Molecular characterization and antibiotics resistance of Aeromonas species isolated from farmed African catfish Clarias gariepinus Burchell, 1822

Characterization and Genome Analyses of the Novel Phages P2 and vB_AhydM-H1 Targeting Aeromonas hydrophila

Background

The emergence of antibiotic-resistant Aeromonas hydrophila strains presents a global health and aquaculture challenge. Bacteriophages offer promise as an alternative to antibiotics for treating drug-resistant Aeromonas infections.

Methods

Two new phages, P2 and vB_AhydM-H1, targeting pathogenic A. hydrophila were isolated from sewage water. Their morphology, growth characteristics, lytic activity, stability, and genomes were analyzed.

Results

Phage P2, a member of genus Ahphunavirus, and vB_AhydM-H1, a novel member of genus Pahsextavirus, exhibited narrow host ranges, extended latent periods, and typical burst sizes. Both phages remained stable at 40°C for 1 h and within a pH range of 4 to 10 for 3 h. The genomes of P2 and vB_AhydM-H1 spanned 42,660 bp with 49 open reading frames (ORFs) and 52,614 bp with 72 ORFs, respectively. Proteomic (ViPTree) and phylogenetic (VICTOR) analyses confirmed that both phages aligned with their respective families. DeepTMHMM predictions suggested that P2 and vB_AhydM-H1 encode three and four ORFs with transmembrane domains, respectively.

Conclusions

Safe for environmental and clinical use because of their lytic nature, and lack of virulence and resistance genes, these newly isolated phages expand the arsenal against antibiotic-resistant Aeromonas infections.

Aeromonas dhakensis: A Zoonotic Bacterium of Increasing Importance in Aquaculture

Aeromonas dhakensis is increasingly recognised to be an important pathogen responsible for disease losses in warm-water aquaculture and, similar to several other Aeromonas species, it can infect humans. Knowledge of A. dhakensis is accumulating, but this species remains relatively under-investigated compared to its close relative, Aeromonas hydrophila. The significance of A. dhakensis may have been overlooked in disease events of aquatic animals due to issues with reliable identification. Critical to appreciating the importance of this pathogen is the application of dependable molecular tools that enable accurate identification and discrimination from A. hydrophila and other motile aeromonads. This review aims to synthesise the key literature on A. dhakensis, particularly with relevance to aquaculture, including knowledge of the bacterium derived from disease case studies in aquatic hosts. Identification methods and strain phylogeny are discussed, with accurate detection important for prompt diagnosis and for distinguishing strains with heightened virulence. Increasing evidence suggests that A. dhakensis may be more virulent than A. hydrophila and correct identification is required to determine the zoonotic risks posed, which includes concerns for antibiotic-resistant strains. This review provides an impetus to improve species identification in the future and screen strain collections of presumptive Aeromonas spp. retrospectively to reveal the true prevalence and impact of A. dhakensis in aquaculture, the environment, and healthcare settings.

Antibacterial Mechanisms and Antivirulence Activities of Oridonin against Pathogenic Aeromonas hydrophila AS 1.1801

Aeromonas hydrophila, a Gram-negative bacterium widely found in freshwater environments, acts as a common conditional pathogen affecting humans, livestock, and aquatic animals. In this study, the impact of oridonin, an ent-kaurane diterpenoid compound derived from Rabdosia rubescens, on the virulence factors of A. hydrophila AS 1.1801 and its antibacterial mechanism was elucidated. The minimum inhibitory concentration (MIC) of oridonin against A. hydrophila AS 1.1801 was 100 μg/mL. Oridonin at inhibitory concentrations could significantly increase the electrical conductivity in the supernatant and escalate nucleic acid leakage (p < 0.01). This effect was concomitant with observed distortions in bacterial cells, the formation of cytoplasmic cavities, cellular damage, and pronounced inhibition of protein and nucleic acid synthesis. Additionally, oridonin at inhibitory levels exhibited a noteworthy suppressive impact on A. hydrophila AS 1.1801 across biofilm formation, motility, hemolytic activity, lipase activity, and protease activity (p < 0.05), demonstrating a dose-dependent enhancement. qRT-PCR analysis showed that the gene expression of luxR, qseB and omp were significantly downregulated after oridonin treatment in A. hydrophila AS 1.1801 (p < 0.05). Our results indicated that oridonin possessed significant antibacterial and anti-virulence effects on A. hydrophila AS 1.1801.