

Background: Rapid and reliable identification of microorganisms is a prerequisite for the diagnosis and subsequent treatment of infectious diseases. The identification of pathogenic bacteria is traditionally based on their isolation from clinical samples and propagation on culture medium in the routine laboratory. However, despite clinical signs of infection, culture of the pathogenic agent often fails. This may be due to a low number of microorganisms, prior antibiotic treatment, nonculturable microorganisms or specific culture requirements for presently unknown pathogens. Amplification and sequencing of the entire prokaryotic 16S-rRNA is time consuming, labor intensive and expensive. Materials and Methods: We describe here a procedure for the identification of a wide range of known and unknown clinically relevant microorganisms by sequencing a small, but highly informative region of the prokaryotic 16S-rRNA gene. This rapid ribosequencing method was evaluated with various reference strains and with clinical samples including eye anterior chamber fluid, cerebrospinal fluid (CSF) and blood cultures. Results: All sequences obtained from the reference strains corresponded to the sequences in databases. We correlated severe eye infection with the isolation of Pseudomonas putida, neurological disorder with Tropheryma whippelii and disseminated visceral abscesses in a child with Blastobacter denitrificans. Conclusion: We consider the rapid ribosequencing method as a promising new tool for the analysis of infectious agents in primarily sterile body fluids where conventional culturing of microorganisms fails. Received: June 14, 2000 · Revision accepted: October 13, 2000     

Nosocomial neonatal outbreak of Serratia marcescens--analysis of pathogens by pulsed field gel electrophoresis and polymerase chain reaction

Background: We investigated an outbreak of Serratia marcescens in the neonatal intensive care unit (NICU) and the pediatric intensive care unit (ICU) of the University Children's Hospital Leipzig, Germany. Patients and Methods: From September to November 1998 15 patients were infected or colonized by S. marcescens. During the outbreak swabs from eye, blood, throat and nose were taken from every patient hospitalized in the ICUs. Results: In 15 cases (14 from the NICU and one from the pediatric ICU) the cultures yielded S. marcescens. All strains were investigated by pulsed field gel electrophoresis (PFGE) as well as by polymerase chain reaction (PCR) fingerprinting. Both molecular typing methods revealed corresponding fingerprint patterns in all of the 15 isolates. Typing results of the outbreak-related isolates demonstrated that two epidemic strains of distinct genotypes were associated with cross-infections of a group of five and a group of ten patients, respectively. The three invasive and seven of the colonizing isolates were related genotypically. Conclusion: This survey shows that PCR and PFGE are comparable in respect to the discrimination and reproducibility for epidemiological studies of S. marcescens strains in nosocomial outbreaks. Genotypic fingerprinting of bacterial isolates is useful and important to limit nosocomial infections. Fingerprinting sources of nosocomial infections can be traced both by PFGE and PCR. All patients infected recovered completely and the nosocomial outbreak could be stopped rapidly. Received: October 10, 2001 · Revision accepted: July 7, 2002 K. Steppberger (corresponding author)     

畜舍环境中魏氏梭菌分离及基因型鉴定

目的从山东省泰安、青岛等地6个市的2个牛舍、4个猪舍、5个兔舍空气及畜舍动物粪便中分离魏氏梭菌,从空气中分离到66株,从畜舍粪便中分离到70株,通过多重聚合酶链反应(multi-PCR)对分离到的魏氏梭菌进行型别的鉴定,结果发现:分离到的魏氏梭菌均为A型。     

Innovative PCR without DNA extraction for African sickle cell disease diagnosis

Objectives: Hemoglobin (Hb) disorders consist of thalassemia and Hb structural variants, of which the major forms are associated with severe anemia and/or vascular occlusion. Current diagnostic techniques are highly accurate and mostly based on isoelectric focusing, high-performance liquid chromatography or mass spectrometry, which often require advanced laboratory equipment. In sub-Saharan Africa, the Hb disorders are mainly associated to the pathological variants hemoglobin S (HbS) and HbC. Unfortunately, until now, it is not easy to get a diagnosis of these disorders in this area. In this study, we tested the performance of a new molecular diagnostic tests on qualified samples.

Methods: The Human Hb S/C Lamp assay is a new polymerase chain reaction test able to detect HbS, HbC and HbA alleles without DNA extraction, directly on fresh or frozen blood samples, or on dried blood spots (DBS). In this study, we compared the genotyping of 248 blood samples (56 whole blood and 192 DBS) with this LAMP assay to the routine diagnostic methods performed in the genetics lab at the university hospital of Liège.

Results: Our results show that the LAMP method can detect HbS and HbC with an accuracy of 100%. Moreover, this test can be used for the neonatal screening because we did not observe any interference with fetal Hb.

Discussion: To our knowledge, this method is the only molecular assay that can be performed directly on dried blood cards without DNA extraction, lowering handling, turnaround time and costs.     

