Mechanisms of antibiotic resistance in bacterial biofilms

Antibiotic resistance in exopolysaccharide-forming Staphylococcus epidermidis clinical isolates from orthopaedic implant infections

The opportunistic pathogen Staphylococcus epidermidis is able to produce biofilm and to frequently cause implant infections. In recent years, it has also exhibited an increasing antimicrobial drug resistance. Here, the resistance to a panel of 16 different antibiotics in 342 clinical strains of S. epidermidis from orthopaedic implant infections has been investigated. The isolates were pheno- and genotyped for extracellular polysaccharide production, relevant to staphylococcal biofilm formation, in order to ascertain possible associations with antibiotic resistance. Approximately 10% of the isolates were found to be sensitive to all screened antibiotics. In all, 37-38% were resistant to beta-lactams such as oxacillin and imipenem, while the resistance to penicillin, ampicillin, cefazolin, cefamandole, was consistently observed in over 80% of the strains. Erythromycin- and clindamycin- resistant strains were approximately 41% and 16%, respectively. Of the isolates, 10% was resistant to chloramphenicol, 23% to sulfamethoxazole and 26% to ciprofloxacin. Resistance to vancomycin was never observed. Interestingly, exopolysaccharide-producing strains exhibited a significantly higher prevalence in the resistance to the four aminoglycosides (gentamicin, amikacin, netilmicin, tobramycin), to sulfamethoxazole and to ciprofloxacin with respect to non-producing isolates. Moreover, multiple resistance to antibiotics was more frequent among exopolysaccharide-forming strains.

Virulence of Burkholderia pseudomallei does not correlate with biofilm formation

Burkholderia pseudomallei is the causative agent of melioidosis but currently the pathogenesis of the disease is still poorly understood. One of the virulent factors of gram-negative bacteria is the ability to produce biofilm to evade host defense. As B. pseudomallei has also been reported to develop the biofilm [1], in the present study, we therefore, quantified the biofilm formation in 50 strains of B. pseudomallei and compared with 50 strains of its avirulent counterpart Burkholderia thailandensis using a modified microtiter-plate test. The results showed that the quantity of biofilm produced by B. pseudomallei was statistically higher (P< 0.01) than that of B. thailandensis (means and SEs of the corrected OD630 were 2.17+/-0.29 and 0.59+/-0.05, respectively). Transmission electron micrographs of the B. pseudomallei strain with high biofilm formation exhibited microcolonies of bacterial cells surrounded by dense extracellular slime matrix comparing with only trace quantity in the low biofilm-producing strain or the biofilm mutants generated by Tn5-OT182 mutagenesis. However, no correlation could be observed between the biofilm formation and virulence, judging from the LD50 values in BALB/c mice. The data obtained with these naturally occurring Burkholderia species and the biofilm mutants are incompatible with the possibility that the biofilm plays a role in the pathogenesis of B. pseudomallei infection.

Persister cells mediate tolerance to metal oxyanions in Escherichia coli

Bacterial cultures produce subpopulations of cells termed ‘persisters’, reputedly known for high tolerance to killing by antibiotics. Ecologically, antibiotics produced by competing microflora are only one potential stress encountered by bacteria. Another pressure in the environment is toxic metals that are distributed ubiquitously by human pollution, volcanic activity and the weathering of minerals. This study evaluated the time- and concentration-dependent killing of Escherichia coli planktonic and biofilm cultures by the water-soluble metal(loid) oxyanions chromate (), arsenate (), arsenite (), selenite (), tellurate () and tellurite (). Correlative to previous reports in the literature, control antibiotic assays indicated that a small proportion of E. coli biofilm populations remained recalcitrant to killing by antibiotics (even with 24 h exposure). In contrast, metal oxyanions presented a slow, bactericidal action that eradicated biofilms. When exposed for 2 h, biofilms were up to 310 times more tolerant to killing by metal oxyanions than corresponding planktonic cultures. However, by 24 h, planktonic cells and biofilms were eradicated at approximately the same concentration in all instances. Coloured complexes of metals and chelators could not be generated in biofilms exposed to or , suggesting that the extracellular polymeric matrix of E. coli may have a low binding affinity for metal oxyanions. Viable cell counts at 2 and 24 h exposure revealed that, at high concentrations, all of the metal oxyanions had killed 99 % (or a greater proportion) of the bacterial cells in biofilm populations. It is suggested here that the short-term survival of <1 % of the bacterial population corresponds well with the hypothesis that a small population of persister cells may be responsible for the time-dependent tolerance of E. coli biofilms to high concentrations of metal oxyanions.

