Bacterial biofilms: a common cause of persistent infections

Bacteria that attach to surfaces aggregate in a hydrated polymeric matrix of their own synthesis to form biofilms. Formation of these sessile communities and their inherent resistance to antimicrobial agents are at the root of many persistent and chronic bacterial infections. Studies of biofilms have revealed differentiated, structured groups of cells with community properties. Recent advances in our understanding of the genetic and molecular basis of bacterial community behavior point to therapeutic targets that may provide a means for the control of biofilm infections.

Antibiotic resistance of bacteria in biofilms

Bacteria that adhere to implanted medical devices or damaged tissue can encase themselves in a hydrated matrix of polysaccharide and protein, and form a slimy layer known as a biofilm. Antibiotic resistance of bacteria in the biofilm mode of growth contributes to the chronicity of infections such as those associated with implanted medical devices. The mechanisms of resistance in biofilms are different from the now familiar plasmids, transposons, and mutations that confer innate resistance to individual bacterial cells. In biofilms, resistance seems to depend on multicellular strategies. We summarise the features of biofilm infections, review emerging mechanisms of resistance, and discuss potential therapies.

Survival strategies of infectious biofilms

Modern medicine is facing the spread of biofilm-related infections. Bacterial biofilms are difficult to detect in routine diagnostics and are inherently tolerant to host defenses and antibiotic therapies. In addition, biofilms facilitate the spread of antibiotic resistance by promoting horizontal gene transfer. We review current concepts of biofilm tolerance with special emphasis on the role of the biofilm matrix and the physiology of biofilm-embedded cells. The heterogeneity in metabolic and reproductive activity within a biofilm correlates with a non-uniform susceptibility of enclosed bacteria. Recent studies have documented similar heterogeneity in planktonic cultures. Nutritional starvation and high cell density, two key characteristics of biofilm physiology, also mediate antimicrobial tolerance in stationary-phase planktonic cultures. Advances in characterizing the role of stress response genes, quorum sensing and phase variation in stationary-phase planktonic cultures have shed new light on tolerance mechanisms within biofilm communities.

Role of antibiotic penetration limitation in Klebsiella pneumoniae biofilm resistance to ampicillin and ciprofloxacin

The penetration of two antibiotics, ampicillin and ciprofloxacin, through biofilms developed in an in vitro model system was investigated. The susceptibilities of biofilms and corresponding freely suspended bacteria to killing by the antibiotics were also measured. Biofilms of Klebsiella pneumoniae were developed on microporous membranes resting on agar nutrient medium. The susceptibilities of planktonic cultures and biofilms to 10 times the MIC were determined. Antibiotic penetration through biofilms was measured by assaying the concentration of antibiotic that diffused through the biofilm to an overlying filter disk. Parallel experiments were performed with a mutant K. pneumoniae strain in which beta-lactamase activity was eliminated. For wild-type K. pneumoniae grown in suspension culture, ampicillin and ciprofloxacin MICs were 500 and 0.18 microgram/ml, respectively. The log reductions in the number of CFU of planktonic wild-type bacteria after 4 h of treatment at 10 times the MIC were 4.43 +/- 0.33 and 4.14 +/- 0.33 for ampicillin and ciprofloxacin, respectively. Biofilms of the same strain were much less susceptible, yielding log reductions in the number of CFU of -0.06 +/- 0.06 and 1.02 +/- 0.04 for ampicillin and ciprofloxacin, respectively, for the same treatment. The number of CFU in the biofilms after 24 h of antibiotic exposure was not statistically different from the number after 4 h of treatment. Ampicillin did not penetrate wild-type K. pneumoniae biofilms, whereas ciprofloxacin and a nonreactive tracer (chloride ion) penetrated the biofilms quickly. The concentration of ciprofloxacin reached the MIC throughout the biofilm within 20 min. Ampicillin penetrated biofilms formed by a beta-lactamase-deficient mutant. However, the biofilms formed by this mutant were resistant to ampicillin treatment, exhibiting a 0.18 +/- 0.07 log reduction in the number of CFU after 4 h of exposure and a 1.64 +/- 0.33 log reduction in the number of CFU after 24 h of exposure. Poor penetration contributed to wild-type biofilm resistance to ampicillin but not to ciprofloxacin. The increased resistance of the wild-type strain to ciprofloxacin and the mutant strain to ampicillin and ciprofloxacin could not be accounted for by antibiotic inactivation or slow diffusion since these antibiotics fully penetrated the biofilms. These results suggest that some other resistance mechanism is involved for both agents.

