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1 Department of Medicine, University at Buffalo, Buffalo, NY, USA
* To whom correspondence should be addressed. E-mail: trusso{at}acsu.buffalo.edu.
Enteric gram-negative bacilli, such as E. coli are the most common cause of nosocomial pneumonia. In this study a wild-type extraintestinal pathogenic strain of E. coli (ExPEC)(CP9) and isogenic derivatives deficient in hemolysin (Hly) and cytotoxic necrotizing factor (CNF) were assessed in vitro and in a rat model of gram-negative pneumonia to test the hypothesis that these virulence factors induce neutrophil apoptosis and/or necrosis/lysis. As ascertained by in vitro caspase-3/7 and LDH activities, and neutrophil morphology; Hly mediated neutrophil apoptosis at lower E. coli titers (1 x 105-6 cfu) and necrosis/lysis at higher titers (
1 x 107 cfu). Data suggested that CNF promotes apoptosis but not necrosis or lysis. We also demonstrated that annexinV/7-AAD staining was an unreliable assessment of apoptosis using live E. coli. The use of caspase-3/7 and LDH activities and neutrophil morphology supported that necrosis, not apoptosis, was the primary mechanism by which neutrophils were affected in our in vivo gram-negative pneumonia model using live E. coli. In addition, in vivo studies demonstrated that Hly mediated lung injury. Neutrophil necrosis was not observed when animals were challenged with purified lipopolysaccharide, demonstrating the importance of using live bacteria. These findings establish that Hly contributes to ExPEC virulence by mediating neutrophil toxicity, with necrosis/lysis being the dominant effect of Hly on neutrophils in vivo, and by lung injury. Whether Hly-mediated lung injury is due to neutrophil necrosis, a direct effect of Hly, or both is unclear.
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