Bacteria may play an important role in treating cancer. Records going back 150 years show bacteria acting against cancer: the German doctors W. Busch and F. Fehleisen separately noticed that certain cancers shrank after patients in hospital caught infections such as Streptococcus pyogenes (erysipelas). In America, William Coley saw a patient with cancer of the neck begin to recover after an erysipelas infection. But the toxicity of such methods was hard to predict, and they were abandoned.
Decades after Coley's work, interest returned, especially for solid tumours. Experiments showed that disease-causing anaerobic clostridia could grow in the dead (necrotic) areas of tumours in animals and shrink the tumour, but with severe toxicity: most animals became very ill or died. So research moved to bacteria that don't cause disease.
Several bacteria were tested, such as bifidobacteria, lactobacilli and clostridia, for their ability to gather in tumours and shrink them. Clostridium novyi proved best, but it makes a deadly toxin, so it is used after the toxin gene has been removed. These genetically modified bacteria are being used against cancer together with standard chemotherapy.
Besides damaging tumour tissue, the bacteria help trigger an immune response that attacks the tumour. Bacillus Calmette–Guérin (BCG) is already used this way, especially to treat bladder cancer. Certain strains of Salmonella with their toxin genes removed have also been tried. Some of these bacteria need purines to multiply; tumour cells are rich in purines while normal cells aren't, so the bacteria prefer tumours.
Bacteria may also be used to carry drugs into tumours, with long-lasting effects against a range of cancers, and possibly the shrinking of metastases.
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