Comparative assessment of antibiotic susceptibility of coagulase-negative staphylococci in biofilm versus planktonic culture as assessed by bacterial enumeration or rapid XTT colorimetry.
OBJECTIVE
To quantitatively compare the antibiotic susceptibility of biofilms formed by the coagulase-negative staphylococci (CoNS) Staphylococcus epidermidis and Staphylococcus haemolyticus with the susceptibility of planktonic cultures.


METHODS
Several CoNS strains were grown planktonically or as biofilms to determine the effect of the mode of growth on the level of susceptibility to antibiotics with different mechanisms of action. The utility of a new, rapid colorimetric method that is based on the reduction of a tetrazolium salt (XTT) to measure cell viability was tested by comparison with standard bacterial enumeration techniques. A 6 h kinetic study was performed using dicloxacillin, cefazolin, vancomycin, tetracycline and rifampicin at the peak serum concentration of each antibiotic.


RESULTS
In planktonic cells, inhibitors of cell wall synthesis were highly effective over a 3 h period. Biofilms were much less susceptible than planktonic cultures to all antibiotics tested, particularly inhibitors of cell wall synthesis. The susceptibility to inhibitors of protein and RNA synthesis was affected by the biofilm phenotype to a lesser degree. Standard bacterial enumeration techniques and the XTT method produced equivalent results both in biofilms and planktonic assays.


CONCLUSIONS
This study provides a more accurate comparison between the antibiotic susceptibilities of planktonic versus biofilm populations, because the cell densities in the two populations were similar and because we measured the concentration required to inhibit bacterial metabolism rather than to eradicate the entire bacterial population. While the biofilm phenotype is highly resistant to antibiotics that target cell wall synthesis, it is fairly susceptible to antibiotics that target RNA and protein synthesis.
Centro de Engenharia Biológica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal.
Nuno
N
Cerca
Silvia
S
Martins
Filipe
F
Cerca
Kimberly K
KK
Jefferson
Gerald B
GB
Pier
Rosário
R
Oliveira
Joana
J
Azeredo
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D018441:Biofilms / Q000187:drug effects* / Q000254:growth & development; D003030:Coagulase / Q000378:metabolism; D003124:Colorimetry / Q000379:methods; D008826:Microbial Sensitivity Tests; D010933:Plankton / Q000187:drug effects* / Q000254:growth & development; D013212:Staphylococcus epidermidis / Q000187:drug effects* / Q000201:enzymology / Q000254:growth & development; D041162:Staphylococcus haemolyticus / Q000187:drug effects* / Q000201:enzymology / Q000254:growth & development; D013778:Tetrazolium Salts
D000900:Anti-Bacterial Agents; D003030:Coagulase; D013778:Tetrazolium Salts