ABSTRACT
BACKGROUND: Procedures for rapid identification and susceptibility testing by direct inoculation (DI) from positive blood culture bottles into an automated system have not been standardized. This study was purposed to evaluate DI from BACTEC 9240 blood culture system (BD, USA) into MicroScan (Dade Behring, USA) or Phoenix (BD, USA). METHODS: From May to June 2006, bacterial pellets from positive aerobic bottles showing gram-positive cocci (GPC) or gram-negative rods (GNR) of single morphology were directly inoculated to MicroScan PosCombo1A and NegCombo32 and to Phoenix PMIC/ID-107 and NMIC/ID-53. In addition, the automated instruments were also inoculated from subcultures (standard inoculations, SI). Species identification and susceptibilities were compared between DI and SI and between MicroScan and Phoenix. RESULTS: A total of 108, 104, and 78 specimens were tested with MicroScan, Phoenix, and both, respectively. When DI and SI were matched, 94.8% of GPC were correctly identified with MicroScan, compared to 80.7% with Phoenix, and 93.9% of GNR were correctly identified with MicroScan, compared to 95.7% with Phoenix. DI with MicroScan and Phoenix showed correct susceptibilities in 94.6% of 1,150 and 96.5% of 660 tests (with very major error [VME] of 1.1% and 1.1%), respectively, among GPC and in 94.4% of 942 and 96.3% of 781 tests (with VME of 0.6% and 0%), respectively, of GNR. Correlation of identification/susceptibilities between MicroScan and Phoenix using DI were 81.8%/98.0% for Staphylococcus aureus and 100.0%/95.6% for Escherichia coli. CONCLUSIONS: DI warrants a reliable method for identification and susceptibility testing of both GPC and GNR in MicroScan, and those of only GNR in Phoenix.
Subject(s)
Humans , Automation , Bacterial Typing Techniques/instrumentation , Culture Media , Gram-Negative Bacteria/classification , Gram-Negative Bacterial Infections/blood , Gram-Positive Bacterial Infections/blood , Gram-Positive Cocci/classification , Microbial Sensitivity Tests/instrumentation , Reagent Kits, Diagnostic , Sensitivity and SpecificityABSTRACT
BACKGROUND: A rapid and sensitive surveillance culture has a pivotal role in infection control of methicillinresistant Staphylococcus aureus (MRSA). This study was aimed to compare the performance of MRSASelect (Bio-Rad, France) to that of mannitol salt agar containing 6 microgram/mL of oxacillin (MSA-OX) for detecting MRSA in surveillance cultures. METHOD: From May to June 2006, 86 nasal swabs and 21 sputum specimens were enrolled. All specimens were inoculated onto MRSASelect and MSA-OX, which were incubated for 2 days and 3 days, respectively, and colonies were read daily by a technologist. Pink colonies on MRSASelect and yellow colonies on MSA-OX were examined with Gram stain, Pastorex(R) Staph-plus (Bio-Rad) and mecA-PCR. After the final reading, both media were re-examined by a superviser. RESULTS: Of the 107 specimens cultured, 32 (29.9%) were positive for MRSA. Of these, 27 were detected by both media, one by MSA-OX only, and 4 by re-examination. The day-1 and day-2 sensitivities/specificities of MRSASelect were 78.1%/97.3% and 84.4%/97.3%, respectively, while those of MSA-OX were 53.1%/100% and 78.1%/92.1%, respectively. With MRSASelect, two more positives were detected at day 2, but their incubation was less than 18 hour at day 1. There were six false positive organisms detected: three Enterobacter spp., one Acinectobacter spp., and two coagulase-negative staphylococci (CNS). But, the two CNS grew on MSA-OX only. CONCLUSION: MRSASelect with 1-day incubation showed a sensitivity equivalent to and a specificity better than MSA-OX with 2-day incubation. MRSASelect should be a useful medium for MRSA surveillance when it is read after an incubation of 18-28 hours with the confirmatory Gram stain of screen-positives.
Subject(s)
Agar , Enterobacter , Infection Control , Mannitol , Methicillin Resistance , Methicillin-Resistant Staphylococcus aureus , Oxacillin , Sensitivity and Specificity , Sputum , Staphylococcus aureusABSTRACT
BACKGROUND: We evaluated the BD Phoenix Automated Microbiology System (Phoenix) for its ability to detect methicillin resistant Staphylococcus aureus (MRSA) and compared the results to those obtained by the Clinical and Laboratory Standards Institute (CLSI) agar dilution method, a mecA gene PCR method, and the MicroScan WalkAway 96 System (MicroScan). METHODS: One hundred seventy S. aureus strains (Group I) isolated from blood and urine cultures were collected from eight university hospitals and 58 strains (Group II) including 20 blood isolates among Group I and 38 isolates from skin lesions of atopic patients were collected from Asan Medical Center. All 208 isolates were tested with Phoenix using PMIC/ID-53 panels, and the tests were repeated when the results were indeterminate. The results by Phoenix were compared to the susceptibility results obtained by reference methods: the CLSI method for oxacillin MIC for Group I strains, and a PCR assay method for detection of the mecA gene and MicroScan tests for oxacillin susceptibility for Group II strains. RESULTS: One hundred strains (58.8%) in Group I were MRSA and 28 strains (48.3%) were mecA positive in Group II. Compared to the CLSI method, Phoenix showed the sensitivity and specificity of 100% and MIC agreement of 99.4% for Group I strains. The level of agreement between Phoenix and MicroScan for oxacillin MIC and their interpretation were 98.3% and 100%, respectively, for Group II strains. Both MicroScan and Phenix failed to detect one mecA-positive strain: its MIC was shown as 2 microgram/mL twice by MicroScan and 2 microgram/mL twice and > 2 microgram/mL once by Phoenix. The frequency of the indeterminate results was 5.5% and the mean time to completion of the tests was 12.8 (10.2-16) hours in Phoenix. CONCLUSION: Phoenix showed a high level of sensitivity and specificity for the detection of MRSA with an excellent correlation with MicroScan. Further evaluation is required for detection of heterogeneous MRSA.