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

Evaluation of new agglutination test for identification of oxacillin-susceptible and oxacillin-resistant Staphylococcus aureus

A new agglutination test (Monostaph +; Bionor, Skien, Norway) has been developed. This new agglutination test has been compared with two other agglutination tests for the identification of 128 isolates of Staphylococcus aureus and 82 coagulase-negative staphylococci. The sensitivities of both Monostaph + and Pastorex Staph-Plus were excellent (98.7 and 97.4%, respectively) in detection of oxacillin-resistant Staphylococcus aureus. The specificity was 96.4% (two Staphylococcus epidermidis isolates and one Staphylococcus hominis isolate were false positive).

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

The performance of a recently introduced Staphylococcus 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-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus 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.

Comparison of various methods and reagents for species identification of Staphylococcus aureus positive or negative for the mecA gene

The reliability of various methods for species identification of Staphylococcus aureus was evaluated. A total of 135 coagulase-positive (SA) or -negative (SS) staphylococcal isolates were tested, including methicillin-resistant (MR) and -susceptible (MS) strains. When the nuc gene which encodes the S. aureus thermonuclease (TNase) was amplified in a multiplex PCR simultaneously with the mecA gene which encodes for the MR-associated penicillin-binding protein 2a of staphylococci, the nuc amplification showed full agreement with the results of the coagulase test. TNase detected by an enzymatic method or as protein in a sandwich ELISA identified S. aureus with nearly the same precision as the PCR. The Staphylase, Monostaph and Staphaurex agglutination kits were all reliable for identification of MSSA, but not for MRSA. Most of the negative MRSA strains were identified by the Pastorex agglutination kit, in which reagents for fibrinogen receptor and protein A detection have been supplemented with antibodies for capsular polysaccharides of the serotypes 5 and 8. These results show that detection of the nuc gene or its TNase product is highly reliable for identification of both MRSA and MSSA strains, while various widely used agglutination kits do not show the same reliability for identification of MRSA strains.

Comparative evaluation of five agglutination techniques and a new miniaturized system for rapid identification of methicillin-resistant strains of Staphylococcus aureus

The speciation of methicillin-resistant Staphylococcus aureus (MRSA) poses a significant diagnostic problem when rapid identification methods such as slide agglutination tests, are used, because of the high proportion of false-negative reactions. 150 perfectly identified MRSA strains were tested on 5 commonly used agglutination reagents ("Bacto staph latex test", "Monostaph", "Pastorex staph", "Staphaurex", and "Staphyslide test") in comparison with a new micromethod ("RAPIDEC staph") which detects a type of staphylocoagulase within 2 hours by a fluorescence test. The "RAPIDEC staph" reagent enabled identification of all the MRSA while the agglutination tests gave poorer results: "Monostaph" correctly identified 64.6% of strains, "Staphyslide", 59.3%, "Bacto staph latex test", 44.6%, "Pastorex staph", 38.6% and "Staphaurex", 28.6%. These results show that agglutination slide tests are not reliable enough for the identification of MRSA which are more and more encountered in hospital wards. The authors recommend not to use slide agglutination methods. They suggest the tube test for coagulase which is the reference technique, although it is time-consuming and not well standardized. The results of this evaluation encourage the use of the "RAPIDEC staph" reagent since it is an easy-to-use, reliable technique for the rapid identification of Staphylococcus aureus.

Coagulase testing compared with commercial kits for routinely identifying Staphylococcus aureus

Five commercial Staphylococcus aureus identification kits--Staphaurex (Wellcome), Staphylase (Oxoid), Staphyslide (bioMèrieux), Biostaph (Medlabs) and Bacto Latex (Difco)--were evaluated for the routine identification of S aureus from primary plates in the routine microbiology laboratory. Comparison was made with two methods of tube coagulase testing and five slide methods for detecting clumping factor (slide coagulase testing). Performances were assessed for two groups of organisms, staphylococcal species alone and a combined staphylococcal and non-staphylococcal species group. The effects of growth on selective media and storage of isolates at room temperature and 4 degrees C were investigated. Selective media cannot be recommended, nor can storage of isolates before testing. Ranked according to efficiency value with the combined staphylococcal and non-staphylococcal species group, the kits and coagulase methods performed as follows (the figures in parentheses are the efficiency values for the staphylococcal group alone): tube coagulase reference method 100% (100%), tube coagulase SJH method 99% (99%), Staphaurex 94% (97%), Staphylase 93% (96%), slide coagulase method No 4 93% (94%), slide coagulase method No 5 93% (93%), Bacto Latex 92% (95%), Staphyslide 92% (95%), and Biostaph 87% (91%). It is concluded that a commercial S aureus identification kit should not replace tube coagulase testing for the routine identification of the organism from primary plates and that, even the kits with the best performances, have little advantage over a good slide coagulase test method.

Staphylococcus aureus strains which are not identified by rapid agglutination methods are of capsular serotype 5

A total of 183 recent Staphylococcus aureus clinical isolates were tested with three commercially available rapid agglutination methods. The capsular polysaccharide type and resistance to oxacillin of these isolates were also determined. Seven isolates were not identified correctly by agglutination methods. All isolates not identified by the rapid methods were of capsular serotype 5, and of these isolates, six were resistant to oxacillin. The results suggest that these agglutination kits can be improved by the use of antibodies reactive with S. aureus capsular polysaccharide.

Evaluation of a direct identification method for Staphylococcus aureus from blood culture broth

We evaluated the reliability of Staphaurex (Wellcome Diagnostics, Dartford, England) for the direct identification of staphylococci from blood culture broth with evidence of positivity and a suggestive Gram-stained smear. Our evaluation indicates that this application is of limited sensitivity, thus reducing the value of a negative test. However, since the test is highly specific, a positive test is significant in predicting the isolation of Staphylococcus aureus.