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Journal of Clinical Microbiology, April 1998, p. 1103-1104, Vol. 36, No. 4
Microbial Diseases Laboratory, Division of
Communicable Disease Control, California Department of Health
Services, Berkeley, California 94704-10111;
Natividad Medical Center, Salinas, California
939062; and
Santa Barbara County
Health Care Services, Santa Barbara, California
931103
Received 28 October 1997/Returned for modification 15 December
1997/Accepted 30 December 1997
Two unusual cases of Aeromonas infection are described,
one associated with bacteremia (Aeromonas schubertii) and
another in which the organism was recovered from an infected gall
bladder (Aeromonas veronii biotype veronii). These strains
were initially identified as Vibrio damsela and
Vibrio cholerae by the Vitek and API 20E systems,
respectively. Use of appropriate screening tests and familiarity with
the newer Aeromonas species could prevent initial
misidentifications and potential public health consequences.
Over the past two decades the number
of recognized species in the genus Aeromonas has expanded
from 4 (Aeromonas hydrophila, Aeromonas sobria,
Aeromonas caviae, and Aeromonas salmonicida) to
at least 13 legitimate genomospecies (2, 3, 6). At least 9 of these taxa have been recovered from clinical material and therefore
could be pathogenic for humans. Despite these taxonomic advances,
commercial identification systems have often been unable to accurately
identify aeromonads to the species level or as part of a complex.
Often, Aeromonas species are mistakenly identified as
vibrios, with which they share many phenotypic characteristics (9). A common example of this misidentification involves
A. caviae being misidentified as Vibrio fluvialis
(6). In this report we describe two cases of infection
caused by unusual Aeromonas species that were originally
mistaken for vibrio infections.
Case 1.
A 64-year-old man with a history of liver cirrhosis
developed diarrhea of 2 to 3 days' duration subsequent to consuming a seafood meal containing shellfish. Subsequent to his diarrheal episode
he developed signs of sepsis, and blood samples were drawn. Two sets of
blood cultures were positive for gram-negative bacilli, and two colony
types were initially detected. One isolate was identified as
Vibrio ("Listonella damsela,"
"Photobacterium damsela") damsela, while the
other isolate was identified as A. sobria by the Vitek
system (bioMérieux Vitek, Inc., Hazelwood, Mo.). The strains were
subsequently forwarded by the Monterey County Health Department to the
Microbial Diseases Laboratory, where both isolates were identified as
Aeromonas schubertii (Table
1).
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Misidentification of Unusual Aeromonas
Species as Members of the Genus Vibrio: a Continuing
Problem
ABSTRACT
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TABLE 1.
Relevant properties of unusual
Aeromonas isolatesa
Case 2. A 56-year-old male who was traveling was seen in the emergency room of a local hospital for acute cholecystitis requiring an immediate cholecystectomy. Cultures of the infected gall bladder yielded a gram-negative bacillus. No further medical information on this patient was available. The organism was presumptively identified as Vibrio cholerae by the local public health laboratory based upon the API 20E (API bioMérieux, Hazelwood, Mo.) profile number, 5347125. The isolate was then forwarded to the Microbial Diseases Laboratory for definitive identification, which revealed the strain in question to actually be Aeromonas veronii biotype veronii (Table 1).
These two cases highlight several important points regarding the accurate identification of aeromonads. First, if key screening reactions that separate members of the Vibrionaceae (Aeromonas, Plesiomonas, and Vibrio) had been used (Table 1), the initial misidentification of these organisms could have been avoided (7). Key screening reactions include growth in nutrient broth in the presence and absence of added salt, resistance to the vibriostatic agent O/129 (2,4-diamino-6,7-diisopropylpteridine), the string test, and growth on thiosulfate-citrate-bile salts-sucrose agar. These can be of particular importance in the case of the ornithine decarboxylase-positive biotype of A. veronii, which can be mistakenly identified as V. cholerae if a complete battery of screening tests are not performed (1, 5). We have previously received several water isolates of A. veronii biotype veronii thought to be V. cholerae by municipal water districts which failed to perform sufficient in vitro biochemical testing to clearly separate these two groups. Such misidentifications can immediately raise red flags leading to mobilization of health care personnel because of the public health significance of V. cholerae. A second point concerns the failure of commercial systems to satisfactorily identify these microorganisms. This has been a recognized problem for over a decade, and it continues to be a weak area of commercial identification systems. While it has not been determined whether the misidentification of one Aeromonas species as another is of any major clinical impact, the identification of aeromonads as vibrios can be an important distinction. In some states, such as California, all cases of Vibrio infections are reportable due to the intimate association these illnesses have with the consumption of raw or undercooked shellfish (4). More importantly, the misidentification of an aeromonad as V. cholerae triggers a series of epidemiologic responses because of the immense public health significance this organism has. Although Vibrio infections are relatively rare in the United States, their public health significance and their clinical significance are well documented, and more accurate and reliable methods to rapidly and accurately identify these organisms need to be developed. Finally, to our knowledge this is the first report linking A. schubertii gastroenteritis and bacteremia with the consumption of shellfish. A. schubertii is one of only five Aeromonas species linked to human cases of septicemia, and all of the documented reports of A. schubertii infection have involved extraintestinal disease (wounds and bacteremia). In vitro laboratory investigations suggest that this is one of the more pathogenic species within the genus based upon 50% lethal dose studies with outbred mice (8). The present case 1 report further supports this view. In summary, then, it is clear that until better and more accurate methods to identify members of the Vibrionaceae are developed, it will be extremely difficult to get an accurate picture of the type of illnesses caused by many of the less well-characterized Aeromonas species and the roles they play in clinical infections. Furthermore, the identification of aeromonads as vibrios by many different commercial systems needs to be addressed in light of its medical and public health importance.| |
FOOTNOTES |
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* Corresponding author. Mailing address: Microbial Diseases Laboratory, 2151 Berkeley Way, Berkeley, CA 94704-1011. Phone: (510) 540-2242. Fax: (510) 540-2374. E-mail: jjanda{at}hw1.cahwnet.gov.
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Journal of Clinical Microbiology, April 1998, p. 1103-1104, Vol. 36, No. 4
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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