Bacterial isolation
The bacterial species isolated from wastewater of the fish ponds are categorized based on their Gram stain characteristics (Fig 3) and bacterial groups (Fig 4).
The results of the distribution are consistent with previous findings
(Sule et al., 2016; Adebami et al., 2020), which also reported a dominance of Gram-negative bacteria in aquaculture wastewater environments.
According to the figure above, the results showed that the bacterial isolates from the wastewater of the fish ponds are categorized into six groups:
Enterobacteriaceae, non-
Enterobacteriaceae,
Streptococcaceae,
Pseudomonadaceae,
Erwiniaceae and
Neisseriaceae, with isolation rates of 41.18%, 38.24%; 8.82%, 7.35%, 2.94% and 1.47%, respectively.
Fish and water are the most reliable sampling sites for pathogen detection in aquaculture environments
(Dronen et al., 2022). The predominance of
Entero-bacteriaceae indicates fecal contamination, likely originating from runoff, contaminated water used in fish farming, fish feed, livestock manure, fish excreta, or direct anthropogenic inputs. This suggests inadequate biosecurity and poor waste management practices, which are commonly observed in local fish farms
(Degefu et al., 2011; Njoku et al., 2015; Sule et al., 2016; Wisnu et al., 2019). Moreover, the failure to implement proper fishpond management practices can pose health risks to fish by facilitating the spread of harmful pathogens, which may sebsequently impact human health (
Ajayi and Okoh, 2014;
Sule et al., 2016; Opiyo et al., 2018).
While similar bacterial groups-belonging to families such as
Staphylococcaceae,
Streptococcaceae,
Pseudomon- adaceae,
Enterobacteriaceae,
non-Enterobacteriaceae,
Morganellaceae,
Neisseriaceae and
Bacillaceae-have been reported in studies conducted in other parts of Africa (
e.g., Nigeria and Zambia) by
Ntengwe and Edema (2008),
Ajayi and Okoh (2014),
Njoku et al. (2015),
Sule et al. (2016),
Eghomwanre et al. (2019) and
Adebami et al. (2020), our findings provide region-specific microbiological insights from southwestern Algeria, a region where such baseline surveillance remains limited.
Bacterial identification
The results of the identification of the fish pond wastewater isolates are presented in Table 2.
At least 21 unique bacterial species were isolated. These included 14 species from the
Enterobacteriaceae family, 3 species from non-
Enterobacteriaceae families and 1 species each from the
Pseudomonadaceae,
Erwiniaceae and
Neisseriaceae families, as well as unidentified species from the
Streptococcaceae group.
According to the Chi-Square Goodness-of-Fit Test, the overall distribution of bacterial groups was highly non-uniform (
p-value < 0.001), indicating significant variation in prevalence among the groups. This reinforces the dominance of
Enterobacteriaceae and
Aeromonadaceae and underlines the selective pressures present in fishpond environments-potentially influenced by temperature, nutrient load and organic matter accumulation.
Antibiotic susceptibility testing (AST)
Streptococcaceae
The results of the antibiotic susceptibility testing for bacterial isolates belonging to the
Streptococcaceae family are presented in Fig 5.
The antibiotic susceptibility test results of
Strepto-coccaceae isolates showed 100% resistance to oxacillin, tetracycline and erythromycin, 75% resistance to ampicillin and 50% resistance to vancomycin. Meanwhile, they were 100% sensitive to chloramphenicol.
Enterobacteriaceae
The results of the antibiotic susceptibility testing for bacterial isolates belonging to the
Enterobacteriaceae family are presented in Fig 6.
As shown in Fig 6, all isolates exhibited 100% resistance to cefoxitin, cefazolin and ampicillin, with variable resistance to amoxicillin-clavulanic acid. Conversely, all isolates were fully sensitive to ciprofloxacin, ofloxacin, gentamicin, azithromycin, amikacin and tobramycin.
