JOHNS HOPKINS
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Department of Pathology
600 N. Wolfe Street / Baltimore MD 21287-7093
(410) 955-5077 / FAX (410) 614-8087
Division of Medical Microbiology
THE JOHNS HOPKINS MICROBIOLOGY NEWSLETTER Vol. 26, No. 01
Tuesday, January 16, 2007
A. Provided by Sharon Wallace, Division of Outbreak Investigation, Maryland Department of
Health and Mental Hygiene.
There is no information available at this time.
B. The Johns Hopkins Hospital, Department of Pathology, Information provided by,
Justin A. Bishop, M.D.
Case presentation: The patient is a 75-year-old man with a history of hypertension and chronic
renal disease. He received a cadaveric renal transplant on 11/15/2006. After three days, the
patient developed atrial fibrillation and was transferred to the medical intensive care unit. He
proceeded to develop symptoms consistent with shock, prompting an exploratory laparotomy.
The patient was found to have infarcted, perforated bowel and underwent a subtotal colectomy on
11/22/2006. The patient has since developed acute cellular rejection of the donated kidney, as
well as bilateral deep venous thromboses, a urinary tract infection, and peritonitis. A 1/4/2007
culture of the patient’s peritoneal fluid grew Aeromonas hydrophila.
Aeromonas: This genus consists of oxidase positive, fermentative Gram negative rods from the
family Aeromonadaceae. Eight of the seventeen species are associated with human disease; these
species are all motile and mesophilic (grow best from 25-42 degrees C). The most common of
these species are A. hydrophila, A. veronii bv. sobria, and A. caviae. Aeromonas grows well on
most routine media. Aeromonads are catalase positive, reduce nitrate to nitrite, and most species
demonstrate beta hemolysis. Aeromonas is most easily confused with another oxidase-positive
genus, Vibrio; resistance to O/129 vibriostatic agent and an inability to grow in 6.5% NaCl
usually indicate Aeromonas. Individual species are differentiated by a battery of biochemical
tests and antimicrobial markers. Aeromonads may produce a multitude of extracellular enzymes,
including enterotoxins, proteases, hemolysins, lipases, adhesins, agglutinins, and peptidases.
Microscopically, Aeromonads appear as straight rods or coccoid forms ranging from 1 to 3.5 µm
long and 0.3 to 1 µm wide, with polar, monotrichous flagella. On CIN agar, colonies appear pink
to red with a clear, uneven apron. Aeromonas species are ubiqitous in various aquatic
environments worldwide; those species associated with human illness have been isolated from
fresh produce, meat, and dairy products as well.
Clinical manifestation: Aeromonas is most often implicated in gastroenteritis, the clinical
spectrum of which can include such symptoms as diarrhea, fever, abdominal pain, nausea, and
vomiting. A. caviae is the most commonly recovered species in such cases. These infections are
usually self-limiting, although children may require hospitalization for fluid replacement.
Rarely, A. veronii bv. sobria has been associated with a severe cholera-like syndrome. In addition,
there have been two reported cases of Aeromonas enterocolitis associated with hemolytic uremic
syndrome.
The most common extra-intestinal sites from which Aeromonas is isolated are blood and wounds.
Aeromonas sepsis may occur in immunocompromised patients, most often those with liver disease
or hematological malignancies. Such infections are associated with a high mortality. A.
hydrophila is the most common species isolated from wounds, which range from cases of
uncomplicated cellulitis to severe myonecrotic infections with high mortality. Interestingly,
Aeromonas infections have also been associated with the use of medicinal leeches; Aeromonas is a
member of the leech's intestinal flora, where it assists in the enzymatic digestion of blood. Finally,
Aeromonads have also been less commonly implicated in infections of the eye, bone, meninges,
ear, bladder, heart, peritoneum, and gallbladder.
Pathogenesis: The virulence of Aeromonas is multifactorial. Factors include the multitude of
extracellular enzymes produced, invasivity, pili formation, an S layer, and the host's own response.
As Aeromonas resides in an aquatic environment, infection is dependent on exposure to water or
food products that have been in contact with such water. Similarly, wound infections are usually
preceded by a traumatic injury that comes in contact with water.
Treatment: Most cases of diarrheal disease are self-limiting; however, antibiotics are generally
administered to the pediatric, geriatric, or immunocompromised. Other forms of infection should
be treated with antimicrobials. Most strains are resistant to penicillin, ampicillin, and ticarcillin,
but susceptible to second and third-generation cephalosporins, aminoglycosides, carbapenems,
tetracyclines, TMP-SMX, and fluoroquinolones. However, there have been shown to be clear
differences in susceptibilities among species, making species identification and susceptibility
testing of all isolates of utmost importance. In addition, many strains produce an inducible betalactamase that may not be detected by commercial susceptibility systems.
References:
1. Horneman A and Morris JG. "Aeromonas infections" on UpToDate. 2006.
2. Munoz P, et al. "Aeromonas peritonitis." Clin Infect Dis. 1994; 18(1):32-7.
3. Murray PR, et al, eds. Manual of Clinical Microbiology, 8th ed. ASM Press, 2003.
4. Snower DP, et al. "Aeromonas hydrophila infection associated with the use of medicinal
leeches." J Clin Microbiol. 1989; 27(6):1421-2.