Plant models for animal pathogenesis

Several bacteria that are pathogenic to animals also infect plants. Mechanistic studies have proven that some human/animal pathogenic bacteria employ a similar subset of virulence determinants to elicit disease in animals, invertebrates and plants. Therefore, the results of plant infection studies are relevant to animal pathogenesis. This discovery has resulted in the development of convenient, cost-effective, and reliable plant infection models to study the molecular basis of infection by animal pathogens. Plant infection models provide a number of advantages in the study of animal pathogenesis. Using a plant model, mutations in animal pathogenic bacteria can easily be screened for putative virulence factors, a process which if done using existing animal infection models would be time-consuming and tedious. High-throughput screening of plants also provides the potential for unravelling the mechanisms by which plants resist animal pathogenic bacteria, and provides a means to discover novel therapeutic agents such as antibiotics and anti-infective compounds. In this review, we describe the developing technique of using plants as a model system to study Pseudomonas aeruginosa, Enterococcus faecalis and Staphylococcus aureus pathogenesis, and discuss ways to use this new technology against disease warfare and other types of bioterrorism.

The effect of a bacteriophage on diversification of the opportunistic bacterial pathogen, Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that colonizes the lungs of cystic fibrosis (CF) patients. CF lungs often contain a diverse range of P. aeruginosa phenotypes, some of which are likely to contribute to the persistence of infection, yet the causes of diversity are unclear. While the ecological heterogeneity of the lung environment and therapeutic regimes are probable factors, a role for parasitic bacteriophage cannot be ruled out. Parasites have been implicated as a key ecological variable driving the evolution of diversity in host populations. PP7 drove cycles of morphological diversification in host populations of P. aeruginosa due to the de novo evolution of small-rough colony variants that coexisted with large diffuse colony morph bacteria. In the absence of phage, bacteria only displayed the large diffuse colony morphology of the wild-type. Further assays revealed there to be two distinct types of resistant bacteria; these had very different ecological phenotypes, yet each carried a cost of resistance.

Effect of pH and antibiotics on microbial overgrowth in the stomachs and duodena of patients undergoing percutaneous endoscopic gastrostomy feeding

Enteral nutrition via a percutaneous endoscopic gastrostomy (PEG) tube is often part of management in patients with dysphagia due to neurological or oropharyngeal disease. Gastrostomy placement can affect normal innate defense mechanisms in the upper gut, resulting in bacterial overgrowth. In this study microbiological investigations were done with gastric and duodenal aspirates from 20 patients undergoing PEG tube placement and PEG tubes from 10 patients undergoing tube replacement. Aspirate and PEG tube microbiotas were assessed by using viable counts and selective solid media followed by aerobic and anaerobic incubation to assess cell viabilities. The antibiotic susceptibility profiles of the isolates were determined by the disk diffusion method, and gas chromatography was used to study the bacterial metabolic products in the aspirates. The aspirates and PEG tubes contained mainly streptococci, staphylococci, lactobacilli, yeasts, and enterobacteria. Enterococci were detected only in PEG tube biofilms and not in aspirates. Gastric pH affected the composition of the aspirate microbiotas but not the total microbial counts. Staphylococci, Escherichia coli, and Candida spp. were isolated only from antibiotic-treated patients, despite the sensitivities of the bacteria to the agents used. Antibiotic treatment had no effect on the incidence of infection or the length of hospital stay in these patients.

Heparin stimulates Staphylococcus aureus biofilm formation

Heparin, known for its anticoagulant activity, is commonly used in catheter locks. Staphylococcus aureus, a versatile human and animal pathogen, is commonly associated with catheter-related bloodstream infections and has evolved a number of mechanisms through which it adheres to biotic and abiotic surfaces. We demonstrate that heparin increased biofilm formation by several S. aureus strains. Surface coverage and the kinetics of biofilm formation were stimulated, but primary attachment to the surface was not affected. Heparin increased S. aureus cell-cell interactions in a protein synthesis-dependent manner. The addition of heparin rescued biofilm formation of hla, ica, and sarA mutants. Our data further suggest that heparin stimulation of biofilm formation occurs neither through an increase in sigB activity nor through an increase in polysaccharide intracellular adhesin levels. These finding suggests that heparin stimulates S. aureus biofilm formation via a novel pathway.

