Introduction
Common carp (Cyprinus carpio) plays an important role in fish production in Hungary. This species accounts for 82% of the annual production of fish (Bojtorné et al., 2023). Successful fish production depends on the overall stable health and favourable reproductive biology of the broodstock. The sector is highly exposed to extreme weather conditions. In addition to this factor, the sorting, transport and handling of individuals may further increase the stress on the fish, which can open the door to infections. An increasingly important area of research today is the study of microbial communities on the surface and in the organism of fish and their impact on health, growth and reproductive parameters. Following research in humans and mammals, it has been described that pathogenic and symbiotic microbes on the surface and within the body of fish influence the internal environment through complex mechanisms (Bernáth et al., 2023). When the balance is disturbed, the overall health of individuals can deteriorate, reducing their growth and thus their reproductive capacity (Amillano-Cisneros et al., 2025). The aim of our study was to investigate the relationship between the reproductive biology of carp and the effects of microbial communities.
Materials and methods
The experimental propagation and sampling were carried out at a Hungarian fish farm (Szabolcsi Halászati Ltd.). During breeding season, 15 male and female individuals were selected from 2 wintering ponds. On the day of sampling individuals (N=10 male and female) were stripped. During sampling, the following parameters were measured and calculated: total volume of milt (TVM), volume of milt per kg of body weight (VOM), spermatozoa per millilitre (SPZ), total sperm production (TSP), total sperm production per kilogram of body weight (TNS), sperm motility and kinetic parameters, osmolality and pH of the seminal plasma and ovarian fluid. The total weight of eggs/individual (kg), the number of eggs/gram (pcs), and the pseudo-gonadosomatic index were recorded. Milt and roe samples were collected from each individual for further microbiological examination. The stripped fish were euthanized and intestinal samples were collected. A tenfold dilution series was prepared from the gut, sperm and egg samples with PBS solution. The dilution series members were spread on tryptic soy agar (TSA), de Man, Rogosa and Sharpe (MRS) agar and incubated for 96 hours at 22 °C. At the end of the incubation period, colony forming units (CFU) were counted, and morphologically distinct colonies were subcultured. The pure cultures were used for MALDI-TOF MS identification. During statistical analysis, correlation matrix and Pearson’s correlation coefficient test were applied (p<0.05).
Results
Negative correlation was observed between the number of CFUs of sperm TSA samples and the total sperm counts (high CFU number - low sperm count, p=0.0226). In addition, a negative correlation was identified between the number of sperm TSA CFU and the amount of semen per body weight (high CFU count, low sperm count; p = 0.0132). Negative correlation was detected between the number of semen CFU on MRSA medium and DSL (straight line distance) kinetic parameter (high CFU count but low DSL, p=0.0481). A negative correlation was identified between semen CFU on MRSA medium and the WOB (wobble coefficient) kinetic parameter (where a high number of CFU was observed, but low WOB kinetic parameter, p=0.0268). Negative correlation was observed between the formation of CFU on the egg MRSA plates and the osmolality of ovarian fluid (high germ count-low osmolality, p=0.0394).
Discussion and conclusion
In addition to the qualification of the gamete samples, several representatives of 17 bacterial genera were isolated and identified from the intestinal tract and gametes of the individuals with traditional culturing and molecular methods as well. The results show that several species within a genus can be found in the organs of a single individual. In the gut, members of the genera Lactococcus and Lactobacillus were abundant, whereas Aeromonas spp. and Pseudomonas spp. were more abundant in the gametes. The number of CFU was associated with negative changes in some gamete quality parameters. Several identified bacterial species are facultative pathogenic (risk group 2) organisms, therefore they may possess a biological risk on fish health.
Acknowledgements
The research was supported by the Ministry of Culture and Innovation through the National Research, Development and Innovation Fund, on the basis of a grant document issued by the National Research, Development and Innovation Office (2022-1.2.6-TÉT-IPARI-TR- 2022-00002). Our work was supported by the National Research, Development and Innovation Fund under the University Research Scholarship Programme 2024 (2024-2. 1.1- EKÖP (EKÖP-24-V/MATE-6)), the Thematic Excellence Programme (TKP2021) of the Ministry of Innovation and Technology, the National Defence and National Security sub-programmes (TKP2021-NVA-22). This work was supported by the Research Excellence Programme and the Flagship Research Groups Programme of the Hungarian University of Agriculture and Life Sciences.
References
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