The acid sphingomyelinase (ASMase) localizes to the lumen of endosomes, 
phagosomes and lysosomes as well as to the outer leaflet of the plasma membrane 
and hydrolyses sphingomyelin to ceramide and phosphorylcholine. Using the 
facultative intracellular bacterium Listeria monocytogenes, we show that 
maturation of phagosomes into phagolysosomes is severely impaired in macrophages 
genetically deficient for ASMase. Unlike in wild-type macrophages, phagosomes 
containing L. monocytogenes in ASMase(-/-) macrophages remained positive for the 
late phagosomal markers mannose-6-phosphate receptor (M6PR) and Rab7 for at 
least 2 h and, correspondingly, showed delayed acquisition of lysosomal markers 
like lysosome associated membrane protein 1 (Lamp1). The transfer of lysosomal 
fluid phase markers into phagosomes containing L. monocytogenes was severely 
impaired in ASMase(-/-) macrophages and decreased with increasing size of the 
cargo. Moreover, phagosomes containing L. monocytogenes from ASMase(-/-) cells 
acquired significantly less listeriocidal proteases cathepsin D, B and L. The 
results of this study suggest that ASMase is required for the proper fusion of 
late phagosomes with lysosomes, which is crucial for efficient transfer of 
lysosomal antibacterial hydrolases into phagosomes.