Performance of four slide agglutination methods for identification of Staphylococcus aureus when testing methicillin-resistant staphylococci.

Four commercially available slide agglutination systems for identifying Staphylococcus aureus were compared with the conventional slide (clumping factor) and tube coagulase tests. The systems evaluated included Bacto Staph Latex (Difco Laboratories, Detroit, Mich.), Staphyloslide (BBL Microbiology Systems, Cockeysville, Md.), Mini ID Accu-Staph (Carr-Scarborough Microbiologicals, Inc., Decatur, Ga.), and Staphaurex (Wellcome Diagnostics, Research Triangle Park, N.C.). A total of 338 clinical isolates, including methicillin-resistant S. aureus (n = 149), methicillin-susceptible S. aureus (n = 78), methicillin-resistant, coagulase-negative staphylococci (n = 45), and methicillin-susceptible, coagulase-negative staphylococci (n = 66), were tested by each method. The slide test for clumping factor, the 4-h tube coagulase test, Bacto Staph Latex, Staphyloslide, Mini ID Accu-Staph, and Staphaurex detected 212 (93.4%), 218 (96%), 223 (98.2%), 223 (98.2%), 221 (97.4%), and 224 (98.7%) of the S. aureus (44% methicillin-resistant) isolates, respectively. There were no false-positive results with any of the methods when the 111 strains of coagulase-negative staphylococci were tested. The results of this evaluation suggest that the four slide identification methods tested can provide rapid and accurate identification of methicillin-resistant S. aureus strains.     

Evaluation of various rapid agglutination methods for the identification of Staphylococcus aureus

A latex agglutination test (SeroSTAT Staph; Scott Laboratories, Fiskeville, R.I.) and two hemagglutination tests (Staphyloslide; BBL Microbiology Systems, Cockeysville, Md.; and Hemastaph; Remel, Lenexa, Kans.) were compared with the slide coagulase (SC) and tube coagulase (TC) tests at room temperature (22 to 25 degrees C) and at 37 degrees C for the rapid identification of Staphylococcus aureus. A total of 380 clinical strains of staphylococci were tested. The TC test performed at room temperature yielded the largest number of TC-positive results (n = 239), and based on this observation 239 organisms were classified as S. aureus and 141 were classified as non-S. aureus. The SC, TC (37 degrees C), SeroSTAT Staph, Staphyloslide, and Hemastaph tests correctly identified 210 (87.9%), 221 (92.5%), 238 (99.6%), 239 (100%), and 236 (98.7%) of the S. aureus isolates, respectively. Of the S. aureus isolates that were TC positive at room temperature 68% required 24 h of incubation before coagulase production was detected. There was one false-negative SeroSTAT Staph result and one false-negative Hemastaph result. The Staphyloslide test yielded two noninterpretable results (both organisms were later confirmed as non-S. aureus), whereas there were six noninterpretable results recorded with the Hemastaph test (four organisms were classified as non-S. aureus, and two were classified as S. aureus). The SeroSTAT Staph, Staphyloslide, and Hemastaph tests were all more sensitive than the conventional SC and TC (37 degrees C) tests and were considerably more rapid than the TC test at either temperature.     

Evaluation of rapid coagulase methods for the identification of Staphylococcus aureus.

