摘要
目标
研究沙门氏菌分离株中fosA3和fosA7的患病率和分子特征。
方法
对2016-19年间从中国食用动物中收集的551株沙门氏菌进行了fos基因筛选。比较了fosA7-和fosA3阳性沙门氏菌的耐药性、血清型、克隆关系和遗传环境。
后果
发现fosA7（9.26%）和fosA3（6.53%）的患病率相对较高。fosA3与高水平磷霉素耐药性相关（≥512 mg/L），而fosA7具有相对低水平的抗性，其与葡萄糖-6-磷酸的存在无关。此外，fosA7可以促进沙门氏菌在氧化应激下的存活。fosA3和fosA7都存在于不同的血清型和ST型中，但被分为不同的组。fosA3阳性鼠伤寒沙门氏菌/印第安纳沙门氏菌菌株表现出密切的遗传关系，而fosA7阳性Meleagridis沙门氏菌/Agona/德比沙门氏菌表现出相对较高的全基因组序列异质性。fosA3位于结合型IncHI2质粒或染色体上，而fosA7严格意义上是染色体。此外，两个菌株在基因组岛内携带大的染色体fosA7区域。来自我们的分离株和NCBI的fosA3和fosA7重叠群可以分离为四个初级和不同的基因组主链。IS26和抗生素抗性基因（ARGs）blaCTX-M、blaTEM-1B和rmtB经常与fosA3相邻，而携带fosA7的重叠群通常缺乏移动元件和ARGs。
结论
在这些沙门氏菌分离株中，fosA3和fosA7是在不同程度上降低磷霉素敏感性的主要因素。fosA7和fosA3的不同分布和分子特征表明，它们在沙门氏菌中的起源和进化很可能是不同的。
Abstract
Objectives
To investigate the prevalence and molecular characteristics of fosA3 and fosA7 among Salmonella isolates.

Methods
Five hundred and fifty-one Salmonella isolates collected from food animals in China during 2016–19 were screened for fos genes. The drug resistance, serovars, clonal relationships and genetic environments of fosA were compared between fosA7- and fosA3-positive Salmonella.

Results
A relatively high prevalence of fosA7 (9.26%) and fosA3 (6.53%) was identified. fosA3 was associated with high-level fosfomycin resistance (≥512 mg/L), while fosA7 conferred relatively low-level resistance that was independent of the presence of glucose-6-phosphate. Additionally, fosA7 could facilitate Salmonella survival under oxidative stress. Both fosA3 and fosA7 were found in diverse serovars and STs, but segregated into distinct groups. The fosA3-positive Salmonella Typhimurium/Salmonella Indiana strains showed close genetic relationships, while fosA7-positive Salmonella Meleagridis/Salmonella Agona/Salmonella Derby showed a relatively high degree of whole-genome sequence heterogeneity. fosA3 was located on conjugative IncHI2 plasmids or chromosomes, while fosA7 was strictly chromosomal. Furthermore, two strains carried large chromosomal fosA7 regions within genomic islands. The fosA3 and fosA7 contigs from our isolates and the NCBI could be segregated into four primary and distinct genomic backbones. IS26 and the antibiotic resistance genes (ARGs) blaCTX-M, blaTEM-1B and rmtB were frequently adjacent to fosA3, while fosA7-carrying contigs generally lacked mobile elements and ARGs.

Conclusions
fosA3 and fosA7 were the primary factors contributing to reduced fosfomycin susceptibility, to different degrees, in these Salmonella isolates. The distinct distributions and molecular characteristics of fosA7 and fosA3 indicated that their origin and evolution in Salmonella were most likely distinct.

https://academic.oup.com/jac/article-abstract/77/5/1286/6549755