Spatial physiological heterogeneity in Pseudomonas aeruginosa biofilm is determined by oxygen availability

The role of oxygen availability in determining the local physiological activity of Pseudomonas aeruginosa growing in biofilms was investigated. Biofilms grown in an ambient-air environment expressed approximately 1/15th the alkaline phosphatase specific activity of planktonic bacteria subjected to the same phosphate limitation treatment. Biofilms grown in a gaseous environment of pure oxygen exhibited 1.9 times the amount of alkaline phosphatase specific activity of air-grown biofilms, whereas biofilms grown in an environment in which the air was replaced with pure nitrogen prior to the inducing treatment did not develop alkaline phosphatase activity. Frozen cross sections of biofilms stained for alkaline phosphatase activity with a fluorogenic stain demonstrated that alkaline phosphatase activity was concentrated in distinct bands adjacent to the gaseous interfaces. These bands were approximately 30 micron thick with biofilms grown in air, 2 micron thick with biofilms grown in pure nitrogen, and 46 micron thick with biofilms grown in pure oxygen. Overall biofilm thickness ranged from approximately 117 to approximately 151 micron. Measurements with an oxygen microelectrode indicated that oxygen was depleted locally within the biofilm and that the oxygen-replete zone was of a dimension similar to that of the biologically active zone, as indicated by alkaline phosphatase induction. These experiments revealed marked spatial physiological heterogeneity within P. aeruginosa biofilms in which active protein synthesis was restricted by oxygen availability to the upper 30 micron of the biofilm. Such physiological heterogeneity has implications for microbial ecology and for understanding the reduced susceptibilities of biofilms to antimicrobial agents.

Quorum sensing in Pseudomonas aeruginosa controls expression of catalase and superoxide dismutase genes and mediates biofilm susceptibility to hydrogen peroxide

Quorum sensing (QS) governs the production of virulence factors and the architecture and sodium dodecyl sulphate (SDS) resistance of biofilm-grown Pseudomonas aeruginosa. P. aeruginosa QS requires two transcriptional activator proteins known as LasR and RhlR and their cognate autoinducers PAI-1 (N-(3-oxododecanoyl)-L-homoserine lactone) and PAI-2 (N-butyryl-L-homoserine lactone) respectively. This study provides evidence of QS control of genes essential for relieving oxidative stress. Mutants devoid of one or both autoinducers were more sensitive to hydrogen peroxide and phenazine methosulphate, and some PAI mutant strains also demonstrated decreased expression of two superoxide dismutases (SODs), Mn-SOD and Fe-SOD, and the major catalase, KatA. The expression of sodA (encoding Mn-SOD) was particularly dependent on PAI-1, whereas the influence of autoinducers on Fe-SOD and KatA levels was also apparent but not to the degree observed with Mn-SOD. beta-Galactosidase reporter fusion results were in agreement with these findings. Also, the addition of both PAIs to suspensions of the PAI-1/2-deficient double mutant partially restored KatA activity, while the addition of PAI-1 only was sufficient for full restoration of Mn-SOD activity. In biofilm studies, catalase activity in wild-type bacteria was significantly reduced relative to planktonic bacteria; catalase activity in the PAI mutants was reduced even further and consistent with relative differences observed between each strain grown planktonically. While wild-type and mutant biofilms contained less catalase activity, they were more resistant to hydrogen peroxide treatment than their respective planktonic counterparts. Also, while catalase was implicated as an important factor in biofilm resistance to hydrogen peroxide insult, other unknown factors seemed potentially important, as PAI mutant biofilm sensitivity appeared not to be incrementally correlated to catalase levels.

Biofilm maturity studies indicate sharp debridement opens a time- dependent therapeutic window

OBJECTIVE

To investigate the hypothesis that newly formed wound biofilms (or bioburdens) are more susceptible to antimicrobial treatment.

