Bacterial contamination of surgical suture resembles a biofilm

Abstract

Background: Although much attention is currently directed to studying microbial biofilms on a variety of surfaces, few studies are designed to study bacterial growth on surgical suture. The purpose of this study was to compare the kinetic development of Staphylococcus aureus and Enterococcus faecalis on five surgical suture materials and to clarify factors that might influence this growth.

Methods: Pure cultures of S. aureus and E. faecalis were incubated with five types of suture for four days using either tissue culture medium or a bacterial growth medium. Suture-associated bacteria were quantified daily. In selected experiments, the bacterial growth medium was supplemented with heparin, a substance known to promote S. aureus biofilm formation. The ultrastructure of S. aureus biofilm developing on braided suture was studied with scanning electron microscopy.

Results: Staphylococcus aureus and E. faecalis were recovered in greater numbers (typically p < 0.01) from braided than from monofilament suture, and the numbers of bacteria were greater (often p < 0.01) on sutures incubated in bacterial growth medium rather than tissue culture medium. Addition of heparin 1,000 U/mL to silk or braided polyglactin 910 suture incubated three days with S. aureus resulted in greater numbers of bacteria on day one but not on subsequent days. Scanning electron microscopy showed a maturing S. aureus biofilm that developed from small clusters of cells among amorphous material and fibrillar elements to larger clusters of cells that appeared covered by more consolidated extracellular material.

Conclusions: Bacterial growth was favored on braided vs. monofilament suture, and heparin enhanced bacterial adherence after day one, but not at subsequent times. Staphylococcus aureus adhered to suture material and formed a structure consistent with a bacterial biofilm.

FIG. 1.

Numbers of viable bacteria adherent to five suture materials (3-0 silk, 3-0 PDS*II, 3-0 polypropylene [Prolene^®], 3-0 braided polyglactin 910 [Vicryl^®], and 4-0 Vicryl^®) after inoculation with Staphylococcus aureus (A, B), Enterococcus faecalis VA1128 (C, D), or E. faecalis OG1RF (E, F) incubated in either tissue culture medium (A, C, E) or 66% tryptic soy broth supplemented with 0.2% glucose (B, D, F). Compared with braided suture (silk and polyglactin 910), fewer bacteria were recovered from monofilament suture. For clarity, significant differences are not highlighted on the graphs, but a difference of 0.7 log₁₀ (reflecting a five-fold difference) typically was significant at p < 0.01. Each data point represents the average of three to six suture segments.

FIG. 2.

Staphylococcus aureus cultivated overnight in 66% tryptic soy broth supplemented with 0.2% glucose in a six-well dish at 37°C with gentle rotation, showing the gross morphologic effect of heparin 1,000 U/mL on growth. The dishes in the upper photographs contain only S. aureus and medium, where unsupplemented broth has a uniform turbidity, and heparin supplementation fosters visible bacterial clumping. Lower photographs show three pieces of 3-0 braided polyglactin 910 suture added to each well, with markedly clumped bacteria obscuring the sutures in medium supplemented with heparin.

FIG. 3.

Effect of heparin 1,000 U/mL on Staphylococcus aureus biofilms cultivated three days on silk suture in serum-supplemented tissue culture medium (A) or 66% tryptic soy broth supplemented with 0.2% glucose (TSB/glu) (B) or on braided polyglactin 910 suture in TSB/glu broth (C). Each bar represents three or four sutures. * Increased compared with corresponding TSB/glu (p < 0.01).

FIG. 4.

Scanning electron microscopy of 3-0 braided polyglactin 910 suture incubated in 66% tryptic soy broth supplemented with 0.2% glucose with Staphylococcus aureus for one day (A, B), two days (C, D), or three days (E, F). (A) Clusters of cocci associated with amorphous material and often connected by fibrillar strands. (B) Cocci adherent to amorphous material; asterisk highlights cluster of cocci difficult to resolve with the electron beam because they reside between strands of braided suture. (C, D) More developed biofilm with cocci embedded in amorphous material and fibrillar elements. Some cocci appear to have a denser covering of extracellular material (D, arrow). (E, F) More consolidated biofilm with some cocci containing relatively smooth cell walls, whereas others appear more embedded in the dense amorphous material (F). All scale bars are 3 micrometers, except panel F, where it is 0.5 micrometers.