Staphylococcus aureus biofilm susceptibility to small and potent β2,2-amino acid derivatives

Small antimicrobial β^2,2-amino acid derivatives (Mw < 500 Da) are reported to display high antibacterial activity against suspended Gram-positive strains combined with low hemolytic activity. In the present study, the anti-biofilm activity of six β^2,2-amino acid derivatives (A1–A6) against Staphylococcus aureus (ATCC 25923) was investigated. The derivatives displayed IC₅₀ values between 5.4 and 42.8 μM for inhibition of biofilm formation, and concentrations between 22.4 and 38.4 μM had substantial effects on preformed biofilms. The lead derivative A2 showed high killing capacity (log R), and it caused distinct ultrastructural changes in the biofilms as shown by electron and atomic force microscopy. The anti-biofilm properties of A2 was preserved under high salinity conditions. Extended screening showed also high activity of A2 against Escherichia coli (XL1 Blue) biofilms. These advantageous features together with high activity against preformed biofilms make β^2,2-amino acid derivatives a promising class of compounds for further development of anti-biofilm agents.     

Chelator-induced dispersal and killing of Pseudomonas aeruginosa cells in a biofilm

Biofilms consist of groups of bacteria attached to surfaces and encased in a hydrated polymeric matrix. Bacteria in biofilms are more resistant to the immune system and to antibiotics than their free-living planktonic counterparts. Thus, biofilm-related infections are persistent and often show recurrent symptoms. The metal chelator EDTA is known to have activity against biofilms of gram-positive bacteria such as Staphylococcus aureus. EDTA can also kill planktonic cells of Proteobacteria like Pseudomonas aeruginosa. In this study we demonstrate that EDTA is a potent P. aeruginosa biofilm disrupter. In Tris buffer, EDTA treatment of P. aeruginosa biofilms results in 1,000-fold greater killing than treatment with the P. aeruginosa antibiotic gentamicin. Furthermore, a combination of EDTA and gentamicin results in complete killing of biofilm cells. P. aeruginosa biofilms can form structured mushroom-like entities when grown under flow on a glass surface. Time lapse confocal scanning laser microscopy shows that EDTA causes a dispersal of P. aeruginosa cells from biofilms and killing of biofilm cells within the mushroom-like structures. An examination of the influence of several divalent cations on the antibiofilm activity of EDTA indicates that magnesium, calcium, and iron protect P. aeruginosa biofilms against EDTA treatment. Our results are consistent with a mechanism whereby EDTA causes detachment and killing of biofilm cells.     

Susceptibility of Staphylococcus aureus biofilms to reactive discharge gases

Formation of bacterial biofilms at solid-liquid interfaces creates numerous problems in both industrial and biomedical sciences. In this study, the susceptibility of Staphylococcus aureus biofilms to discharge gas generated from plasma was tested. It was found that despite distinct chemical/physical properties, discharge gases from oxygen, nitrogen, and argon demonstrated very potent and almost the same anti-biofilm activity. The bacterial cells in S. aureus biofilms were killed (>99.9%) by discharge gas within minutes of exposure. Under optimal experimental conditions, no bacteria and biofilm re-growth from discharge gas treated biofilms was found. Further studies revealed that the anti-biofilm activity of the discharge gas occurred by two distinct mechanisms: (1) killing bacteria in biofilms by causing severe cell membrane damage, and (2) damaging the extracellular polymeric matrix in the architecture of the biofilm to release biofilm from the surface of the solid substratum. Information gathered from this study provides an insight into the anti-biofilm mechanisms of plasma and confirms the applications of discharge gas in the treatment of biofilms and biofilm related bacterial infections.     

Chelated iron sources are inhibitors of Pseudomonas aeruginosa biofilms and distribute efficiently in an in vitro model of drug delivery to the human lung

Aims:  To determine whether chelated sources of ferric iron were efficient inhibitors of biofilm formation in Pseudomonas aeruginosa and might be suitable for drug delivery to the lungs of cystic fibrosis (CF) patients via nebulization.
Methods and Results:  The response of P. aeruginosa biofilms to elevated iron concentrations in the form of eight structurally varied iron chelators in a microtitre plate assay for biofilm production was examined in the lab. Among these iron chelates, picolinic acid and acetohydroxamic acid-chelated iron were able to effectively thwart biofilm production in P. aeruginosa PA14 and in 20 clinical isolates of P. aeruginosa from a local hospital. The chelated iron sources showed excellent distribution in an Anderson cascade impactor model of particle size distribution in the human lung.
Conclusions:  Ferric picolinate and ferric acetohydroxamate are effective anti-biofilm compounds against both lab and clinical strains of P. aeruginosa and are readily nebulized into particles of suitable size for lung delivery.
Significance and Impact of the Study:  The data herein serve both to solidify the growing base of literature correlating high iron levels with biofilm inhibition in P. aeruginosa and to highlight the potential of these chelators as nebulized agents to combat biofilms of P. aeruginosa in CF patients.     