Discrepancy between viable counts and light output as viability measurements, following ciprofloxacin challenge of self-bioluminescent Pseudomonas aeruginosa biofilms

OBJECTIVES

To utilize bioluminescence to follow the effect of ciprofloxacin challenge on Pseudomonas aeruginosa biofilms.

METHODS

The Sorbarod continuous perfusion culture system was used for the cultivation of biofilms of a self-bioluminescent strain of P. aeruginosa PAO1. Biofilms were challenged with ciprofloxacin (5 mg/L) in the perfusing medium for 3 h and allowed to recover to pre-challenge population levels before initiation of a second 3 h challenge. In addition to determining eluate and biofilm cell survival by conventional viable plate counts, light output was monitored via a luminometer and a low-light-level ICCD camera, to give an indication of metabolism. The effect of drug challenge on biofilm structure was investigated using an environmental scanning electron microscope, which allowed discernment of changes to the three-dimensional biofilm architecture.

RESULTS

On challenge with ciprofloxacin, eluate light output measurements declined to a lesser extent than viable counts for the same samples and also indicated that post-challenge recovery of the biofilm metabolism did not occur as rapidly as suggested by viable count data. Photon detection by ICCD camera allowed real-time, non-invasive imaging of metabolic activity within intact biofilms.

CONCLUSIONS

The application of a bioluminescent reporter strain to biofilm research provides valuable real-time positional data on the efficacy of anti-biofilm treatment strategies.

Persister cells, the biofilm matrix and tolerance to metal cations in biofilm and planktonic Pseudomonas aeruginosa

In this study, we examined Pseudomonas aeruginosa ATCC 27853 biofilm and planktonic cell susceptibility to metal cations. The minimum inhibitory concentration (MIC), the minimum bactericidal concentration (MBC) required to eradicate 100% of the planktonic population (MBC 100), and the minimum biofilm eradication concentration (MBEC) were determined using the MBEC trade mark-high throughput assay. Six metals - Co(2+), Ni(2+), Cu(2+), Zn(2+), Al(3+) and Pb(2+)- were each tested at 2, 4, 6, 8, 10 and 27 h of exposure to biofilm and planktonic cultures grown in rich or minimal media. With 2 or 4 h of exposure, biofilms were approximately 2-25 times more tolerant to killing by metal cations than the corresponding planktonic cultures. However, by 27 h of exposure, biofilm and planktonic bacteria were eradicated at approximately the same concentration in every instance. Viable cell counts evaluated at 2 and 27 h of exposure revealed that at high concentrations, most of the metals assayed had killed greater than 99.9% of biofilm and planktonic cell populations. The surviving cells were propogated in vitro and gave rise to biofilm and planktonic cultures with normal sensitivity to metals. Further, retention of copper by the biofilm matrix was investigated using the chelator sodium diethlydithiocarbamate. Formation of visible brown metal-chelates in biofilms treated with Cu(2+) suggests that the biofilm matrix may coordinate and sequester metal cations from the aqueous surroundings. Overall, our data suggest that both metal sequestration in the biofilm matrix and the presence of a small population of 'persister' cells may be contributing factors in the time-dependent tolerance of both planktonic cells and biofilms to high concentrations of metal cations.

Effects of the twin-arginine translocase on the structure and antimicrobial susceptibility ofEscherichia colibiofilms

In this descriptive study, we used Escherichia coli twin-arginine translocase (tat) mutants to distinguish antibiotic tolerance from the formation of mature biofilm structure. Biofilm formation by wild-type and Δtat strains of E. coli was evaluated using viable cell counts, scanning electron microscopy, and confocal laser-scanning microscopy. Escherichia coli Δtat mutants had an impaired ability to form biofilms when grown in rich or minimal media. These mutants produced disorganized layers and cell aggregates with significantly decreased cell density relative to the wild-type strain. In contrast, wild-type E. coli grown under similar test conditions formed highly structured, surface-adherent communities. We thus determined if this decreased biofilm formation by E. coli Δtat mutants may result in lowered tolerance to antimicrobials. When grown in rich media, planktonic Δtat mutants were hypersensitive to some metals, detergents, and antibiotics. However, the corresponding biofilms were about as resilient as the wild-type strain. In contrast, both planktonic cells and biofilms of the ΔtatABC strain grown in minimal media were hypersensitive to many antimicrobials. Remarkably, these biofilms remained up to 365 times more tolerant to β-lactams than corresponding planktonic cells. Our data suggest that the twin-arginine translocase may play a contributing role in the antimicrobial tolerance, structural organization, and formation of mature E. coli biofilms under nutrient-limited conditions. However, the high tolerance of the ΔtatABC strain to bactericidal concentrations of antimicrobials indicates that mature biofilm structure may not be required for surface-adherent E. coli to survive exposure to these lethal factors.Key words: biofilm structure, twin-arginine translocase (tat), Escherichia coli, antimicrobial susceptibility/tolerance.

