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Category: Food Microbiology
Aeromonas, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555819972/9781555819965.ch15-1.gif /docserver/preview/fulltext/10.1128/9781555819972/9781555819965.ch15-2.gifAbstract:
Gram-negative Aeromonas species are ubiquitous in both aquatic and terrestrial environments. Their adaptability to various ecosystems has resulted in their isolation from a wide variety of organisms, spanning mammals to teleosts. As the awareness of this genus grows, its prevalence and economic impact continue to increase. Because of their aquatic nature, aeromonads have been isolated from most agricultural food products, whether directly or as a result of contamination within the food processing system. Additionally, seafood, especially finfish, is vulnerable to Aeromonas-associated diseases. Most food- and water-related human illnesses caused by aeromonads are due to the species Aeromonas hydrophila, A. veronii, A. caviae, and A. dhakensis. This genus has demonstrated its pathogenic nature in conditions ranging from gastroenteritis to wound infections to severe life-threatening septicemia due to a myriad of virulence factors, including adhesion molecules (i.e., lateral flagella and pili), capsules, cytotonic and cytotoxic enterotoxins (i.e., Alt, Ast, Act and AerA), hemolysins, and degradative enzymes, as well as the formation of biofilms. Thanks to their ubiquitous nature, in combination with overuse of antibiotics agriculturally and clinically, aeromonads have acquired an alarming resistance to a plethora of antibiotics. Therefore, this genus can serve as biological reservoirs of antibiotic resistance genes; intergenus gene exchange between members of the Enterobacteriaceae and the Aeromonadaceae has been documented. Together, Aeromonas spp. present multiple risks: they are foodborne pathogens; they impose economic burdens on the food industry due to contamination, resulting in food spoilage; and they act as reservoirs of antibiotic resistance, resulting in clinical infections that are more resilient to treatment.
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Virulence factors associated with Aeromonas spp. causing gastroenteritis. Aeromonads encode numerous virulence factors, though their pathogenic roles vary with disease. The image presents the most influential virulence factors associated with gastroenteritis. Adherence to intestinal epithelial cells and mucosa to initiate infection is mediated via lateral flagella. Additionally, adherence is involved in aggregative behavior with other aeromonads, resulting in a “stacked-brick” formation and biofilm formation (a). Once bound to the epithelial cells of the small intestine, Aeromonas spp. have an arsenal of enterotoxins they can secrete. Two of the enterotoxins, Alt and Ast, act in a cholera toxin-like fashion, elevating adenylate cAMP in the host. The exact mechanisms are not fully elucidated, but this induces an efflux of chloride ions (b), leading to osmotic leakage of water into the intestinal lumen and causing diarrhea. Other virulence factors associated with gastroenteritis include pore-forming aerolysin-related cytotoxic enterotoxin (Act) and aerolysin (AerA). These enterotoxins cause direct damage to the intestinal wall via enterocyte toxicity (c). The release of enterotoxins during disease distorts enterocyte morphology, including loss of microvilli (c) as well as detachment, providing passages through submucosa and tight junctions (d) ( 26 ). Some clinical strains of Aeromonas produce Shiga toxin homologs (encoded by stx1 and stx2), causing a ribotoxic stress response, which in turn leads to inflammation and apoptosis (e), though the exact mechanisms of action of these Aeromonas homologs have yet to be determined.
Potential role of virulence factors in Aeromonas-associated bacteremia. Aeromonads produce a single polar flagellum to aid in motility (a). T2SS releases proaerolysin and proteases, which may either act as activators of aerolysin or break down complement proteins (b). Mature aerolysin is a pore-forming enzyme causing complete lysis of red blood cells (c). Hydrolysis of lymphocytic membranes occurs via secreted lipases mediated by T6SS (d) and can cause irreversible erythrocyte lysis (e). Additionally, T6SS secrete iron-binding proteins (e.g., amonabactin) that enable acquisition of extracellular iron (f). Reversible lysis of erythrocytes may occur following α-hemolysin production. CM, cell membrane. Illustration by Cody Rasmussen-Ivey, Tibor Král, and S. Králová.
Preliminary identification scheme for Aeromonas. This flow chart can be used for rapid preliminary identification of aeromonads. Aeromonads are Gram-negative, facultative anaerobes, ferment glucose, and are positive for oxidase, which differentiates them from oxidase-negative Enterobacteriaceae. Distinguishing Aeromonas spp. from other oxidase-positive Gram-negative bacilli (i.e., Vibrio and Plesiomonas) is possible with the following test results: resistant to vibriostatic compound O/129 (150 μg 2,4-diamino-6,7-diisopropylpteridine), negative for ornithine decarboxylase (ORN-decarboxylase), and resistant to ampicillin (10 μg). +, positive reaction; −, negative reaction; v, variable result. a, some strains are sensitive to O/129 (150 µg) ( 161 ); b, except for rare ornithine decarboxylase-positive strains ( 162 ); c, except for A. trota ( 22 ).
Cycle of antibiotic resistance within aeromonad reservoirs. The widespread occurrence of aeromonads contributes to their ability to colonize cold- and warm-blooded animals, both aquatic and terrestrial. The use of antibiotics prophylactically and clinically provides environments with subinhibitory concentrations of antibiotics, creating stress that can lead to resistance. Solid arrows represent the entrance of antibiotics, and dashed arrows represent pathways of transmission of antimicrobial resistance (AbR). Illustration by Andrew Albers and T. Skwor.