Biofilm-Based Infections in Long-term Care Facilities

Gianfranco Donelli; Claudia Vuotto

Disclosures

Future Microbiol. 2014;9(2):175-188. 

In This Article

Catheter-Related Bloodstream Infections

The early transfer of severely injured patients from intensive care units to LTCFs has given a new dynamic to the incidence of CRBSIs, mostly caused by the development of a microbial biofilm on both the inner and outer surfaces of vascular catheters. In fact, extraluminal biofilm is the major source of bloodstream infections in intravascular catheters within the first week of catheterization while, in both short- and long-term catheters, the intraluminal biofilm becomes the major cause after 1 week of implantation (Figure 3). The formation of this biofilm is considered the result of a close interaction among microorganisms, device surfaces and host proteins, and arise from two sequential events: skin colonization around the catheter insertion site by bacteria and/or fungi, and their migration along the catheter, where microbes attach to its polymeric surfaces and start to develop as biofilm.[89–91] In intravascular catheter-associated infections, the most involved microorganisms are coagulase-negative staphylococci, accounting for approximately 40%, Staphylococcus aureus (~20%) and fungi, particularly Candida species (~10%). The remaining 30% are represented by other Gram-positive (Enterococcus faecalis) and -negative (P. aeruginosa, E. coli, and so on) bacteria.[92]

Figure 3.

Field emission scanning electron microscopy micrograph (10,000×) of a polymicrobial biofilm grown in the lumen of a long-term central venous catheter removed from a patient recovered at the research hospital for neuromotor rehabilitation Fondazione Santa Lucia in Rome. The species identified by culture methods were Escherichia coli and Enterococcus faecalis.

Staphylococcus epidermidis, constituting a relevant component of the normal human skin flora and mucosal epithelia, is regarded as a harmless commensal inhabitant of these tissues, even if it can be easily introduced as a contaminant agent during the surgical implantation of an intravascular catheter,[93] meaning that it is the causative agent most frequently isolated from CRBSIs.

Apart from the prevalence of coagulase-negative staphylococci, S. aureus also has a pre-eminent role as a causative agent of CRBSIs. The emergence, in recent years, of methicillin-resistant S. aureus (MRSA) in general hospitals, particularly in intensive care units, has drawn attention to its spread in LTCFs. In fact, in the framework of the MOSAR study, data were provided on the prevalence of MRSA and associated risk factors for colonization in four European rehabilitation centers.[94] Of a total of 1204 patients, 105 (8.7%) were positive for MRSA and, among them, 74.3% had been implanted with an invasive medical device in the previous month. Since the early 1980s, electron microscopy observations showed that multilayered bacterial communities of staphylococci were encased in a slimy matrix[95] and adhered to intravascular catheters removed from patients.[96]

Molecular biology investigations added more detail to the different steps of biofilm development in both S. epidermidis and S. aureus. In S. aureus, and particularly in MRSA strains, the initial attachment to host extracellular matrix proteins is mediated by microbial surface components recognizing adhesive matrix molecules, such as FnBPs, while in S. epidermidis, and in most of the methicillin-resistant Staphylococcus aureus strains, the production of the exopolysaccharide poly-β-1,6-N acetyl-D-glucosamine is required for polysaccharide intercellular adhesin-dependent biofilm formation and encoded by the accessory intercellular adhesion operon.[97–102] Poly-β-1,6-N acetyl-D-glucosamine has been used as a target in an antibiofilm strategy based on the enzyme Dispersin B, which is able to disrupt the matrix of S. epidermidis, thus promoting antibiotic diffusion into the biofilm and then its killing activity.[103]S. aureus Bap is another surface protein found to be critical for biofilm formation in 5% of 350 S. aureus isolates from bovine mastitis, even if not found in all 75 S. aureus human strains analyzed.[104] So far, Bap has never been reported in S. aureus human clinical isolates.

Furthemore, the staphylococcal accessory regulator (sarA) and accessory gene regulator (agr) have been implicated in biofilm formation: the first induces attachment and early biofilm formation[105] and the latter leads to the upregulation of a number of virulence factors, and supports seeding dispersal in mature biofilms.[106,107]

In a very recent study on the molecular epidemiology of S. epidermidis, a set of 33 isolates causing CRBSI in hospitalized patients has been compared with a set of 33 commensal isolates, demonstrating that strains from CRBSI patients were more resistant to the majority of tested antibiotics and more often ica positive than the commensal ones, the majority therefore being strong biofilm producers.[108]

Enterococcus spp. are increasingly considered emerging causative agents of intravascular CRBSIs. In fact, a significant increase in nosocomial infections caused by enterococci has been recently observed in many countries,[109] this bacterial species being the fourth most common cause of bloodstream infections in Europe.[110] Hospital-acquired enterococcal bacteremias associated with central venous catheters have increased dramatically since the early 1980s.[111,112] All over the world, the ability of enterococci to form a biofilm on the polymeric surfaces of intravascular catheters, together with their intrinsic resistance to several antibiotics, resulted in the increasing spread of MDR enterococcal strains in clinical settings. As the biofilm grows and becomes mature, single or clustered bacteria detach from the top of the biofilm and, re-entering their planktonic mode of growth, spread to uncolonized areas, so repeating the biofilm development cycle on new surfaces.

The adherence factors involved in enterococcal colonization of intravascular catheters are:

  • Carbohydrates of the cell surface;[113]

  • Efa A protein;[114]

  • Ace (adhesin of collagen from enterococci) protein acting as a collagen-binding adhesin;[115]

  • AS surface protein.[116,117]

Furthermore, the cell surface protein (Esp) has been demonstrated to play a pivotal role in enterococcal biofilm formation,[118] its sequence being similar to the S. aureus surface protein Bap, critical in biofilm formation.[104]

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