Use of confocal microscopy to analyze the rate of vancomycin penetration through Staphylococcus aureus biofilms

When bacteria assume the biofilm mode of growth, they can tolerate levels of antimicrobial agents 10 to 1,000 times higher than the MICs of genetically equivalent planktonic bacteria. The properties of biofilms that give rise to antibiotic resistance are only partially understood. Inhibition of antibiotic penetration into the biofilm may play a role, but this has not been proven directly. In this report, penetration of the glycopeptide antibiotic vancomycin into viable Staphylococcus aureus biofilms was analyzed by confocal scanning laser microscopy using a fluorescently labeled derivative of the drug. We found that while vancomycin bound to free-floating bacteria in water within 5 min, it took more than 1 h to bind to cells within the deepest layers of a biofilm. These results indicate that the antibiotic is transported through the depth of the biofilm but that the rate is significantly reduced with respect to its transport through flowing water. This suggests that, whereas planktonic bacteria were rapidly exposed to a full bolus of vancomycin, the bacteria in the deeper layers of the biofilm were exposed to a gradually increasing dose of the drug due to its reduced rate of penetration. This gradual exposure may allow the biofilm bacteria to undergo stress-induced metabolic or transcriptional changes that increase resistance to the antibiotic. We also investigated the role of poly-N-acetylglucosamine, an important component of the S. aureus biofilm matrix, and found that its production was not involved in the observed decrease in the rate of vancomycin penetration.

Lessons from DNA microarray analysis: the gene expression profile of biofilms

DNA microarray technology has been used to identify the global gene expression profile of biofilm cells. This is an interesting case study in how DNA microarray technology has advanced the molecular understanding of an understudied research area. DNA microarray analyses have suggested that there may be common responses upon biofilm formation, such as the repression of flagella genes and hyper-expression of genes for adhesion and ribosomal protein. They have also assisted in the identification of transcription factors that affect the formation of biofilms and indicated that there may not be biofilm-specific genes, arguing against biofilm formation being a developmental process. Instead, the DNA microarray data suggest that biofilms may have a unique pattern of gene expression, in which sub-sets of genes expressed in biofilms are also expressed under different planktonic conditions, but only in the biofilm are they all expressed simultaneously.

Involvement of Mycobacterium smegmatis undecaprenyl phosphokinase in biofilm and smegma formation

We describe a Mycobacterium smegmatis mutant with impaired biofilm and smegma formation. A gene homologous to Escherichia coli bacA, which has been proposed to play a role as undecaprenyl phosphokinase (Upk) was unmarked in-frame deleted from M. smegmatis. Though Upk is involved in cell wall synthesis, the surface of the mutant strain appeared virtually comparable to that of the wild type by electron microscopy. The absence of Upk influenced colony morphology and bacitracin resistance. The M. smegmatis Deltaupk mutant developed a biofilm characterized by scattered islands of bacteria distinct from the completely covered biofilm surface observed for wild-type bacteria. We further demonstrate biological consequences of upk deletion for smegma development in an in vivo model. These results suggest the upk gene to be essential in biofilm and smegma development.

