Serratia marcescens - Infectious Disease and Antimicrobial Agents http://www.antimicrobe.org/b26.asp Microbes Home Editorial Board Reviews Contact Us Search Serratia marcescens Authors: Celine Herra, M.Sc., Frederick R Falkiner, B.A., Ph.D, FTCD. Table of Contents Monograph Tables What's New Reviews History Collapse All | Expand All Microbiology MICROBIOLOGY Epidemiology Clinical Manifestations Once considered a harmless saprophyte, Serratia marcescens is now recognized as an important opportunistic pathogen Pathogenesis combining a propensity for healthcare-associated infection and antimicrobial resistance. Susceptibility In Vitro and In Serratia marcescens is a member of the genus Serratia, which is a part of the family Enterobacteriaceae. Currently 14 species Vivo ofSerratia are recognized within the genus, eight of which are associated with human infection (67). Of the eight species implicated Antimicrobial Therapy in clinical infection S. marcescens, S. liquefaciens and S. odorifera are best known (31, 67). Of Prevention all Serratia species, S. marcescens is the most common clinical isolate and the most important human pathogen. Conclusion S. marcescens is credited with a long fanatical history dating back to antiquity, when, because of its ability to produce a red pigment it was described as having ‘masqueraded’ as blood (36). Early in this century, this distinctive red pigmentation of S. marcescens, combined with an apparent low level of virulence, led to its use as a biological marker of infection. Consequently, S. marcescens was used in a number of classic bacterial transmission experiments, which led to improved understanding of the epidemiology of infection (124). Under more controversial settings, S. marcescens was also used by the US military in a series of biological warfare test experiments conducted on the general population (124). From 1960 onwards, however, non-pigmented isolates of S. marcescenspredominated over pigmented strains in the clinical setting and were increasingly implicated in healthcare-associated infection (24,30) particularly among compromised patients. As members of the Enterobacteriaceae family, Serratia spp are motile, non-endospore forming Gram-negative rods. In the laboratorySerratia are routinely isolated from bloodstream and wound sites using blood agar culture or from respiratory and urinary sites using selective culture methods. Common selective agar cultures include MacConkey agar which categorizes Serratia isolates with the other non-lactose fermenting Enterobacteriaceae or chromogenic agars, which classifies them into a broad Klebsiella, Enterobacter,Serratia and Citrobacter (KESC) grouping (13, 17). Phenotypically Serratia is one of the easiest genera to differentiate within the Enterobacteriaceae family. Unlike other enterobacteria, strains of Serratia usually produce extracellular deoxyribonuclease (DNase), gelatinase and lipase and are resistant to the antibiotics colistin and cephalothin. Traditionally, Serratia spp were identified using the Analytical Profile Index (API) 20E (bio-Merieux) phenotypic microbial identification system (40). With the widespread introduction of automated identification systems, laboratory identification of Serratia spp. is now routinely performed using systems including Vitek 2 (bio-Merieux), Microscan Walk-Away (Dade-Behring, Siemens), or BD Phoenix (BD Diagnostics, Sparks) (71). More recently, species level identification of Serratia isolates has been successfully achieved using rapid, automated MALDI-TOF MS (matrix-assisted laser desorbtion/ionization time-of-flight mass spectrometry) identification systems (86, 95). EPIDEMIOLOGY Serratia marcescens is an opportunistic pathogen whose clinical significance has been appreciated only in the last four decades. While S. marcescens is a rare cause of community-acquired infections, it has emerged as an important nosocomial healthcare- associated pathogen and a frequent source of outbreaks of hospital infection (72), in both adult (122) and paediatric patients (115). Results from a recent surveillance programme in the US and Europe, indicate that Serratia spp., accounts for an average of 6.5% of all Gram negative infection in Intensive Care Units (ranked 5th amongst Gram negative organisms in ICU) and an average of 3.5% in non-ICU patients (91). Currently Serratia is the seventh most common cause of pneumonia with an incidence of 4.1% in the US, 3.2% in Europe and 2.4% in Latin America (51), and the tenth most common cause of bloodstream infection with an incidence of 2.0% amongst hospitalized patients (2). S. marcescens is rarely associated with primary invasive infection. It operates as a true opportunist producing infection whenever it gains access to a suitably compromised host. Patients