In the bustling realm of cleanliness and germ-fighting, biocides became the unsung heroes in hospitals, households, farms, and food industries. Picture a scenario where these magical concoctions, armed with the power to banish microbes, ensured a sparkling world of hygiene. Yet, behind the scenes, a tale of unintended consequences unfolded. The biocides, with concentrations strong enough to make even the toughest bacteria surrender, lingered on the surfaces post-cleanup, creating an environment of unintentional continuous exposure.  This gave rise to an unexpected twist – bacteria, once vulnerable, started to progressively develop tolerance to the biocides. Some scientific studies suggest that the residual biocides might influence bacterial adaptability, orchestrating the emergence of multi-drug resistant (MDR) pathogens [1].

In our lab, we are highly interested in the opportunistic human pathogen Acinetobacter baumannii. This organism’s remarkable ability to adapt to antimicrobials [2], makes it a compelling subject for our investigation. Our scientific curiosity is piqued by the persistent nature of A. baumannii in nosocomial environments, where biocides play a pivotal role in routine disinfection.  Currently, there is inadequate regulation regarding the use of biocides, and researchers have a limited understanding of how these compounds exert their antimicrobial effects. In this context, a rigorous analysis of the impacts of residual biocide amounts on this multidrug-resistant (MDR) pathogen becomes imperative. With this in mind, we systematically studied the mechanism of action of ten structurally and chemically diverse biocides, and their potential impact on the antibiotic tolerance capacity of A. baumannii. Our goal was simple: unravel the connection between these agents and the emergence of antimicrobial resistance.

We used a global genomics approach called transposon-directed insertion-site sequencing (TraDIS) to map the genomic response of A. baumannii upon exposure to sub-inhibitory concentrations of silver nitrate, benzalkonium, cetyltrimethylammonium bromide (CTAB), chlorhexidine, triclosan, chloroxylenol, polyvidone iodine, bleach, glutaraldehyde, and ethanol – all of which are biocides either commonly used in clinical or household settings or are classified by WHO as “essential medicines”. TraDIS allowed us to simultaneously assay the fitness of individual cells within a saturated transposon mutant library upon biocide exposure.

Our findings made it clear: when exposed to low levels of biocides, crucial genes responsible for vital cellular functions, like outer membrane oligosaccharide biosynthesis, cell division, cell respiration, electron transport chain, and TCA cycle were significantly affected. What also caught our attention is the impact on multidrug efflux, a superhero mechanism in antimicrobial resistance [3]. Our study indicated that the multidrug efflux pump AdeABC in A. baumannii potentially mediates resistance to biocides like benzalkonium, cetrimonium, chlorhexidine, triclosan, and chloroxylenol. Additionally, benzalkonium and chlorhexidine, even at low concentrations, were shown to induce the expression of the AdeABC efflux pump through its transcriptional activator AdeRS. This is quite an alarming impact of biocide exposure, as AdeABC is one of the most clinically significant drug transporters that mediate tolerance to several classes of antibiotics.

Our research also directed to another noteworthy finding: even at low concentrations, biocides can disrupt bacterial cell-membrane potential, thereby reducing the effectiveness of several key antibiotics. Cell division, cell respiration and the tricarboxylic acid cycle genes are essential for bacterial fitness in the presence of multiple biocides. This led us to predict that most biocides used in this study would impact the cell membrane potential. Also, as membrane transport in cells is an energy-dependent procedure, biocide-induced dissipated membrane potential would likely mean that these compounds lower the efficacy of efflux pumps that rely on membrane potential as energy sources.

To put our hypothesis to the test, we directly measured cell membrane potential and energy-dependent membrane transport activities in the presence of all ten biocides. The results spoke volumes – seven biocides dissipated membrane potential without causing membrane damage or permeabilization. This is a big deal because many antibiotics, especially aminoglycosides, rely on intact membrane potential to penetrate bacterial cells. This was mirrored in our findings, as low concentrations of membrane-potential-dissipating biocides impacted the intake and effectiveness of antibiotics with intracellular targets. Benzalkonium, chlorhexidine, CTAB, and polyvidone iodine, known for disrupting membrane potential, also acted as antagonists, weakening the potency of antibiotics like amikacin, ciprofloxacin, and tigecycline. Interestingly, antibiotics targeting the cell envelope, such as colistin and imipenem, remained unaffected by these biocides. This discovery therefore confirms the critical insight that residual levels of biocides on surfaces might indeed be a driving force in the emergence and propagation of bacteria that can evade antibiotics.

To sum up, our study delves into the dangerous impact of lingering biocide residues on clinically relevant bacterial species at the genomic level. It emphasizes the need to explore clinical environmental factors that could contribute to the spread of drug resistance. With multidrug resistance on the rise, relying solely on antibiotic stewardship might not be enough. It suggests that some level of regulation and control for biocides and other essential medicines vital in infection control may be necessary. While our research unveils evidence that even low biocide levels can compromise antibiotic effectiveness, it's crucial to note that this was a lab-induced phenomenon. The next step is real-world research to see if residual biocides on hospital surfaces indeed create selective pressure, leading to the emergence of multidrug-resistant bacteria. This exploration is vital for a more comprehensive understanding of the dynamics between biocides, antibiotics, and the alarming growth of drug-resistant bacteria.

References:

1. Webber, M.A., et al., Parallel evolutionary pathways to antibiotic resistance selected by biocide exposure. Journal of antimicrobial chemotherapy, 2015. 70(8): p. 2241-2248.
2. Peleg, A.Y., H. Seifert, and D.L. Paterson, Acinetobacter baumannii : Emergence of a Successful Pathogen. Clinical microbiology reviews : CMR., 2008. 21(3): p. 538-582.
3. Coyne, S., P. Courvalin, and B. Périchon, Efflux-Mediated Antibiotic Resistance in Acinetobacter spp. Antimicrobial Agents and Chemotherapy, 2011. 55(3): p. 947-953.