Non-disruptive, real-time analyses of the metabolic status and viability of Streptococcus mutans cells in response to antimicrobial treatments

Streptococcus mutans is one of a small number of recognized pathogens that lives among the hundreds of other bacterial species which comprise the oral flora. The virulence of this organism is intimately associated with its ability to live as an attached biofilm community on the tooth surface, and consequently, there is a great interest in its biofilm lifestyle. Currently, there are no established protocols that facilitate drug screening against this organism while it is entrenched in the biofilm. Furthermore, greater complications arise when attempting to perform these experiments in a multi-species setting. In an effort to circumvent these problems, we developed a quick, real-time, and non-disruptive method to probe the metabolic status of S. mutans growing as either a planktonic culture or a biofilm community. This assay takes advantage of the proven utility of luciferase measurements for drug screening. We placed the luciferase gene under the control of the S. mutans lactate dehydrogenase promoter (ldh) and integrated the construct onto its native position on the chromosome. We found this construct to be both highly expressed (<10000 cells easily detectable) and insensitive to many different growth parameters. When testing this reporter in both planktonic and biofilm cultures receiving either bacteriostatic or bactericidal antibiotics, we found the ldh-luc reporter to be a very accurate measurement of cell viability. Furthermore, we also demonstrated that this assay can generate useful information about the characteristics of intoxication caused by antibiotic activity. In addition, we modified the biofilm assay into the 96-well format and demonstrated the feasibility of high throughput drug screening of biofilm embedded S. mutans.

Self-generated diversity produces "insurance effects" in biofilm communities

Diversity generally protects communities from unstable environmental conditions. This principle, known as the "insurance hypothesis," has been tested in many different ecosystems. Here we show that the opportunistic pathogen Pseudomonas aeruginosa undergoes extensive genetic diversification during short-term growth in biofilm communities. The induced genetic changes are produced by a recA-dependent mechanism and affect multiple traits, including the behavior of the bacteria in biofilms. Some biofilm-derived variants exhibit an increased ability to disseminate, whereas others manifest accelerated biofilm formation. Furthermore, the presence of these functionally diverse bacteria increases the ability of biofilms to resist an environmental stress. These findings suggest that self-generated diversity in biofilms provides a form of biological insurance that can safeguard the community in the face of adverse conditions.

Global gene expression in Staphylococcus aureus biofilms

We previously demonstrated that mutation of the staphylococcal accessory regulator (sarA) in a clinical isolate of Staphylococcus aureus (UAMS-1) results in an impaired capacity to form a biofilm in vitro (K. E. Beenken, J. S. Blevins, and M. S. Smeltzer, Infect. Immun. 71:4206-4211, 2003). In this report, we used a murine model of catheter-based biofilm formation to demonstrate that a UAMS-1 sarA mutant also has a reduced capacity to form a biofilm in vivo. Surprisingly, mutation of the UAMS-1 ica locus had little impact on biofilm formation in vitro or in vivo. In an effort to identify additional loci that might be relevant to biofilm formation and/or the adaptive response required for persistence of S. aureus within a biofilm, we isolated total cellular RNA from UAMS-1 harvested from a biofilm grown in a flow cell and compared the transcriptional profile of this RNA to RNA isolated from both exponential- and stationary-phase planktonic cultures. Comparisons were done using a custom-made Affymetrix GeneChip representing the genomic complement of six strains of S. aureus (COL, N315, Mu50, NCTC 8325, EMRSA-16 [strain 252], and MSSA-476). The results confirm that the sessile lifestyle associated with persistence within a biofilm is distinct by comparison to the lifestyles of both the exponential and postexponential phases of planktonic culture. Indeed, we identified 48 genes in which expression was induced at least twofold in biofilms over expression under both planktonic conditions. Similarly, we identified 84 genes in which expression was repressed by a factor of at least 2 compared to expression under both planktonic conditions. A primary theme that emerged from the analysis of these genes is that persistence within a biofilm requires an adaptive response that limits the deleterious effects of the reduced pH associated with anaerobic growth conditions.

Osteomyelitis

Bone and joint infections are painful for patients and frustrating for both them and their doctors. The high success rates of antimicrobial therapy in most infectious diseases have not yet been achieved in bone and joint infections owing to the physiological and anatomical characteristics of bone. The key to successful management is early diagnosis, including bone sampling for microbiological and pathological examination to allow targeted and long-lasting antimicrobial therapy. The various types of osteomyelitis require differing medical and surgical therapeutic strategies. These types include, in order of decreasing frequency: osteomyelitis secondary to a contiguous focus of infection (after trauma, surgery, or insertion of a joint prosthesis); that secondary to vascular insufficiency (in diabetic foot infections); or that of haematogenous origin. Chronic osteomyelitis is associated with avascular necrosis of bone and formation of sequestrum (dead bone), and surgical debridement is necessary for cure in addition to antibiotic therapy. By contrast, acute osteomyelitis can respond to antibiotics alone. Generally, a multidisciplinary approach is required for success, involving expertise in orthopaedic surgery, infectious diseases, and plastic surgery, as well as vascular surgery, particularly for complex cases with soft-tissue loss.

