Bacillus velezensis WLYS23 strain possesses antagonistic activity against hybrid snakehead bacterial pathogens

AIM

The aims of this study were to screen an antagonistic probiotic for the prevention and control of bacterial diseases in snakehead fish and to evaluate the antimicrobial activities, biosafety and biocontrol effect of the antagonistic strain.

METHODS AND RESULTS

In this study, the WLYS23 strain exhibiting the most effective antagonistic properties against several fish pathogens was selected from the intestine of healthy snakehead fish. The strain was identified as Bacillus velezensis based on morphological, physiological and biochemical characteristics, and phylogenetic analysis. This strain showed amylase, cellulase, protease and lipase activities according to extracellular enzyme activity assays. Moreover, the cell-free extract of the WLYS23 strain exhibited strong antibacterial activity, with MICs of ≤64 μg ml^-1 for most fish pathogens. Additionally, the cell-free extract was heat and pH stable, and resistant to protease, whereas their antimicrobial activities were significantly influenced by metal ions at high concentration. The WLYS23 strain and its cell-free extract were safe for snakehead and zebrafish. The disease resistance of snakehead fish was significantly increased following administration of the WLYS23 strain and its cell-free extract respectively. The complete genome of the WLYS23 strain was sequenced and found to harbour seven gene clusters encoding secondary metabolites with antimicrobial activity.

CONCLUSION

The WLYS23 strain possesses great potential as a biocontrol agent, which can be commercially developed to improve disease control in freshwater aquaculture.

SIGNIFICANCE AND IMPACT OF THE STUDY

Snakehead fish are important commercially farmed fish worldwide. However, the bacterial diseases caused by Aeromonas sp. and Nocardia seriolae in farmed snakehead fish lead to huge economic losses. Safe, economical and efficient probiotics are limited to prevent and control these diseases. Here, we provide a promising biocontrol agent with antagonistic activity against bacterial diseases of snakehead.

[Relationship between occupational stress and working ability of workers in a petroleum processing enterprise in high altitude area]

Objective: To investigate the relationship between occupational stress and working ability of workers in a petroleum processing enterprise in a high altitude area. Methods: A total of 728 workers in a petroleum processing enterprise at an altitude of 2850 m were subjected to a survey using Occupational Stress Inventory (OSI) , Work Ability Index (WAI) Scale, Occupational Role Questionnaire (ORQ) , Personal Strain Questionnaire (PSQ) , and Personal Resource Questionnaire (PRQ) from May 2014 to August 2016. Results: Of the 728 workers, 55 (7.6%) had a poor working ability, moderate in 262 (35.9%) , and good in 411 (56.5%). There were significant differences in WAI between the workers with different types of work, sexes, ages, and working years (P<0.05). There was a significant difference in WAI between different occupational stress groups (P<0.05). WAI was negatively correlated with ORQ score and PSQ score (r(s)=-0.387, P<0.05; r(s)=-0.467, P<0.05) and positively correlated with PRQ score (r(s)=0.343, P<0.05). The multiple linear regression analysis showed that high ORQ score and PSQ score were the inhibitory factors for high WAI (B=-0.058; B=-0.082) and high PRQ score was a contributing factor for high WAI (B=0.029) . Conclusion: Occupational stress is an influencing factor for the working ability of workers in the petroleum processing enterprise in the high altitude area. Hypoxia in high altitude area may further reduce the working ability. In order to reduce occupational stress and improve work ability, it should be considered to strengthen skills training, improve the working environment, and pay attention to mental health.

Aeromonas shuberti as a cause of multi-organ necrosis in internal organs of Nile tilapia, Oreochromis niloticus

A disease with white spots in internal organs of Nile tilapia occurred in Zhanjiang, southern China. Multiple, white nodules, 0.8-2.2 mm in diameter, were scattered throughout the liver, spleen and kidney of diseased fish. Signs of nodules reproduced after artificial infection with the isolated strain. Isolated bacteria were Gram-negative, facultative anaerobic, motile, short rod-shaped, with a length of 1.2-2.2 μm. Morphological and biochemical tests, as well as phylogenetic analysis, all strongly indicated that the isolate from tilapia is identical to Aeromonas schubertii (A. schubertii) which temporary named LF1708 strain. Antibiotic sensitivity assays showed the LF1708 is sensitive to 24 of 27 tested antibiotics. Pathogenicity test revealed that the isolate at the dose of 3.75 × 10⁶ CFU/g killed 100% of experimental tilapia within 2 days and the dose of 1 × 10⁷ CFU/g killed 100% of experimental zebrafish within 1 day. Histopathology of diseased tilapia infected with A. schubertii showed numerous necrotic lesions widely distributed in spleen, liver and kidney, and infiltration with a large number of bacteria. To our knowledge, this was the first report that associated A. schubertii with mortality in tilapia.

Effects of High O2 Pretreatment and Gibberellic Acid on Sensorial Quality and Storability of Table Grapes

Kyoho grapes ( Vitis vinifera X V. labrusca) were stored under three different treatments at 0°C in 95% relative humidity for 60 days. Grapes in the first treatment were stored in to air (control), in the second treatment the grapes were stored in 80% O2 for 15 days and then transferred into air for 45 days, and in the third treatment the grapes were sprayed with 100ppm of gibberellic acid (GA)3 containing 100ppm of Tween 20 and then kept in air. Compared with the air control, high oxygen pretreatment (HOP) and GA3 maintained higher fruit detachment force and membrane integrity, markedly reduced berry drop, rachis browning and decay and delayed the decrease of soluble solids content (SSC) and vitamin C. In addition, HOP and GA3 retarded the conversion of protopectin to water-soluble pectin and improved berry firmness. HOP resulted in a significantly firmer berry, less berry drop, and more soluble solids content, total acidity and protopectin than those treated with GA3, but did not markedly reduce rachis browning and membrane integrity loss. There were no significant differences in fruit detachment force and water-soluble pectin levels between HOP and GA3. In conclusion, high O2 pretreatment and GA3application retained effectively the quality of harvested grapes.