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Background: Salmonella enterica subsp. enterica serovar Newport (antigenic formula 6,8,20:e,h:1,2) is a serovar of the O:8 (C₂-C₃) serogroup. According to the National Veterinary Services Laboratories (NVSL), serovar Newport became one of the top 10 most frequently identified Salmonella serotypes from US cattle from July 1998 through June 1999. Ranked as one of the top three Salmonella serotypes causing US foodborne outbreaks, serovar Newport has seen a substantial increase in human clinical cases since 1995, with an estimated 100,000 annual infections. In recent years, multidrug resistant Salmonella Newport has emerged as an epidemic, rapidly spreading through animal and human populations in US. In 2019, serovar Newport ranked as the fifth most prevalent Salmonella serotype in the EU/EEA, representing 1.1% of reported human salmonellosis cases.

Genetic characteristics: Serovar Newport has been found to be polyphyletic with four lineages identified. Similarly, Sangal et al. analyzed 384 serovar Newport isolates from various sources and revealed that these serovar Newport isolates grouped into three distinct lineages—Newport-I, Newport-II, and Newport-III—each comprising multiple sequence types (STs). Newport-I included relatively few STs, suggesting a more recent emergence, and was more commonly found in humans in Europe than in North America. In contrast, Newport-II was mainly linked to animal sources. Notably, two STs within Newport-II contained all multidrug-resistant AmpC (MDR-AmpC) isolates, indicating a recent global expansion following acquisition of the bla_CMY-2 gene. Most Newport-III isolates were from humans in North America and were generally susceptible to antibiotics. Overall, serovar Newport showed an intermediate population structure compared to other serovars, ranging from a single lineage in Salmonella Enteritidis and Typhimurium to four separate lineages in Salmonella Paratyphi B. They found that both mutation and homologous recombination contributed to the genetic diversity within these lineages, though their relative impact varied by lineage. Cao et al. found that Asian strains differed genetically from those found in the Americas. To distinguish between different lineages of serovar Newport, they focused on regions near the mutS gene, including segments between the invH and mutS genes at the 3′ end of SPI-1, the ste fimbrial operon, and CRISPR-associated (cas) proteins. Outgroup genomes showed high similarity to either serovar Newport Lineage II or III at these loci. Their analysis revealed that lineages II and III have distinct evolutionary backgrounds in this region, with evidence of genetic exchange and homologous recombination near mutS. These results indicate that serovar Newport Lineages II and III diverged early in the serotype’s evolution and have since followed largely separate evolutionary paths.

Commichaux et al. studied serovar Newport strains associated with 2020 onion outbreak and found that the clinical clade harbored at least 20 plasmid types (e.g., IncFII(S) and IncI1-I(Gamma)), exceeding previously documented diversity for Salmonella Newport. Notably, 14 plasmids from 13 clinical isolates and 17 from 8 farm isolates exhibited >95% sequence similarity, suggesting possible horizontal transfer among these strains. In 2015, CDC detected MDR Salmonella Newport strain related to multiple illnesses and outbreaks, which has since been designated as a persistent strain (REPJJP01) under ongoing surveillance. Most REPJJP01 isolates are resistant to sulfisoxazole, tetracycline, trimethoprim-sulfamethoxazole, azithromycin, chloramphenicol, ciprofloxacin, streptomycin, and ampicillin but susceptible to ceftriaxone. Zhao et al. also found the bla_CMY gene was identify in every Newport MDR-AmpC isolate analyzed. Additionally, this plasmid-borne gene could be transferred through conjugation to an Escherichia coli strain, and the resulting transconjugant exhibited the same MDR-AmpC resistance pattern. Among the isolates, 35 (40%) carried class 1 integrons. Sequencing revealed that these integrons harbored either aadA (conferring streptomycin resistance) or both aadA and dhfr (providing resistance to trimethoprim-sulfamethoxazole). Notably, one integron from a swine-derived isolate included sat-1, which confers resistance to streptothricin—an antibiotic never approved for use in the US.

Animal reservoir: Cattle is the main reservoir for serovar Newport. This serovar was also isolated from other animals, such as pigs, poultry, horses, dogs, and pigeons.

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