Characteristics of Biofilms on Indwelling Medical Devices
Biofilms on indwelling medical devices may be composed of gram-positive or gram-negative bacteria or yeasts. Bacteria commonly isolated from these devices include the gram-positive Enterococcus faecalis,Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus viridans; and the gram-negative Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, and Pseudomonas aeruginosa. These organisms may originate from the skin of patients or health-care workers, tap water to which entry ports are exposed, or other sources in the environment. Biofilms may be composed of a single species or multiple species, depending on the device and its duration of use in the patient. Urinary catheter biofilms may initially be composed of single species, but longer exposures inevitably lead to multispecies biofilms. A distinguishing characteristic of biofilms is the presence of extracellular polymeric substances, primarily polysaccharides, surrounding and encasing the cells. These polysaccharides, which have been visualized by scanning electron microscopy (Figure 1), appear either as thin strands connecting the cells to the surface and one another or as sheets of amorphous material on a surface. Most biofilm volume is actually composed of this extracellular polymeric substance rather than cells, a fact that has been confirmed by ruthenium red staining and transmission electron microscopy. This biofilm matrix may act as a filter, entrapping minerals or host-produced serum components. Biofilms are both tenacious and highly resistant to antimicrobial treatment; Anwar et al. showed that treatment with levels of tobramycin far in excess of the MIC reduced biofilm cell counts for P. aeruginosa by approximately 2 logs, while the same dosage provided a >8-log decrease in planktonic cells of this organism.
Figure 1. Scanning electron micrograph of Staphylococcus biofilm on the inner surface of a needleless connector.
Photograph by Janice Carr, Centers for Disease Control and Prevention, Antlanta, GA USA.
Emerging Infectious Diseases. 2001;7(2) © 2001 Centers for Disease Control and Prevention (CDC)