Four rapid latex agglutination assays, StaphAurex (Wellcome Diagnostics, Research Triangle Park, N.C.), Bacto Staph (Difco Laboratories, Detroit, Mich.), SeroSTAT (Scott Laboratories, Inc., Fiskeville, R.I.), Veri-Staph (Zeus Technologies, Raritan, N.J.), and two hemagglutination tests, Staphyloslide (BBL Microbiology Systems, Cockeysville, Md.) and Hemastaph (Remel, Lenexa, Kans.), were compared with the conventional slide coagulase, tube coagulase (TC), and thermonuclease (TNase) tests for the identification of Staphylococcus aureus. A total of 118 clinical isolates of S. aureus (52 methicillin resistant), 50 S. epidermidis, 5 S. capitis, 2 S. hominis, 3 S. simulans, 6 S. saprophyticus, and 2 S. warneri were tested. The slide coagulase, TC and TNase tests detected 115 (97.5%), 117 (99.2%), and 118 (100%) of the S. aureus isolates, respectively. All showed 100% specificity. The StaphAurex, Veri-Staph, Staphyloslide, Hemastaph, SeroSTAT, and Bacto Staph assays correctly identified 117 (99.2%), 117 (99.2%), 116 (98.3%), 110 (93.2%), 108 (91.5%), and 107 (90.7%) of the S. aureus isolates, respectively. For methicillin-resistant S. aureus isolates, StaphAurex, Veri-Staph, Staphyloslide, Hemastaph, SeroSTAT, and Bacto Staph showed 1 (2%), 1 (2%), 2 (4%), 7 (13.5%), 7 (13.5%), and 8 (15.4%) false-negative results, respectively. All the commercial agglutination assays demonstrated false-positive results with strains of S. capitis, S. saprophyticus and S. warneri. The overall accuracy of the commercial agglutination assays compared with TC and TNase ranged from 90.7 to 99.2%. We recommend that negative reactions with the rapid commercial test kits for methicillin-resistant Staphylococcus isolates be confirmed with the TC or TNase test.     

Comparison of rapid identification assays for Staphylococcus aureus.

A total of 137 strains of Staphylococcus species were blindly tested by four rapid serological assays, and the results were compared with those of the tube coagulase assay. For the S. aureus isolates, the Sero-STAT Staph assay gave six false-negative results, four of which were for methicillin-resistant strains. The Accu -Staph, Staphylatex , and Staphyloslide assays identified all the coagulase-positive strains as Staphylococcus aureus. Among the coagulase-negative staphylococci, false-positive results were seen with strains of S. capitis. S. saprophyticus, and S. cohnii. The overall accuracy of the kits compared with the tube coagulase test ranged from 95.1 to 100%.     

Comparison of five agglutination tests for identification of Staphylococcus aureus.

Various commercially produced agglutination kits are widely used for the identification of Staphylococcus aureus. These kits detect the presence of protein A and/or clumping factor on S. aureus. The literature has shown that methicillin-resistant S. aureus (MRSA) isolates which are deficient in both clumping factor and protein A may be misidentified. Two products, Slidex and Staphaurex Plus, utilize specific anti-S. aureus antibodies, potentially giving them greater sensitivity compared to products without these antibodies. We report a prospective study designed to compare the performance characteristics of Fastaph, Slidex, Staphaurex, Staphaurex Plus, Staphyloslide, and the tube coagulase test for the identification of staphylococcal isolates. All discrepant isolates were tested with the Gen-Probe AccuProbe S. aureus test and were identified to the species level with conventional reference biochemicals. A total of 1,193 isolates were tested, including 33 MRSA and 423 methicillin-sensitive S. aureus isolates. The sensitivities and specificities of the tests, respectively, were as follows: Fastaph, 99.1 and 98.9%; Slidex, 99.6 and 96.4%; Staphaurex, 98.9 and 99.9%; Staphaurex Plus, 99.6 and 93.9%; Staphyloslide, 99.1 and 98.9%; and tube coagulase, 99.3 and 100%. Sensitivity was excellent for all of the products tested. The specificities of Fastaph, Staphaurex, and Staphyloslide were excellent, while Staphaurex Plus and Slidex demonstrated less optimal results.     

Reliability of latex agglutination tests for identification of Staphylococcus aureus resistant to oxacillin.

