Aeromonas is a genus of gram-negative bacilli that are similar in morphology to the Enterobacteriaceae. Numerous species have been described, the most frequent of which are A. caviae, A. veronii and, above all, A. hydrophila. These bacteria are transmitted through contaminated fresh and salt waters. In humans, they are most commonly involved in wound infections, systemic disease (usually in patients with underlying conditions) and especially acute gastroenteritis (AGE).1 In Spain, Aeromonas is considered the third most frequent enteropathogenic bacterium identified in patients with AGE, following Campylobacter and Salmonella, accounting for 6–7% of cases2; in the Valencian metropolitan area, the incidence of AGE due to Aeromonas is of 20 cases per 1,000,000 inhabitants.3 Recently, the European Society for Paediatric Gastroenterology Hepatology and Nutrition (ESPGHAN) and the European Society for Paediatric Infectious Diseases (ESPID) have recommended oral azithromycin for the treatment of moderate to severe gastroenteritis based on the bacterial enteropathogens that are most commonly involved.4 However, due to the lack of data on Aeromonas species, we do not have any evidence of the actual usefulness of this antimicrobial. The aim of our study was to assess the in vitro activity of azithromycin against clinical isolates of Aeromonas.

We conducted a prospective descriptive study of clinical isolates of A. hydrophila from patients with AGE during 2015. We isolated the bacterial strains by culturing stool samples in selective CIN agar (Becton-Dickinson). Bacterial identification and antibiotic susceptibility testing were performed by means of standardised and widely used techniques, specifically with NC70 panels of the commercially available MicroScan WalkAway® system (Siemens). Furthermore, we used Etest® strips (bioMérieux) to determine the minimum inhibitory concentration (MIC) in a McFarland 0.5 bacterial suspension; and interpreted the results applying the epidemiological cut-off values established by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) for Shigella and Salmonella (sensitivity: MIC<16mg/L). We analysed the cases by on time, age and sex as well as the microbiological characteristics of the isolated strains.

During the period under study, A. hydrophila was isolated from a total of 50 clinical samples. The median age of the patients was 8 years (range, 0–89 years). Fifty-four percent were children (78% aged less than 4 years) and 32% elderly. Fifty-two percent were male. All isolates came from patients in primary care, and no patients reported having been hospitalised. The isolates did not exhibit a significant seasonal pattern, although 58% of detections occurred in summer or spring. The percentage of isolates that showed in vitro sensitivity to antibiotics commonly used to treat AGE were: 0% for ampicillin, 10% for amoxicillin/clavulanic acid, 92% for cefuroxime, 100% for cefotaxime, 100% for ciprofloxacin, 100% for gentamicin and 96% for cotrimoxazol. All isolates were considered sensitive to azithromycin; the determined MICs ranged between 0.5 and 8.0mg/L, and the MIC50 (concentration required to inhibit 50% of bacterial strains) and the MIC90 (for inhibition of 90% of strains) were 3.0–6.0mg/L, respectively.

Antibiotic treatment is not always indicated for AGE caused by Aeromonas, so the use and choice of antimicrobials should be based on the symptoms and age of the patient, the site of infection (intestinal or extraintestinal), the immune status of the patient and the local prevalence of different pathogens and patterns of drug resistance. The antibiotic sensitivity of Aeromonas has been studied mainly in environmental bacterial strains,5 but few studies have analysed clinical isolates and the use of azithromycin. In our experience, A. hydrophila showed a high sensitivity to azithromycin in vitro, although it would be advisable to have international committees on microbiology to establish clinical breakpoints to determine the sensitivity or resistance to azithromycin, as has been done for Salmonella and Shigella. Given its easy dosage (one dose a day), low toxicity and evidence of the high sensitivity of Campylobacter and also Salmonella6 to it, azithromycin has been established as the empirical treatment of choice in cases of bacterial AGE that require antibiotherapy.