Antibiotic resistance and polymyxin B resistance mechanism of Aeromonas spp. isolated from yellow catfish, hybrid snakeheads and associated water from intensive fish farms in Southern China

Antimicrobial resistance, biofilm formation, and presence of colistin resistance mcr genes in Escherichia coli and Aeromonas spp. isolated from water samples

This study investigated the presence of Klebsiella pneumonia, Salmonella spp., Aeromonas spp., and Escherichia coli. Within the scope of the study, between November 2020 and September 2022, 600 water samples (springs, ponds, and drinking waters) were used as materials. Isolation and identification tests were performed to investigate the presence of Salmonella spp., Aeromonas spp., and E. coli through MALDI-TOF (Matrix-assisted Laser Desorption Ionization Time-of-Flight). K. pneumonia, P. aeruginosa, and Salmonella spp. were not found in any isolate. However, Aeromonas spp., and E. coli were found at 1 % (8/600) and 2 % (14/600), respectively. Aeromonas spp. isolates were identified as 1.5 (4/600) A. veronii, 0.5 (3/600) A. media and 0.1 (1/600) A. caviae. The biofilm-forming ability was observed in 14 (100 %) of 14 E. coli strains with the microplate method. Out of 14 biofilm-forming E. coli strains, 2 (14 %) demonstrated strong, 10 (71 %) moderate, and 2 (14 %) weak production. Moreover, out of 8 biofilms forming Aeromonas spp. strains, 3 (37.5 %) demonstrated strong, 3 (37.5 %) moderate, and 2 (25 %) weak production. Colistin-related genes (mcr-3/7, 1/2/6, mcr-4, mcr-5 and mcr-8) were identified by Real-Time PCR. As a result of real time PCR, Aeromonas spp. it was determined that 37 % (3/8) of the isolates carried the mcr-3/7 gene. It was determined that A. veronii and A. media carried the mcr-3/7 gene at rates of 66 % (2/3) and 33 % (1/3). However, mcr-1/2/6, mcr-4, mcr-5 and mcr-8 genes were not found in the examined Aeromonas spp. isolates. Hereby, factors that play a role in the emergence and spread of antibiotic-resistant strains should be identified, and necessary precautions should be taken.

Evaluation of public health impact risks associated with bacterial antimicrobial resistome in tropical coastal environments

This study investigated antimicrobial resistance genes (ARGs) and microbial communities in Singapore's tropical coastal environments, comparing natural marine waters with aquaculture sites using metagenomic analysis. Results show consistently low ARG levels in natural ecosystems, whereas aquaculture sites exhibit elevated ARG concentrations, particularly those relevant to human pathogens, with considerably temporal fluctuations likely driven by human activities and seasonal factors. Additionally, aquaculture sites were found to host mobile genetic elements (MGEs) that may facilitate ARG spread, identifying aquaculture as a key reservoir of resistance genes with potential public health implications. These findings underscore distinct prevalence of ARG between natural and managed marine environments and highlight the need for continued surveillance to monitor ARG dynamics in coastal areas. By elucidating the potential for ARGs to transfer from aquaculture environments to the human food chain, this research emphasizes the importance of sustainable aquaculture practices and proactive ARG management strategies to mitigate antimicrobial resistance risks posed by tropical coastal ecosystem.

Environmental concentrations of antibiotics induced polymyxin B tolerance in Aeromonas hydrophila

