Features included bacterial persistence and histopathological evidence of bronchopneumonia, including extensive tissue consolidation and diminished lung function. occurred early after contamination. Mice depleted of neutrophils were exquisitely susceptible to an normally nonlethal inoculum, thereby demonstrating the requirement for neutrophils in host protection. Mutation of the genes encoding the cytolysin ShlA and its transporter ShlB resulted in attenuatedS. marcescensstrains that failed to cause profound excess weight loss, extended illness, hemorrhage, and prolonged lung pathology in mice. This study explains a model ofS. marcescenspneumonia that mimics known clinical features of human illness, identifies neutrophils and the toxin ShlA as a key factors important for defense and contamination, respectively, and provides a solid foundation Synephrine (Oxedrine) for future studies of novel therapeutics for this important opportunistic pathogen. == INTRODUCTION == Serratia marcescensis a facultative anaerobic, rod-shaped, Gram-negative bacterium that is ubiquitous in water, in ground, and on herb surfaces.S. marcescensis also an antibiotic-resistant opportunistic pathogen and is among the top 10 10 causative brokers of bloodstream bacterial infections in North America, with a mortality rate of 41% (13). In newborns and immunocompromised and rigorous care patients,S. marcescenscan cause severe infections such as pneumonia (4), bloodstream infections (5), and urinary tract infections, surgical site infections, and ocular infections (6).S. marcescensinfections are most often associated with the hospital environment (5), SOCS-1 Synephrine (Oxedrine) but community-acquired infections are now progressively diagnosed (7). S. marcescenshas acquired notoriety in the last 20 years because of its resistance to multiple antibiotics (7,8). An 8-12 months surveillance study in Taiwan recognized multipleS. marcescensstrains that were resistant to the antibiotics ciprofloxacin and levofloxacin (9). Multiple studies have revealed an alarming increase inS. marcescensresistance to carbapenem and other -lactam antibiotics (8,10,11). As such,S. marcescensis considered a member of the carbapenem-resistantEnterobacteriaceae(CRE) (12). A recent study found thatS. marcescenshas an intrinsic resistance to polymyxin, which is considered to be the final option for treating CRE pathogens (13). These studies underscore the importance of understanding howS. marcescenscauses disease in order to identify prophylactic vaccine candidates or adjunct therapeutic approaches for prevention or treatment of nosocomial infections. Since the 1950s, multiple case reports have explained the pathological patterns of pneumonia caused byS. marcescens(1418), yet little is known in regard to the molecular basis for the observed pathology and the host factors that are required to resolve contamination in the lungs. To address this space in knowledge, we developed and characterized a mouse model ofS. marcescenspneumonia. Our model strongly recapitulated the clinical features of human disease, such as bronchopneumonia, loss of lung function, neutrophil infiltration, edema, and hemorrhage. What is more, by using this model, we ascertained the importance of the neutrophil responses to limit severe contamination and generated findings that suggest that the bacterial cytolysin ShlA was critical for Synephrine (Oxedrine) the observed pathology. These studies enhance our knowledge and understanding ofS. marcescenspneumonia and lay a foundation for future studies focused on this important emerging pathogen. == MATERIALS AND METHODS == Synephrine (Oxedrine) == Bacterial strains. == The tetracycline-resistantSerratia marcescensclinical isolate UT-383 was obtained from Jan Patterson (Division of Infectious Disease, Department of Medicine, The University or college of Texas Health Sciences Center at San Antonio [UTHSCSA]) and is defined here as wild type (WT). Isogenic, unmarkedshlAandshlBdeletion mutants were constructed by deleting the majority of the corresponding coding regions via allelic exchange, leaving behind the first 5 and the last 5 codons. Briefly, fragments transporting 750 nucleotides upstream and 750 nucleotides downstream of either theshlAorshlBopen reading frame were PCR amplified and cloned into the kanamycin resistance-encoding suicide vector pSR47s. Clones were originally isolated inEscherichia coliDH5 pir, and then, after sequence confirmation, a plasmid made up of the deletion allele was relocated into UT-383 via triparental conjugation withE. coliMT607/pRK600, as previously explained (19). After isolation of TetrKanrbacteria, bacteria were produced in the absence of kanamycin selection, and sucrose-resistant isolates were selected and confirmed by sequencing for the deletion of theshlAorshlBcoding sequence with no accompanying upstream or downstream mutations. == Culture conditions. == S. marcescenswas produced on.