Biofilms: survival mechanisms of clinically relevant microorganisms

Candida biofilms and their role in infection

Pathogenic fungi in the genus Candida can cause both superficial and serious systemic disease, and are now recognized as major agents of hospital-acquired infection. Many Candida infections involve the formation of biofilms on implanted devices such as indwelling catheters or prosthetic heart valves. Biofilms of Candida albicans formed in vitro on catheter material consist of matrix-enclosed microcolonies of yeasts and hyphae, arranged in a bilayer structure. The biofilms are resistant to a range of antifungal agents currently in clinical use, including amphotericin B and fluconazole, and there appear to be multiple resistance mechanisms. Recent studies with mixed biofilms containing Candida and bacterial species suggest that extensive and striking interactions occur between the prokaryotic and eukaryotic cells in these adherent populations.

Bacterial outer membrane and cell wall penetration and cell destruction by polluting chemical agents and physical conditions

In the environment, bacteria and other microorganisms are subjected to a variety of constantly changing chemical and physical agencies. Chemical ones include antimicrobial compounds (both biocides and antibiotics), pollutants, drugs, cosmetic and pharmaceutical ingredients and pesticides. The physical agents include desiccation and drying, osmotic pressure, hydrostatic pressure, temperature and pH changes and radiations (ultraviolet, sunlight, ionizing). Bacteria must thus adapt to survive these inimicable conditions. Organisms such as bacterial spores usually survive, whereas other types of microorganisms may be much more susceptible. Depending on the type of organism, the bacterial cell wall, outer membrane or the spore outer layers may act as permeability barriers to the intracellular uptake of antibiotics and biocides. Some antibacterial agents interact with, and damage or modify, the outer components. Physical agencies are known to damage the cytoplasmic membrane or to produce alterations in DNA or proteins or enzymes. Nevertheless, significant damage to the cell wall or outer membrane may also occur. Four types of organisms are considered: cocci, mycobactria, Gram-negative bacteria and bacterial spores. The nature of the damage inflicted on, or in some cases prevented by, their outer cell layers is discussed for each type of organism.

Are there biofilm-specific physiological pathways beyond a reasonable doubt?

The development of surface-attached communities called biofilms is often thought to be a regulated developmental process involving profound biofilm-specific modifications. Despite intense scrutiny and a growing body of evidence correlating changes in gene expression with the switch from planktonic-to-biofilm lifestyle, an integrated view of biofilm formation is still missing.

Candida biofilms

In response to attachment to a surface, fungal cells produce biofilms, three-dimensional structures composed of cells surrounded by exopolymeric matrices. Surface attachment causes Candida albicans cells to enter a special physiological state in which they are highly resistant to antifungal drugs and express the drug efflux determinants CDR1, CDR2 and MDR1. C. albicans biofilms produced under different conditions differ in their cellular morphology and matrix content, which suggests that biofilms formed within a host, for example on indwelling medical devices, would also differ depending on the nature of the device and its location. The mechanisms by which surface attachment leads to biofilm formation are presently not understood.

Biofilm dispersal of Neisseria subflava and other phylogenetically diverse oral bacteria

Polystyrene petri dishes containing liquid medium were inoculated with single-cell suspensions of a fresh clinical isolate of Neisseria subflava and were incubated under conditions of low vibration. N. subflava colonies grew firmly attached to the surface of the dish, while the broth remained clear. Growing colonies released cells into the medium, resulting in the appearance of 10(2) to 10(4) small satellite colonies attached to the surface of the dish in an area adjacent to each mature colony after 24 h. Satellite colonies grew in patterns of streamers shaped like jets and flares emanating from mature colonies and pointing toward the center of the dish. This dispersal pattern evidently resulted from the surface translocation of detached biofilm cells by buoyancy-driven convection currents that were generated due to slight temperature gradients in the medium. Streamers of satellite colonies ranged from 2 to >40 mm in length. Satellite colonies in very long streamers were relatively uniform in size regardless of their distance from the mature colony, suggesting that mature colonies released single cells or small clusters of cells into the medium and that the detachment, surface translocation, and subsequent surface reattachment of released cells were a transitory process. Incubation of N. subflava single cells in a perfused biofilm fermentor resulted in a large spike of the number of CFU in the perfusate after 9.5 h of growth, consistent with a rapid release of cells into the medium. Biofilm colonies of several other phylogenetically diverse oral bacteria, including Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus, Streptococcus mitis, and a prevalent but previously uncultured oral Streptococcus sp., exhibited similar temperature-dependent dispersal patterns in broth culture. This in vitro spreading phenotype could be a useful tool for studying biofilm dispersal in these and other nonflagellated bacteria and may have physiological relevance to biofilm dispersal in the oral cavity.

Molecular interactions in biofilms

A biofilm may be defined as a microbially derived, sessile community characterized by cells that attach to an interface, embed in a matrix of exopolysaccharide, and demonstrate an altered phenotype. This review covers the current understanding of the nature of biofilms and the impact that molecular interactions may have on biofilm development and phenotype using the motile gram-negative rod Pseudomonas aeruginosa and the nonmotile gram-positive cocci Staphylococcus aureus as examples.

Antibiofilm approaches: prevention of catheter colonization

Bacteria frequently attach to medical devices such as intravascular catheters by forming sessile multicellular communities known as biofilms, which can be the source of persistent infections that are recalcitrant to systemic antibiotic therapy. As a result of this persistence, a number of technologies have been developed to prevent catheter-associated biofilm formation. Whereas the most straightforward approaches focus on impregnating catheter material with classical antimicrobial agents, these approaches are not universally effective, thereby underscoring the need for more potent and more sophisticated approaches to the prevention of catheter-related biofilm infections.