Research Article

Evaluation of Biofilm Formation in Coagulase Negative Staphylococci on Various Congo Red Agar Medium


  • Duygu Öcal
  • Alper Tekeli
  • İştar Dolapçı

Received Date: 26.09.2021 Accepted Date: 27.10.2021 J Ankara Univ Fac Med 2022;75(1):8-15


In this study, it was aimed to evaluate the performance of different Congo red agar (CRA) media and its modifications in comparison with the microplate method in the demonstration of biofilm formation in coagulase negative staphylococcus (CoNS) isolates.

Materials and Methods:

A total of 80 CoNS isolates, including 24 (30%) catheter colonizers, 13 (16.3%) bloodstream infection (BSI) agents, 20 (25%) catheter-related bloodstream infection (CR-BSI) agents and 23 (28.8%) obtained from the nasal cavities of hospital staff, were evaluated in the study. In the determination of biofilm formation, CRA mediums (CRA G (2% glucose added), CRA GN (2% glucose and 1.5% NaCl added), CRA GNV (2% glucose, 1.5% NaCl 0.5 µg/mL vancomycin added) and microplate method were used. Sensitivity, specificity, positive predictive and negative predictive values were calculated by comparing with the microplate method.


Staphylococcus hominis was the most frequently detected species (36.25%), followed by Staphylococcus epidermidis (35%), Staphylococcus haemolyticus (13.75%), Staphylococcus capitis (11.25%), Staphylococcus saprophyticus (2.5%), and Staphylococcus warneri (1.25%). Thirty-four (42.5%) isolates were detected by CRA method, 57 (71.25%) [18 (22.5%) weak, 24 (30%) medium strength, 15 (18.75%) strong biofilm] isolates were detected by microplate method. Six (7.5%) of CoNS agents of BSI, 20 (25%) of CoNS agents of CRBSI, 18 (22.5%) of those with catheter colonization, 13 (16.3%) of those isolated from nasal cavities formed a biofilm of varying strengths. The sensitivities of the CRA, CRA G, CRA GN and CRA GNV methods were 59.6%, 59.6%, 59.6% and 58.2%, and their specificities were 100%, respectively.


Biofilm formation in staphylococcal species is affected by various environmental conditions and media content. To assess biofilm formation in all staphylococcal isolates, no superiority of modified CRA media was found over the original CRA. Microplate method can be recommended as a general screening method for the detection of biofilm-producing bacteria, compared to CRA and its modified forms.

Keywords: Biofilm, Coagulase Negative Staphylococci, Congo Red Agar and Modified Forms, Microplate Method

Full Text (Turkish)