Salmonella is an older pathogen, well-known in the swine industry. However, clinical signs of salmonellosis are still seen on the farm, sometimes despite the use of vaccines. Dr. Matheus Costa and graduate student Mariana Meneguzzi explain why this might be due to the great variety of Salmonella found in pigs in an article for the National Hog Farmer.
A genetically distinct clade of Salmonella 4,,12:i:- (also referred to as S. monophasic), harboring multiple antimicrobial resistance genes (including to ampicillin,streptomycin, sulfonamides, and tetracyclines) became the predominant S. monophasic type in swine in the U.S. during 2014-2016.
Phenotypic resistance to enrofloxacin (fluoroquinolone) and ceftiofur (3rd generation cephalosporin) was present in a proportion these isolates, and whole genome sequencing revealed the presence of the plasmid-mediated genes.
These plasmid-mediated resistance genes could potentially transfer horizontally to other microorganisms and augment the problem of antimicrobial resistance to these critically important antibiotics.
S. monophasic emerged globally in the recent years and pig products have been identified as a source in some foodborne outbreaks. The prevalence of S. monophasic, and phenotypic resistance (minimum inhibitory concentration (MIC) above the cut-off value for this bacteria) to enrofloxacin increased in swine clinical samples in the Midwest during 2006 and 2016.
During this period, injectable enrofloxacin was approved by the Food and Drug Administration (FDA) for treatment of swine respiratory disease and colibacillosis in piglets (in 2008 and 2014, respectively); therefore, the objective of the study was to characterize the S. monophasic in swine in the U.S Midwest.
We used whole genome sequencing to compare S. monophasic isolates collected from livestock in the Midwest with isolates collected from different sources in the U.S. and Europe. We then determined the antimicrobial resistance genotypes and presence of other virulence factors that could help to explain the emergence of this variant.
Salmonella monophasic formed two main genetic clades regardless of source and geographical origin (Figure 1). Most (84%) isolates recovered in the U.S. during 2014-2016, including 50 isolates (out of 51) originating mainly from swine in the Midwest, were part of an emerging clade genotypically resistant to ampicillin, streptomycin, sulphonamides and tetracyclines. In the Midwest samples, phenotypic resistance to enrofloxacin (11 out of 50; 22%) and ceftiofur (9 out of 50; 18%) was found in conjunction with plasmid-mediated resistance genes. This is of particular concern because fluoroquinolones and 3rd generation cephalosporins are often used to treat invasive Salmonella infections in people. Furthermore, because the genes were plasmid borne there is greater likelihood for horizontal transfer of these genes to other bacterial strains.
challenged with both S. Typhimurium and L. intracellularis,
challenged with S. Typhimurium and vaccinated against L. intracellularis,
challenged with both S. Typhimurium and L. intracellularis and vaccinated against L. intracellularis
a non-infected control.
The greatest difference in shedding level between groups was found at 7 days post-infection. At this time point, the co-challenged animals from the vaccinated group shed statistically less S. Typhimurium per gram of feces than the animals from the non-vaccinated, co-challenged group. The co-challenged vaccinated group also shed significantly less S. Typhimurium than the singly infected S. Typhimurium group.
L. intracellularis vaccination did not have a significant impact on S. Typhimurium shedding when animals were singly infected with S. Typhimurium.
At 7 days post-infection, different treatment groups had significant differences in their microbiome community structure. The co-infected vaccinated group clustered apart from all other treatment groups.
These results indicate that vaccination against L. intracellularis impacts the microbiome and reduces shedding of S. Typhimurium in co-infected animals.
A new study by Dr. Julio Alvarez‘s team from the STEMMA laboratory, published in Clinical infectious Diseases suggests that a Salmonella strain circulating in pigs in the US Midwest is part of an emerging clade from Europe that is resistant to multiple antibiotics and may pose a public health risk.
The strain, Salmonella 4,,12:i:-, causes many foodborne disease outbreaks mostly tied to pigs and pork products and is expanding in the United States, according to the report by researchers from Minnesota and the United Kingdom.
The team used whole-genome sequencing to assess the relatedness of 659 S 4,,12:i:- isolates and 325 S Typhimurium isolates from various sources and locations in the United States and Europe. They also searched for resistance genes and other virulence factors and, for 50 livestock isolates and 22 human isolates, determined the antimicrobial resistance phenotypes.
The researchers found that the S 4,,12:i:- isolates fell into two main clades, regardless of their host or place of origin. Eighty-four percent of the US isolates recovered from 2014 through 2016, including nearly all those from pigs in the Midwest, were part of an emerging clade. This clade carried multiple genetic markers for antimicrobial resistance, including resistance to ampicillin, streptomycin, sulphonamides, and tetracyclines.
In addition, phenotypic (actual) resistance to enrofloxacin and ceftiofur was found in 11 of the 50 tested livestock isolates and 9 of the 22 human isolates. This was accompanied by plasmid-mediated resistance genes.
The authors conclude that S 4,,12:i:- strains circulating in Midwestern swine herds “are likely part of an emerging multidrug resistant clade first reported in Europe, and can carry plasmid-mediated resistance genes that may be transmitted horizontally to other bacteria and thus could represent a public-health concern.”
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Background Salmonella 4,,12:i:-, a worldwide emerging pathogen that causes many foodborne outbreaks mostly attributed to pig and pig products, is expanding in the U.S
Whole genome sequencing was applied to conduct multiple comparisons of 659 S. 4,,12:i:- and 325 S. Typhimurium from different sources and locations (i.e. U.S. and Europe) to assess their genetic heterogeneity, with a focus on strains recovered from swine in the U.S. Midwest. In addition, presence of resistance genes and other virulence factors was detected and the antimicrobial resistance phenotype of 50 and 22 isolates of livestock and human origin, respectively, was determined.
The S. 4,5,12:i:- strains formed two main clades regardless of their source and geographical origin. Most (84%) of the U.S. isolates recovered in 2014–2016, including those (50/51) recovered from swine in the U.S. Midwest, were part of an emerging clade. In this clade, multiple genotypic resistance determinants were predominant, including resistance against ampicillin, streptomycin, sulphonamides and tetracyclines (ASSuT). Phenotypic resistance to enrofloxacin (11/50) and ceftiofur (9/50) was found in conjunction with the presence of plasmid-mediated resistance genes (qnrB19/qnrB2/qnrS1 and blaCMY-2/blaSHV-12, respectively). Also, higher similarity was found between S. 4,,12:i:- from the emerging clade and S. Typhimurium from Europe than with S. Typhimurium from the U.S.
Conclusions Salmonella 4,,12:i:- currently circulating in swine in the U.S. Midwest are likely part of an emerging multidrug resistant clade first reported in Europe, and can carry plasmid-mediated resistance genes that may be transmitted horizontally to other bacteria and thus could represent a public-health concern.