Aeromonas hydrophila is a Gram-negative, facultative anaerobic, motile, mesophilic, oxidase-positive bacillus with global distribution in aquatic environments. It has several virulence factors such as proteases, phospholipases, lipases, enterotoxins, adhesins, invasins and aerolysins,1 and can be the aetiological agent of gastrointestinal infections, wound infections, biliary system infections, post-traumatic cellulitis and bacteremia.2
The treatment of choice is third-generation cephalosporins, although there are reports of some resistant strains due to the production of extended-spectrum ®-lactamases (ESBL) and inducible AmpC ®-lactamases. In addition, they can present selective resistance to carbapenems by production of a class B CphA chromosomal carbapenemase.3 Resistance to quinolones,4 tetracyclines,5 colistin (mcr-3-type plasmid) and sulfonamides6 has also been described.
We present a clinical case of an infection caused by a strain of A. hydrophila with an unusual sensitivity profile.
This was a 72-year-old patient with a history of adenocarcinoma of the colon and chronic pulmonary embolism, who attended the Accident and Emergency Department of our centre with partial bowel obstruction and portal peripheral subsegmental thrombosis, for which, after being assessed, he was admitted to General Surgery. Subsequently, after suddenly developing speech problems, he was transferred to Neurology where, following a biopsy, he was diagnosed with cerebral aspergillosis.
Due to a fever spike of 38°C, two peripheral blood cultures were taken and the femoral catheter tip was cultured. The blood cultures were incubated in the BD BACTEC FX® incubator and were positive on the first day. In the catheter culture, 7 CFU were isolated using the Maki technique. After seeding the flasks, growth was observed of honey-coloured ®-haemolytic colonies on blood agar and oxidase-positive and lactose-fermenting colonies on MacConkey agar. Using MALDI-TOF, it was unanimously identified as A. hydrophila, with scores above 2; maximum of 2.34. In addition, a battery of biochemical tests was carried out in which the esculin, Voges-Proskauer and L-arabinose fermentation tests were positive, and cellobiose and sorbitol fermentation negative. Antibiotic susceptibility was determined by the broth microdilution method in MicroScan® (Beckman Coulter Inc., Brea, CA, USA) (Table 1). We used the EUCAST 2021 breakpoints corresponding to Aeromonas spp. and those corresponding to Enterobacterales for the rest of the antibiotics not contemplated. Some type of plasmid resistance mechanism was suspected, as the sensitivity to ceftazidime/avibactam was not consistent with the resistance mechanism to carbapenems most associated with this species,3 and subsequently the presence of a KPC carbapenemase was detected using immunochromatography (NG-Test CARBA 5, NG Biotech®, Guipry, France). A specific PCR and a Sanger sequencing of the fragment were then performed. After comparing the sequence obtained in The Comprehensive Antibiotic Resistance Database® (CARD), the presence of a KPC-2-like coding gene was confirmed.
Sensitivity of the KPC carbapenemase-producing strain of Aeromonas hydrophila (MIC in mg/l).
| Antibiotic | MIC | Status |
|---|---|---|
| Ampicillin | >8 | Resistant |
| Mecillinam | >8 | Resistant |
| Piperacillin | >16 | Resistant |
| Amoxicillin/clavulanic acid | >32 | Resistant |
| Piperacillin/tazobactam | >16 | Resistant |
| Cefazoline | >16 | Resistant |
| Cefuroxime | >8 | Resistant |
| Cefoxitin | >16 | Resistant |
| Cefotaxime | >32 | Resistant |
| Ceftazidime | 8 | Resistant |
| Ceftazidime/avibactam | ≤2 | Sensitive |
| Cefepime | >8 | Resistant |
| Aztreonam | >4 | Resistant |
| Imipenem | >8 | Resistant |
| Meropenem | >32 | Resistant |
| Ertapenem | >1 | Resistant |
| Ceftolozane/tazobactam | 4 | Resistant |
| Amikacin | 16 | Resistant |
| Nitrofurantoin | ≤64 | Sensitive |
| Colistin | 4 | Resistant |
| Co-trimoxazole | ≤2/38 | Sensitive |
| Levofloxacin | >1 | Resistant |
| Ciprofloxacin | >1 | Resistant |
At the time the isolation was made, the patient was being treated with moxifloxacin and voriconazole, although he had previously received antibiotic treatment with acyclovir, linezolid, amphotericin and meropenem. After the microbiological report, he was administered ceftazidime/avibactam. As this treatment was effective, he was placed under the care of Hospital at Home.
KPC carbapenemases are class A serine-carbapenemases first described in Klebsiella pneumoniae. These plasmid enzymes have been transmitted from Klebsiella spp. to other genera and species, from Enterobacterales to Pseudomonas aeruginosa. They show a wide range of MIC to carbapenems and are inhibited to varying degrees by classic ®-lactamase inhibitors; they are usually completely inhibited by the new inhibitors. As a result, strains that carry KPC are sensitive to combinations such as ceftazidime/avibactam, unlike class B carbapenemase-producing strains, such as those of the CphA type.
Although it is not a common nosocomial pathogen, multidrug resistance in Aeromonas spp. should be taken into account. A study in Bangladesh showed that Aeromonas spp. was the enteric pathogen with the highest rate of multidrug resistance (81.5%) among immunocompromised patients and in critical care units.7 Moreover, although we have not found any references in the literature to clinical isolates of KPC-producing Aeromonas spp., strains carrying the blaKPC-2 gene have been reported in a wastewater treatment plant in Japan, so we should be alert to the emergence of these plasmid resistance mechanisms in Aeromonas spp.8