Phosphorus limitation enhances biofilm formation of the plant pathogen Agrobacterium tumefaciens through the PhoR-PhoB regulatory system

The plant pathogen Agrobacterium tumefaciens forms architecturally complex biofilms on inert surfaces. Adherence of A. tumefaciens C58 was significantly enhanced under phosphate limitation compared to phosphate-replete conditions, despite slower overall growth under low-phosphate conditions. Replacement of Pi with sn-glycerol-3-phosphate and 2-aminoethylphosphonate yielded similar results. The increase in surface interactions under phosphate limitation was observed in both static culture and continuous-culture flow cells. Statistical analysis of confocal micrographs obtained from the flow cell biofilms revealed that phosphate limitation increased both the overall attached biomass and the surface coverage, whereas the maximum thickness of the biofilm was not affected. Functions encoded on the two large plasmids of A. tumefaciens C58, pTiC58 and pAtC58, were not required for the observed phosphate effect. The phosphate concentration at which increased attachment was observed triggered the phosphate limitation response, controlled in many bacteria by the two-component regulatory system PhoR-PhoB. The A. tumefaciens phoB and phoR orthologues could only be disrupted in the presence of plasmid-borne copies of the genes, suggesting that this regulatory system might be essential. Expression of the A. tumefaciens phoB gene from a tightly regulated inducible promoter, however, correlated with the amount of biofilm under both phosphate-limiting and nonlimiting conditions, demonstrating that components of the Pho regulon influence A. tumefaciens surface interactions.

Characterization of the hipA7 allele of Escherichia coli and evidence that high persistence is governed by (p)ppGpp synthesis

The ability of a high frequency (10(-2)) of Escherichia coli to survive prolonged exposure to penicillin antibiotics, called high persistence, is associated with mutations in the hipA gene. The hip operon is located in the chromosomal terminus near dif and consists of two genes, hipA and hipB. The wild-type hipA gene encodes a toxin, whereas hipB encodes a DNA-binding protein that autoregulates expression of the hip operon and binds to HipA to nullify its toxic effects. We have characterized the hipA7 allele, which confers high persistence, and established that HipA7 is non-toxic, contains two mutations (G22S and D291A) and that both mutations are required for the full range of phenotypes associated with hip mutants. Furthermore, expression of hipA7 in the absence of hipB is sufficient to establish the high persistent phenotype, indicating that hipB is not required. There is a strong correlation between the frequency of persister cells generated by hipA7 strains and cell density, with hipA7 strains generating a 20-fold higher frequency of persisters as cultures approach stationary phase. It is also demonstrated that relA knock-outs diminish the high persistent phenotype in hipA7 mutants and that relA spoT knock-outs eliminate high persistence altogether, suggesting that hipA7 facilitates the establishment of the persister state by inducing (p)ppGpp synthesis. Consistent with this proposal, ectopic expression of relA' from a plasmid was shown to increase the number of persistent cells produced by hipA7 relA double mutants by 100-fold or more. A model is presented that postulates that hipA7 increases the basal level of (p)ppGpp synthesis, allowing a significantly greater percentage of cells in a population to assume a persistent, antibiotic-insensitive state by potentiating a rapid transition to a dormant state upon application of stress.

Oxidative stress involved in the antibacterial action of different antibiotics

Staphylococcus aureus and Escherichia coli sensitive to chloramphenicol incubated with this antibiotic suffered oxidative stress with increase of anion superoxide (O2-). This reactive species of oxygen was detected by chemiluminescence with lucigenin. S. aureus, E. coli, and Enterococcus faecalis sensitive to ciprofloxacin exhibited oxidative stress when they were incubated with this antibiotic while resistant strains did not show stimuli of O2-. Other bacteria investigated was Pseudomonas aeruginosa, strains sensitive to ceftazidime and piperacillin presented oxidative stress in presence of these antibiotics while resistant strains were not stressed. Higher antibiotic concentration was necessary to augment O2- in P. aeruginosa biofilm than in suspension, moreover old biofilms were resistant to oxidative stress caused by antibiotics. A ceftazidime-sensitive mutant of P. aeruginosa, coming from a resistant strain, exhibited higher production of O2- than wild type in presence of this antibiotic. There was relation between antibiotic susceptibility and production of oxidative stress.

