Detection of slime production by means of an optimised Congo red agar plate test based on a colourimetric scale in Staphylococcus epidermidis clinical isolates genotyped for ica locus

• Published in: Biomaterials 2002-11, Vol.23 (21), p.4233-4239
• Main Authors: Arciola, Carla Renata, Campoccia, Davide, Gamberini, Simonetta, Cervellati, Marina, Donati, Elena, Montanaro, Lucio
• Format: Article
• Language: English
• Subjects: Biological Assay - methods; Biomaterial-associated infections; Colorimetry; Coloring Agents - metabolism; Congo Red - metabolism; Congo red agar; Genotype; ica Genes; Microtiter plate method; N-Acetylglucosaminyltransferases - analysis; N-Acetylglucosaminyltransferases - genetics; Polymerase chain reaction (PCR); Polymerase Chain Reaction - methods; Slime; Staphylococcus epidermidis; Staphylococcus epidermidis - genetics; Staphylococcus epidermidis - metabolism
• ISSN: 0142-9612; 1878-5905
• DOI: 10.1016/S0142-9612(02)00171-0

This investigation was conduced on a collection of 113 S. epidermidis strains isolated from biomaterial-associated infections. All strains were examined both for the presence of icaA and icaD genes responsible for slime synthesis by a PCR method and for the in vitro slime production ability by the Congo red agar (CRA) plate test. In the present study, the original CRA test was optimised adopting a six-colour reference scale for a fine classification of colonies colours. The six-colour tones of the scale were as follows: very black (vb), black (b), almost black (ab), which were considered as positive results, and bordeaux (brd), red (r), and very red (vr), interpreted as negative. 57.5% of all the strains were found to be icaA icaD-positive as well as slime-forming onto CRA, exhibiting the following colonies colours: vb (35.4%); b (15.9%); ab (6.2%). The percentage of icaA icaD-negative strains was 42.5% and all of them were negative onto CRA: brd (19.5%), r (14.2%), vr (8.8%). The comparison of colour classification with the information on ica genes confirmed the validity of the scale adopted, providing support to the criteria used for a correct interpretation of the colonies colour during the execution of the CRA test. Overall these results indicate a fine consistency between these two experimental methods and a good reliability of CRA plate test, especially when this is supported by a colourimetric scale.