AFM study of the effect of metronidazole on surface structures of sulfate-reducing bacteria

Atomic force microscopy investigation of the morphology and topography of colistin-heteroresistant Acinetobacter baumannii strains as a function of growth phase and in response to colistin treatment

The prevalence of infections caused by multidrug-resistant gram-negative Acinetobacter baumannii strains and the lack of novel antibiotics under development are posing a global dilemma, forcing a resurgence of the last-line antibiotic colistin. Our aim was to use atomic force microscopy (AFM) to investigate the morphology and topography of paired colistin-susceptible and -resistant cells from colistin-heteroresistant A. baumannii strains as a function of bacterial growth phase and colistin exposure. An optimal AFM bacterial sample preparation protocol was established and applied to examine three paired strains. Images revealed rod-shaped colistin-susceptible cells (1.65 +/- 0.27 microm by 0.98 +/- 0.07 microm) at mid-logarithmic phase, in contrast to spherical colistin-resistant cells (1.03 +/- 0.09 microm); the latter were also more diverse in appearance and exhibited a rougher surface topography (7.05 +/- 1.3 nm versus 11.4 +/- 2.5 nm for susceptible versus resistant, respectively). Cellular elongation up to approximately 18 microm at stationary phase was more commonly observed in susceptible strains, although these "worm-like" cells were also observed occasionally in the resistant population. The effects of colistin exposure on the cell surface of colistin-susceptible and -resistant cells were found to be similar; topographical changes were minor in response to 0.5 microg/ml colistin; however, at 4 microg/ml colistin, a significant degree of surface disruption was detected. At 32 microg/ml colistin, cellular clumping and surface smoothening were evident. Our study has demonstrated for the first time substantial morphological and topographical differences between colistin-susceptible and -resistant cells from heteroresistant A. baumannii strains. These results contribute to an understanding of colistin action and resistance in regard to this problematic pathogen.

Microélectrophorèse et microscopie à force atomique: deux nouveaux outils d'évaluation de l'effet pariétal d'antibactériens

Microbial cell surface properties play a central role in controlling phenomena such as bacterial adhesion and biofilm formation (on stent or on prosthesis for example). The quantification of these properties and the understanding of interactions with antibacterial compounds remain difficult, in view of the complex and dynamic nature of the cell wall constituents. Various approaches, macroscopic, microscopic or molecular, have been developed. Two of them interest us today: (i) microelectrophoresis, which permits to evaluate surface modifications by measuring eletrophoretic mobility; and (ii) atomic force microscopy (AFM), a high resolution imaging device, which allows investigations at nanometric scale. After brief presentation of principles and instrumentations, the aim of this article is to present the different applications of these techniques in Microbiology, and to discuss interest of these tools in order to investigate mechanism of action of antibacterial compounds.