Biocide use and antibiotic resistance: the relevance of laboratory findings to clinical and environmental situations

Antibiotics are used as chemotherapeutic drugs, and biocides are used as antiseptics, disinfectants, and preservatives. Several factors affect biocidal activity, notably concentration, period of contact, pH, temperature, the presence of interfering material, and the types, numbers, location, and condition of microorganisms. Bacterial cells as part of natural or artificial (laboratory) biofilm communities are much less susceptible than planktonic cells to antibiotics and biocides. Assessment of biocidal activity by bactericidal testing is more relevant than by determination of minimum inhibitory concentrations. Biocides and antibiotics may show some similarities in their mechanisms of action and common mechanisms of bacterial insusceptibility may apply, but there are also major differences. In the laboratory, bacteria can become less susceptible to some biocides. Decreased resistance may be stable or unstable and may be accompanied by a low-level increase in antibiotic resistance. Laboratory studies are useful for examining stress responses and basic mechanisms of action and of bacterial insusceptibility to antibacterial agents. Translation of such findings to the clinical and environmental situations to provide evidence of a possible relation between biocide use and clinical antibiotic resistance is difficult and should be viewed with caution.

Antimicrobial resistance of Pseudomonas aeruginosa biofilms

Resistance to antimicrobial agents is the most important feature of biofilm infections. As a result, infections caused by bacterial biofilms are persistent and very difficult to eradicate. Although several mechanisms have been postulated to explain reduced susceptibility to antimicrobials in bacterial biofilms, it is becoming evident that biofilm resistance is multifactorial. The contribution of each of the different mechanisms involved in biofilm resistance is now beginning to emerge.

Bacterial biofilms: an emerging link to disease pathogenesis

The role of biofilms in the pathogenesis of some chronic human infections is now widely accepted. However, the criteria used to determine whether a given infection is caused by biofilms remain unclear. In this chapter we discuss three infections that are caused by biofilms--infectious kidney stones, bacterial endocarditis, and cystic fibrosis lung infections--and focus on the role of the biofilm in disease pathogenesis. Biofilms are also important as environmental reservoirs for pathogens, and the biofilm growth mode may provide organisms with survival advantages in natural environments and increase their virulence. The consequences of pathogens living in environmental biofilms and an analysis of some specific environmental biofilm systems are presented.

Swarming populations of Salmonella represent a unique physiological state coupled to multiple mechanisms of antibiotic resistance

Salmonella enterica serovar Typhimurium is capable of swarming over semi-solid surfaces. Although its swarming behavior shares many readily observable similarities with other swarming bacteria, the phenomenon remains somewhat of an enigma in this bacterium since some attributes skew away from the better characterized systems. Swarming is quite distinct from the classic swimming motility, as there is a prerequisite for cells to first undergo a morphological transformation into swarmer cells. In some organisms, swarming is controlled by quorum sensing, and in others, swarming has been shown to be coupled to increased expression of important virulence factors. Swarming in serovar Typhimurium is coupled to elevated resistance to a wide variety of structurally and functionally distinct classes of antimicrobial compounds. As serovar Typhimurium differentiates into swarm cells, the pmrHFIJKLM operon is up-regulated, resulting in a more positively charged LPS core. Furthermore, as swarm cells begin to de-differentiate, the pmr operon expression is down-regulated, rapidly reaching the levels observed in swim cells. This is one potential mechanism which confers swarm cells increased resistance to antibiotics such as the cationic antimicrobial peptides. However, additional mechanisms are likely associated with the cells in the swarm state that confer elevated resistance to such a broad spectrum of antimicrobial agents.

