Aeromonas hydrophila bacterium is found in all freshwater environments as well as brackish, chlorinated, and unchlorinated water.¹ The bacterium is both a gram-negative rod and a facultative anaerobe. The bacterium is about 0.3–1.0 µm in diameter and 1.0–3.5 µm in length. It achieves optimal growth at 28^ºC (82^ºF), but can also grow at temperatures ranging from 4C (39^ºF) to 37^ºC (99^ºF).^2,3 The bacterium was originally discovered in 1962 while researchers were looking at the causes of “red fin,” a disease of eel and fish.⁴ In fish and other marine life the infection has been associated with several diseases—tail rot, fin rot, and hemorrhagic septicemias.^4,5 The bacterium is transmitted in humans through oral contact with contaminated water, food, soil, feces, and/or ingestion of contaminated fish or reptiles. The bacterium is commonly acquired through an open wound that is exposed to contaminated water.^3,6

Case Report

A 64-year-old man presented at the emergency department of the Medical Institute, Ministry of Interior. Three days prior to presentation, the patient had taken a mud bath in brackish water on the seaside. While in the water, he experienced a burning pain in the lower portion of his right leg. A few hours later he became febrile, experiencing chills and diarrhea. On the third day post exposure to the brackish water, the patient experienced increased edema of the right lower extremity, which was tender to palpation and manifested blue streaks up the thigh along with redness, bullous lesions, and ecchymosis. For more than 10 years he suffered from type 2 diabetes and postphlebitic syndrome with varicose ulcers. Upon arrival at the authors’ emergency department, the patient was agitated, with blood pressure of 60/40 mmHg, heart rate 110/min, respiratory rate 30/min, and temperature 37.5^ºC (100^ºF). Examination of the right lower extremity revealed edema with ecchymosis, blue discoloration, and bullae extending up the thigh. Capillary refill of the hands was delayed. White blood cell count was 7,400 mm³, hematocrit 36%, sodium 137 mEq/L, potassium 5.1 mEq/L, blood urea nitrogen (BUN) 18 mg/dL, creatinine 3.58 mg/dL, LDH 286 U/L, CPK 880 U/L, and MB 41 U/L. The platelet count was 174,000 mm³, INR 3.10, and partial thromboplastin time was 42 seconds. Doppler examination revealed normal circulation in arterial and venous vessels of the right lower extremity. Chest radiograph results were normal.
The patient was initially treated for toxic shock in the emergency department and then transferred to the surgical clinic. He continued to receive vasopressors to stabilize blood pressure and broad-spectrum antibiotics to control infection. The patient’s condition was not stable enough for surgery and exploration of the wound.
Three days after hospitalization, the patient displayed signs indicative of renal damage. The BUN level had risen to 44.5 mg/dL and the creatinine value to 4.15 mg/dL despite supportive therapy including vasopressors, intravenous fluids, and diuretics. The other laboratory values were ASAT 200 U/L, ALAT 105 U/L, LDH 845 U/L, CPK 4435 U/L, and MB 190 U/L.
The condition of the lower extremity had worsened with increased edema and dermatoneurosis of the entire leg. An incision revealed necrotizing fasciitis and myonecrosis (Figure 1).
Tissue biopsy showed subcutaneous abscess formation, acute inflammation, hemorrhage, and necrosis in the soft tissue, muscles, and fascia. Tissue culture revealed mixed aerobic and anaerobic bacteria. Aeromonas hydrophila was found along with gram-positive cocci Enterococcus sp, and S aureus, as well as gram-negative rods Klebsiella oxytoca, Morganella morganii, and E coli. The hemoculture revealed only Aeromonas hydrophila.
After the operation, the general condition of the patient slowly improved. The patient was discharged from the hospital on day 43 after admission.

Discussion

Aeromonas hydrophila is a heterotrophic, gram-negative bacterium, mainly found in areas where the climate is very warm. This bacterium can also be found in fresh, salt, marine, estuarine, chlorinated, and unchlorinated water. Aeromonas hydrophila is resistant to chlorine, refrigeration, or cold temperatures (Aeromonas hydrophila has been known to survive in temperatures as low as 4^ºC).^7,8 Aeromonas hydrophila contains a gene called Aerolysin Cytotoxic Enterotoxin (ACT) that releases a toxin that can cause tissue damage. The aerolysin toxin is produced by some strains of Aeromonas hydrophila. It is an extracellular, soluble, hydrophilic protein that exhibits both hemolytic and cytolytic properties. Aerolysin binds to specific glycoprotein receptors on the surface of eucaryotic cells before inserting into the lipid bilayer and forms holes.^9,10
In the presented case (the first reported in Bulgaria), Aeromonas hydrophila was cultured from the necrotic tissue and from hemoculture. Only single reports of this rare infection are available in the medical literature.³ In all reported cases, the contamination occurred in an aquatic environment mostly upon biting.^3,6 Aeromonas hydrophila releases Aerolysin and was presumed to be the cause of the necrotizing fasciitis and myonecrosis. The other bacteria cultured from biopsy tissue (Enterococcus sp, S aureus, Klebsiella oxytoca, Morganella morganii, and E coli) may have been a result of the wound contamination. Most likely these bacteria act synergistically with Aeromonas hydrophila although they have not been characterized in the literature as independent agents of necrotizing fasciitis and myonecrosis. This particular interaction has been confirmed by other reports.³
Gastroenteritis is a disease associated with Aeromonas hydrophila. This disease can infect anyone, but it occurs mostly among young children and individuals who have compromised immune systems and growth problems.^8,11 Immunocompromised hosts can develop Aeromonas pneumonia, sepsis or meningitis, and both immunocompetent and immunocompromised hosts can suffer from Aeromonas-infected wounds.^12–15
The 3 types of wound infections that can stem from Aeromonas hydrophila in humans are cellulitis, myonecrosis with necrotizing fasciitis, and ecthyma gangrenosum. Cellulitis is the most common infection associated with Aeromonas hydrophila.⁸ Myonecrosis with necrotizing fasciitis and ecthyma gangrenosum are less common but have more damaging results.³ Cellulitis, with the proper medication, will pass with minimal damage, while the others can result in amputation and sometimes death.^16,17
This microbe is resistant to penicillin, ampicillin, carbenicillin, and ticarcillin but is susceptible to broad-spectrum cephalosporins, aminoglycosides, carbapenems, chloramphenicol, tetracycline, trimethoprim-sulfamethoxazole, and quinolones.¹⁸
Necrotizing fasciitis must be promptly recognized and aggressively treated, since the resulting rates of morbidity and mortality are high if treatment is delayed. Treatment of necrotizing fasciitis is first and foremost surgical intervention that includes antibiotic therapy and supportive care.^3,16,17

Conclusion

Reports of wounds infected with Aeromonas have been increasingly reported in the literature. Unlike gastroenteritis, these infections can have fatal or seriously debilitating outcomes, such as amputation.
Aeromonas hydrophila infection is frequently the cause of necrotizing fasciitis in patients with suppressed immune systems, diabetes, burns, and trauma in an aquatic setting. These patients require aggressive antimicrobial therapy and debridement. Individuals that fail to respond to these treatment measures may require amputation.