Susceptibility of Staphylococcus aureus biofilms to reactive discharge gases

Formation of bacterial biofilms at solid–liquid interfaces creates numerous problems in both industrial and biomedical sciences. In this study, the susceptibility of Staphylococcus aureus biofilms to discharge gas generated from plasma was tested. It was found that despite distinct chemical/physical properties, discharge gases from oxygen, nitrogen, and argon demonstrated very potent and almost the same anti-biofilm activity. The bacterial cells in S. aureus biofilms were killed (>99.9%) by discharge gas within minutes of exposure. Under optimal experimental conditions, no bacteria and biofilm re-growth from discharge gas treated biofilms was found. Further studies revealed that the anti-biofilm activity of the discharge gas occurred by two distinct mechanisms: (1) killing bacteria in biofilms by causing severe cell membrane damage, and (2) damaging the extracellular polymeric matrix in the architecture of the biofilm to release biofilm from the surface of the solid substratum. Information gathered from this study provides an insight into the anti-biofilm mechanisms of plasma and confirms the applications of discharge gas in the treatment of biofilms and biofilm related bacterial infections.     

Pseudomonas aeruginosa-plant root interactions. Pathogenicity, biofilm formation, and root exudation

Pseudomonas aeruginosa is an opportunistic human pathogen capable of forming a biofilm under physiological conditions that contributes to its persistence despite long-term treatment with antibiotics. Here, we report that pathogenic P. aeruginosa strains PAO1 and PA14 are capable of infecting the roots of Arabidopsis and sweet basil (Ocimum basilicum), in vitro and in the soil, and are capable of causing plant mortality 7 d postinoculation. Before plant mortality, PAO1 and PA14 colonize the roots of Arabidopsis and sweet basil and form a biofilm as observed by scanning electron microscopy, phase contrast microscopy, and confocal scanning laser microscopy. Upon P. aeruginosa infection, sweet basil roots secrete rosmarinic acid (RA), a multifunctional caffeic acid ester that exhibits in vitro antibacterial activity against planktonic cells of both P. aeruginosa strains with a minimum inhibitory concentration of 3 microg mL(-1). However, in our studies RA did not attain minimum inhibitory concentration levels in sweet basil's root exudates before P. aeruginosa formed a biofilm that resisted the microbicidal effects of RA and ultimately caused plant mortality. We further demonstrated that P. aeruginosa biofilms were resistant to RA treatment under in vivo and in vitro conditions. In contrast, induction of RA secretion by sweet basil roots and exogenous supplementation of Arabidopsis root exudates with RA before infection conferred resistance to P. aeruginosa. Under the latter conditions, confocal scanning laser microscopy revealed large clusters of dead P. aeruginosa on the root surface of Arabidopsis and sweet basil, and biofilm formation was not observed. Studies with quorum-sensing mutants PAO210 (DeltarhlI), PAO214 (DeltalasI), and PAO216 (DeltalasI DeltarhlI) demonstrated that all of the strains were pathogenic to Arabidopsis, which does not naturally secrete RA as a root exudate. However, PAO214 was the only pathogenic strain toward sweet basil, and PAO214 biofilm appeared comparable with biofilms formed by wild-type strains of P. aeruginosa. Our results collectively suggest that upon root colonization, P. aeruginosa forms a biofilm that confers resistance against root-secreted antibiotics.     

