Characterisation of Pellicles Formed by <i>Acinetobacter baumannii</i> at the Air-Liquid Interface

The clinical importance of Acinetobacter baumannii is partly due to its natural ability to survive in the hospital environment. This persistence may be explained by its capacity to form biofilms and, interestingly, A. baumannii can form pellicles at the air-liquid interface more readily than other less pathogenic Acinetobacter species. Pellicles from twenty-six strains were morphologically classified into three groups: I) egg-shaped (27%); II) ball-shaped (50%); and III) irregular pellicles (23%). One strain representative of each group was further analysed by Brewster’s Angle Microscopy to follow pellicle development, demonstrating that their formation did not require anchoring to a solid surface. Total carbohydrate analysis of the matrix showed three main components: Glucose, GlcNAc and Kdo. Dispersin B, an enzyme that hydrolyzes poly-N-acetylglucosamine (PNAG) polysaccharide, inhibited A. baumannii pellicle formation, suggesting that this exopolysaccharide contributes to pellicle formation. Also associated with the pellicle matrix were three subunits of pili assembled by chaperon-usher systems: the major CsuA/B, A1S_1510 (presented 45% of identity with the main pilin F17-A from enterotoxigenic Escherichia coli pili) and A1S_2091. The presence of both PNAG polysaccharide and pili systems in matrix of pellicles might contribute to the virulence of this emerging pathogen.鲍曼不动杆菌（Acinetobacter baumannii）的临床重要性，部分源于其在医院环境中存活的天然能力。这种持久定植特性可归因于其形成生物被膜（biofilm）的能力；值得注意的是，相较于其他致病力较弱的不动杆菌属物种，鲍曼不动杆菌更易在气液界面形成菌膜（pellicle）。本研究对26株菌株的菌膜进行形态学分类，可划分为三类：Ⅰ类为卵圆形菌膜（占比27%）；Ⅱ类为球形菌膜（占比50%）；Ⅲ类为不规则形菌膜（占比23%）。随后选取每类菌株的1株代表株，通过布鲁斯特角显微镜（Brewster’s Angle Microscopy）追踪菌膜的发育过程，结果证实其形成无需锚定至固体表面。对菌膜基质的总碳水化合物分析显示，其主要包含三种组分：葡萄糖、N-乙酰葡糖胺（GlcNAc）以及3-脱氧-D-甘露糖-辛酮糖酸（Kdo）。分散酶B（Dispersin B）是一种可水解聚-N-乙酰葡糖胺（poly-N-acetylglucosamine, PNAG）多糖的酶，该酶能够抑制鲍曼不动杆菌菌膜的形成，提示此种胞外多糖参与了菌膜的构建过程。菌膜基质中还存在由分子伴侣- usher系统（chaperone-usher systems）组装的菌毛的三个亚基：主要菌毛亚基CsuA/B、A1S_1510（与产肠毒素大肠杆菌（enterotoxigenic Escherichia coli）的主要菌毛亚基F17-A的同源性达45%）以及A1S_2091。菌膜基质中同时存在PNAG多糖与菌毛系统，这可能增强这种新发病原体的致病力。