Clostridium innocuum

Clostridium innocuum is an anaerobic, non-motile, gram-positive bacterium that reproduces by sporulation. While there are over 130 species of Clostridium, "C. innocuum" is the third most commonly isolated. Although it is not normally considered an aggressive human pathogen, it has been isolated in some disease processes. "C. innocuum" and other Clostridium line the oropharynx and gastrointestinal tract, and are considered normal gut flora.
Characteristics and identification.
Anaerobic gram-positive bacilli affecting human beings are generally divided into two distinct groups, those that form spores ("Clostridium" spp) and those that do not form spores. Within the spore-forming group of "Clostridium" species, some are very pathogenic or toxigenic ("C. perfringens") while others are rarely pathogenic. Identification and differentiation between anaerobic gram-positive bacteria in a clinical laboratory can be a very difficult task.
"C. innocuum" forms white, glossy, raised colonies and exhibits a chartreuse fluorescence. It is a small, non-motile rod that does not swarm. "C. innocuum" falls into the saccharolytic, non-proteolytic group of Clostridium spp. and is negative for gelatin hydrolysis, lecithinase, lipase, indole, urea, and nitrate. It is positive for glucose fermentation, esculin and produces oval, terminal spores.
Pathology.
Although "Clostridium innocuum" are often present and harmless in healthy people, they have been isolated in various infections and predominantly in patients that are immunocompromised as an opportunistic bacteria. In one case, "C. innocuum" was isolated in an infected hematoma adjacent to a transplanted kidney in a 38-year-old, Hepatitis C patient. Originally, the bacteria was misidentified as another "Clostridium" species but was eventually correctly identified. This led the authors to state that "C. innocuum" infection post transplant may be severely underestimated. "C. innocuum" has also been implicated in cases of fatal bacteremia as reported in a retrospective study done by Newark Beth Israel Medical Center. Patients that succumbed to the infection (100% mortality) had underlying disease states (malignancy, diabetes or liver disease). While clostridial species only account for 2% of clinical bacteremia, "C innocuum" was among the three most commonly isolated bacteria in this study. "C. innocuum" has also been isolated in patients with recurrent diarrhea who had prior "C. difficile" associated diarrhea. One case of fatal bacterial endocarditis was also found in the literature. This is possibly the only confirmed case of "C. innocuum" endocarditis involving the pulmonary and tricuspid valves resulting in multiple emboli and death.
Drug resistance.
In a study that identified clostridial species using commercial identification kits, it was found that as many as 10% of "C. innocuum" isolates were somewhat resistant to clindamycin. Most strains were found to be only moderately susceptible to vancomycin (MIC at which 90% of strains are inhibited, 4 micrograms/mL). In another study, where "C innocuum" was positively identified by gas-liquid chromatography, resistance to vancomycin was seen in all 28 strains isolated. All other clostridial species were at least 8 times more susceptible to vancomycin than "C. innocuum", suggesting an intrinsic vancomycin resistance mechanism in "C. innocuum".