Previous studies, such as those by
Sule et al., (2016), Adebami et al., (2020) and
Abedin et al., (2020), have similarly reported resistance to amoxicillin–clavulanic acid, amoxicillin and ceftazidime among bacterial isolates from aquaculture wastewater, while sensitivity to ciprofloxacin and ofloxacin was maintained-findings that align with our results. These patterns underscore the role of aquaculture environments as reservoirs of bacteria carrying mobile resistance genes (
Del Castillo et al., 2013), which pose a risk of horizontal gene transfer to human pathogens or direct infection
(Apenteng et al., 2017). Notably, multidrug resistance-including to critical antibiotics such as cephalos- porins and carbapenems-has been detected in
Entero-bacteriaceae from aquaculture systems
(Hamza et al., 2020; Custodio et al., 2023), highlighting the potential for interspecies gene transfer within these settings.
Non-enterobacteriaceae (Aeromonadaceae)
The results of the antibiotic susceptibility testing for bacterial isolates belonging to the non-
Enterobacteriaceae family (
Aeromonadaceae) are presented in Fig 7.
The results obtained showed that
Aeromonadaceae strains were 100% resistant to ticarcillin-clavulanic acid and, to a lesser extent, to imipenem, ceftazidime and cefotaxime. However, they were 100% sensitive to ciprofloxacin, amikacin, tobramycin, ofloxacin, piperacillin and gentamicin and showed 96% sensitivity to aztreonam and chloramphenicol.
Non-enterobacteriaceae (Vibrionaceae)
The results of the antibiotic susceptibility testing for bacterial isolates belonging to the non-
Enterobacteriaceae family (
Vibrionaceae) are presented in Fig 8.
The results obtained showed that the
Vibrionaceae strains exhibited resistant ranging from 40% to 80% to amikacin, cefotaxime, ceftazidime, aztreonam, imipenem and ticarcillin-clavulanic acid, while the isolates remained 100% sensitive to ciprofloxacin, tetracycline, sulfamethoxazole- trimethoprim, chloramphenicol and gentamicin.
Previous studies have documented varying antibiotic resistance patterns in aquaculture environments. For example, while
Manjusha et al. (2005) found that
Vibrio spp. isolated from coastal and tissue samples (shrimp, mussels and cuttlefish) exhibited higher resistance than those from aquaculture farms-likely due to greater antimicrobial use and pollution-our results indicate a similar trend, in that bacterial isolates from inland fish ponds display comparatively lower resistance levels.
In addition, the resistance pattern observed for
Aeromonas spp. in our study is consistent with earlier reports
(Fosse et al., 2003; Henriques et al., 2006; Daood, 2012), which attributed β-lactam resistance to the presence of β-lactamase genes such as
blaTEM, despite maintained efficacy for antibiotics like ciprofloxacin, gentamicin, amikacin and trimethoprim-sulfamethoxazole. This suggests that reduced antimicrobial application and lower environmental pressures in inland aquaculture may mitigate the development of resistance. Moreover, the widespread presence of mobile genetic elements (MGEs), reported in previous studies (
Lamy, 2012;
Del Castillo et al., 2013), supports our hypothesis that horizontal gene transfer is an important driver in the dissemination of resistance determinants in these ecosystems.
Pseudomonadaceae
The results of the antibiotic susceptibility testing for bacterial isolates belonging to the
Pseudomonadaceae family are presented in Fig 9.
The results obtained showed that the
Pseudomona-daceae strains were 100% resistant to ticarcillin-clavulanic acid and to a lesser degree, resistant to aztreonam and ceftazidime, with resistance rates of 80% and 40%, respectively. However, they were 100% sensitive to ciprofloxacin, amikacin, tobramycin, ofloxacin, piperacillin and gentamicin.
All
P. aeruginosa isolates formed biofilms (Fig 10), which enhances resistance and survival in aquatic environments. This biofilm-forming capacity may explain their persistence in low-flow, sediment-rich pond systems.
