Biofilms--matrix-enclosed microbial accretions that adhere to biological or non-biological surfaces--represent a significant and incompletely understood mode of growth for bacteria. Biofilm formation appears early in the fossil record (approximately 3.25 billion years ago) and is common throughout a diverse range of organisms in both the Archaea and Bacteria lineages, including the 'living fossils' in the most deeply dividing branches of the phylogenetic tree. It is evident that biofilm formation is an ancient and integral component of the prokaryotic life cycle, and is a key factor for survival in diverse environments. Recent advances show that biofilms are structurally complex, dynamic systems with attributes of both primordial multicellular organisms and multifaceted ecosystems. Biofilm formation represents a protected mode of growth that allows cells to survive in hostile environments and also disperse to colonize new niches. The implications of these survival and propagative mechanisms in the context of both the natural environment and infectious diseases are discussed in this review.

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Bacterial biofilms: from the natural environment to infectious diseases.

Though biofilms were first described by Antonie van Leeuwenhoek, the theory describing the biofilm process was not developed until 1978. We now understand that biofilms are universal, occurring in aquatic and industrial water systems as well as a large number of environments and medical devices relevant for public health. Using tools such as the scanning electron microscope and, more recently, the confocal laser scanning microscope, biofilm researchers now understand that biofilms are not unstructured, homogeneous deposits of cells and accumulated slime, but complex communities of surface-associated cells enclosed in a polymer matrix containing open water channels. Further studies have shown that the biofilm phenotype can be described in terms of the genes expressed by biofilm-associated cells. Microorganisms growing in a biofilm are highly resistant to antimicrobial agents by one or more mechanisms. Biofilm-associated microorganisms have been shown to be associated with several human diseases, such as native valve endocarditis and cystic fibrosis, and to colonize a wide variety of medical devices. Though epidemiologic evidence points to biofilms as a source of several infectious diseases, the exact mechanisms by which biofilm-associated microorganisms elicit disease are poorly understood. Detachment of cells or cell aggregates, production of endotoxin, increased resistance to the host immune system, and provision of a niche for the generation of resistant organisms are all biofilm processes which could initiate the disease process. Effective strategies to prevent or control biofilms on medical devices must take into consideration the unique and tenacious nature of biofilms. Current intervention strategies are designed to prevent initial device colonization, minimize microbial cell attachment to the device, penetrate the biofilm matrix and kill the associated cells, or remove the device from the patient. In the future, treatments may be based on inhibition of genes involved in cell attachment and biofilm formation.

Biofilms: Survival Mechanisms of Clinically Relevant Microorganisms

Guideline for Prevention of Surgical Site Infection, 1999 ☆ ☆☆ ★ ★★

The “Guideline for Prevention of Surgical Site Infection, 1999” presents the Centers for Disease Control and Prevention (CDC)'s recommendations for the prevention of surgical site infections (SSIs), formerly called surgical wound infections. This two-part guideline updates and replaces previous guidelines.Part I, “Surgical Site Infection: An Overview,” describes the epidemiology, definitions, microbiology, pathogenesis, and surveillance of SSIs. Included is a detailed discussion of the pre-, intra-, and postoperative issues relevant to SSI genesis.

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Guideline for Prevention of Surgical Site Infection, 1999

Biomaterials are being used with increasing frequency for tissue substitution. Complex devices such as total joint replacements and the total artificial heart represent combinations of polymers and metal alloys for system and organ replacement. The major barriers to the extended use of these devices are the possibility of bacterial adhesion to biomaterials, which causes biomaterial-centered infection, and the lack of successful tissue integration or compatibility with biomaterial surfaces. Interactions of biomaterials with bacteria and tissue cells are directed not only by specific receptors and outer membrane molecules on the cell surface, but also by the atomic geometry and electronic state of the biomaterial surface. An understanding of these mechanisms is important to all fields of medicine and is derived from and relevant to studies in microbiology, biochemistry, and physics. Modifications to biomaterial surfaces at an atomic level will allow the programming of cell-to-substratum events, thereby diminishing infection by enhancing tissue compatibility or integration, or by directly inhibiting bacterial adhesion.

https://science.sciencemag.org/content/sci/237/4822/1588.full.pdf

Biomaterial-centered infection: microbial adhesion versus tissue integration.

