Insight into the Role of Chlorhexidine in Delftia acidovorans Biofilm Formation

LETTER

We read with great interest the recently published work of Rema and colleagues (1). In that work, the authors verify the role of chlorhexidine (CHX) within the structure of Delftia acidovorans biofilm in two strains: one wild strain, tolerant to CHX, and a mutant with a Tn5 mutation.

As their main results, Rema et al. demonstrate by confocal laser scanning microscopy (CLSM) that a subinhibitory concentration of chlorhexidine (CHX) (10 μg/ml) increased the biofilm thickness of the wild strain WT15 (MIC = 15 μg/ml) and that “the proportion of viable cells increased with the distance from the attachment surface,” suggesting that a subinhibitory concentration of CHX stimulates biofilm production in WT15 and does not affect the cells' viability. On the other hand, the dose of 10 μg/ml significantly reduced the thickness of the biofilm in the MT51 strain (MIC = 1.0 μg/ml), but “total viable cells decreased significantly (P < 0.05) with depth following both 10 μg/ml and 30 μg/ml CHX treatment,” indicating a possible impact on the intercellular arrangement of MT51 biofilms but not on isolated cells. That is, exposure to CHX did not influence cell viability, even at inhibitory doses, as was also observed for WT15 with subinhibitory CHX dosages, which is different from what was reported in the discussion: “MT51 cells in direct contact with CHX were affected by the biocide to a greater extent than were the cells located deep within the biofilm matrix.”

With their scanning transmission X-ray microscopy (STXM) results, the authors verify that CHX at subinhibitory levels increased the protein amount in some regions of WT15 biofilms and also state that this phenomenon may be explained by the presence of two distinguishable patterns in WT15 biofilms (as in biofilms not treated with CHX). In MT51 strains, however, CHX did not affect the biofilm protein composition. Taken together, these findings may indicate the absence of a role for CHX in affecting biofilm protein proportions.

In addition, infrared spectroscopy (IR) demonstrated that a prominent peak at 1,492 cm⁻¹ might explain the higher levels of accumulation of CHX in MT51 biofilm strains and not a disruption of cells.

Taking all those findings together, the authors attributed to the paradigm that biofilm growth renders cells more resistant than planktonic counterparts and conclude that “Delftia acidovorans biofilms provided an ideal model system for integrating analysis of CLSM, STXM, and IR data to gain insights into bacterial interactions with CHX at the micro- to nanoscales of resolution.”

In their concluding remarks, indeed, the authors speculate about a potential role of the cell membrane in CHX resistance. In our view, it seems that the authors neglected to consider a particular feature of the D. acidovorans cell membrane that can contribute to its resistance or tolerance to CHX. The major protein compounds of the outer membrane of D. acidovorans are outer membrane protein 32 (Omp32) (2) and two less prominent proteins, Omp37 and Omp21 (3). Previous characterization of Omp32 demonstrated that this protein presents strong anion selectivity and positive surface potential at both the external and the periplasmic surface of the outer membrane (4), repulsing the penetration of positively charged compounds, as in the case of CHX. Although this theory has not been proved so far, antimicrobial resistance speculated to be intrinsic to this bacterium, in particular aminoglycosides and polymyxins, endorses our assertion because both of those drug categories are polycationic, i.e., positively charged. On the other hand, antimicrobial agents with zwitterionic, neutral, or negatively charged species may preserve their activities against D. acidovorans isolates (5). Considering both the data found by Rema et al. (1) and the information about the characteristics of outer membrane proteins of D. acidovorans, we may suppose that CHX actually acts only at the intercellular level, disrupting the biofilm but not killing the cells, suggesting the need for multiple approaches in order to reach an effective clearance of D. acidovorans biofilms.

Ed. Note: The authors of the published article declined to respond.