Polymyxin B is one of the last lines of defense in infections caused by multidrug-resistant Gram-negative bacteria. Aeromonas hydrophila are important fish pathogens and the occurrence of polymyxin B-resistant A. hydrophila isolates is increasing. While investigating 14 environmental chemical pollutants that may affect bacterial polymyxin B sensitivity in aquatic bacteria, we discovered that tolerance of A. hydrophila to polymyxin B is increased by short-term (90 min) concurrent exposure to tetracyclines, tigecycline or gentamicin at environmentally relevant concentrations (0.0625 μg/mL) and persists as long as the inducer is present. The exposure increased the growth of A. hydrophila at an inhibitory concentration of polymyxin B. The increased polymyxin B tolerance was attributed to changes in gene expression, without alterations in genotype and independent of cell surface charge variations. Such changes are relate to six KEGG pathways, including ribosome, nucleotide metabolism, pyrimidine metabolism, glyoxylate and dicarboxylate metabolism, purine metabolism, starch and sucrose metabolism. The dysregulated genes were involved in broad physiological changes, such as cell motility, flagella biosynthesis, quorum sensing, biofilm formation, and chemotaxis. Furthermore, the up-regulation of genes encoding Mg2+ transport, biotin synthesis, lipoprotein, glycerol phospholipid metabolism, phospholipid transport, and the down-regulation of genes, such as ompK, yidD and ompA related to enhanced cell membrane barrier, may contribute to the increased polymyxin B tolerance in A. hydrophila. In summary, the study results revealed the impact of environmental antibiotics in promoting microbial polymyxin B tolerance. Our findings underscore the role of environmental antibiotics in promoting polymyxin B tolerance and provide insights into the mechanisms of polymyxin B tolerance evolution in A. hydrophila.

Emergence of polymyxin-resistant Yersinia enterocolitica strains in natural aquatic environments

Aquatic environments serve as ideal reservoirs for antibiotic-resistant bacteria and resistance genes. However, the presence of polymyxin-resistant Yersinia enterocolitica, the pathogen responsible for human yersiniosis, in aquatic environments remains poorly understood. Herein, we isolated polymyxin-resistant Y. enterocolitica strains from natural water for the first time. In addition to intrinsic resistance to ampicillin and cefazolin, the strains demonstrated high resistance to polymyxin B and polymyxin E. All isolates were capable of biofilm production and exerted high virulent effects in Galleria mellonella, with 90% mortality occurring within 48 h post-infection. Furthermore, whole genome sequencing identified 26 antibiotic resistance genes, including polymyxin resistance determinants (arnA and PmrF), beta-lactam resistance determinants (vatF and blaA), and 60 virulence genes such as yaxA and yaxB in Y. enterocolitica isolates. Notably, phylogenetic analysis revealed that Y. enterocolitica involved multilocus sequence types ST937 and ST631, which were clustered with strains isolated from a human in the United States or swine in China. The close relatedness to clinical isolates suggests that polymyxin-resistant Y. enterocolitica may pose considerable health risk to humans. Our findings provide evidence of the presence of polymyxin-resistant Y. enterocolitica in aquatic environments and raise concerns about health risks due to their potential high virulence.

Emergence of colistin resistance and characterization of antimicrobial resistance and virulence factors of Aeromonas hydrophila, Salmonella spp., and Vibrio cholerae isolated from hybrid red tilapia cage culture

Background

Tilapia is a primary aquaculture fish in Thailand, but little is known about the occurrence of antimicrobial resistance (AMR) in Aeromonas hydrophila, Salmonella spp., and Vibrio cholerae colonizing healthy tilapia intended for human consumption and the co-occurrence of these AMR bacteria in the cultivation water.

Methods

This study determined the phenotype and genotype of AMR, extended-spectrum β-lactamase (ESBL) production, and virulence factors of A. hydrophila, Salmonella spp., and V. cholerae isolated from hybrid red tilapia and cultivation water in Thailand. Standard culture methods such as USFDA's BAM or ISO procedures were used for the original isolation, with all isolates confirmed by biochemical tests, serotyping, and species-specific gene detection based on PCR.

Results

A total of 278 isolates consisting of 15 A. hydrophila, 188 Salmonella spp., and 75 V. cholerae isolates were retrieved from a previous study. All isolates of A. hydrophila and Salmonella isolates were resistance to at least one antimicrobial, with 26.7% and 72.3% of the isolates being multidrug resistant (MDR), respectively. All A. hydrophila isolates were resistant to ampicillin (100%), followed by oxytetracycline (26.7%), tetracycline (26.7%), trimethoprim (26.7%), and oxolinic acid (20.0%). The predominant resistance genes in A. hydrophila were mcr-3 (20.0%), followed by 13.3% of isolates having floR, qnrS, sul1, sul2, and dfrA1. Salmonella isolates also exhibited a high prevalence of resistance to ampicillin (79.3%), oxolinic acid (75.5%), oxytetracycline (71.8%), chloramphenicol (62.8%), and florfenicol (55.3%). The most common resistance genes in these Salmonella isolates were qnrS (65.4%), tetA (64.9%), bla _TEM (63.8%), and floR (55.9%). All V. cholerae isolates were susceptible to all antimicrobials tested, while the most common resistance gene was sul1 (12.0%). One isolate of A. hydrophila was positive for int1, while all isolates of Salmonella and V. cholerae isolates were negative for integrons and int _SXT. None of the bacterial isolates in this study were producing ESBL. The occurrence of mcr-3 (20.0%) in these isolates from tilapia aquaculture may signify a serious occupational and consumer health risk given that colistin is a last resort antimicrobial for treatment of Gram-negative bacteria infections.