Involvement of nitric oxide in biofilm dispersal of Pseudomonas aeruginosa

Bacterial biofilms at times undergo regulated and coordinated dispersal events where sessile biofilm cells convert to free-swimming, planktonic bacteria. In the opportunistic pathogen Pseudomonas aeruginosa, we previously observed that dispersal occurs concurrently with three interrelated processes within mature biofilms: (i) production of oxidative or nitrosative stress-inducing molecules inside biofilm structures, (ii) bacteriophage induction, and (iii) cell lysis. Here we examine whether specific reactive oxygen or nitrogen intermediates play a role in cell dispersal from P. aeruginosa biofilms. We demonstrate the involvement of anaerobic respiration processes in P. aeruginosa biofilm dispersal and show that nitric oxide (NO), used widely as a signaling molecule in biological systems, causes dispersal of P. aeruginosa biofilm bacteria. Dispersal was induced with low, sublethal concentrations (25 to 500 nM) of the NO donor sodium nitroprusside (SNP). Moreover, a P. aeruginosa mutant lacking the only enzyme capable of generating metabolic NO through anaerobic respiration (nitrite reductase, DeltanirS) did not disperse, whereas a NO reductase mutant (DeltanorCB) exhibited greatly enhanced dispersal. Strategies to induce biofilm dispersal are of interest due to their potential to prevent biofilms and biofilm-related infections. We observed that exposure to SNP (500 nM) greatly enhanced the efficacy of antimicrobial compounds (tobramycin, hydrogen peroxide, and sodium dodecyl sulfate) in the removal of established P. aeruginosa biofilms from a glass surface. Combined exposure to both NO and antimicrobial agents may therefore offer a novel strategy to control preestablished, persistent P. aeruginosa biofilms and biofilm-related infections.     

Eradication of Pseudomonas aeruginosa Biofilms by Atmospheric Pressure Non-Thermal Plasma

Bacteria exist, in most environments, as complex, organised communities of sessile cells embedded within a matrix of self-produced, hydrated extracellular polymeric substances known as biofilms. Bacterial biofilms represent a ubiquitous and predominant cause of both chronic infections and infections associated with the use of indwelling medical devices such as catheters and prostheses. Such infections typically exhibit significantly enhanced tolerance to antimicrobial, biocidal and immunological challenge. This renders them difficult, sometimes impossible, to treat using conventional chemotherapeutic agents. Effective alternative approaches for prevention and eradication of biofilm associated chronic and device-associated infections are therefore urgently required. Atmospheric pressure non-thermal plasmas are gaining increasing attention as a potential approach for the eradication and control of bacterial infection and contamination. To date, however, the majority of studies have been conducted with reference to planktonic bacteria and rather less attention has been directed towards bacteria in the biofilm mode of growth. In this study, the activity of a kilohertz-driven atmospheric pressure non-thermal plasma jet, operated in a helium oxygen mixture, against Pseudomonas aeruginosa in vitro biofilms was evaluated. Pseudomonas aeruginosa biofilms exhibit marked susceptibility to exposure of the plasma jet effluent, following even relatively short (～10's s) exposure times. Manipulation of plasma operating conditions, for example, plasma operating frequency, had a significant effect on the bacterial inactivation rate. Survival curves exhibit a rapid decline in the number of surviving cells in the first 60 seconds followed by slower rate of cell number reduction. Excellent anti-biofilm activity of the plasma jet was also demonstrated by both confocal scanning laser microscopy and metabolism of the tetrazolium salt, XTT, a measure of bactericidal activity.     

Inhibition and dispersal of Agrobacterium tumefaciens biofilms by a small diffusible Pseudomonas aeruginosa exoproduct(s)

Environmental biofilms often contain mixed populations of different species. In these dense communities, competition between biofilm residents for limited nutrients such as iron can be fierce, leading to the evolution of competitive factors that affect the ability of competitors to grow or form biofilms. We have discovered a compound(s) present in the conditioned culture fluids of Pseudomonas aeruginosa that disperses and inhibits the formation of biofilms produced by the facultative plant pathogen Agrobacterium tumefaciens. The inhibitory activity is strongly induced when P. aeruginosa is cultivated in iron-limited conditions, but it does not function through iron sequestration. In addition, the production of the biofilm inhibitory activity is not regulated by the global iron regulatory protein Fur, the iron-responsive extracytoplasmic function σ factor PvdS, or three of the recognized P. aeruginosa quorum-sensing systems. In addition, the compound(s) responsible for the inhibition and dispersal of A. tumefaciens biofilm formation is likely distinct from the recently identified P. aeruginosa dispersal factor, cis-2-decenoic acid (CDA), as dialysis of the culture fluids showed that the inhibitory compound was larger than CDA and culture fluids that dispersed and inhibited biofilm formation by A. tumefaciens had no effect on biofilm formation by P. aeruginosa.     