METHOD

Four separate and distinct models were performed by four separate biofilm research laboratories to evaluate the resistance of biofilms to antimicrobial treatments over time. These included a drip-flow biofilm model along with a hydrodebridement study, a porcine skin punch biopsy ex vivo model, a mouse chronic wound model and clinical longitudinal debridement study.

RESULTS

All four models showed that, within the first 24 hours, the biofilm community was more susceptible to the selected antibiotics, and after maturing for up to 48 hours became increasingly tolerant. In each model, there was at least a 24-hour period in which the biofilms were more resistant to antibiotics. Each of the models utilised showed a significant decrease in the resistance of the biofilm/ burden to gentamicin for up to 24 hours with a confidence interval of at least 95%. The resistance increased in each of the models by 48 hours and reached original resistance levels by 72 hours.

CONCLUSION

These data suggest the principles of biofilm-based wound care, along with the use of serial debridement to continually remove mature biofilm, followed by biofilm wound management strategies, including topical antibiotics while the bioburden is still immature and more susceptible, are valid.

Theoretical aspects of antibiotic diffusion into microbial biofilms

Antibiotic penetration into microbial biofilm was investigated theoretically by the solution of mathematical equations describing various combinations of the processes of diffusion, sorption, and reaction. Unsteady material balances on the antibiotic and on a reactive or sorptive biomass constituent, along with associated boundary and initial conditions, constitute the mathematical formulations. Five cases were examined: diffusion of a noninteracting solute; diffusion of a reversibly sorbing, nonreacting solute; diffusion of an irreversibly sorbing, nonreacting solute; diffusion of a stoichiometrically reacting solute; and diffusion of a catalytically reacting solute. A noninteracting solute was predicted to penetrate biofilms of up to 1 mm in thickness relatively quickly, within a matter of seconds or minutes. In the case of a solute that does not sorb or react in the biofilm, therefore, the diffusion barrier is not nearly large enough to account for the reduced susceptibility of biofilms to antibiotics. Reversible and irreversible sorption retards antibiotic penetration. On the basis of data available in the literature at this point, the extent of retardation of antibiotic diffusion due to sorption does not appear to be sufficient to account for reduced biofilm susceptibility. A catalytic (e.g., enzymatic) reaction, provided it is sufficiently rapid, can lead to severe antibiotic penetration failure. For example, calculation of beta-lactam penetration indicated that the reaction-diffusion mechanism may be a viable explanation for failure of certain of these agents to control biofilm infections. The theory presented in this study provides a framework for the design and analysis of experiments to test these mechanisms of reduced biofilm susceptibility to antibiotics.

Direct measurement of chlorine penetration into biofilms during disinfection

Transient chlorine concentration profiles were measured in biofilms during disinfection by use of a microelectrode developed for this investigation. The electrode had a tip diameter of ca. 10 microm and was sensitive to chlorine in the micromolar range. The biofilms contained Pseudomonas aeruginosa and Klebsiella pneumoniae. Chlorine concentrations measured in biofilms were typically only 20% or less of the concentration in the bulk liquid. Complete equilibration with the bulk liquid did not occur during the incubation time of 1 to 2 h. The penetration depth of chlorine into the biofilm and rate of penetration varied depending on the measurement location, reflecting heterogeneity in the distribution of biomass and in local hydrodynamics. The shape of the chlorine profiles, the long equilibration times, and the dependence on the bulk chlorine concentration showed that the penetration was a function of simultaneous reaction and diffusion of chlorine in the biofilm matrix. Frozen cross sections of biofilms, stained with a redox dye and a DNA stain, showed that the area of chlorine penetration overlapped with nonrespiring zones near the biofilm-bulk fluid interface. These data indicate that the limited penetration of chlorine into the biofilm matrix is likely to be an important factor influencing the reduced efficacy of this biocide against biofilms as compared with its action against planktonic cells.