Conclusions

Findings from this study on AMR bacteria in hybrid red tilapia suggest that aquaculture as practiced in Thailand can select for ubiquitous AMR pathogens, mobile genetic elements, and an emerging reservoir of mcr and colistin-resistant bacteria. Resistant and pathogenic bacteria, such as resistance to ampicillin and tetracycline, or MDR Salmonella circulating in aquaculture, together highlight the public health concerns and foodborne risks of zoonotic pathogens in humans from cultured freshwater fish.

Hiding in Plain Sight: Characterization of Aeromonas Species Isolated from a Recreational Estuary Reveals the Carriage and Putative Dissemination of Resistance Genes

Antimicrobial resistance (AMR) has become one of the greatest challenges worldwide, hampering the treatment of a plethora of infections. Indeed, the AMR crisis poses a threat to the achievement of the United Nations' Sustainable Development Goals and, due to its multisectoral character, a holistic approach is needed to tackle this issue. Thus, the investigation of environments beyond the clinic is of utmost importance. Here, we investigated thirteen strains of antimicrobial-resistant Aeromonas isolated from an urban estuary in Brazil. Most strains carried at least one antimicrobial resistance gene and 11 carried at least one heavy metal resistance gene. Noteworthy, four (30.7%) strains carried the bla_KPC gene, coding for a carbapenemase. In particular, the whole-genome sequence of Aeromonas hydrophila strain 34SFC-3 was determined, revealing not only the presence of antimicrobial and heavy metal resistance genes but also a versatile virulome repertoire. Mobile genetic elements, including insertion sequences, transposons, integrative conjugative elements, and an IncQ1 plasmid were also detected. Considering the ubiquity of Aeromonas species, their genetic promiscuity, pathogenicity, and intrinsic features to endure environmental stress, our findings reinforce the concept that A. hydrophila truly is a "Jack of all trades'' that should not be overlooked under the One Health perspective.

Evaluation of Trophic Structure and Energy Flow in a Pelteobagrus fulvidraco Integrated Multi-Trophic Aquaculture System

An integrated multi-trophic aquaculture system (IMTA) combined muti-trophic organism cultivation with ecological engineering facilities effectively improves energy utilization efficiency and reduces pollution emission, which promotes the development of the aquaculture industry. In this study, an Ecopath model was used to analyze the Pelteobagrus fulvidraco-integrated multi-trophic aquaculture system (FMRP). The results showed that the effective trophic level range of FMRP was low (1~2.566), and the energy throughput was mainly concentrated in trophic level I (65.39%). The utilization rate of commercial fish feed was high. Due to the lack of predators for detritus and primary producers (Oryza sativa L. and hydrophyte), the energy throughput of detritus and the primary production were not fully utilized. The ascendency/total development capacity (A/TDC) and overhead/total development capacity (O/TDC) were 0.29 and 0.59, respectively, which indicated that the aquaculture system had high elasticity and strong anti-perturbation ability, but the stability could be substantially improved. The results of the carrying capacity assessment showed that the maximal single increments of Pelteobagrus fulvidraco fry and juvenile were 0.12 g/m² and 0.42 g/m², respectively, and the maximal common increments of Pelteobagrus fulvidraco fry and juvenile were 0.10 g/m² and 0.10 g/m², respectively, which indicated that there was insufficient space for increment. The study showed that the FMRP still needed to be improved in the aspects of polyculture species, energy consumption and stability. It would be necessary for the FMRP to perform further optimization and enhancement on the energy utilization efficiency, system stability and comprehensive benefits.