Role of Pseudomonas aeruginosa and Pasteurella multocida outer membrane permeability properties in triclosan susceptibility

Triclosan is commonly used antimicrobial compound with a wide spectrum of activity against many bacteria however Pseudomonas aeruginosa is intrinsically resistant to high concentrations while Pasteurella multocida is markedly susceptible to the biocide. Although P. aeruginosa and P. multocida possess typical gram-negative cell envelope ultrastructure includes a gram-negative outer membrane that normally presents a permeability barrier for nonpolar compounds, the former is intrinsically resistant to triclosan, while the latter is highly susceptible. The purpose of this study was to investigate the role of the outer membrane in triclosan susceptibility for these bacteria. We show in this study that intrinsic triclosan resistance in P. aeruginosa is due, in part, to the inability of triclosan to permeate the outer membrane to reach its protoplasmic targets. Furthermore, turbidimetric growth kinetic and minimal inhibitory concentration determinations demonstrated that treatment of P. aeruginosa with compounds known to disrupt outer membrane integrity sensitized the bacterium to low concentrations of triclosan. Viable cell plating along with a chloramphenicol susceptibility bioassay revealed a dual role for outer membrane impermeability as well as triclosan-recognizing multidrug efflux pumps in triclosan resistance. Agar disk diffusion, turbidimetric growth kinetic, and viable cell plating bioassays revealed that P. multocida is susceptible to low concentrations of triclosan in a dose-dependent manner in the absense of outer membrane permeabilizers. Results obtained using 1-N-phenylnaphthylamine uptake analysis suggested that areas of phospholipid exist in the P. multocida outer membrane which provides a hydrophobic pathway for nonpolar molecules such as triclosan to partition into regions of phospholipid bilayer. In summary, these data demonstrate the necessity of the gram-negative outer membrane to form an effective permeability barrier for nonpolar compounds, thereby rendering most gram-negative bacteria intrinsically resistant to the biocide.