Cancer has become the second ranking cause of death in the industrialized world. 
Conventional anti-cancer therapies such as surgery, radiotherapy, and 
chemotherapy are effective in the treatment of solid tumors only to some extent. 
Furthermore, they are often associated with severe side effects. Use of bacteria 
as alternative cancer therapeutics has sporadically been followed over more than 
a century. The potential to target and colonize solid tumors could be shown for 
many different bacteria in the meantime. Such bacteria are either obligate 
anaerobic bacteria like Clostridium or Bifidobacterium or facultative anaerobic 
like Escherichia coli or Salmonella. Here we describe bacterial strains that 
were successfully applied mostly in animals bearing model tumors, although first 
clinical trials have been reported as well. Our review mainly concentrates on 
Salmonella enterica serovar Typhimurium (S. Typhimurium) since these bacteria 
were studied most intensively thus far. Importantly, S. Typhimurium were shown 
not only to colonize large established tumors but also exhibit the property to 
invade and affect metastases. We report on a potential mechanism by which such 
bacteria can invade solid tumors. Furthermore, we describe several successful 
attempts in which the bacteria have been used as carriers for recombinant 
therapeutic molecules that render bacteria more powerful in eradication of the 
established tumor. Such attempts should be considered starting points on the way 
to an effective and safe tumor therapy with the help of bacteria.