Densely adherent growth mode, rather than extracellular polymer substance matrix build-up ability, contributes to high resistance of Staphylococcus epidermidis biofilms to antibiotics.
OBJECTIVE
(i) To evaluate the role of the adherent growth mode and extracellular polymer substance build-up in biofilm resistance to antibiotics. (ii) To re-assess various mechanisms leading to biofilm resistance to antibiotics.


METHODS
We compared the biofilm MICs, biofilm MBCs using the viable count method, biofilm MBCs based on broth recovery methods and minimum biofilm eradication concentrations (MBECs) of antistaphylococcal antibiotics for multilayer biofilms formed by 'biofilm-positive' S. epidermidis strains and monolayer biofilms formed by their 'biofilm-negative' mutants/variants. Bacterial densities and the quantity of persister cells in both multilayer and monolayer biofilms were assessed to evaluate their roles in biofilm resistance.


RESULTS
Monolayer and multilayer biofilms presented similar susceptibilities to multiple antibiotics, based on biofilm MIC, broth recovery-based biofilm MBC and MBEC results. Multilayer biofilms demonstrated higher viable count-based MBCs than monolayer biofilms. Both monolayer and multilayer biofilms had very high bacterial densities of approximately 10(11-12) cfu/mL. Persister cells were found in both monolayer and multilayer biofilms, but not in planktonic cultures at log phase. The presence of persister cells in monolayer and multilayer biofilms appeared to be strain and antibiotic dependent.


CONCLUSIONS
The adherent growth mode, rather than the ability to build up a typical multilayer biofilm structure, contributes to the high resistance of biofilms to antibiotics, and therefore might be the main virulence factor of coagulase-negative staphylococci (CoNS) with respect to antibiotic resistance. The presence of persister cells in CoNS biofilms plays an important role in antibiotic resistance. Growth at high bacterial densities is another significant factor in biofilm resistance.
Qu
Yue
Y
School of Applied Sciences, RMIT University, Plenty Road, Bundoora 3083, Victoria, Australia.
Daley
Andrew J
AJ
Istivan
Taghrid S
TS
Rouch
Duncan A
DA
Deighton
Margaret A
MA
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001422:Bacterial Adhesion; D001426:Bacterial Proteins; D018441:Biofilms / Q000187:drug effects* / Q000254:growth & development; D001704:Biopolymers / Q000378:metabolism*; D024881:Drug Resistance, Bacterial; D006801:Humans; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D013212:Staphylococcus epidermidis / Q000187:drug effects* / Q000502:physiology*; D014157:Transcription Factors
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D001704:Biopolymers; C101050:ModE protein, bacteria; D014157:Transcription Factors