The adherence of coagulase-negative staphylococci to smooth surfaces was assayed by measuring the optical densities of stained bacterial films adherent to the floors of plastic tissue culture plates. The optical densities correlated with the weight of the adherent bacterial film (r = 0.906; P less than 0.01). The measurements also agreed with visual assessments of bacterial adherence to culture tubes, microtiter plates, and tissue culture plates. Selected clinical strains were passed through a mouse model for foreign body infections and a rat model for catheter-induced endocarditis. The adherence measurements of animal passed strains remained the same as those of the laboratory-maintained parent strain. Spectrophotometric classification of coagulase-negative staphylococci into nonadherent and adherent categories according to these measurements had a sensitivity, specificity, and accuracy of 90.6, 80.8, and 88.4%, respectively. We examined a previously described collection of 127 strains of coagulase-negative staphylococci isolated from an outbreak of intravascular catheter-associated sepsis; strains associated with sepsis were more adherent than blood culture contaminants and cutaneous strains (P less than 0.001). We also examined a collection of 84 strains isolated from pediatric patients with cerebrospinal fluid (CSF) shunts; once again, pathogenic strains were more adherent than were CSF contaminants (P less than 0.01). Finally, we measured the adherence of seven endocarditis strains. As opposed to strains associated with intravascular catheters and CSF shunts, endocarditis strains were less adherent than were saprophytic strains of coagulase-negative staphylococci. The optical densities of bacterial films adherent to plastic tissue culture plates serve as a quantitative model for the study of the adherence of coagulase-negative staphylococci to medical devices, a process which may be important in the pathogenesis of foreign body infections. Images

Adherence of coagulase-negative staphylococci to plastic tissue culture plates: a quantitative model for the adherence of staphylococci to medical devices.

On occasion, a patient may have two or more clinical cultures yielding a coagulase-negative staphylococcus If these multiple isolates have the same phenotype, one might conclude that the same strain was reisolated from the patient, indicating its persistent and pathological presence. We examined the validity of this conclusion when we applied a number of characterizing systems to a collection of 143 isolates of coagulase-negative staphylococci collected during an outbreak of intravascular catheter-associated sepsis. The probability of classifying two random isolates as the same phenotype or species was as follows: P = 0.356 for phage typing, P = 0.348 for Baird-Parker biotyping, P = 0.346 for the API STAPH-IDENT (Analytab Products) system, P = 0.327 for Bentley et al. biotyping, and P = 0.077 for antimicrobial susceptibility patterns. Although antimicrobial susceptibility patterns had the lowest probability, a variability in test results of 7.7% and a tendency for strains to have similar antibiograms effectively raised the probability to P = 0.897. The combination of the API STAPH-IDENT with antibiograms resulted in a probability of P = 0.037 to P = 0.147. When all of the above methods were used together a probability of P = 0.014 was achieved. Five patients had isolates from two or more blood cultures spaced more than 1 day apart that were identical by all of the above criteria, thus confirming prolonged bacteremia. The collection was also examined for the incidence of slime production. Slime production was not associated with any of the above groups, but was associated with symptomatic infections (P less than 0.05) and gentamicin resistance (P less than 0.01). Slime production was strain stable and was of assistance in typing strains of coagulase-negative staphylococci.

Characterization of clinically significant strains of coagulase-negative staphylococci

Clinical studies performed by us and others have found an association between slime production and strains of coagulase-negative staphylococci that infect indwelling medical devices. By serial low-speed centrifugation of broth cultures we have isolated a stable, weakly adherent strain (RP62A-NA) from a strongly adherent, slime-producing, pathogenic strain of Staphylococcus epidermidis sensu stricto (RP62A, ATCC 35984). We obtained a second strain from RP62A-NA (RP62A-NAR) by serial subculture of glass-adherent cells of RP62A-NA. All three strains had the same pattern of biochemical reactions, antimicrobial susceptibilities, and plasmid analysis. Transmission electron micrograph sections stained with the mucopolysaccharide-specific stain alcian blue demonstrated that the adherent strains RP62A and RP62A-NAR were covered with an extracellular coat of polysaccharide-rich material. In contrast, the nonadherent RP62A-NA strain lacked this external coat. All three strains were used in a mouse model of foreign body infection and a rat model of catheter-induced infective endocarditis. The adherence characteristics of isolates of RP62A and RP62A-NA recovered from experimental animals were relatively stable, although we noted a slight but a significant increase in the adherence of RP62A-NA isolates recovered from the foreign body model. The adherence characteristics of RP62A-NAR isolates recovered from infected animals were variable; in general these isolates were less adherent than the laboratory strain of RP62A-NAR. In both models the 50% infective dose (calculated by the Reed and Muench method) was three times greater for the RP62A-NA strain than for the RP62A strain. The phenotypic expression of slime production is subject to both in vitro and in vivo variation and could play a role in the pathogenesis of foreign body infection.

