Cell surface physiology and outer cell envelope impermeability for hydrophobic substances in Burkholderia multivorans

Influence of outer membrane permeabilization on intrinsic resistance to the hydrophobic biocide triclosan in opportunistic Serratia species

Triclosan is a hydrophobic antimicrobial agent commonly employed in health care settings. While it exhibits broad-spectrum antibacterial properties, the gram-negative nosocomial opportunists Pseudomonas aeruginosa and Serratia marcescens are atypically refractory. Intrinsic resistance to triclosan in P. aeruginosa is largely due to its outer membrane impermeability properties for hydrophobic and bulky substances. The present study was undertaken to determine the relationship between triclosan and the outer cell envelopes of thirteen strains of ten Serratia species reported to be opportunistic pathogens in humans. General intrinsic resistance to hydrophobic and other outer membrane impermeant compounds was assessed using cultural selection, disk agar diffusion, and macrobroth dilution bioassays. Uptake of the hydrophobic fluorescent probe 1-N-phenylnapthylamine was assessed in four disparate strains of S. marcescens. Batch culture kinetics in the presence of combinations of triclosan and outer membrane permeabilizer compound 48/80 allowed analysis of outer membrane involvement in intrinsic resistance. Aggregate results revealed that individual species ranged in response to hydrophobic and bulky molecules from generally refractory to extremely susceptible. Moreover, susceptivity to triclosan sensitization by chemical disruption of outer membrane exclusionary properties differed markedly among species which exhibited intrinsic resistance to triclosan. These data suggest that disparate opportunistic pathogens within the genus Serratia differ phenotypically regarding the degree to which outer membrane exclusion contributes to intrinsic resistance for impermeant molecules in general, and triclosan specifically. Ancillary resistance mechanisms appear to contribute in some species and may involve constitutive multi-drug efflux systems. Importance A paucity of knowledge exists regarding the cellular and molecular mechanisms by which opportunistically pathogenic members of the genus Serratia are able to infect immunocompromised and otherwise susceptible individuals, and then evade chemotherapy. This is especially true for species other than Serratia marcescens and Serratia liquefaciens, although much remains to be learned with regard to the nature of key virulence factors and infection mechanisms which allow for the typically nosocomial acquisition of even these species. The research described in the present study will provide a better understanding of the contribution of outer cell envelope permeability properties to the pathogenicity of these opportunistic species in an ever-increasing susceptible patient population. It is our hope that greater knowledge of the basic biology of these organisms will contribute to the mitigation of suffering they cause in patients with underlying diseases.

Disparate properties of Burkholderia multivorans and Pseudomonas aeruginosa regarding outer membrane chemical permeabilization to the hydrophobic substances novobiocin and triclosan

Burkholderia multivorans causes opportunistic pulmonary infections and is intrinsically resistant to many antibacterial compounds including the hydrophobic biocide triclosan. Chemical permeabilization of the Pseudomonas aeruginosa outer membrane affects sensitization to hydrophobic substances. The purpose of the present study was to determine if B. multivorans is similarly susceptive suggesting that outer membrane impermeability properties underlie triclosan resistance. Antibiograms and conventional macrobroth dilution bioassays were employed to establish baseline susceptibility levels to hydrophobic antibacterial compounds. Outer membrane permeabilizers compound 48/80, polymyxin B, polymyxin B-nonapeptide, and ethylenediaminetetraacetic acid were used in attempts to sensitize disparate B. multivorans isolates to the hydrophobic agents novobiocin and triclosan, and to potentiate partitioning of the hydrophobic fluorescent probe 1-N-phenylnapthylamine (NPN). The lipophilic agent resistance profiles for all B. multivorans strains were essentially the same as that of P. aeruginosa except that they were resistant to polymyxin B. Moreover, they resisted sensitization to hydrophobic compounds and remained inaccessible to NPN when treated with outer membrane permeabilizers. These data support the notion that while both phylogenetically-related organisms exhibit general intrinsic resistance properties to hydrophobic substances, the outer membrane of B. multivorans either resists permeabilization by chemical modification or sensitization is mitigated by a supplemental mechanism not present in P. aeruginosa.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Disparate outer membrane exclusionary properties underlie intrinsic resistance to hydrophobic substances in Pseudomonas spp. isolated from surface waters under triclosan selection

Representative members of surface water microbiota were obtained from three unrelated municipal sites in Oklahoma by direct plating under selection by the hydrophobic biocide triclosan. Multiple methods were employed to determine if intrinsic triclosan resistance reflected resistance to hydrophobic molecules by virtue of outer membrane impermeability. While all but one organism isolated in the absence of triclosan were able to initiate growth on MacConkey agar, only one was able to initiate significant growth with triclosan present. In contrast, all bacteria selected with triclosan were identified as Pseudomonas spp. using 16S RNA gene sequencing and exhibited growth comparable to Pseudomonas aeruginosa controls in the presence of hydrophobic antibacterial agents to include triclosan. Two representative bacteria isolated in the absence of triclosan allowed for greater outer membrane association with the fluorescent hydrophobic probe 1-N-phenylnapthylamine than did two triclosan-resistant isolates. Compound 48/80 disruption of outer membrane impermeability properties for hydrophobic substances either partially or fully sensitized nine of twelve intrinsically resistant isolates to triclosan. These data suggest that outer membrane exclusion underlies intrinsic resistance to triclosan in some, but not all Pseudomonas spp. isolated by selection from municipal surface waters and implicates the involvement of concomitant triclosan resistance mechanisms.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.