diff --git a/sloth/annotations/container.py b/sloth/annotations/container.py
index da423f5..43f7938 100644
--- a/sloth/annotations/container.py
+++ b/sloth/annotations/container.py
@@ -314,7 +314,7 @@ class JsonContainer(AnnotationContainer):
         """
         # TODO make all image filenames relative to the label file
         f = open(fname, "w")
-        json.dump(annotations, f, indent=4, sort_keys=True)
+        json.dump(annotations, f, indent=4, separators=(',', ': '), sort_keys=True)
 
 
 class YamlContainer(AnnotationContainer):