Comparative proteomics analyses of Acinetobacter baumannii strains ATCC 17978 and AB5075 reveal the differential role of type II secretion system secretomes in lung colonization and ciprofloxacin resistance

Acinetobacter baumannii is an emerging nosocomial pathogen with alarming antibiotic resistance profiles. A better understanding of the virulence and resistance mechanisms of this pathogen is necessary for identifying new methods to combat its infections in a more efficient way. In this regard, the type II secretion system (T2SS) of A. baumannii is an attractive target majorly secreting lipid-metabolizing enzymes and contributes significantly to its virulence. No attempts have been made to study the differential role, and the nature of T2SS secreted proteins among different strains of A. baumannii. In this study, we compare T2SS substrates and functions between A. baumannii strains ATCC 17978, and the MDR highly virulent strain AB5075. The functional categories of the T2-secreted proteins were analyzed, and the virulence potential of the tested strains was compared in vivo using a murine pneumonia model. Biofilm formation was compared using crystal violet assay in micro-titer plates. The contribution to antibiotic resistance was measured by determining the minimum inhibitory concentration (MIC) of different classes of antibiotic. Results indicate that the T2SS secretome gives a colonization advantage to AB5075 over ATCC 1797 but is more important for biofilm formation by the latter. Transposon insertional inactivation of the general secretory pathway protein D (gspD), which is a key component in the structure of the T2SS, significantly increased the MIC of AB5075 to ciprofloxacin. Our report is the first to describe the strain-dependent evolution of the T2SS secretome in relation to the virulence and antibiotic resistance attributes of Gram-negative species.