抗生物膜肽研究进展

细菌生物膜导致的细菌耐药性增加受到了广泛关注。抗生物膜肽是一类具有抑制和杀灭细菌生物膜独特优势的抗微生物肽,有望成为理想的抗细菌生物膜的新型抗菌药物。就抗生物膜肽与生物膜各组分间的相互作用、抗生物膜肽对生物膜形成的干预作用及其调控、抗生物膜肽目前存在的问题及其解决思路以及抗生物膜肽未来的应用领域等展开综述。     

Influence of aptamer-targeted antibiofilm agents for treatment of <Emphasis Type="Italic">Pseudomonas aeruginosa</Emphasis> biofilms

Biofilms are bacterial communities consisting of numerous extracellular polymeric substances. Infections caused by biofilm-forming bacteria are considered to be a major threat to health security and so novel approaches to control biofilm are of importance. Aptamers are single-strand nucleic acid molecules that have high selectivity to their targets. Single-walled carbon nanotubes (SWNTs) are common nanomaterials and have been shown to be toxic to bacterial biofilms. The aim of this study was to test whether an aptamer could play a role as targeting agents to enhance the efficiency of anti-biofilm agents. Hence, two complexes (aptamer–SWNTs and aptamer–ciprofloxacin–SWNTs) based on an aptamer which targets Pseudomonas aeruginosa and SWNTs were constructed. Both complexes were assessed against P. aeruginosa biofilms. In vitro tests demonstrated that the aptamer–SWNTs could inhibit ~36% more biofilm formation than SWNTs alone. Similarly, the aptamer–ciprofloxacin–SWNTs had a higher anti-biofilm efficiency than either component or simple mixtures of two components. Our study underscores the potential of aptamers as targeting agents for anti-biofilm compounds, as well as providing a new strategy to control biofilms.     

Glucose Starvation-Induced Dispersal of Pseudomonas aeruginosa Biofilms Is cAMP and Energy Dependent

Carbon starvation has been shown to induce a massive dispersal event in biofilms of the opportunistic pathogen Pseudomonas aeruginosa; however, the molecular pathways controlling this dispersal response remain unknown. We quantified changes in the proteome of P. aeruginosa PAO1 biofilm and planktonic cells during glucose starvation by differential peptide-fingerprint mass-spectrometry (iTRAQ). In addition, we monitored dispersal photometrically, as a decrease in turbidity/opacity of biofilms pre-grown and starved in continuous flow-cells, in order to evaluate treatments (e.g. inhibitors CCCP, arsenate, chloramphenicol, L-serine hydroxamate) and key mutants altered in biofilm development and dispersal (e.g. nirS, vfr, bdlA, rpoS, lasRrhlR, Pf4-bacteriophage and cyaA). In wild-type biofilms, dispersal started within five minutes of glucose starvation, was maximal after 2 h, and up to 60% of the original biomass had dispersed after 24 h of starvation. The changes in protein synthesis were generally not more than two fold and indicated that more than 100 proteins belonging to various classes, including carbon and energy metabolism, stress adaptation, and motility, were differentially expressed. For the different treatments, only the proton-ionophore CCCP or arsenate, an inhibitor of ATP synthesis, prevented dispersal of the biofilms. For the different mutants tested, only cyaA, the synthase of the intracellular second messenger cAMP, failed to disperse; complementation of the cyaA mutation restored the wild-type phenotype. Hence, the pathway for carbon starvation-induced biofilm dispersal in P. aeruginosa PAO1 involves ATP production via direct ATP synthesis and proton-motive force dependent step(s) and is mediated through cAMP, which is likely to control the activity of proteins involved in remodeling biofilm cells in preparation for planktonic survival.     

Fitness landscape of antibiotic tolerance in Pseudomonas aeruginosa biofilms

Bacteria in biofilms have higher antibiotic tolerance than their planktonic counterparts. A major outstanding question is the degree to which the biofilm-specific cellular state and its constituent genetic determinants contribute to this hyper-tolerant phenotype. Here, we used genome-wide functional profiling of a complex, heterogeneous mutant population of Pseudomonas aeruginosa MPAO1 in biofilm and planktonic growth conditions with and without tobramycin to systematically quantify the contribution of each locus to antibiotic tolerance under these two states. We identified large sets of mutations that contribute to antibiotic tolerance predominantly in the biofilm or planktonic setting only, offering global insights into the differences and similarities between biofilm and planktonic antibiotic tolerance. Our mixed population-based experimental design recapitulated the complexity of natural biofilms and, unlike previous studies, revealed clinically observed behaviors including the emergence of quorum sensing-deficient mutants. Our study revealed a substantial contribution of the cellular state to the antibiotic tolerance of biofilms, providing a rational foundation for the development of novel therapeutics against P. aeruginosa biofilm-associated infections.     