Commercial latex agglutination tests (LATs) for the simultaneous detection of clumping factor and protein A are gaining increased acceptance as a means of identifying Staphylococcus aureus. We evaluated two LATs (Accu-Staph; Carr-Scarborough, Decatur, Ga.; Staphaurex; Wellcome, Dartford, England) with particular emphasis on their ability to correctly identify oxacillin-resistant S. aureus. We tested 59 oxacillin-resistant S. aureus, 136 oxacillin-susceptible S. aureus, and 92 coagulase-negative staphylococcal strains with the two LATs and with thermonuclease, slide clumping factor, tube coagulase, and protein A hemagglutination tests. Clumping factor and protein A were present in 96.9 and 82.1% of our S. aureus strains, respectively. Accu-Staph correctly identified 92.8% and Staphaurex correctly identified 91.3% of S. aureus strains. No significant difference in LAT positivity rates, presence of clumping factor, or presence of protein A was found between oxacillin-resistant and -susceptible S. aureus. Overall, there were 31 false-negative LATs for 20 S. aureus strains, 14 with Accu-Staph and 17 with Staphaurex. Ninety-five percent of these strains possessed either clumping factor or protein A or both when these factors were determined independently. There were five false-positive LATs for four strains of coagulase-negative staphylococci (three Staphylococcus epidermidis and one Staphylococcus warneri), four with Accu-Staph and one with Staphaurex. Clumping factor was present in one S. warneri strain. Thus, the specificities of Accu-Staph, Staphaurex, and the clumping factor test were 95.6, 98.9, and 98.9%, respectively. Our results indicated that LATs identify oxacillin-resistant and -susceptible S. aureus equally well; however, they offer no greater sensitivity or specificity than the clumping factor test for identification of S. aureus.     

International multicenter evaluation of latex agglutination tests for identification of Staphylococcus aureus

A newly marketed rapid agglutination kit for the identification of Staphylococcus aureus, Slidex Staph Plus (bioMérieux), was compared to Staphaurex Plus (Murex Diagnostics) and Pastorex Staph-Plus (Sanofi Diagnostics Pasteur). The study took place in three clinical microbiology laboratories in three different European countries. A total of 892 staphylococcal isolates, including 278 methicillin-sensitive S. aureus (MSSA) isolates, 171 methicillin-resistant S. aureus (MRSA) isolates, and 443 coagulase-negative staphylococcal isolates, were analyzed. The sensitivities (MSSA/MRSA) and specificities, respectively, were 98. 2% (98.9%/97.1%) and 98.9% for Slidex Staph Plus, 98.2% (98.2%/98. 2%) and 96.2% for Staphaurex Plus, and 98.7% (98.6%/98.8%) and 95.7% for Pastorex Staph Plus. The specificity of the Slidex Staph Plus kit was statistically significantly higher than the specificities of Staphaurex Plus and Pastorex Staph-Plus. The Slidex Staph Plus is a very reliable test for the identification of S. aureus.     

Use of commercial particle agglutination systems for the rapid identification of methicillin-susceptible and methicillin-resistantStaphylococcus aureus

The performance of a recently introducedStaphylococcus aureus identification system (Slidex Staph-Kit) was compared with that of currently available systems (Immuno Scan Staphlatex, Staphyloslide, Staphaurex and SeroSTAT II) for the identification of methicillin-resistantStaphylococcus aureus (MRSA) and methicillin-susceptibleStaphylococcus aureus. The new system, which detects a capsular antigen common in MRSA, performed with equal or greater sensitivity than the other systems. None of the commercial systems was adversely affected by the methicillin susceptibility of the staphylococci when isolates were recovered from non-selective media. The greatest advantage of the anti-capsular monoclonal reagent was its improved performance on isolates recovered from selective media.     

Efficacies of rapid agglutination tests for identification of methicillin-resistant staphylococcal strains as Staphylococcus aureus.

Four commercially available rapid agglutination tests for the identification of Staphylococcus aureus were compared with the tube coagulase test for the identification of 300 methicillin-resistant isolates of staphylococci. Isolates tested included 207 methicillin-resistant S. aureus and 93 coagulase-negative staphylococci, collected from five medical centers. Strain variability was documented by phage typing and antimicrobial susceptibility patterns. Results of rapid identification tests ranged between 82 and 86% sensitivity, significantly poorer than the 98% sensitivity which the tube coagulase test provided.     