https://iai.asm.org/content/iai/55/12/2870.full.pdf

Phenotypic variation of Staphylococcus epidermidis slime production in vitro and in vivo.

Inactivation of fibronectin (Fn) binding by insertional mutagenesis of Streptococcus sanguis with Tn916 reduces virulence of this bacterium in the rat model of infective endocarditis (IE). Transconjugants were screened for Fn adherence using an ELISA adherence test. One transconjugant had a decreased adherence to immobilized Fn. Southern hybridization demonstrated that the insertion occurred only once in this mutant. The parent strain and mutant strain JL113 were used as challenge strains in a rat endocarditis model. These experiments demonstrated that the mutant had a reduced ability (P less than 0.05) to produce IE. Spontaneous excision of Tn916 from JL113 produced strains identical to both the parental and mutant phenotypes. One strain (JLR-19) that retained the mutant phenotype and one (JLR-15) that regained the parental phenotype for Fn binding were tested for their ability to produce IE. These strains demonstrated that the ability to bind Fn and to produce IE were correlated after Tn916 excision. The reduced virulence of the mutant suggested that adherence of S. sanguis to immobilized Fn plays an important role in the production of IE.

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The role of fibronectin binding in the rat model of experimental endocarditis caused by Streptococcus sanguis.

The adherence of Streptococcus sanguis to specific receptors exposed or deposited at the site of endothelial damage may play an important role in the development of infective endocarditis. Adherence of the Challis strain of S. sanguis to gelatin (or collagen) and gelatin-binding components of plasma was examined with an enzyme-linked immunosorbent assay. S. sanguis adhered poorly to immobilized gelatin and to molecular or fibrillar collagen. However, in the presence of fresh human plasma, the adherence of S. sanguis to all three substrates increased as much as eightfold. Removal of gelatin-binding proteins eliminates the ability of plasma to enhance adherence of S. sanguis to the substrates. Addition of purified human plasma fibronectin (Fn) to the absorbed plasma restored the adherence-promoting ability in a dose-dependent manner. A similar dose-dependent increase in S. sanguis adherence was observed when increasing concentrations of Fn alone were added to the gelatin-coated assay wells. S. sanguis adherence to immobilized fibronectin could not be inhibited by preincubating either the bacteria or the gelatin-coated assay wells with Fn or by including excess soluble Fn in the assay mixture. Studies with peptides purified from trypsin digests of Fn indicated that the 160- to 180-kilodalton (kDa) fragments which retain both the gelatin-binding and the cell-binding regions of the intact molecule support adherence of S. sanguis to gelatin. The 160- to 180-kDa fragments inhibited the interaction of S. sanguis with immobilized Fn. In contrast, intact Fn and the 31-kDa amino-terminal fragment were unable to inhibit the adherence when used in equivalent or greater molar amounts. These in vitro results suggest that in the presence of whole plasma, S. sanguis binds to immobilized gelatin or collagen via Fn bound to the immobilized substrates. Our finding that adherence of S. sanguis to immobilized Fn can occur in the presence of large concentrations of Fn, whether in plasma or purified, indicates that a S. sanguis-binding domain is cryptic in the Fn molecule while in solution and is exposed by a conformational change when the Fn becomes bound to gelatin-coated plastic. The ability of peptide fragments of Fn to inhibit S. sanguis adherence is consistent with this hypothesis. Images

W A Simpson

Papers

Adherence of coagulase-negative staphylococci to plastic tissue culture plates: a quantitative model for the adherence of staphylococci to medical devices.

Characterization of clinically significant strains of coagulase-negative staphylococci

Phenotypic variation of Staphylococcus epidermidis slime production in vitro and in vivo.

The role of fibronectin binding in the rat model of experimental endocarditis caused by Streptococcus sanguis.

Adherence of Streptococcus sanguis to conformationally specific determinants in fibronectin.