THE GENUS AEROMONAS

The genus Aeromonas was initially placed in the family Vibrionaceae with the genera Plesiomonas and Vibrio. Subsequent phylogenetic studies resulted in transfer of Aeromonas to the new family Aeromonadaceae, as the sole genus within the order Aeromonadales. There are currently 15 validly published species, which are catalase positive, reduce nitrate to nitrite, and are resistant to the vibriostatic agent O129. Most species possess flagella, although fish strains of Aeromonas salmonicida are exceptions.

Aeromonads grow well at temperatures ranging from 10° to 42° C. Most strains pathogenic for fish and shellfish are psychrophilic and seldom grow above 28° C, with optimal growth occurring at 22° to 25° C. Cells are straight, may be coccoid, and range from 1.0 to 3.5 μm long and 0.3 to 1 μm wide (Figure 20-1).

FIGURE 20-1 Aeromonas hydrophila Gram stain.

(Courtesy Public Health Image Library, PHIL #1255, Centers for Disease Control and Prevention, Atlanta, 1977, W.A. Clark.)

DISEASES AND EPIDEMIOLOGY

Aeromonads are found in water, soil, and sewage. Their pathogenicity was first established in poikilothermic animals, in association with massive outbreaks of septicemia, with high mortality in aquatic species. Aeromonas spp. inhabit the gastrointestinal tract of many aquatic species, and are important pathogens of reptiles and amphibians. Rare infections have been reported in swine, cattle, birds, and marine mammals.

Aeromonas hydrophila is an opportunistic pathogen associated with hemorrhagic septicemia in cold-blooded animals, including amphibians, reptiles, fish, and shellfish. It is best known as the etiologic agent of “red leg” disease in frogs, so named because of the hemorrhages observed in the leg muscles of affected animals (Figure 20-2). Fish disease has worldwide distribution in warm-water cultured fish such as catfish, carp, and bass. Aeromonas hydrophila is also associated with tail or fin rot. Hemorrhagic septicemia, sometimes called motile aeromonad septicemia, is characterized by the presence of small surface lesions that progress to skin sloughing and localized hemorrhages. Necrotic stomatitis of snakes is also attributed to A. hydrophila infection.

FIGURE 20-2 Frog with lesions of “red leg.”

(Courtesy Raymond E. Reed.)

Aeromonas salmonicida ssp. salmonicida, the etiologic agent of furunculosis in salmonid fish, is one of the oldest fish pathogens and is found in most regions of the world. The disease was so named because, in the subacute or chronic form, lesions in the musculature resemble boils or furuncles. Although once considered to be a pathogen of salmonids, the bacterium has increasingly been recovered from other species, such as perch and pike, and may on occasion be associated with disease outbreaks in cultured fish. Infection occurs by fish-to-fish contact either through the skin or by ingestion, and asymptomatic carriers have been implicated in transmission. Risk factors include skin trauma, stress, and presence of concurrent disease. Furunculosis is most prevalent in late spring or summer, when water temperatures are warmest.

Aeromonas spp. have recently emerged as important human pathogens associated with food-borne disease outbreaks and traveler’s diarrhea, with A. hydrophila, Aeromonas caviae, and Aeromonas veronii biovar sobria being the most frequently isolated species. Infection is generally through consumption of contaminated water and food. Sporadic cases of septicemia, peritonitis, urinary tract infections, and a hemolytic-uremic syndrome have been attributed to A. hydrophila. The ability of aeromonads to survive and multiply at a variety of temperatures, over a wide pH range, and at salt concentrations up to 4% have contributed to their ability to cause food-borne illness.

PATHOGENESIS

Aeromonads invade host cells and disseminate to virtually any organ. Death is usually attributed to massive septicemia and toxic extracellular products of the organism, which interfere with host blood supply and result in massive tissue necrosis.

Virulence factors of Aeromonas spp. include toxins, surface proteins or structures, and extracellular degradative enzymes. A cytotoxin encoded by a plasmid-based gene is similar to Shiga toxin and may be responsible for gastroenteritis. Distinct hemolysins causing fluid accumulation in ileal loop assays have been detected in β-hemolytic strains of A. hydrophila. Aerolysin, another hemolysin, is a pore-forming toxin that causes ion leakage and, ultimately, eukaryotic cell lysis. The cytotoxic enterotoxin, the heat-stable cytotoxic enterotoxin, and the heat-labile cytotoxic entero toxin all contribute to enteropathogenicity. Certain Aeromonas spp. produce polar and lateral flagella; the former mediate swimming, allowing the organisms to reach their target cells, whereas the latter facilitate swarming over surfaces and adhesion to and invasion of host cells. Pili have been described in several Aeromonas spp., and type IV pili are known virulence determinants in A. salmonicida. An outer-membrane protein in A. caviae functions as an adhesin. A capsule, produced by some species, imparts resistance to bacteriolysis and phagocytosis.

Aeromonads produce a wide range of enzymes that cause tissue damage and aid in the establishment of infection by overwhelming host defenses and by providing nutrients for bacterial proliferation. These include serine protease, metalloproteases, aminopeptidases, lipases, and phospholipases. Aeromonas spp. produce siderophores to obtain their supply of iron.