Comparative performances of six agglutination kits assessed by using typical and atypical strains of Staphylococcus aureus.

Six commercial agglutination tests designed for the identification of Staphylococcus aureus were compared by using a strain collection which included 512 staphylococci representing 33 species (318 isolates of Staphylococcus aureus [including 144 oxacillin resistant], 46 S. epidermidis isolates, 15 S. haemolyticus isolates, 12 S. saprophyticus isolates, 29 S. schleiferi isolates, 30 S. lugdunensis isolates, and 62 other coagulase-negative staphylococci). This group also included a proportion of strains with unusual phenotypes (e.g., 19 coagulase-negative S. aureus isolates, 26 clumping factor-negative S. aureus isolates, and 4 S. aureus isolates each with a double deficiency). The overall sensitivity for identification of typical and atypical S. aureus was high with the Staphaurex Plus test (Murex Biotech) (99.7%), the Pastorex Staph Plus test (Sanofi Diagnostics Pasteur) (99.7%), and the Slidex Staph Plus test (bioMérieux) (100%). The overall rate of specificity was affected by the unusual inclusion in this study of a high proportion of non-S. aureus species, such as S. lugdunensis and S. schleiferi, which express a clumping factor and therefore produce a positive result with the agglutination tests.     

Isolation of methicillin-resistant coagulase-negative staphylococci from chickens.

Methicillin-resistant coagulase-negative staphylococci were isolated from the nares and skin of 1- to 8-week-old healthy chickens in three flocks from a farm. Isolation of methicillin-resistant coagulase-negative staphylococci was positive for 72 (25.7%) of the 280 chickens tested, with the frequency varying from 2.2 to 100% according to flock. A total of 45 appropriate isolates were selected and subjected to identification. Of the 45 methicillin-resistant coagulase-negative staphylococcal isolates selected, 37 were identified as Staphylococcus sciuri, 5 were identified as Staphylococcus epidermidis, and 3 were identified as Staphylococcus saprophyticus. The distribution of the species was different among the flocks. Comparative analysis of the SmaI-digested chromosomal DNA by pulsed-field gel electrophoresis revealed that the isolates could have originated from a single clone of each of S. sciuri and S. saprophyticus and three clones of S. epidermidis. By two methods based on the PCR technique, the mecA gene was detected in all five representative isolates of each methicillin-resistant coagulase-negative staphylococcal clone. The nucleotide sequence of a PCR fragment obtained from an isolate of S. sciuri was completely identical to the corresponding region of mecA genes reported in human methicillin-resistant Staphylococcus aureus isolates and Staphylococcus epidermidis isolates. The representative methicillin-resistant coagulase-negative staphylococcal isolates were resistant to many beta-lactam antibiotics, and some isolates were also resistant to macrolide and aminoglycoside antibiotics. This is the first evidence of the existence of methicillin-resistant coagulase-negative staphylococci from animals possessing the mecA gene.     

Evaluation of a new Staphylococcus aureus latex agglutination kit, Prolex Staph Xtra, against other third-generation kits

Staphylococcus aureus, including methicillin-resistant strains, continues to be a common cause of infection/colonisation, which necessitates accurate and prompt diagnosis in the laboratory. Several rapid agglutination tests that aid this function are available, and some have been modified to improve their performance. One such kit, Prolex Staph Xtra, has been released recently. This study aims to compare this kit with other improved kits (i.e., Pastorex Staph-Plus, Staphaurex Plus and Staphytect Plus) and investigate their ability to confirm the identity of 100 strains of S. aureus. Results showed that 50 were resistant to methicillin. Specificity was checked against 30 strains of coagulase-negative staphylococci and 20 Enterococcus species isolates. Of the four kits tested, Prolex Staph Xtra and Pastorex Staph-Plus proved superior in terms of sensitivity and speed.     