Predictive factors in chronic graft-versus-host disease in patients with aplastic anemia treated by marrow transplantation from HLA-identical siblings

One hundred ten of 175 patients with aplastic anemia conditioned by cyclophosphamide had sustained engraftment of marrow from human leukocyte antigen (HLA)-identical siblings and lived for more than 6 months. Forty-nine of the 110 patients developed chronic graft-versus-host disease between 85 and 464 days. Ninety-seven patients are alive from 1.4 to 11 years after engraftment; 13 died between 208 and 726 days. Twenty of the 36 surviving patients with chronic graft-versus-host disease have Karnofsky performance scores of 100%, 7 of 90%, 5 of 80%, 1 of 70%, 2 of 60%, and 1 of 40%. Our analysis, using a binary logistic regression model, identified three factors predicting chronic graft-versus-host disease: moderate to severe acute graft-versus-host disease with an estimated relative risk of 11.65; increasing patient age; and the use of viable donor buffy coat cells in addition to the marrow to prevent graft rejection. The last two factors were significant only in patients without acute graft-versus-host disease.

A review of experimental measurements of effective diffusive permeabilities and effective diffusion coefficients in biofilms

Experimental measurements of effective diffusive permeabilities and effective diffusion coefficients in biofilms are reviewed. Effective diffusive permeabilities, the parameter appropriate to the analysis of reaction-diffusion interactions, depend on solute type and biofilm density. Three categories of solute physical chemistry with distinct diffusive properties were distinguished by the present analysis. In order of descending mean relative effective diffusive permeability (De/Daq) these were inorganic anions or cations (0.56), nonpolar solutes with molecular weights of 44 or less (0.43), and organic solutes of molecular weight greater than 44 (0.29). Effective diffusive permeabilities decrease sharply with increasing biomass volume fraction suggesting a serial resistance model of diffusion in biofilms as proposed by Hinson and Kocher (1996). A conceptual model of biofilm structure is proposed in which each cell is surrounded by a restricted permeability envelope. Effective diffusion coefficients, which are appropriate to the analysis of transient penetration of nonreactive solutes, are generally similar to effective diffusive permeabilities in biofilms of similar composition. In three studies that examine diffusion of very large molecular weight solutes (>5000) in biofilms, the average ratio of the relative effective diffusion coefficient of the large solute to the relative effective diffusion coefficient of either sucrose or fluorescein was 0.64, 0.61, and 0.36. It is proposed that large solutes are effectively excluded from microbial cells, that small solutes partition into and diffuse within cells, and that ionic solutes are excluded from cells but exhibit increased diffusive permeability (but decreased effective diffusion coefficients) due to sorption to the biofilm matrix.

Protective role of catalase in Pseudomonas aeruginosa biofilm resistance to hydrogen peroxide

The role of the two known catalases in Pseudomonas aeruginosa in protecting planktonic and biofilm cells against hydrogen peroxide (H(2)O(2)) was investigated. Planktonic cultures and biofilms formed by the wild-type strain PAO1 and the katA and katB catalase mutants were compared for their susceptibility to H(2)O(2). Over the course of 1 h, wild-type cell viability decreased steadily in planktonic cells exposed to a single dose of 50 mM H(2)O(2), whereas biofilm cell viability remained at approximately 90% when cells were exposed to a flowing stream of 50 mM H(2)O(2). The katB mutant, lacking the H(2)O(2)-inducible catalase KatB, was similar to the wild-type strain with respect to H(2)O(2) resistance. The katA mutant possessed undetectable catalase activity. Planktonic katA mutant cultures were hypersusceptible to a single dose of 50 mM H(2)O(2), while biofilms displayed a 10-fold reduction in the number of culturable cells after a 1-h exposure to 50 mM H(2)O(2). Catalase activity assays, activity stains in nondenaturing polyacrylamide gels, and lacZ reporter genes were used to characterize the oxidative stress responses of planktonic cultures and biofilms. Enzyme assays and catalase activity bands in nondenaturing polyacrylamide gels showed significant KatB catalase induction occurred in biofilms after a 20-min exposure to H(2)O(2), suggesting that biofilms were capable of a rapid adaptive response to the oxidant. Reporter gene data obtained with a katB::lacZ transcriptional reporter strain confirmed katB induction and that the increase in total cellular catalase activity was attributable to KatB. Biofilms upregulated the reporter in the constant presence of 50 mM H(2)O(2), while planktonic cells were overwhelmed by a single 50 mM dose and were unable to make detectable levels of beta-galactosidase. The results of this study demonstrated the following: the constitutively expressed KatA catalase is important for resistance of planktonic and biofilm P. aeruginosa to H(2)O(2), particularly at high H(2)O(2) concentrations; KatB is induced in both planktonic and biofilm cells in response to H(2)O(2) insult, but plays a relatively small role in biofilm resistance; and KatB is important to either planktonic cells or biofilm cells for acquired antioxidant resistance when initial levels of H(2)O(2) are sublethal.

