title: Identification of Staphylococcus aureus: DNase and Mannitol salt agar improve the efficiency of the tube coagulase test.
abstract: BACKGROUND
  The ideal identification of Staphylococcus aureus clinical isolates requires a battery of tests and this is costly in resource limited settings. In many developing countries, the tube coagulase test is usually confirmatory for S. aureus and is routinely done using either human or sheep plasma. This study evaluated Mannitol salt agar and the deoxyribonuclease (DNase) test for improving the efficiency of the tube coagulase test in resource limited settings. The efficiency of human and sheep plasma with tube coagulase tests was also evaluated.
  
  
  METHODS
  One hundred and eighty Gram positive, Catalase positive cocci occurring in pairs, short chains or clusters were subjected to growth on Mannitol salt agar, deoxyribonuclease and tube coagulase tests. Of these, isolates that were positive for at least two of the three tests (n = 60) were used to evaluate the performance of the tube coagulase test for identification of S. aureus, using PCR-amplification of the nuc gene as a gold standard.
  
  
  RESULTS
  Human plasma was more sensitive than sheep plasma for the tube coagulase test (sensitivity of 91% vs. 81% respectively), but both plasmas had very low specificity (11% and 7% respectively). The sensitivity and specificity of the tube coagulase test (human plasma) was markedly improved when Mannitol salt agar and DNase were introduced as a tri-combination test for routine identification of Staphylococcus aureus (100% specificity and 75% sensitivity). The specificity and sensitivity of Mannitol salt agar/DNase/tube coagulase (sheep plasma) combination was 100% and 67%, respectively.
  
  
  CONCLUSIONS
  The efficiency of the tube coagulase test can be markedly improved by sequel testing of the isolates with Mannitol salt agar, DNase and Tube coagulase. There is no single phenotypic test (including tube coagulase) that can guarantee reliable results in the identification of Staphylococcus aureus.
authors: Department of Medical Microbiology, School of Biomedical Sciences, Makerere University College of Health Sciences, Upper Mulago hill road, Kampala, Uganda.
authors: David P
authors: DP
authors: Kateete
authors: Cyrus N
authors: CN
authors: Kimani
authors: Fred A
authors: FA
authors: Katabazi
authors: Alfred
authors: A
authors: Okeng
authors: Moses S
authors: MS
authors: Okee
authors: Ann
authors: A
authors: Nanteza
authors: Moses L
authors: ML
authors: Joloba
authors: Florence C
authors: FC
authors: Najjuka
mesh_terms: D000362:Agar; D000818:Animals; D001769:Blood; D003030:Coagulase / Q000378:metabolism*; D003851:Deoxyribonucleases / Q000378:metabolism*; D003906:Developing Countries; D006801:Humans; D008353:Mannitol / Q000378:metabolism*; D016133:Polymerase Chain Reaction; D011237:Predictive Value of Tests; D012756:Sheep; D013203:Staphylococcal Infections / Q000175:diagnosis / Q000382:microbiology; D013211:Staphylococcus aureus / Q000145:classification / Q000302:isolation & purification* / Q000502:physiology; D014454:Uganda
chemical_list: D003030:Coagulase; D008353:Mannitol; D000362:Agar; D003851:Deoxyribonucleases