Water quality of swimming pools and prevalence of antibiotic resistance genes in Haikou in 2019

Background Water environment is a reservoir of antibiotic resistance genes. It may pose a risk of horizontal transfer of antibiotic resistance genes due to its close contact with human body in swimming pools.

Objective This study investigates the types and spatial distributions of antibiotic resistance genes, and the correlations between antibiotic resistance genes and the bacteriological or disinfection indicators of water in swimming pools in Haikou.

Methods A total of 96 samples of water and wall swabs were collected in 16 swimming pools by multi-stage stratified random sampling methods in Haikou from January to June 2019. The aerobic bacterial count, free chlorine residual, and combined chlorine residual were tested in accordance with the Test methods for hygienic standards for public places (GB/T 18204.6-2013), and E. coli was detected in chromogenic medium. The DNA extracted from drug-resistant genes were amplified by PCR and confirmed by sequencing. Correlation analysis of drug-resistant genes and water quality indicators was performed.

Results The qualified rates of aerobic bacterial count, free chlorine residual, combined chlorine residual, and E. coli were 77.08% (37/48), 79.17% (38/48), 85.42% (41/48), and 97.92% (47/48), respectively. Eight antibiotic resistance genes such as mcr-1, mecA, floR, sul I, sul II, aadA, strA, and tetS were found, and the overall positive rate was 25.00% (24/96); the frequency of floR was the highest (7.29%, 7/96), and the frequencies of tetS and strA were both the lowest (1.04%, 1/96). The positive rates of antibiotic resistance genes in swimming pools were different in the four districts of the city (χ²=12.178, P=0.007). The positive rate of swimming pool walls was higher than that of swimming pool water (35.42% vs 14.58%; χ²=5.556, P=0.018), and mcr-1 were detected in both kinds of samples. There were negative correlations of free chlorine residual with aerobic bacterial count and the positive rate of antibiotic resistance genes (r=-0.395, P=0.005; r=-0.434, P=0.002).

Conclusion The detection frequency of antibiotic resistance genes, such as mcr-1, is high in the swimming pools of Haikou. Free chlorine residual in swimming pool water affects aerobic bacterial count and antibiotic resistance genes.