Lipopolysaccharide structure modulates cationic biocide susceptibility and crystalline biofilm formation in Proteus mirabilis

Chlorhexidine (CHD) is a cationic biocide used ubiquitously in healthcare settings. Proteus mirabilis, an important pathogen of the catheterized urinary tract, and isolates of this species are often described as “resistant” to CHD-containing products used for catheter infection control. To identify the mechanisms underlying reduced CHD susceptibility in P. mirabilis, we subjected the CHD tolerant clinical isolate RS47 to random transposon mutagenesis and screened for mutants with reduced CHD minimum inhibitory concentrations (MICs). One mutant recovered from these screens (designated RS47-2) exhibited ~ 8-fold reduction in CHD MIC. Complete genome sequencing of RS47-2 showed a single mini-Tn5 insert in the waaC gene involved in lipopolysaccharide (LPS) inner core biosynthesis. Phenotypic screening of RS47-2 revealed a significant increase in cell surface hydrophobicity and serum susceptibility compared to the wildtype, and confirmed defects in LPS production congruent with waaC inactivation. Disruption of waaC was also associated with increased susceptibility to a range of other cationic biocides but did not affect susceptibility to antibiotics tested. Complementation studies showed that repression of smvA efflux activity in RS47-2 further increased susceptibility to CHD and other cationic biocides, reducing CHD MICs to values comparable with the most CHD susceptible isolates characterized. The formation of crystalline biofilms and blockage of urethral catheters was also significantly attenuated in RS47-2. Taken together, these data show that aspects of LPS structure and upregulation of the smvA efflux system function in synergy to modulate susceptibility to CHD and other cationic biocides, and that LPS structure is also an important factor in P. mirabilis crystalline biofilm formation.

De-repression of the smvA efflux system arises in clinical isolates of Proteus mirabilis and reduces susceptibility to chlorhexidine and other biocides

Proteus mirabilis is a common pathogen of the catheterised urinary tract and often described as intrinsically resistant to the biocide chlorhexidine (CHD). Here we demonstrate that de-repression of the smvA efflux system has occurred in clinical isolates of P. mirabilis and reduces susceptibility to CHD and other cationic biocides. Compared to other isolates examined, P. mirabilis RS47 exhibited a significantly higher CHD MIC (≥512 μg/ml) and significantly greater expression of smvA. Comparison of the RS47 smvA and cognate smvR repressor with sequences from other isolates, indicated that RS47 encodes an inactivated smvR. Complementation of RS47 with a functional smvR from isolate RS50a (which exhibited the lowest smvA expression and lowest CHD MIC) reduced smvA expression by ∼59-fold, and markedly lowered the MIC of CHD and other cationic biocides. Although complementation of RS47 did not reduce MICs to concentrations observed in isolate RS50a, the significantly lower polymyxin B MIC of RS50a indicated that differences in LPS structure are also a factor in P. mirabilis CHD susceptibility. To determine if exposure to CHD can select for mutations in smvR, clinical isolates with the lowest CHD MICs were adapted to grow at increasing concentrations of CHD up to 512 μg/ml. Analysis of the smvR in adapted populations indicated that mutations predicted to inactivate smvR occurred following CHD exposure in some isolates. Collectively, our data show that smvA de-repression contributes to reduced biocide susceptibility in P. mirabilis, but differences in LPS structure between strains are also likely to be an important factor.

Bacterial biofilm formation on indwelling urethral catheters

Urethral catheters are the most commonly deployed medical devices and used to manage a wide range of conditions in both hospital and community care settings. The use of long-term catheterization, where the catheter remains in place for a period >28 days remains common, and the care of these patients is often undermined by the acquisition of infections and formation of biofilms on catheter surfaces. Particular problems arise from colonization with urease-producing species such as Proteus mirabilis, which form unusual crystalline biofilms that encrust catheter surfaces and block urine flow. Encrustation and blockage often lead to a range of serious clinical complications and emergency hospital referrals in long-term catheterized patients. Here we review current understanding of bacterial biofilm formation on urethral catheters, with a focus on crystalline biofilm formation by P. mirabilis, as well as approaches that may be used to control biofilm formation on these devices. SIGNIFICANCE AND IMPACT OF THE STUDY: Urinary catheters are the most commonly used medical devices in many healthcare systems, but their use predisposes to infection and provide ideal conditions for bacterial biofilm formation. Patients managed by long-term urethral catheterization are particularly vulnerable to biofilm-related infections, with crystalline biofilm formation by urease producing species frequently leading to catheter blockage and other serious clinical complications. This review considers current knowledge regarding biofilm formation on urethral catheters, and possible strategies for their control.

Prototype Development of the Intelligent Hydrogel Wound Dressing and Its Efficacy in the Detection of Model Pathogenic Wound Biofilms

The early detection of wound infection in situ can dramatically improve patient care pathways and clinical outcomes. There is increasing evidence that within an infected wound the main bacterial mode of living is a biofilm: a confluent community of adherent bacteria encased in an extracellular polymeric matrix. Here we have reported the development of a prototype wound dressing, which switches on a fluorescent color when in contact with pathogenic wound biofilms. The dressing is made of a hydrated agarose film in which the fluorescent dye containing vesicles were mixed with agarose and dispersed within the hydrogel matrix. The static and dynamic models of wound biofilms, from clinical strains of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Enterococcus faecalis, were established on nanoporous polycarbonate membrane for 24, 48, and 72 h, and the dressing response to the biofilms on the prototype dressing evaluated. The dressing indicated a clear fluorescent/color response within 4 h, only observed when in contact with biofilms produced by a pathogenic strain. The sensitivity of the dressing to biofilms was dependent on the species and strain types of the bacterial pathogens involved, but a relatively higher response was observed in strains considered good biofilm formers. There was a clear difference in the levels of dressing response, when dressings were tested on bacteria grown in biofilm or in planktonic cultures, suggesting that the level of expression of virulence factors is different depending of the growth mode. Colorimetric detection on wound biofilms of prevalent pathogens (S. aureus, P. aeruginosa, and E. faecalis) is also demonstrated using an ex vivo porcine skin model of burn wound infection.