Chinese researchers warn that some aquaculture probiotics may contain cytotoxins and antibiotic resistance genes that can be transferred to other organisms.
Farmers use probiotics as a sustainable way to prevent disease, promote growth, and improve the health of aquatic animals, but a recent review in Aquaculture Reports has raised concerns about the safety of many commercially available probiotics. After conducting genetic analysis of several aquaculture probiotics, researchers noted that many formulations were mislabeled and did not contain the specific bacteria indicated on the label.
When researchers screened probiotic bacteria (Bacillus and its subspecies) for antibiotic resistance (AMR), they found that most isolated bacterial strains were resistant to beta-lactam antibiotics such as penicillin, and a smaller number were resistant to ampicillin, oxacillin, cefuroxime, and ceftriaxone. Some isolated strains also showed resistance to minocycline, a tetracycline antibiotic. In 3 cases, genetic screening revealed the presence of virulence genes. These genes can produce cytotoxins - compounds capable of killing living cells.
The researchers suggest that these results indicate safety issues with conventional probiotics. Mislabeled containers mean that manufacturers cannot be sure of their biological composition. The fact that commercial probiotics in this study bypassed this crucial first step raises an alarm.
Along with the presence of antibiotic resistance and virulence genes, it is recommended that “Bacillus species should be carefully selected for use in commercial probiotics”. The aquaculture industry should focus more on ensuring the safety of probiotics as aquaculture production continues to expand.
Shifting focus from “efficacy” to “safety”
Many studies have verified the benefits of probiotics. Probiotic bacteria such as baker's yeast (Saccharomyces cerevisiae) and other Bacillus species have been used in aquaculture for water treatment, increasing feed absorption, improving growth rates, and enhancing health and immune function in aquatic animals. They have also been used as an alternative to antibiotics to prevent pathogenic bacteria in the farming environment. The use of probiotics has minimized the risks caused by antibiotic resistance, while bacteria are often affordable and sustainable.
Despite the numerous benefits, researchers are raising concerns about the safety of some commercially available probiotics – suggesting that they may cause some side effects in aquatic animals. Issues such as mislabeling, microbial contamination, and harmful probiotics have been identified in other food production sectors.
Currently, the global microbial market lacks monitoring and detection systems, meaning that cases of contaminated and mislabeled products often go unreported. A study by Hong et al. showed that a significant number of probiotics produced in Asia are not clearly identified at the species level, and farmers cannot always be sure what type of microbes they are adding to their ponds.
Other studies have found that some Bacillus probiotics contain virulence and AMR genes that adversely affect human health. These genes can be transferred from bacteria to the host body after they are ingested.
Commercial Microbial Assessment
Researchers purchased 32 Bacillus probiotic formulations used in aquaculture. They isolated microbial samples and characterized them based on morphological, biochemical, and PCR tests. PCR screening was used to detect enterotoxin genes, and reagents were used to assess the antibiotic susceptibility of the isolates.
Results showed that some probiotics contained fewer microorganisms than stated on the label. In some cases, samples did not contain any of the listed beneficial bacteria and instead contained different types of bacteria. The researchers stated that some of these bacteria are considered pathogenic, affecting product safety.
An antibiotic assessment showed that almost all isolated strains were resistant to penicillin (90.9%), ampicillin (47.7%), oxacillin (25%), erythromycin (18.2%), cefuroxime (11.4%), and tetracycline (2.3% - 4.5% depending on the drug type). This result raises concerns because Bacillus bacteria can act as intermediaries and transfer resistance genes into the environment. They can also transfer these resistance genes to other bacteria in the same habitat.
PCR tests indicated that some probiotic samples contained enterotoxin genes, capable of transferring genes from probiotic bacteria into the habitat and the bacteria themselves, causing disease and complications for the host. The researchers stated that using probiotics with these genes could harm the farming environment, compromising the probiotics' ability to prevent disease.
Conclusion
The researchers concluded that “Safety assessment for probiotics, especially mixed probiotic formulations for aquaculture, needs to be established, and the product's ingredients are the basis for establishing its safety.”
Source: thefishsite.com
Translated by: Trần Thị Thúy Quyên
