On the microbiological side, the only phenotypic characteristic that classically distinguishes L. monocytogenes from L. innocua is hemolysis (4, 8). However, the hemolytic activity of L. monocytogenes may be weak, especially with low-producing strains, and questionable hemolytic reaction has been reported with some L. innocua strains when nonselective culture with brain heart infusion agar was used (5). Moreover, nonhemolytic L. monocytogenes strains have been recently described (2). The use of the CAMP test, proposed to enhance hemolytic activity of L. monocytogenes, does not always resolve the problem: ambiguous hemolysis has been noticed with several L. monocytogenes strains, even with the use of conventional sheep blood agar plates (6). Thus, distinguishing between L. monocytogenes and L. innocua on the basis of hemolytic activity is a risk. Rapid slide test with Listeria O polyvalent antiserum (recommended for a rapid identification of L. monocytogenes, since it is able to detect Listeria serovars 1, 4, 2, and 3) (Difco) should be avoided because of lack of specificity. Indeed, a distinct agglutination was noticed, although the strain was of serovar 6a, which was incompatible with L. monocytogenes. Since a few years ago, the Api Listeria system (Biomerieux) has provided a useful help for differentiating between L. monocytogenes and L. innocua on the basis of the absence of arylamidase (differentiation of L. innocua and L. monocytogenes [DIM] test) from the former (4). In our case, the Api Listeria system has given, easily and rapidly, the correct identification.
The clinical case reported here is unusual for several reasons. Bacteria encountered in cases of acute cholangitis are usually gram-negative rods such as Escherichia coli or Klebsiella spp. or sometimes gram-positive cocci (Enterococcus or Streptococcus), seldom anaerobes (7), and the initial antibiotherapy given was active against most of these species except Enterococcus. Gram-positive rods have never been reported. Most of these rods are susceptible to cefotaxime, except those of the genus Listeria, which are naturally resistant to this antibiotic. Second, among the Listeria species, only L. monocytogenes is widely known to be able to cause severe disease. L. seeligeri has been documented recently to have caused acute meningitis in an immunocompetent host (9). As for L. innocua, this is, to our knowledge, the first description of a human infection caused by this bacterium. Third, our patient was not known to be immunocompromised. Only inhaled corticoids taken to treat asthma could have led to some immunodeficiency. Thus, that L. innocua infection could lead to a fatal outcome was totally unexpected.