Changes in antimicrobial resistance and molecular attributes of Salmonellae causing enteric fever in Kolkata, India, 2014-2018

Defying the odds: Determinants of the antimicrobial response of Salmonella Typhi and their interplay

Salmonella Typhi, the invasive serovar of S. enterica subspecies enterica, causes typhoid fever in healthy human hosts. The emergence of antibiotic-resistant strains has consistently challenged the successful treatment of typhoid fever with conventional antibiotics. Antimicrobial resistance (AMR) in Salmonella is acquired either by mutations in the genomic DNA or by acquiring extrachromosomal DNA via horizontal gene transfer. In addition, Salmonella can form a subpopulation of antibiotic persistent (AP) cells that can survive at high concentrations of antibiotics. These have reduced the effectiveness of the first and second lines of antibiotics used to treat Salmonella infection. The recurrent and chronic carriage of S. Typhi in human hosts further complicates the treatment process, as a remarkable shift in the immune response from pro-inflammatory Th1 to anti-inflammatory Th2 is observed. Recent studies have also highlighted the overlap between AP, persistent infection (PI) and AMR. These incidents have revealed several areas of research. In this review, we have put forward a timeline for the evolution of antibiotic resistance in Salmonella and discussed the different mechanisms of the same availed by the pathogen at the genotypic and phenotypic levels. Further, we have presented a detailed discussion on Salmonella antibiotic persistence (AP), PI, the host and bacterial virulence factors that can influence PI, and how both AP and PI can lead to AMR.

Emergence of ceftriaxone resistant Salmonella enterica serovar Typhi in Eastern India

In view of widespread isolation of fluoroquinolone (FQ) resistant Salmonella enterica serovar Typhi globally, third generation cephalosporins (ceftriaxone) are used as alternative drugs for treatment of typhoid fever in recent years. But reports on emergence of third generation cephalosporin resistant S. Typhi have been documented from various countries including India posing threat in future use of this drug for typhoid treatment. Here, we report on genomic analysis of a third generation cephalosporin resistant S. Typhi strain isolated for the first time from Eastern India, Kolkata during 2019. The study strain was phenotypically resistant to ceftriaxone, ampicillin. Whole genome sequencing revealed the presence of conjugative IncX3 plasmid carrying bla_SHV-12 gene on it. The study strain belongs to H58 haplotype (4.3.1.2) and ST1 type. Comparison of phylogenetic analysis of the study strain with other cephalosporin resistant S. Typhi strains across the world revealed that three strains isolated from Western part of India during 2016 were its closest neighbours. Hence close monitoring of cephalosporin resistant S. Typhi strains are of great importance to control the furure use of this antibiotic for the treatment of typhoid fever.