Researchers in Brazil have found Salmonella is resistant to different classes of antibiotics and have identified genes responsible for that resistance. It was the first study of Salmonella Typhimurium strains isolated in Brazil that used Whole Genome Sequencing to access the genetic diversity and molecular bases of antimicrobial resistance.

The variety and prevalence of resistant genes found in Salmonella Typhimurium strains reinforces their potential hazard for humans and the risk in foods in Brazil, scientists reported in the journal PLOS ONE.

Ninety Salmonella Typhimurium strains had their genome sequenced to uncover the diversity of isolates collected between 1983 and 2013 from seven states in Brazil based on single nucleotide polymorphism (SNP) analysis. Strains were taken from patients with foodborne infections or from contaminated food such as poultry, pork, or lettuce and other vegetables.

A total of 39 genes responsible for resistance to antibiotics were identified, such as aminoglycoside, tetracycline, sulfonamide, trimethoprim, beta-lactam, fluoroquinolone, phenicol and macrolide.

When the action of antibiotics in each of the 90 strains was tested, it was discovered the majority were resistant to different classes. The strains of Salmonella Typhimurium were sequenced at the U.S. Food and Drug Administration.

The study found 65 of 90 Salmonella Typhimurium strains were phenotypically resistant to sulfonamides, 44 were streptomycin resistant, 27 were resistant to tetracycline, 21 gentamicin resistant and seven were resistant to ceftriaxone. Streptomycin is not frequently employed to treat Salmonella enterica infections but has been used as a growth promoter in food-producing animals.

Use of antimicrobials is not recommended in non-invasive Salmonella infections but in some cases antibiotic therapy is necessary. The drug to treat Salmonella infections is typically ciprofloxacin because of its broad spectrum antimicrobial activity.

Extensive use of antimicrobials has led to increasing non-typhoidal Salmonella strains that are resistant to quinolones and have reduced susceptibility to fluoroquinolones which can lead to treatment failure.

Antimicrobial susceptibility was tested by the disc diffusion method of the Clinical and Laboratory Standards Institute (CLSI).

Brazil’s Ministry of Health received reports of 11,524 outbreaks of foodborne diseases between 2000 and 2015, with 219,909 individuals sickened and 167 deaths. The top causes were Salmonella spp., with 31,700 cases diagnosed (14.4 percent of the total), Staphylococcus aureus (7.4 percent), and E. coli (6.1 percent).

According to a Ministry of Social Development survey, Salmonella was the etiological agent in 42.5 percent of the laboratory-confirmed foodborne disease outbreaks in Brazil between 1999 and 2009.

Phylogenetic results placed the 90 Salmonella Typhimurium strains into two major clades. Of 34 strains in Clade A, 15 (14 foods, one human) were resistant to three or more antimicrobial classes being multidrug-resistant (MDR). Of the 56 strains in Clade B, 23 (18 humans, five foods) were MDR.

Food isolates were distributed in Clades A and B in relatively similar numbers suggesting there is more than one subtype in circulation in foods in Brazil, according to researchers. Amanda Aparecida Seribelli reported the samples collected from food differed from the group from humans.

“Food isolates were distributed between groups A and B in relatively similar numbers, suggesting that more than one subtype is circulating in foods in Brazil. Human isolates were more prevalent in group B, suggesting that a specific subtype has probably adapted to humans,” she said.

Researchers found a difference between the strains’ resistance in the 30-year sample collection period.

“The samples of Salmonella Typhimurium collected in the mid-1990s showed more resistance to antibiotics than samples from later years. This could be explained by the emergence in the early 1990s of the serovar Salmonella Enteritidis, which has since become one of the main causes of salmonella infection,” Seribelli said.

Another researcher at the University of São Paulo’s Ribeirão Preto School of Pharmaceutical Sciences is currently sequencing and analyzing samples containing Salmonella Enteritidis.

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