most at risk include those with debilitating or immunocompromising disorders, those treated with broad-spectrum antibiotics and patients in ICU who are subjected to invasive instrumentation. The indwelling urinary catheter is a major risk factor for infection. The risk of a catheterized patient becoming infected with S. marcescens has been directly related to the proximity of other catheterized patients colonized or infected with the organism (68). The respiratory tract is also recognized as a major portal of entry with S. marcescens being isolated from the respiratory tract of up to 80% of post-operative patients developing S. marcescens bacteremia (125). Not surprisingly, common infections include urinary tract infection in patients with indwelling catheters, respiratory tract infection in intubated patients and bloodstream infection in post- surgical patients, especially in those with intravenous catheters. CLINICAL MANIFESTATIONS S. marcescens is implicated in a wide range of serious infections including pneumonia (51), lower respiratory tract infection (112), urinary tract infection (53), bloodstream infection, wound infection and meningitis (72, 74). The organism has also been described 1 of 12 10/7/2019, 4:57 PM Serratia marcescens - Infectious Disease and Antimicrobial Agents http://www.antimicrobe.org/b26.asp as an important cause of ocular infection with high incidence in contact lens-related keratitis (4, 21, 92). S. marcescens is also a rare cause of endocarditis. In the 1970s, S. marcescens was the most frequent cause of Gram-negative endocarditis among intravenous drug addicts in San Francisco (73). The frequency has since subsided, although sporadic cases ofSerratia endocarditis still occasionally occur with two of the highest risk groups including intravenous drug users and patients undergoing prosthetic valve surgery. Skin and soft tissue infections are also unusual although rare cases of invasive cellulitis and necrotizing fasciitis have been reported (60). Septic arthritis has also been reported following diagnostic and therapeutic intra- articular injections (76). Over the years, S. marcescens infection has been attributed to many different sources. Outbreaks of infection have been traced to medical equipment including nebulisers (87), bronchoscopes (82), electrocardiogram leads (100), laryngoscopes (20) and contaminated solutions such as inhalation medications (112), prefilled heparin syringes (8, 103), saline solutions (105), parenteral nutrition (3) and antiseptics (76). Many diverse environmental sources such as air conditioning units (82), urine-collecting basins (122), bed-pan macerators (42), liquid soap dispensers (104) and even tap water (46) have also been implicated. Outside of environmental sources, hospital patients have also been recognized as a reservoir for infection. The gastrointestinal tract is recognized as the predominant site of colonization for S. marcescens, as is the case for most members of the Enterobacteriaceaefamily. Studies have shown, however, that rates of S. marcescens gastrointestinal carriage are largely dependent on the population screened (11). In the healthy host and non-infected immunocompetent patient, carriage rates remain low (<1% and 3%, respectively). However, in affected wards carriage rates of 21% have been reported in non-infected patients, whilst over 30% of patients infected with S. marcescens carry the organism in the gut (11, 123). Once established, carriage is persistent and patients are likely to carry the organism at multiple sites, with the throat and nose identified as common sites in 59% and 31% of colonized patients, respectively (11). Regardless of the source or reservoir, the predominant mode of spread of S. marcescens is thought to be hand-to-hand transmission by hospital personnel (23, 68). The recovery of epidemic strains of S. marcescens from hand cultures of hospital staff has been reported consistently in epidemiologic studies (39, 49). In one hospital, almost 50% of hand cultures from staff were positive at the end of their working shift (113). Factors such as debilitating clinical condition, lengthy ward-stay and frequent exposure to medical interventions, most likely act by necessitating increased frequency and intensity of direct contact with staff hands (111). PATHOGENESIS In keeping with its role as an agent of opportunistic infection, S. marcescens was traditionally associated with low intrinsic pathogenicity. Whilst almost all isolates produce extracellular products such as DNase, chitinase, lecithinase, lipase, gelatinase and siderophores, it appears that in S. marcescens, these products do not act as potent virulence factors (5). Nevertheless, ongoing studies indicate that S. marcescens may produce other invasive