The
Pseudomonas aeruginosa strains isolated were 100% sensitive to 6 out of 10 (
i.e., 60%) antibiotics tested and exhibited lower resistance compared to the
P. aeruginosa strains isolated in our previous study (
Benyagoub, 2023a), which also found that all isolated strains were capable of forming biofilms, a characteristic that enhances the pathogen’s adaptability, survival and resistance in various environments
(Moradali et al., 2017).
However, due to the absence of standardized guidelines for antibiotic selection and interpretation, we did not include the AST results for
Pantoea spp. and
Chromobacterium violaceum strains in this study.
Multidrug-resistant bacterial strains
The strains exhibiting resistance to more than three classes of antibiotics, along with the antibiotic resistance patterns categorized by bacterial group, are presented in Table 3 and Fig 11.
Out of 68 bacterial isolates, 17 (25%) were resistant to more than three classes of antibiotics and exhibited h
igh MAR indices. The variability in MAR indices among bacterial groups was statistically significant (ANOVA,
p = 0.0498), suggesting that resistance levels differ across bacterial families. The elevated MAR values observed in
Enterobacteriaceae (above 0.4) suggest sustained antibiotic pressure and fecal contamination, potentially originating from farm runoff, livestock waste, or human sources
(Sarkar et al., 2019). Similarly,
Rezaul Karim et al. (2023) reported non-
Enterobacteriaceae isolates with MAR indices above 0.2, indicative of antibiotic misuse in both human and veterinary contexts. These findings highlight the urgent need for the implementation of best practice codes among fish farmers and routine monitoring of fish pond wastewater prior to environmental discharge, in order to prevent the transmission of potential pathogens
(Sule et al., 2016).
Comparable results have been reported elsewhere.
Snoussi et al. (2011) found that
Aeromonas hydrophila and
Vibrio alginolyticus strains isolated from marine farm environments were resistant to at least three antimicrobial agents, with MAR indices of 0.71 and 0.68, respectively. Likewise,
Abedin et al. (2020), identified
Aeromonas spp.,
Pseudomonas spp. and
Vibrio spp. as dominant bacteria in various fish species in Bangladesh, reinforcing concerns over antibiotic-resistant pathogens in aquaculture systems.
These results point to a concerning level of resistance to several antibiotic classes, which, according to
Adebami et al. (2020), could increase the prevalence of infections in fish and potentially trigger disease outbreaks.
Gram-positive bacteria can develop resistance through two main strategies: one involves the enzymatic breakdown of the antibiotic
via β-lactamase production, while the other reduces the binding affinity of the antibiotic to its target, the penicillin-binding protein (PBP)
(Benyagoub et al., 2020b; Jubeh et al. 2020;
Benyagoub et al., 2022; Benyagoub, 2024). However, Gram-negative bacteria can resist antibiotics through various mechanisms, such as reducing drug uptake, altering the drug target, inactivating the drug and actively pumping it out. Additionally, these bacteria can acquire resistance factors from other microorganisms (
Reygaert, 2018;
Benyagoub et al., 2020a; Benyagoub et al., 2021a, 2021b;
Gauba and Rahman, 2023).
Search for antibiotic residues
The qualitative test (Fig 12) indicated the absence of detectable antibiotic residues in the samples.
This may reflect either the absence of recent antibiotic use or the limited sensitivity of the test method. However, indirect selection pressures-such as low-dose residues from reused water sources or contamination from surrounding agricultural activities-cannot be excluded.
Manjusha et al. (2005) and
Thiang et al. (2021) underscore the threat posed by low-level antibiotic exposure, which can still drive the propagation of resistance genes. The lack of residues should not be interpreted as the absence of selective pressure, especially considering the prevalence of multidrug-resistant bacteria. Effective management of aquaculture wastewater is crucial to reduce antibiotic pollution and prevent the transfer of resistance genes to humans through the food chain. This highlights the need for a deeper understanding of how drug resistance determinants are spread and transmitted, as well as the importance of enhancing antimicrobial stewardship (
Del Castillo et al., 2013,
Schar et al., 2020).