Biofilm penetration and disinfection efficacy of alkaline hypochlorite and chlorosulfamates

AIMS

The purpose of this study was to compare the efficacy, in terms of bacterial biofilm penetration and killing, of alkaline hypochlorite (pH 11) and chlorosulfamate (pH 5.5) formulations.

METHODS AND RESULTS

Two species biofilms of Pseudomonas aeruginosa and Klebsiella pneumoniae were grown by flowing a dilute medium over inclined stainless steel slides for 6 d. Microelectrode technology was used to measure concentration profiles of active chlorine species within the biofilms in response to treatment at a concentration of 1000 mg total chlorine l(-1). Chlorosulfamate formulations penetrated biofilms faster than did hypochlorite. The mean penetration time into approximately 1 mm-thick biofilms for chlorosulfamate (6 min) was only one-eighth as long as for the same concentration of hypochlorite (48 min). Chloride ion penetrated biofilms rapidly (5 min) with an effective diffusion coefficient in the biofilm that was close to the value for chloride in water. Biofilm bacteria were highly resistant to killing by both antimicrobial agents. Biofilms challenged with 1000 mg l(-1) alkaline hypochlorite or chlorosulfamate for 1 h experienced 0.85 and 1.3 log reductions in viable cell numbers, respectively. Similar treatment reduced viable numbers of planktonic bacteria to non-detectable levels (log reduction greater than 6) within 60 s. Aged planktonic and resuspended laboratory biofilm bacteria were just as susceptible to hypochlorite as fresh planktonic cells.

CONCLUSION

Chlorosulfamate transport into biofilm was not retarded whereas hypochlorite transport clearly was retarded. Superior penetration by chlorosulfamate was hypothesized to be due to its lower capacity for reaction with constituents of the biofilm. Poor biofilm killing despite direct measurement of effective physical penetration of the antimicrobial agent into the biofilm demonstrates that bacteria in the biofilm are protected by some mechanism other than simple physical shielding by the biofilm matrix.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study lends support to the theory that the penetration of antimicrobial agents into microbial biofilms is controlled by the reactivity of the antimicrobial agent with biofilm components. The finding that chlorine-based biocides can penetrate, but fail to kill, bacteria in biofilms should motivate the search for other mechanisms of protection from killing by antimicrobial agents in biofilms.

Spatial variations in growth rate within Klebsiella pneumoniae colonies and biofilm

The use of acridine orange to visualize and quantify spatial variations in growth rate within Klebsiella pneumoniae colonies and biofilm was investigated. Bacterial colonies supported on polycarbonate filter membranes were grown on R2A agar plates. Some colonies were sampled for cell enumeration, while others were cryoembedded, sectioned, and stained with the fluorescent nucleic acid stain acridine orange. Spatial patterns of fluorescent color and intensity with depth in the colony were quantified using confocal microscopy and image analysis of stained cross sections. Colonies sampled in the midexponential phase were thin (20 microns), had high average specific growth rates (> 1 h-1), and had all the cells stained bright orange. Colonies sampled after more than 24 h of growth were thick (> 200 microns) and were growing slowly (mu < 0.15 h-1). These older colonies were characterized by distinct bands of orange at the colony edges and a dark green center. Stained biofilm cross sections displayed a similar orange band at the biofilm-bulk fluid interface and a green interior. Colony-average specific growth rates, determined by calculating the local slope of the cell accumulation versus time data, were correlated with colony-average fluorescence intensities. There was no correlation between average specific growth rate and orange or green intensity individually, but growth rate did correlate with the orange:green intensity ratio (r2 = 0.57). The resulting regression was used to predict specific growth rate profiles within colonies. These profiles indicated that bacteria were growing rapidly near the air and agar interfaces and more slowly in the center of the colonies when thicker than about 30 microns. The dimension of the orange bands ranged from 10 to 30 microns, which may indicate the thickness of growing regions. The inherent variability associated with this technique suggests that it is best applied in single species systems and that the results should be regarded as qualitative in nature.