Interspecies biofilms of Pseudomonas aeruginosa and Burkholderia cepacia.

The leading cause of morbidity and mortality in cystic fibrosis (CF) continues to be lung infections with Pseudomonas aeruginosa biofilms. Co-colonization of the lungs with P aeruginosa and Burkholderia cepacia can result in more severe pulmonary disease than P. aeruginosa alone. The interactions between P. aeruginosa biofilms and B. cepacia are not yet understood; one possible association being that mixed species biofilm formation may be part of the interspecies relationship. Using the Calgary Biofilm Device (CBD), members of all genomovars of the B. cepacia complex were shown to form biofilms, including those isolated from CF lungs. Mixed species biofilm formation between CF isolates of P. aeruginosa and B. cepacia was readily achieved using the CBD. Oxidation-fermentation lactose agar was adapted as a differential agar to monitor mixed biofilm composition. Scanning electron micrographs of the biofilms demonstrated that both species readily integrated in close association in the biofilm structure. Pseudomonas aeruginosa laboratory strain PAO1, however, inhibited mixed biofilm formation of both CF isolates and environmental strains of the B. cepacia complex. Characterization of the soluble inhibitor suggested pyocyanin as the active compound.     

Common plant flavonoids prevent the assembly of amyloid curli fibres and can interfere with bacterial biofilm formation

Like all macroorganisms, plants have to control bacterial biofilm formation on their surfaces. On the other hand, biofilms are highly tolerant against antimicrobial agents and other stresses. Consequently, biofilms are also involved in human chronic infectious diseases, which generates a strong demand for anti-biofilm agents. Therefore, we systematically explored major plant flavonoids as putative anti-biofilm agents using different types of biofilms produced by Gram-negative and Gram-positive bacteria. In Escherichia coli macrocolony biofilms, the flavone luteolin and the flavonols myricetin, morin and quercetin were found to strongly reduce the extracellular matrix. These agents directly inhibit the assembly of amyloid curli fibres by driving CsgA subunits into an off-pathway leading to SDS-insoluble oligomers. In addition, they can interfere with cellulose production by still unknown mechanisms. Submerged biofilm formation, however, is hardly affected. Moreover, the same flavonoids tend to stimulate macrocolony and submerged biofilm formation by Pseudomonas aeruginosa. For Bacillus subtilis, the flavonone naringenin and the chalcone phloretin were found to inhibit growth. Thus, plant flavonoids are not general anti-biofilm compounds but show species-specific effects. However, based on their strong and direct anti-amyloidogenic activities, distinct plant flavonoids may provide an attractive strategy to specifically combat amyloid-based biofilms of some relevant pathogens.     

Stratified growth in Pseudomonas aeruginosa biofilms

In this study, stratified patterns of protein synthesis and growth were demonstrated in Pseudomonas aeruginosa biofilms. Spatial patterns of protein synthetic activity inside biofilms were characterized by the use of two green fluorescent protein (GFP) reporter gene constructs. One construct carried an isopropyl-beta-d-thiogalactopyranoside (IPTG)-inducible gfpmut2 gene encoding a stable GFP. The second construct carried a GFP derivative, gfp-AGA, encoding an unstable GFP under the control of the growth-rate-dependent rrnBp(1) promoter. Both GFP reporters indicated that active protein synthesis was restricted to a narrow band in the part of the biofilm adjacent to the source of oxygen. The zone of active GFP expression was approximately 60 microm wide in colony biofilms and 30 microm wide in flow cell biofilms. The region of the biofilm in which cells were capable of elongation was mapped by treating colony biofilms with carbenicillin, which blocks cell division, and then measuring individual cell lengths by transmission electron microscopy. Cell elongation was localized at the air interface of the biofilm. The heterogeneous anabolic patterns measured inside these biofilms were likely a result of oxygen limitation in the biofilm. Oxygen microelectrode measurements showed that oxygen only penetrated approximately 50 microm into the biofilm. P. aeruginosa was incapable of anaerobic growth in the medium used for this investigation. These results show that while mature P. aeruginosa biofilms contain active, growing cells, they can also harbor large numbers of cells that are inactive and not growing.     