Phenotypic and genotypic mupirocin resistance among Staphylococci causing prosthetic joint infection

Mupirocin MICs and mupA presence were determined in 108 staphylococci causing prosthetic joint infection. Zero of 35 isolates (0%) of methicillin-susceptible Staphylococcus aureus, 4/15 (27%) methicillin-resistant S. aureus isolates, 3/16 (19%) methicillin-susceptible coagulase-negative staphylococci, and 11/42 (26%) methicillin-resistant coagulase-negative staphylococci were mupirocin resistant. mupA was detected in all five high-level mupirocin-resistant staphylococci and one mupirocin-susceptible staphylococcus.     

Evaluation of several commercial biochemical and immunologic methods for rapid identification of gram-positive cocci directly from blood cultures.

To develop laboratory methods for the rapid identification of gram-positive cocci from blood cultures, several commercial immunological and biochemical tests for identifying staphylococci and streptococci from two different blood culture systems (Thiol/TSB, Difco Laboratories; BACTEC, Johnston Laboratories, Inc.) were evaluated and compared with conventional identification methods. A total of 44 cultures contained Staphylococcus aureus as determined by conventional methods. Commercial immunological methods (six tested) ranged in overall sensitivity from 38.6 (Staphyloslide; BBL Microbiology Systems) to 77.3% (Staphaurex; Wellcome Diagnostics). All methods tested had 100% specificity. A total of 30 isolates of group D streptococci were tested with immunological and biochemical identification systems. The overall sensitivity ranged from 14.2% (Phadebact Group D; Pharmacia Diagnostics) to 100% with Streptex (Wellcome; immunological) and Identicult A-E (Scott Laboratories, Inc.; biochemical). The results of this study suggest that some reagents can be used to provide rapid identification of gram-positive cocci from blood cultures 24 h earlier than standard methods.     

Comparison of a yellow latex reagent with other agglutination methods for the identification of Staphylococcus aureus.

A new commercial yellow latex agglutination reagent (Bacto-Staph) was compared with the slide and tube coagulase tests and three other commercial reagents for the identification of 283 Staphylococcus aureus and 54 non-S. aureus staphylococcal strains. Test sensitivities for the identification of S. aureus were as follows: tube coagulase, 99.6%; slide coagulase, 98.6%; Bacto-Staph, 99.6%; Staphylatex, 98.6%; Sero STAT Staph, 98.2%; and Staphyloslide, 97.5%. No false-positive reactions were observed with any of the commercial reagents.     

Evaluation of three commercial agglutination tests for the identification of Staphylococcus aureus.

Three commercially available rapid slide agglutination tests for the identification of Staphylococcus aureus were evaluated with 354 recent clinical isolates (165 strains of S. aureus). The test results of two latex agglutination products, SeroSTAT Staph (Scott Laboratories, Inc.) and Staphylatex (American Micro Scan), and one hemagglutination product, Staphyloslide (BBL Microbiology Systems), were compared with the results of the tube coagulase test, which was read at 4 h (4-h tube coagulase test) and, if negative, again after overnight incubation at room temperature (24-h tube coagulase test). Discrepancies between agglutination and tube coagulase identifications were resolved by use of the thermonuclease, mannitol fermentation, and slide coagulase tests. All sensitivities, specificities, predictive values of a positive result, and predictive values of a negative result for the three agglutination tests were at least 98.8% and comparable with the 4-h tube coagulase test. Best results were obtained with the 24-h tube coagulase test, which yielded one false-negative and no false-positive tests. Agglutination identifications may be performed on organisms taken directly from a primary plate when sufficient growth is present. Kit agglutination procedures yield rapid and reliable identifications and are easy to perform. This study also demonstrates the usefulness of the 24-h tube coagulase test.     

Problems with rapid agglutination methods for identification of Staphylococcus aureus when Staphylococcus saprophyticus is being tested.

Six rapid agglutination tests for identification of Staphylococcus aureus were evaluated by using 62 strains of S. aureus, 63 strains of S. saprophyticus, and 67 strains of other coagulase-negative staphylococci. S. saprophyticus was responsible for 19 of 26 false-positive results and 20 uninterpretable reactions. Thus, urinary staphylococcal isolates that are positive by rapid agglutination tests may require other confirmatory tests for the identification of possible S. saprophyticus.     

