The recovery of an unusual organism in the clinical microbiology laboratory may be an indicator of an immunological abnormality in the patient. For instance, an important relationship between Clostridium septicum and colon carcinoma as well as between leukemia or lymphoma with species frequently considered contaminants (Bacillus spp., Corynebacterium spp.) or others rarely isolated from different contexts (Capnocytophaga spp.) were described. Some bacteria are almost exclusively isolated from AIDS patients (Rhodococcus equi). Campylobacter spp., Aeromonas spp., group G and mitis group streptococci were more frequently isolated in individuals suffering from any type of cancer than in other patients. Furthermore, some other bacteria can be considered markers of an undetected cancer that can be found mostly in neutropenic patients rather than in immunologically normal individuals. Possible mechanisms of bacterial oncogenesis include a modification of the inflammatory response, antigen-derived lymphoproliferation, and induction of hormones that increase epithelial cell proliferation.

Typical examples of the above are:

gastric adenocarcinoma induced by Helicobacter pylori, the association between group bovis bacteremia and colon carcinoma and the mucosa-associated lymphoid tissue (MALT) related to Helicobacter species (gastric MALT) and Chlamydophila spp. (ocular MALT). Isolation of any of these pathogens should require a thorough search for possible malignant diseases.