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.
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.
Research Laboratory on Biocompatibility of Implant Materials, Rizzoli Orthopedic Institute, via di Barbiano 1110, Bologna 40136, Italy. lucio.montanaro@alma.unibo.it
Carla Renata
CR
Arciola
Davide
D
Campoccia
Simonetta
S
Gamberini
Marina
M
Cervellati
Elena
E
Donati
Lucio
L
Montanaro
D001681:Biological Assay / Q000379:methods*; D003124:Colorimetry; D004396:Coloring Agents / Q000378:metabolism*; D003224:Congo Red / Q000378:metabolism*; D005838:Genotype; D017351:N-Acetylglucosaminyltransferases / Q000032:analysis* / Q000235:genetics; D016133:Polymerase Chain Reaction / Q000379:methods; D013212:Staphylococcus epidermidis / Q000235:genetics / Q000378:metabolism*
D004396:Coloring Agents; D003224:Congo Red; D017351:N-Acetylglucosaminyltransferases; C037649:N-acetyllactosaminide beta-1,6-N-acetylglucosaminyltransferase