Evaluation of different disk diffusion/media combinations for detection of methicillin resistance in Staphylococcus aureus and coagulase-negative staphylococci

In order to find a disk diffusion method with both high sensitivity and specificity for determination of methicillin resistance primarily for S. aureus but also for coagulase-negative staphylococci we screened several methodological variants using a material of 66 S. aureus comprising of 11 methicillin-susceptible, 18 borderline-resistant, and 37 methicillin-resistant strains. Only four of the combinations studied performed with both high sensitivity and specificity. Two of these, the Columbia agar +4.5% NaCl and Mueller Hinton agar +2% NaCl combined with a 5 microg oxacillin disk, confluent inoculum and 24 h incubation at 35 degrees C were further evaluated using 105 MRSA and 91 mecA-negative S. aureus and 193 clinical isolates of coagulase-negative staphylococci. The Columbia agar +4.5% NaCl performed excellently for both S. aureus and coagulase-negative staphylococci. For Columbia agar +4.5% NaCl using a 5 microg oxacillin disk we suggest an interpretive zone diameter of R < or =15 mm and S > or =16 mm for S. aureus and R < or =24 mm and S >or =26 mm for coagulase-negative staphylococci. The Mueller Hinton agar +2% NaCl performed well for coagulase-negative staphylococci but for S. aureus at least three (3%) very major errors were found, making this method less attractive.     

Sensitivity and Specificity of an Improved Rapid Latex Agglutination Test for Identification of Methicillin-Sensitive and -Resistant Staphylococcus aureus Isolates

The performance of a second-generation rapid agglutination kit, Slidex Staph Plus (SSP; bioMérieux), was compared to those of the Slidex Staph (SS; bioMérieux), Staphaurex (SRX; Murex Diagnostics), and BBL Staphyloslide (BBL; Becton Dickinson) kits by using 508 clinical isolates composed of 150 methicillin-sensitive Staphylococcus aureus (MSSA) organisms, 154 methicillin-resistant S. aureus (MRSA) organisms, and 204 non-S. aureus Staphylococcus spp. Of the 508 isolates tested, 75% were fresh clinical isolates, with the remainder taken from five different freezer collections. All four agglutination tests had comparable sensitivities for MSSA and MRSA. However, the SS kit was significantly less specific (93.1%) than the three other tests (P > 0.05, McNemar test). These results demonstrate that the new rapid latex agglutination kit, SSP, was more specific for the identification of S. aureus than the previous version and performed comparably to the SRX and BBL kits.     

An evaluation of three rapid coagglutination tests: Sero-STAT, Accu-Staph, and Staphyloslide, for differentiating Staphylococcus aureus from other species of staphylococci

Three commercial coagglutination tests--Sero-STAT, Accu-Staph, and Staphyloslide--were performed in parallel with slide coagulase, tube coagulase, and thermostable nuclease tests on 100 methicillin-susceptible Staphylococcus aureus (MSS) strains, 100 methicillin-resistant S. aureus (MRS) strains, and 100 non-S. aureus staphylococcal strains (NSA). All three coagglutination tests showed sensitivities of 100% for MSS strains. For MRS strains, sensitivities were, respectively, 99%, 100%, and 99%. False-positive reactions were, respectively, 10%, 2%, and 2%. A marked difference in slide coagulase test sensitivity was found for MSS strains (79%) and MRS strains (14%). These findings suggest that the coagglutination tests may be less sensitive for detecting MRS strains than for detecting MSS strains and that these properties may be related to clumping factor reactivity. The high false-positive rate for Sero-STAT and even the 2% false-positive rate for Accu-Staph and Staphyloslide make clinical usefulness at this time somewhat problematic and debatable. In view of these findings, the authors prefer to retain the tube coagulase test and thermostable nuclease test for differentiation of S. aureus from non-S. aureus strains in their laboratory.