Mechanism of fluconazole resistance in Candida albicans biofilms: phase-specific role of efflux pumps and membrane sterols

Candida albicans biofilms are formed through three distinct developmental phases and are associated with high fluconazole (FLU) resistance. In the present study, we used a set of isogenic Candida strains lacking one or more of the drug efflux pumps Cdr1p, Cdr2p, and Mdr1p to determine their role in FLU resistance of biofilms. Additionally, variation in sterol profile as a possible mechanism of drug resistance was investigated. Our results indicate that parent and mutant strains formed similar biofilms. However, biofilms formed by double and triple mutants were more susceptible to FLU at 6 h (MIC = 64 and 16 microg/ml, respectively) than the wild-type strain (MIC > 256 microg/ml). At later time points (12 and 48 h), all the strains became resistant to this azole (MIC > or = 256 microg/ml), indicating lack of involvement of efflux pumps in resistance at late stages of biofilm formation. Northern blot analyses revealed that Candida biofilms expressed CDR and MDR1 genes in all the developmental phases, while planktonic cells expressed these genes only at the 12- and 48-h time points. Functionality of efflux pumps was assayed by rhodamine (Rh123) efflux assays, which revealed significant differences in Rh123 retention between biofilm and planktonic cells at the early phase (P = 0.0006) but not at later stages (12 and 48 h). Sterol analyses showed that ergosterol levels were significantly decreased (P < 0.001) at intermediate and mature phases, compared to those in early-phase biofilms. These studies suggest that multicomponent, phase-specific mechanisms are operative in antifungal resistance of fungal biofilms.

Swarm-cell differentiation in Salmonella enterica serovar typhimurium results in elevated resistance to multiple antibiotics

Although a wealth of knowledge exists about the molecular and biochemical mechanisms governing the swimming motility of Salmonella enterica serovar Typhimurium, its surface swarming behavior has not been extensively characterized. When inoculated onto a semisolid agar medium supplemented with appropriate nutrients, serovar Typhimurium undergoes a morphological differentiation whereby single cells hyperflagellate and elongate into nonseptate, multinucleate swarm cells. Swarm migration is a collective behavior of groups of cells. We have isolated a MudJ insertion mutant of serovar Typhimurium 14028 that failed to swarm under any conditions. The site of the MudJ insertion was determined to be in the pmrK locus within the pmrHFIJKLM operon, which was previously demonstrated to confer resistance to cationic antimicrobial peptides. beta-Galactosidase assays, using the pmrK::lacZ transcriptional fusion, showed increased expression of the pmr operon in swarm cells compared to that in vegetative cells. In concurrence with the expression data, swarm cells exhibited greater tolerance to polymyxin. To compare the profiles of vegetative and swarm-cell resistance to other antibiotics, E-test strips representing a wide range of antibiotic classes were used. Swarm cells exhibited elevated resistance to a variety of antibiotics, including those that target the cell envelope, protein translation, DNA replication, and transcription. These observations, in addition to the dramatic morphological changes associated with the swarming phenotype, provide an intriguing model for examining global differences between the physiological states of vegetative and swarm cells of serovar Typhimurium.

Contributions of antibiotic penetration, oxygen limitation, and low metabolic activity to tolerance of Pseudomonas aeruginosa biofilms to ciprofloxacin and tobramycin

The roles of slow antibiotic penetration, oxygen limitation, and low metabolic activity in the tolerance of Pseudomonas aeruginosa in biofilms to killing by antibiotics were investigated in vitro. Tobramycin and ciprofloxacin penetrated biofilms but failed to effectively kill the bacteria. Bacteria in colony biofilms survived prolonged exposure to either 10 micro g of tobramycin ml(-1)or 1.0 micro g of ciprofloxacin ml(-1). After 100 h of antibiotic treatment, during which the colony biofilms were transferred to fresh antibiotic-containing plates every 24 h, the log reduction in viable cell numbers was only 0.49 +/- 0.18 for tobramycin and 1.42 +/- 0.03 for ciprofloxacin. Antibiotic permeation through colony biofilms, indicated by a diffusion cell bioassay, demonstrated that there was no acceleration in bacterial killing once the antibiotics penetrated the biofilms. These results suggested that limited antibiotic diffusion is not the primary protective mechanism for these biofilms. Transmission electron microscopic observations of antibiotic-affected cells showed lysed, vacuolated, and elongated cells exclusively near the air interface in antibiotic-treated biofilms, suggesting a role for oxygen limitation in protecting biofilm bacteria from antibiotics. To test this hypothesis, a microelectrode analysis was performed. The results demonstrated that oxygen penetrated 50 to 90 micro m into the biofilm from the air interface. This oxic zone correlated to the region of the biofilm where an inducible green fluorescent protein was expressed, indicating that this was the active zone of bacterial metabolic activity. These results show that oxygen limitation and low metabolic activity in the interior of the biofilm, not poor antibiotic penetration, are correlated with antibiotic tolerance of this P. aeruginosa biofilm system.