Synergistic Efficacy of Doxycycline and Florfenicol Against Aeromonas hydrophilia and Morganella morganii Infections in Pelodiscus sinensis with Skin Ulcer Disease

Abstract

The intensive farming of Pelodiscus sinensis (P. sinensis) for its medicinal and economic value has led to an increased incidence of bacterial skin infections, commonly treated with antibiotics. However, rising antibiotic resistance has reduced the effectiveness of these treatments, highlighting an urgent need for alternative therapeutic approaches. In this study, we isolated and identified pathogenic bacteria (Aeromonas and Morganella) from P. sinensis affected by skin ulcer disease at a turtle farm in Beijing. Antibiotic resistance was evaluated using the checkerboard microdilution method and the in vitro antibacterial efficacy of a florfenicol-doxycycline combination therapy was assessed. A total of 30 bacterial isolates were obtained, primarily Morganella and Aeromonas species. Among these, 83.3% of Morganella and 57.14% of Aeromonas isolates demonstrated multidrug-resistance. The florfenicol-doxycycline combination yielded lower fractional inhibitory concentration (FIC) values, indicating synergistic or additive effects. Animal experiments have shown that florfenicol in combination with doxycycline can achieve better therapeutic effects with less drug. These findings support that combination antibiotic therapies help in managing skin ulcer disease in P. sinensis and offer valuable insights for clinical applications.

Keywords: Pelodiscus sinensis; antibiotic resistance; combination therapy; skin ulcer disease.

Figure 1

Bacterial isolation rate from skin ulcer disease in P. sinensis. Aeromonas and Morganella were the most prevalent.

Figure 2

Phylogenetic trees of Morganella (A) and Aeromonas (B). Phylogenetic network generated using the neighbor-net algorithm with unselected P distances (bar = 0.020).

Figure 3

Antibacterial effect of combined doxycycline and florfenicol treatment on multidrug-resistant Morganella and Aeromonas strains. (a–h) Images correspond to bacterial isolates numbered 7, 11, 19, 27, 34, 2, 6, and 13, respectively.

Figure 4

Therapeutic effects of doxycycline, florfenicol, and their combination on skin ulcers in turtles infected with Aeromonas (a–c) and Morganella (d–f). (a,d) Representative images of P. sinensis with skin ulcer disease in each treatment group. Groups include doxycycline, florfenicol, doxycycline + florfenicol combination, povidone–iodine, and positive control. (b,e) The Kaplan–Meier survival curves of P. sinensis in each treatment group during the 30-day observation period. (c,f) Changes in wound area (mm²) at days 0, 10, 20, and 30. Data are presented as mean ± standard deviation. Different letters indicate statistically significant differences between groups (p < 0.05).

Figure 5

Histological analysis and bacterial load in skin lesions of turtles treated with different therapies. (a–j) Representative H&E-stained histological sections showing the epidermal and dermal layers of skin lesions from untreated and treated turtles (bar = 100 μm). (k) Negative control. (l,m) Bacterial load in the wound (CFU/mL) over 30 days of treatment, measured at different time points. Data are presented as mean ± standard deviation. Different letters indicate statistically significant differences between groups (p < 0.05).