By Stan Deresinski, MD, FACP, FIDSA
Synopsis: Infection with Aeromonas mostly involves skin and soft tissue and traumatic wounds occurring with exposure to water contaminated by the organism. The organism often is resistant to carbapenems because of the presence of a chromosomal carbapenemase, while often remaining susceptible to third-generation cephalosporins.
Source: Pineda-Reyes R, Neil BH, Orndorff J, et al. Clinical presentation, antimicrobial resistance, and treatment outcomes of Aeromonas human infections: A 14-year retrospective study and comparative genomics of 2 isolates from fatal cases. Clin Infect Dis. 2024;79(5):1144-1152.
Pineda-Reyes reviewed cases of infection due to Aeromonas at University of Texas Medical Branch Hospitals in Galveston Bay seen over 14 years ending on Dec. 31, 2021. A total of 112 isolates of the organism were recovered from 105 patients. Skin and soft tissue infections (SSTIs) were involved in 70.5% and traumatic wounds (TWIs) were involved in 27.6%, with 24.8% having sepsis, 20% having intraabdominal infection, and 20% having burns. The infection was community-acquired in 90 cases (82.5%). Thirty-four percent of SSTIs had documented water exposure, as did 65% of those with TWI. The infection was considered complicated because of the presence of bacteremia, necrotizing fasciitis, need for amputation, or sepsis in 34 cases (32.4%). Only two patients had necrotizing fasciitis, but five patients (4.8%) required amputation.
Wounds were the most common source of positive cultures, accounting for 65.7%, followed by blood (12.4%). Of 105 isolates, 52 were identified as A. hydrophila, 17 as A. caviae, 13 as A. hydrophila/caviae, 12 as A. sobria, three as A. veronii, and one as A. hydrophila/andaei. In addition, culture results were polymicrobial in 74 cases (70.5%).
Meropenem resistance was detected in 27 (58.7%) of the 46 isolates tested, while 30% of 50 were resistant to piperacillin-tazobactam. Only one (1%) of 99 cases was resistant to cefepime, as were 8.3% of 79 to cefotaxime and 3.7% of 109 to levofloxacin. Of the 106 isolates tested, 9.4% were resistant to trimethoprim-sulfamethoxazole and 15.6% of the 32 tested were resistant to tetracycline.
Appropriate empiric antibiotic administration was initiated within 24 hours of suspected infection in 42 (40%) patients but was delayed for more than three days in 19 patients (18.1%). The following empiric antibiotics were administered: vancomycin (n = 53, 50.5%), piperacillin-tazobactam (n = 44, 41.9%), meropenem (n = 13, 12.4%), doxycycline (n = 11, 10.5%), and trimethoprim-sulfamethoxazole (n = 10, 9.5%). Overall, 27 patients (25.7%) experienced treatment failure and seven patients (6.7%) died, while 10 of 70 (14.3%) required readmission.
The investigators investigated four isolates in detail — two bloodstream isolates from fatal cases that proved by whole genome sequencing to be A. dhakensis (A. hydrophila by standard methods), one A. dhakensis reference isolate, and one A. hydrophila. The isolates carried multiple virulence factors as well as many resistance-associated genes, including mcr-3 and cphA in all four.
Commentary
Aeromonas is an organism that is prevalent in freshwater and estuarine environments, and exposure to water is a frequent source of infection. Of the 36 known species, at least 19 species may cause human infection.1 Fernandez-Bravo and colleagues have reported that just four species were among 1,852 recent clinical isolates: A. caviae (37.3%), A. dhakensis (23.5%), A. veronii (21.5%), and A. hydrophila (13.1%).1 However, there are some difficulties with accurate speciation, as demonstrated by Pineda-Reyes and colleagues in their finding that the two bloodstream isolates they selected for whole genome sequencing were A. dhakensis, not A. hydrophila/caviae and Aeromonas spp. as originally identified. They point out that, although A. dhakensis is “increasingly recognized for invasive infections and high rates of antimicrobial resistance” and poor outcomes, its identification is omitted from commercially available matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) databases. A. dhakensis also has become increasingly important as a pathogen in aquaculture.2
The carriage of multiple virulence factors by Aeromonas accounts for the severity of many infections and, at the same time, its panoply of antimicrobial resistance genes can lead to failure of empiric therapy. Furthermore, several resistance genes may have limited baseline expression and may be inducible, making detection of phenotypic resistance to some antibiotics, especially beta-lactams, potentially problematic. The prevalence of mcr is consistent with the hypothesis that Aeromonas was the original source of this polymyxin resistance gene.3 Of great interest is the high prevalence of cphA, a chromosomal zinc-containing metallo-beta-lactamase (Ambler 2b) that specifically hydrolyzes carbapenems while leaving third-generation cephalosporins intact.
In cases in which Aeromonas infection may be suspected, such as TWI with water exposure, empiric use of a carbapenem as a single agent should be avoided.
Stan Deresinski, MD, FACP, FIDSA, is Clinical Professor of Medicine, Stanford University.
References
1. Fernández-Bravo A, Figueras MJ. An update on the genus Aeromonas: Taxonomy, epidemiology, and pathogenicity. Microorganisms. 2020;8(1):129.
2. Bartie KL, Desbois AP. Aeromonas dhakensis: A zoonotic bacterium of increasing importance in aquaculture. Pathogens. 2024;13(6):465.
3. McDonald NL, Wareham DW, Bean DC. Aeromonas and mcr-3: A critical juncture for transferable polymyxin resistance in gram-negative bacteria. Pathogens. 2024;13(11):921.
Infection with Aeromonas mostly involves skin and soft tissue and traumatic wounds occurring with exposure to water contaminated by the organism. The organism often is resistant to carbapenems because of the presence of a chromosomal carbapenemase, while often remaining susceptible to third-generation cephalosporins.
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