Bacterial Meningitis in an Urban Area: Etiologic Study and Prognostic Factors

Abstract Objectives: The study of clinical features, diagnostic methods and prognostic factors of bacterial meningitis, in an urban area. Patients and Methods: All patients admitted between June 2001 and July 2004 in the emergency departments of a few hospitals, with the diagnosis of bacterial meningitis were included. CSF and blood cultures were performed in every case. Phenotypic characterization of strains of Streptococcus pneumoniae and Neisseria meningitidis identified by culture were performed. In order to detect the three most common agents it was done a PCR assay in culture negative CSF samples. Results: Bacterial meningitis was diagnosed in 201 patients. Etiologic definition was based on culture in 142 patients (70.6%), done by CSF PCR assay in 33 (16.4%) other patients and exclusively by latex agglutination test results in two cases. Thus, an etiologic diagnosis was established in 177 (88%) cases. Antigenic characterization showed a slight prevalence of N. meningitidis phenotype C:2b:P1; the S. pneumoniae serotype characterization showed that 43.8% of identified serotypes are not included in any of the available vaccines. Eighteen patients died (8.9%). The statistic analysis found that factors associated with an adverse outcome were age older than 50 years (OR 7.07; IC 95% 1.1–27.4), the presence of comorbidities (OR 3.3; IC 95% 1.1–9.6) and the occurrence of systemic complications (OR 5.8; IC 95% 2.1–16.0). Conclusions: This epidemiologic pattern is similar to that found in other countries after the introduction of Haemophilus influenzae b conjugated vaccine. The association of culture and noncultural methods of diagnosis had a better performance in defining the etiology. Comparing to other series, in-patients mortality rate was lower (8.9%) than usually referred to, being considered unfavourable prognostic factors the age more than 50 years, the presence of comorbidities and of systemic complications.     

Sequence analysis of VP4 genes of wild type and culture adapted human rotavirus G1P[8] strains

ObjectiveTo conduct a comparative analysis of the VP4 gene sequences of Indian wild type (06361, 0613158, 061060 and 0715880) and cell culture adapted (06361-CA, 0613158-CA, 061060-CA and 0715880-CA) G1P[8] rotavirus strains.MethodsFull-length VP4 genes of each of the four wild type G1P[8] rotavirus strains and their cell culture adapted counterparts displaying consistent cytopathic effect were subjected to RT-PCR amplification and nucleotide sequencing.ResultsAll four cell culture adapted G1P[8] rotavirus strains showed nucleotide and amino acid substitutions in the VP4 gene as compared to their wild type strains. The number of substitutions however, varied from 1-64 and 1-13 respectively. The substitutions were distributed in both VP5* and VP8* subunits of VP4 gene respectively of permeabilization and hemagglutinating activity. The presence of unique amino acid substitutions was identified in two of the four wild type (V377G, S387N in 061060 and I644L in 0715880) and all four cell culture adapted (A46V in 0613158-CA, T60R in 06361-CA, L237V, G389V and Q480H in 061060-CA and S615G and T625P in 0715880-CA) strains for the first time in the VP4 gene of P[8] specificity. Amino acid substitutions generated increase in the hydrophilicity in the cell culture adapted rotavirus strains as compared to their corresponding wild type strains.ConclusionsAmino acid substitutions detected in the VP4 genes of G1P[8] rotavirus strains from this study together with those from other studies highlight occurrence of only strain and/or host specific substitutions during cell culture adaptation. Further evaluation of such substitutions for their role in attenuation, immunogenicity and conformation is needed for the development of newer rotavirus vaccines.     

Evaluation of S. aureus ID, a Chromogenic Agar Medium for the Detection of Staphylococcus aureus

Abstract Background: Screening for staphylococci among various patient populations has become important for appropriate therapeutic management and for control of nosocomial infections. The purpose of this study is to evaluate the in vitro sensitivity and specificity of a chromogenic agar medium, S. aureus ID (bioMérieux, France), for the identification of Staphylococcus aureus. Materials and Methods: A well-defined collection of S. aureus and coagulase-negative staphylococci (CNS) was used. The methicillin-resistant S. aureus (MRSA) isolates were collected in The Netherlands and all had a unique typing pattern. The methicillin-susceptible S. aureus (MSSA) and CNS were isolated from cultures of blood. The isolates were inoculated on Columbia agar plates with 5% sheep blood and incubated for 24 h at 35 °C. From the resulting cultures, a suspension of 0.5 McFarland was made and subsequently 10 μl was streaked on a S. aureus ID plate using a sterile loop. The results were read after 24 h and 48 h of incubation at 35 °C. Growth of colonies showing green coloration was considered to be positive (indicating S. aureus). Results: A total of 519 S. aureus strains were tested (249 MSSA, 270 MRSA). The sensitivity to detect S. aureus was 96.5% (501/519) after 24 h and 97.5% (506/519) after 48 h. A total of 478 CNS were tested. The specificity was 98.5% (471/478) after 24 h and 98.3% (470/478) after 48 h. The differences between 24 h and 48 h incubation were not statistically significant. Conclusion: S. aureus ID is highly sensitive and specific to differentiate between S. aureus and CNS in vitro. Since the performance does not significantly differ between 24 h or 48 h of incubation, samples need only 1 day of incubation before optimal results can be obtained.