Synthesis,Biological Evaluation and in Silico Study of N-(2- and 3-Pyridinyl)benzamide Derivatives as Quorum Sensing Inhibitors against Pseudomonas aeruginosa

The effectiveness of treating bacterial infections is seriously threatened by the emergence of bacterial resistance to chemical treatment. Growth of microbes in biofilm is one of the main causes of resistance to antimicrobial drugs. Quorum sensing (QS) inhibition, which targets the QS signalling system by obstructing cell-cell communication, was developed as an alternative treatment by creating innovative anti-biofilm drugs. Therefore, the goal of this study is to develop novel antimicrobial drugs that are effective against Pseudomonas aeruginosa by inhibiting QS and acting as anti-biofilm agents. In this study, N-(2- and 3-pyridinyl)benzamide derivatives were selected to design and syntheses. Antibiofilm activity was revealed by all the synthesized compounds and the biofilm was visibly impaired, and the OD_595nm readings of solubilized biofilm cells presented a momentous difference between the treated and untreated biofilms. The best anti-QS zone was observed for compound 5d and found to be 4.96 mm. Through in silico research, the physicochemical characteristics and binding manner of these produced compounds were examined. For the purpose of understanding the stability of the protein and ligand complex, molecular dynamic simulation was also carried out. The overall findings showed that N-(2- and 3-pyridinyl)benzamide derivatives could be the key to creating effective newer anti-quorum sensing drugs that are effective against different bacteria.     

Rhamnolipids mediate detachment of Pseudomonas aeruginosa from biofilms

The process of detachment, through which bacteria use active mechanisms to leave biofilms and return to the planktonic (free-living) state, is perhaps the least understood aspect of the biofilm life cycle. Like other stages of biofilm development, detachment is a dynamic, regulated process, controlled by specific genes, and induced by particular environmental cues. In previous work we discovered Pseudomonas aeruginosa variants that exhibit accelerated biofilm detachment. These hyper-detaching variants arise spontaneously from biofilms at a high frequency, and they exhibit robust detachment under different biofilm growth conditions. Here we show that these variants detach by a mechanism requiring the biosurfactant rhamnolipid and that this detachment mechanism rapidly restores antibiotic sensitivity to separating bacteria. We also show that rhamnolipids can bring about detachment in wild-type P. aeruginosa biofilms. These findings raise the possibility that this detachment mechanism may be useful as a treatment to disrupt established biofilms. Interestingly, the rhamnolipid-mediated detachment mechanism involves the formation of cavities within the centre of biofilm structures. Our data suggest a model to explain detachment that occurs via this pattern.     

The exopolysaccharide alginate protects Pseudomonas aeruginosa biofilm bacteria from IFN-gamma-mediated macrophage killing

The ability of Pseudomonas aeruginosa to form biofilms and cause chronic infections in the lungs of cystic fibrosis patients is well documented. Numerous studies have revealed that P. aeruginosa biofilms are highly refractory to antibiotics. However, dramatically fewer studies have addressed P. aeruginosa biofilm resistance to the host's immune system. In planktonic, unattached (nonbiofilm) P. aeruginosa, the exopolysaccharide alginate provides protection against a variety of host factors yet the role of alginate in protection of biofilm bacteria is unclear. To address this issue, we tested wild-type strains PAO1, PA14, the mucoid cystic fibrosis isolate, FRD1 (mucA22+), and the respective isogenic mutants which lacked the ability to produce alginate, for their susceptibility to human leukocytes in the presence and absence of IFN-gamma. Human leukocytes, in the presence of recombinant human IFN-gamma, killed biofilm bacteria lacking alginate after a 4-h challenge at 37 degrees C. Bacterial killing was dependent on the presence of IFN-gamma. Killing of the alginate-negative biofilm bacteria was mediated through mononuclear cell phagocytosis since treatment with cytochalasin B, which prevents actin polymerization, inhibited leukocyte-specific bacterial killing. By direct microscopic observation, phagocytosis of alginate-negative biofilm bacteria was significantly increased in the presence of IFN-gamma vs all other treatments. Addition of exogenous, purified alginate to the alginate-negative biofilms restored resistance to human leukocyte killing. Our results suggest that although alginate may not play a significant role in bacterial attachment, biofilm development, and formation, it may play an important role in protecting mucoid P. aeruginosa biofilm bacteria from the human immune system.