PREVALENCE AND DRUG-RESISTANCE PATTERNS OF SALMONELLA ENTERICA SEROVAR TYPHI IN CLINICAL ISOLATES FROM KARACHI, PAKISTAN USING MALDI-TOF MS AND VITEK 2 SYSTEM

Abstract

Background Salmonella enterica serovar Typhi is a causative agent of typhoid fever. In Pakistan, it remains a major public health burden due to the increasing prevalence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains. Karachi, being one of the largest metropolitan regions of the country, suffers to experience a significant burden of enteric fever caused by antimicrobial-resistant strains. Objective This study aimed to determine the prevalence of Salmonella enterica serovar Typhi in different clinical specimens collected from diagnostic laboratories in Karachi, Pakistan, and to evaluate the distribution of MDR, XDR, and non-MDR/non-XDR isolates using advanced automated identification and antimicrobial susceptibility testing systems. Methodology A cross-sectional study was conducted using clinical isolates (n = 115) obtained from multiple diagnostic laboratories during 2023 in Karachi after informed consent and ethical consideration of available anonymized patient records. Clinical information regarding specimen source and laboratory history was documented. High throughput identification and biochemical characterization of isolates were carried out using Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry (MALDI-TOF MS) and the VITEK 2 automated microbial identification system. Antimicrobial susceptibility testing (AST) was carried out using the VITEK 2 system according to the Clinical & Laboratory Standards Institute (CLSI). Isolates were categorized as multidrug-resistant (MDR), extensively drug-resistant (XDR), or non-MDR/non-XDR based on observed resistance profiles against standard antibiotics. Results The study demonstrated that blood samples constituted the highest proportion of Salmonella enterica serovar Typhi isolates (74.78%), followed by stool (16.52%), urine (7.83%), and bone marrow (0.87%) specimens, confirming bloodstream infection as the predominant clinical presentation of typhoid fever in the studied population. MALDI-TOF MS and VITEK 2 showed high concordance for species-level and serotype-level identification, respectively. Serotypes were successfully confirmed by antigen-antibody latex agglutination test. Antimicrobial susceptibility profiling revealed an alarming prevalence of resistant strains among clinical isolates. Our findings demonstrated a predominant prevalence of XDR Salmonella enterica serovar Typhi isolates in blood samples, while multi-drug-resistant strains were found mainly among blood and stool specimens. Non-MDR/non-XDR isolates were significantly fewer in number as compared to XDR. Collectively, amicrobial-resistant phenotypes surpassed susceptible isolates. Thus, indicating a high occurrence of antimicrobial-resistant Salmonella enterica serovar Typhi strains in Karachi. Conclusion: Our study displayed substantial prevalence of MDR and XDR Salmonella enterica serovar Typhi among clinical isolates in Karachi, Pakistan. The blood culture appeared as the primary source of recovery. The growing problem of Salmonella enterica serovar Typhi infection poses a serious therapeutic and public health challenge. There is a need for continuous surveillance, authentic and rapid diagnostic facilities, antibiotic stewardship programs, and enhanced infection prevention stratagems at the national level for the prevention and control of resistant phenotypes of Salmonella enterica serovar Typhi.