2.4. Statistical AnalysisData were presented as count and percentage. Sensitivity and specificity of CRA method were calculated according to Ilstrup, [22] using 2 �� 2 table and MTP method as gold standard. The kappa coefficient test was also used to determine the agreement between the results obtained by the CRA method.3. Results3.1. Bacterial Isolation and IdentificationThe selleck kinase inhibitor 108 Staphylococcus isolates were identified to be 40 S. aureus and 68 CNS. 3.2. Biofilm Formation3.2.1. Slime Production on Congo Red Agar (CRA) The results of biofilm production by S. aureus and CNS using CRA method are demonstrated in Table 2. About 70.4% of the isolates were positive for biofilm production with varied degree. Out of 40 S. aureus strains, 32.5%, 35%, and 32.5% were strong, intermediate, and negative biofilm producers, respectively.

Among the 68 CNS strains, 29.5%, 42.6%, and 27.9% were strong, intermediate, and negative biofilm producers, respectively. Totally, from the 108 Staphylococcus isolates, 33 (30.6%), 27 (25%), 16 (14.8%), and 32 (29.6%) were dry black, smooth black, dry red, and smooth red, respectively. Morphology of all types of colonies is showed in Figure 1. Figure 1Screening of biofilm (slime) producer Staphylococci using Congo red agar plate method ((a) dry black colonies; (b) smooth black colonies; (c) dry red colonies; (d) smooth red colonies).Table 2Biofilm formation in S. aureus and CNS isolates according to CRA method.3.2.2. Biofilm Formation by MTP Method As shown in Figure 2 and Table 3, approximately 96.3% of the isolates were biofilm producers with MTP method, although production level varied.

The biofilm production of S. aureus isolates (100%) was slightly higher than that of CNS isolates (94.1%). In the biofilm positive S. aureus strains, 52.5% of isolates were strong producers, while 27.5% and 20% were moderate and weak, respectively. Out of the biofilm positive CNS isolates, 44%, 30.9%, and 19.2% were strong, moderate, and weak, respectively.Figure 2 2 Microtiter plate method showing none, strong, moderate, and weak biofilm producers differentiated by crystal violet stain in 96-well tissue culture plate.Table 3Biofilm formation in S. aureus and CNS isolates according to MTP method.Comparison of results obtained by CRA method versus that of MTP method is declared in Table 4.

Out of 32 biofilm negative Staphylococcus isolates by CRA method, 28 isolates were positive by MTP method but with different degrees of production (4 strong, 8 moderate, 16 weak) and only 4 CNS isolates were true negative by both methods. CRA method Brefeldin_A showed little correlation with MTP assay where only 76 (70.4%) of the isolates were positive by both methods, kappa coefficient 0.167 slight agreement. The sensitivity and specificity of CRA method versus MTP method as gold standard were calculated to be 73.1% and 100%, respectively.Table 4CRA method versus MTP method for detection of biofilm formation by S. aureus and CNS isolates.3.2.3.