Malaysian Journal of Microbiology, Vol 19(4) 2023, pp. 392-401 DOI: http://dx.doi.org/10.21161/mjm.220165 Malaysian Journal of Microbiology Published by Malaysian Society for Microbiology (In since 2011) 392 ISSN (print): 1823-8262, ISSN (online): 2231-7538 *Corresponding author Effect of Salmonella enterica ser. Typhi on the gut population of Lactobacillus spp. among typhoid patients in Ondo State, Nigeria Enabulele Onaiwu Idahosa 1 and Fadare Olalekan Shadrach 1,2 * 1 Microbiology Department, Faculty of Life Sciences, University of Benin, Benin City, Edo State, Nigeria. 2 Department of Biological Sciences (Microbiology Unit), Faculty of Basic and Applied Sciences, Elizade University, Wuraola Ade Ojo Avenue, P. M. B. 002, Ilara-Mokin, Ondo State, Nigeria. Email: olalekan.fadare@elizadeuniversity.edu.ng Received 30 December 2022; Received in revised form 27 February 2023; Accepted 13 June 2023 ABSTRACT Aims: Aberrant gut microbiota has been linked to frequent exposure to enteric pathogens, a condition capable of causing various adverse effects on human health. In this study, we looked into how the typhoid fever condition might affect the lactobacillus population in the human gut. Methodology and results: A total of 424 faecal samples were collected from consented participants, who included 191 patients and 233 apparently healthy individuals. Lactobacilli and Salmonella enterica ser. Typhi (S. Typhi) was isolated from samples cultured on de Man Rogosa and Sharpe agar (MRS) and xylose lysine deoxycholate agar (XLD), respectively. The overall prevalence of S. Typhi was 104(24.5%), of which 83(43.5%) were patients and 21(9.0%) were healthy controls. Lactobacillus spp. load in patients was significantly lower among the febrile, S. Typhi-positive patients with an average population of 5.5 ± 0.96 log10 cfu/g compared with the febrile, but S. Typhi-negative patients with 6.41 ± log10 cfu/g and the apparently healthy respondents with 7.34 ± 1.1 log10 cfu/g. All the randomly selected S. Typhi strains obtained from both groups of respondents were sensitive to tetracycline and trimethoprim but resistant to chloramphenicol. Resistance to ciprofloxacin (18.2%) and ofloxacin (9.1%) was observed among the strains isolated from the febrile typhoid patients. Conclusion, significance and impact of study: This study has demonstrated an association between the population of Lactobacillus spp. and the presence of S. Typhi in the human gut. In order to ensure the recovery of beneficial bacteria during and after the treatment of infections, it is crucial to promote critical research into new treatment methods. Keywords: Lactobacillus spp., microbiota, Salmonella Typhi, typhoid fever INTRODUCTION The gut microbiota is regarded as the most varied and abundant microbial assemblage, interacting with the host in a variety of ways and so impacting the host's health and well-being (Senghor et al., 2018). Lactobacilli have recently been identified as dominant taxa in the normal microbiota, with the greatest abundance in the small bowel (Duar et al., 2017). Its beneficial effect on human gut health is undoubtedly one of the most extensively researched topics in probiotic research. It has been established that Lactobacillus probiotics help prevent infection by enhancing the production of proteins related to the intestinal barrier and mucus, secreting anti- microbial agents like short-chain fatty acids (SCFA), bacteriocins and hydrogen peroxide. These anti-microbial agents are known to either stop the growth of pathogens or kill them by regulating the host's immune response to pathogens, preventing pathogen adhesion and competing with them for binding sites (Dempsey and Corr, 2022). Homeostasis within the host may be disturbed by dysbiosis, an altered intestinal microbiota state. This has been associated with numerous detrimental effects on human health as well as long-term effects that have the potential to cause a wide range of diseases, including inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), diabetes mellitus, obesity and colorectal cancer (Humphrey, 2009; Baothman et al., 2016; Bhattarai et al., 2017). Frequent exposure to enteric pathogens like Salmonella enterica serovar Typhi has been linked to aberrant gut microbiota (Humphrey, 2009). According to a recent study by Haak et al. (2020), patients with typhoid fever had significantly different patterns of compositional and functional disruption of the gut microbiota when compared to patients with non- typhoidal febrile illness and healthy local controls. In healthy individuals, a balanced relationship with the microbes, with no resulting disease has been reported