Annals of R.S.C.B., ISSN: 1583-6258, Vol. 25, Issue 1, 2021, Pages. 5274 - 5285 Received 15 December 2020; Accepted 05 January 2021. 5274 http://annalsofrscb.ro 3 Molecular Detection of SpnI/SpnR and AHL‐Mediated Quorum Sensing Signals Genes in Serratia Marcescens Mohammed S. Abbas*, Munim R. Ali and Ali H. Alsakini Department of Biology, College of Science, Mustansiriyah University *Cooresponding author email is :mohammedsalehabbas@gmail. Com ABSTRACT Quorum sensing process, bacteria connect with each other using secreted chemical signaling molecules called autoinducers. a quorum-sensing signaling molecule suggested to be involved in related species communication. Twelve isolates Serratia marcescens were collected from Baghdad hospitals. Of the 12 S. marcescens isolates examined, only 8 (66.66%) produced prodigiosin pigment. This work describes a high frequency of isolation of pigmented S. marcescens from clinical specimens, indicating that pigmented strains become clinically significant. The antibiotic sensitivity test showed multi drug resistant against (8- 18) antibiotics, all isolates were resistant (%100) against Beta lactam group, an indication of the comprehensive resistance to beta-lactam antibiotics, as a result of the irresponsible use of these antibiotics, which made them lose the effective therapeutic role that they previously required. In another hand S. marcescens 100% sensitive for to Meropenem and Imipenem and varied in their response to other antibiotics used in this study. The results showed a different distribution in S. marcescens (AI-2 kinase, Tsq, rpoS and SpnI/SpnR) genes. Eleven out of (92%) isolates namely were negative for AI-2 kinase gene, while (100%) 12/12 isolates observed positive for Tsq gene. 1\12 (8%) were positive for the presence of rpoS gene, whereas 12/12 isolates were positive, spnI/spnR gene was found with the highest percentage ratio (100%) when only one isolate were harboring all sets of genes. Our study suggested that expression of spnI/spnR by S. marcescens is independent of AHL signals synthesis. Key words: Serratia marcescens, Quorum sensing, AI-2 kinase, Tsq, rpoS and SpnI/SpnR INTRODUCTION Serratia marcescens is a Gram-negative bacterium that causes infections in many organisms including humans [1]. There are two types of Serratia marcescens; pigmented (red) and non pigmented (white) strains, Pigmented Serratia marcescens strains have been shown to cause infections in much less frequency than non-pigmented strains, thus reducing the risk of infection during mass production of pigment [2].The ability of S. marcescens to adapt to and survive in both hostile and changing environments also relates to the bacterial capacity to express a wide range of secreted enzymes, including lipases, phospholipases, chitinases, proteases, and nucleases [3].S. marcescens is able to develop biofilm associated with either