A model of biofilm detachment

A general mathematical framework for modeling biofilm detachment is presented. The approach is founded on a material balance on biomass that equates the detachment rate to the product of a detachment frequency and a detaching particle mass. The model provides a theoretical basis for deriving many of the empirical detachment rate expressions in common use and can thus lend some insight into their physical and biological significance. By allowing for variation in the detachment frequency with depth in the biofilm, the model permits derivation of detachment expressions that reflect a dependence on chemical or physiological gradients in the biofilm. Analysis of literature data sets from two different biofilm systems suggests, in both cases, that detachment is a growth-associated phenomenon.

Nonuniform spatial patterns of respiratory activity within biofilms during disinfection

Fluorescent stains in conjunction with cryoembedding and image analysis were applied to demonstrate spatial gradients in respiratory activity within bacterial biofilms during disinfection with monochloramine. Biofilms of Klebsiella pneumoniae and Pseudomonas aeruginosa grown together on stainless steel surfaces in continuous-flow annular reactors were treated with 2 mg of monochloramine per liter (influent concentration) for 2 h. Relatively little biofilm removal occurred as evidenced by total cell direct counts. Plate counts (of both species summed) indicated an average 1.3-log decrease after exposure to 2 mg of monochloramine per liter. The fluorogenic redox indicator 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and the DNA stain 4',6-diamidino-2-phenylindole (DAPI) were used to differentiate respiring and nonrespiring cells in biofilms. Epifluorescence micrographs of frozen biofilm cross sections clearly revealed gradients of respiratory activity within biofilms in response to monochloramine treatment. These gradients in specific respiratory activity were quantified by calculating the ratio of CTC and DAPI intensities measured by image analysis. Cells near the biofilm-bulk fluid interface lost respiratory activity first. After 2 h of biocide treatment, greater respiratory activity persisted deep in the biofilm than near the biofilm-bulk fluid interface.

Effect of catalase on hydrogen peroxide penetration into Pseudomonas aeruginosa biofilms

The penetration of hydrogen peroxide into biofilms formed by wild-type and catalase-deficient Pseudomonas aeruginosa strains was measured using microelectrodes. A flowing stream of hydrogen peroxide (50 mM, 1 h) was unable to penetrate or kill wild-type biofilms but did penetrate and partially kill biofilms formed by an isogenic strain in which the katA gene was knocked out. Catalase protects aggregated bacteria by preventing full penetration of hydrogen peroxide into the biofilm.

Reduced susceptibility of thin Pseudomonas aeruginosa biofilms to hydrogen peroxide and monochloramine

Pseudomonas aeruginosa attached to alginate gel beads in sparse, thin biofilms exhibited reduced susceptibility to monochloramine and hydrogen peroxide compared with planktonic cells of the same micro-organism. Disinfection rate coefficients for planktonic bacteria averaged 0.551 mg(-1)min(-1) for monochloramine and 3.1 x 10(-4)l mg(-1) min(-1) for hydrogen peroxide. The corresponding values for 24-h-old biofilm cells were 0.291 mg min(-1) and 9.2 x 10(-5) 1 mg(-1) min(-1) for monochloramine and hydrogen peroxide, respectively. Several pieces of evidence support the interpretation that the reduced susceptibility of biofilm was not due simply to inadequate delivery of the antimicrobial agent to the local environment of the attached cells. No correlation between biofilm susceptibility and biofilm initial areal cell density was observed. Rapid delivery of hydrogen peroxide to the attachment surface, and subsequently to the interior, of the alginate gel beads was visualized by a direct experimental technique. Theoretical analysis of unsteady diffusion and diffusion reaction interactions also argued against any significant delay or barrier to antimicrobial or oxygen delivery. It was hypothesized that new genes are expressed when bacteria attach to a surface and begin to form a biofilm and that some of the resulting gene products reduce the susceptibility of the cell to antimicrobial agents including oxidative biocides such as monochloramine and hydrogen peroxide.

