Susceptibility of Acinetobacter baumannii isolates from different sources to biocides: a matter of biofilm

Acinetobacter baumannii is an opportunistic pathogen causing severe infections in patients residing in intensive care units. The ability of A. baumannii to acquire carbapenem-resistance makes these infections difficult to treat and poses a serious risk for global health. In fact, carbapenem-resistant A. baumannii isolates belonging to eleven international clones (IC1-11), especially IC2, have disseminated globally, generating outbreaks and epidemics. Persistence of A. baumannii in nosocomial settings is favored by its ability to produce biofilm and resist to biocides. However, investigations analyzing the efficacy of biocides by determining the minimum bactericidal concentration (MBC) on field A. baumannii isolates are rare. We determined the MBC of five commonly used biocides (hydrogen peroxide, ethanol, sodium hypochlorite, chlorhexidine digluconate, and benzalkonium chloride) adapting the EN1040 and EN1276 standard methods on 64 A. baumannii isolates collected from human hospital settings and diseased animals (multidrug resistant) and from food (multidrug susceptible). The 64 isolates belonged to diverse sequence types, some of them being part of ICs (IC2, IC7, IC9 and IC11). Efficacy of biocides was evaluated in presence of an interfering substance (bovine serum albumin, BSA) to simulate dirtiness, but also on in vitro formed biofilms. MBC₅₀ and MBC₉₀ values of the five biocides on bacteria in planktonic phase were lower than concentrations used in commercial formulations. Presence of BSA increased the MBC values of sodium hypochlorite and benzalkonium chloride. Isolates presenting the highest MBC values, mostly belonged to IC2 (multidrug resistant) and other expanding clonal complexes (multidrug susceptible). On biofilm matrix, none of the tested biocides could eradicate bacteria neither demonstrated bactericidal activity at the concentrations and exposure time tested. Based on a standardized methodology, our investigation provides data that could contribute to define epidemiological cut offs to interpret bacterial response to biocides. Investigations determining the efficacy of biocides on field isolates both in planktonic phase and in biofilms matrix are necessary to understand if hygienic measure to counteract environmental contamination are adapted and unveil bacterial adaptive strategies to these substances.