Capacity for biofilm formation by Methicilin Resistant Staphylococcus aureus may be linked to genotypic, clinical, and antibiotic resistance characteristics

Background:  Biofilm formation by Staphylococcus aureus is an important virulence factor.  Preliminary data from other investigators suggest that biofilm production may depend on the clonal lineage of the S. aureus isolate (BMC Microbiol 2009; 9:229).   Clinically, increased capacity for biofilm formation likely leads to enhanced capacity to cause device related infections and may increase antibiotic resistance and delayed clinical response to therapy.      

Objective:  To evaluate the biofilm forming capacity of clinical MRSA isolates based on molecular genotyping and correlate this with clinical characteristics and phenotypic drug resistance patterns.   

Methods: Clinical MRSA isolates from the Ohio State Health Network MRSA Surveillance Project were screened for biofilm forming capacity.  The quantity of biofilm formation was quantified using crystal violet method.  Briefly, MRSA isolates were grown overnight in TSB without glucose and incubated.   The resulting biofilms were washed and stained with crystal violet.   Absorbance of crystal violet was then quantified at 540 nm using a mictrotiter plate reader.   Cut off values for weak biofilm was <=0.3 A540nm, intermediated >0.3 to <0.7 A540nm, and strong is >=0.7 A540nm. Additional data regarding S. aureus isolate antibiotic susceptibilities, molecular genotypes were collected as well as characteristics of patient clinical status infected with MRSA. 

Results:  A total of 50 clinical MRSA isolates (blood 26, skin and soft tissue 10, respiratory 7, and other 7) were evaluated. Number of isolates in each PFGE types included were USA 100 (9), 200 (2), 300 (13), 500 (3), 600 (2), 800 (4), 900 (1), 1000 (3), 1100 (1), and Brazilian (8).   Of these, 24% (7/50) isolates were weak biofilm producers; 48% (24/50) were intermediate and 38% (19/50) were strong biofilm producers. Among the 19 strong biofilm producers, 12 (63%) were blood isolates, 3 (16%) were respiratory isolates, 2 (11%) were skin and soft tissue isolates and 2 (11%) were others.  Comparison of A540 values for bloodstream isolates compared to non-bloodstream isolates was statistically different at p=0.0136 using a t test.   The MRSA isolates varied in the number of antibiotic classes that the isolates were resistant to (range from 1 to 7 antibiotic classes).  A trend towards increased antibiotic resistance was observed in the intermediate and strong biofilm producer group.

Conclusions: There was a clear correlation between MRSA isolated from the bloodstream and biofilm forming capacity.  MRSA isolates from bloodstream infections appear to be more efficient biofilm producers than non-bloodstream isolates. The more drug resistant MRSA isolates may also be stronger biofilm producers.  PFGE and SCC mec A types appeared not to correlate with biofilm formation and more sensitive sequencing methods may be required to distinguish clonal differences.