Résumé : Bacterial infection is a frequent trigger of acute-onchronic liver failure (ACL F), syndrome that could also increase the risk of infection. This investigation evaluated prevalence and characteristics of bacterial and fungal infections causing and complicating ACL F, predictors of follow-up bacterial infections and impact of bacterial infections on survival. Patients 407 patients with ACL F and 235 patients with acute decompensation (AD). Results 152 patients (37%) presented bacterial infections at ACL F diagnosis; 46%(n=117) of the remaining 255 patients with ACL F developed bacterial infections during follow-up (4 weeks). The corresponding figures in patients with AD were 25% and 18% (p<0.001). Severe infections (spontaneous bacterial peritonitis, pneumonia, severe sepsis/shock, nosocomial infections and infections caused by multiresistant organisms) were more prevalent in patients with ACL F. Patients with ACL F and bacterial infections (either at diagnosis or during follow-up) showed higher grade of systemic inflammation at diagnosis of the syndrome, worse clinical course (ACL F 2-3 at final assessment: 47% vs 26%; p<0.001) and lower 90-day probability of survival (49% vs 72.5%;p<0.001) than patients with ACL F without infection. Bacterial infections were independently associated with mortality in patients with ACL F-1 and ACL F-2. Fungal infections developed in 9 patients with ACL F (2%) and in none with AD, occurred mainly after ACL F diagnosis (78%) and had high 90-day mortality (71%). Conclusion Bacterial infections are extremely frequent in ACL F. They are severe and associated with intense systemic inflammation, poor clinical course and high mortality. Patients with ACL F are highly predisposed to develop bacterial infections within a short follow-up period and could benefit from prophylactic strategies.