The smooth black and dry red colonies were considered as indeterminate result. As displayed in Table 2, slime production was detected in 33 (30.6%) isolates that produced characteristic black colonies of dry crystalline consistency while 32 isolates (29.6%) produced smooth red colonies. Also, 27 isolates Tivantinib (25%) produced black colonies with smooth shiny surfaces were interpreted as indeterminate result. Furthermore, 16 isolates (14.8%) produced red colonies with dry rough consistency were described as indeterminate result. Totally, 76 isolates (70.4%) were found to be slime producers with variable degrees.MTP method was reported to have high specificity, sensitivity, and positive predictive value [13]. As shown in Table 3, using MTP method, 96.3% of the isolates were biofilm producers with variable production levels.

This high prevalence agreed with that reported by Seo et al. [29] who noted that approximately 80% of their isolates produced slime with the MTP method but with variable degrees of production. On the contrary, lower prevalence rates of biofilm producers among S. aureus and S. epidermidis bovine mastitis isolates were also reported to be 29.41% [30] and 37.5% [8]. Comparison of results obtained by CRA method versus that of MTP method was declared in Table 4. Out of 32 biofilm negative Staphylococcus isolates (13 S. aureus and 19 CNS) by CRA method, 28 isolates were positive by MTP method but with different degrees of production (4 strong, 8 moderate, and 16 weak) while only 4 CNS isolates were negative by both methods.

Congo red agar (CRA) method showed little correlation with MTP method where only 76 (70.4%) of the isolates were positive by both methods with 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. Although CRA method was easy to perform and less time consuming, however, our findings confirm what was reported by Mathur et al. [13] that CRA method cannot be recommended for detection of biofilm formation by Staphylococci alone. Difference between results of CRA and MTP methods can be attributed to the fact that phenotypic expression of biofilm formation is highly sensitive to in vitro conditions and hence can be detected variably by different methods. Also, both tests measure the same phenomenon but in different ways.

CRA has been Cilengitide used as indirect indicator of polysaccharide production [16, 31].Additionally, Indian ink was used to stain wet preparations of Staphylococcus isolates; all the 108 isolates except 4 biofilm negative CNS showed distinct halo transparent zones denoting to EPS layer surrounding the cells (Figure 3). The size of the hallo zones enlarged with the ability of the isolates to produce biofilms.