Spatial patterns of alkaline phosphatase expression within bacterial colonies and biofilms in response to phosphate starvation

The expression of alkaline phosphatase in response to phosphate starvation was shown to be spatially and temporally heterogeneous in bacterial biofilms and colonies. A commercial alkaline phosphatase substrate that generates a fluorescent, insoluble product was used in conjunction with frozen sectioning techniques to visualize spatial patterns of enzyme expression in both Klebsiella pneumoniae and Pseudomonas aeruginosa biofilms. Some of the expression patterns observed revealed alkaline phosphatase activity at the boundary of the biofilm opposite the place where the staining substrate was delivered, indicating that the enzyme substrate penetrated the biofilm fully. Alkaline phosphatase accumulated linearly with time in K. pneumoniae colonies transferred from high-phosphate medium to low-phosphate medium up to specific activities of 50 mumol per min per mg of protein after 24 h. In K. pneumoniae biofilms and colonies, alkaline phosphatase was initially expressed in the region of the biofilm immediately adjacent to the carbon and energy source (glucose). In time, the region of alkaline phosphatase expression expanded inward until it spanned most, but not all, of the biofilm or colony depth. In contrast, expression of alkaline phosphatase in P. aeruginosa biofilms occurred in a thin, sharply delineated band at the biofilm-bulk fluid interface. In this case, the band of activity never occupied more than approximately one-sixth of the biofilm. These results are consistent with the working hypothesis that alkaline phosphatase expression patterns are primarily controlled by the local availability of either the carbon and energy source or the electron acceptor.

Role of electrostatic interactions in cohesion of bacterial biofilms

Significant decreases in the apparent viscosity of a bacterial biofilm suspension were measured following addition of sodium, potassium, magnesium, or calcium salts, whereas iron salts increased the viscosity. Electrostatic interactions contribute to biofilm cohesion and iron cations are potent crosslinkers of the biofilm matrix.

Biofilm accumulation model that predicts antibiotic resistance of Pseudomonas aeruginosa biofilms

A computer model of biofilm dynamics was adapted to incorporate the activity of an antimicrobial agent on bacterial biofilm. The model was used to evaluate the plausibility of two mechanisms of biofilm antibiotic resistance by qualitative comparison with data from a well-characterized experimental system (H. Anwar, J. L. Strap, and J. W. Costerton, Antimicrob. Agents Chemother. 36:1208-1214, 1992). The two mechanisms involved either depletion of the antibiotic by reaction with biomass or physiological resistance due to reduced bacterial growth rates in the biofilm. Both mechanisms predicted the experimentally observed resistance of 7-day-old Pseudomonas aeruginosa biofilms compared with that of 2-day-old ones. A version of the model that incorporated growth rate-dependent killing predicted reduced susceptibility of thicker biofilms because oxygen was exhausted within these biofilms, leading to very slow growth in part of the biofilm. A version of the model that incorporated a destructive reaction of the antibiotic with biomass likewise accounted for the relative resistance of thicker biofilms. Resistance in this latter case was due to depletion of the antibiotic in the bulk fluid rather than development of a gradient in the antibiotic concentration within the biofilm. The modeling results predicted differences between the two cases, such as in the survival profiles within the biofilm, that could permit these resistance mechanisms to be experimentally distinguished.

Risk factors for chronic biofilm-related infection associated with implanted medical devices

BACKGROUND

The use of implanted medical devices is associated with a small but clinically important risk of foreign body infection. A key question is: why do some patients develop chronic infection associated with an implanted device, but most do not?

AIMS

The literature on patient-specific risk factors for chronic infections associated with five types of implants was surveyed to glean clues about the etiology of these infections.

SOURCES

Data were collected from 47 articles through calendar year 2017 for five categories of device-related infections: cardiovascular implantable electronic devices (CIEDs), hernia meshes, prosthetic hip and knee joints, prosthetic shoulder joints and breast implants.

CONTENT

Important risk factors include immunomodulation/steroid therapy, diabetes, smoking, and renal disease/haemodialysis-findings that point to a critical role of a compromised innate immune response in determining vulnerable subpopulations.

IMPLICATIONS

A model of biofilm-related device infection is presented that posits defects in the innate immune response both systemically and locally, in the immediate vicinity of an abiotic biomaterial. The limitations of in vitro and animal models of chronic device-related infections are discussed in this context as are implications for research and clinical practice.