Morganella morganii, a non-negligent opportunistic pathogen

Morganella morganii is a facultative anaerobic rod Gram-negative enteric bacterium, which was first isolated in 1906 by Morgan et al. from a pediatric fecal culture. The genome size of M. morganii is about 4,000,000 bp, and the number of its protein-coding sequences (CDSs) is about 4,000. M. morganii was formerly classified as Proteus morganii and later assigned to the genus Morganella, which belongs to the tribe Proteeae of the Enterobacteriaceae family on the basis of DNA–DNA hybridization determinations. Although members of the tribe Proteeae, including Proteus, Providencia and Morganella, share homologous genes acquired from horizontal gene transfer via mobile transposition or conjugative integration, the overall G+C contents in the genomes of other Proteeae members range from 39% to 43%, which are lower than that of the M. morganii (51%); therefore, the G+C contents provides genetic evidence for distinguishing M. morganii from other species.

The genus Morganella currently consists of a single species (M. morganii) with two subspecies, namely, morganii and sibonii. Biologically, M. morganii is a motile, non-lactose fermenting bacterium, which shares with the Proteus members on the capacity for urease production and presence of phenylalanine deaminase. M. morganii is widely distributed in nature. This bacterium is commonly found in the environment and intestinal tracts of humans, mammals, and reptiles as part of the normal flora. The drug resistance of M. morganii is increasing in recent years, and this resistance is mainly introduced via extra genetic^,  and mobile elements.^,  The infections caused by multidrug-resistant (MDR) or even the extensively drug-resistant (XDR) M. morganii often result in clinical treatment failure.^,  Generally, M. morganii can produce virulence factors, such as urease, hemolysins, and lipopolysaccharide (LPS); these virulence factors pose M. morganii an opportunistic pathogen that mainly causes wound and urinary tract infections.^{,  ,}  Comparative genome analysis revealed several pathogenicity-related genes, and novel genes carried by M. morganii genome are not found in the genomes of other Proteeae members, which may provide important information concerning the virulence and fitness determinants in M. morganii. The disease spectrum of M. morganii infection varies and is changeable according to its virulence evolution. This review aims to summarize the epidemiology of M. morganii, focus on its resistance profile and resistant genes, and discuss its disease spectrum and risk factors for infection.

As a member of the family Enterobacteriaceae, M. morganii is considered as a rare cause of nosocomial infection. Farmer et al. classified the bacteria of Enterobacteriaceae among 11 levels according to the relative frequency of a certain bacterium isolated from the clinical specimen. The relative frequency increases from 0 (not known to occur) to 10 (most common), in which that of M. morganii is 4, i.e., an opportunistic pathogen that causes rare infection.

Originally, M. morganii was thought to be a cause of summer diarrhea and considered to be a very unimportant pathogen. This bacterium was first found to be a cause of urinary tract infection in 1939. In the 1970s, M. morganii was shown to be a primary cause of nosocomial infection in adults and a rare cause of bacteremia. Adler et al. isolated six M. morganii strains from 71 cases of Proteusbacillus vulgaris bacteremia through investigating the characteristics of P. vulgaris infections and epidemiology in a general hospital. In the early 1980s, Tucci and Isenberg reported 13 M. morganii infections scattered over four services and five floors of a hospital; this outbreak was eventually resolved when strict aseptic techniques, i.e., hand washing, were reinforced. Following that incident, M. morganii has been identified as a significant cause of nosocomial infection, but recognized as an increasingly important pathogen in recent years. Over a six-year period (2006–2011), samples from all patients who presented symptoms of Gram-negative bacterial infections at Changhua Christian Hospital, Taiwan, were collected. Of the 82,861 samples, 1,219 (1.47%) are positive for M. morganii, which is the ninth prevalent cause of clinical infections in patients at the said hospital. In addition to Taiwan, other regions including Japan, USA, and Spain are also the most frequent areas with reported M. morganii-associated infections. However, the M. morganii-associated case reports are scattered and often present in immunocompromised patients.^{,  ,  ,  ,  ,  ,  ,}  No link exists between case reported areas and economic status, sanitary condition, natural environment, and population mobility.

Given the wide distribution of M. morganii in nature, M. morganii can commendably adapt to the environment for survival. Therefore, M. morganii dissemination may be advanced, including the mechanisms for M. morganii to cause diseases in both humans and animals. To assess the carriage of Enterobacteriaceae in the anterior nares in pig-exposed persons, Fischer et al. demonstrated that 66.7% (76/114) of the participants are positive for Enterobacteriaceae bacteria, with the predominant species of Proteus mirabilis (14.9%,17/114), followed by Pantoea agglomerans (11.4%, 13/114), M. morganii (7.9%, 9/114), Citrobacter koseri (7.9%, 9/114), Klebsiella pneumoniae, Escherichia coli, and P. vulgaris (each 7.0%, 8/114). Their studies suggest a possible transmission pathway between human, and the closely contiguous animal may exist; further investigation is also needed.

The intensive selection pressure of the widely used antibiotics results in a considerable acceleration of the evolution and spread of resistant genes in bacteria; moreover, drug resistance has posed a significant challenge for bacterial infection control. The bacterial isolates with MDR, XDR, and pandrug-resistant phenotypes are increasingly observed. Various mechanisms can theoretically lead to antibiotic resistance; these mechanisms include intrinsic, acquired, and adaptive resistances.^,  Intrinsic resistance is the innate ability of a certain bacterial species to resist the activity of a particular antimicrobial agent through its inherent structural or functional characteristics. Such intrinsic insensitivity can be due to the lacking affinity of the drug for the bacterial target, inaccessibility of the drug into the bacterial cell, or extrusion of the drug by chromosomally encoded molecules with active exportation activities. Acquired resistance occurs when a particular bacterial cell obtains the ability to resist the activity of a particular antimicrobial agent to which it was susceptible previously. This phenomenon can result from the acquisition of foreign resistance genes that are horizontally transferred between different strains or even species via conjugation, or/and from the mutation in certain genes involved in normal physiological processes and cellular structures. However, adaptive resistance happens when the bacterial population is subjected to gradual antibiotic increases; this resistance is characterized by a rapid emergence of resistance and reversibility to the normal phenotype when the antibiotic is removed. Adaptive resistance may require epigenetic inheritance and modification of gene expression patterns in the particular bacterial population.

All three kinds of resistances may occur in a particular bacterial species. M. morganii has intrinsic resistance to oxacillin, ampicillin, amoxicillin, most of the first- and second-generation cephalosporins, macrolides, lincosamides, glycopeptides, fosfomycin, fusidic acid, and colistin; this pathogen is also normally sensitive to aztreonam, aminoglycosides, antipseudomonal penicillins, third- and fourth-generation cephalosporins, carbapenems, quinolones, trimethoprim/ sulfamethoxazole, and chloramphenicol. A unique biochemical character of M. morganii is that this organism has the capability for extracellular biosynthesis of crystalline silver nanoparticles, which was found independent of environmental changes. Three chromosomal gene homologues (silE, silP and silS) identified in M. morganii were characterized to be responsible for the biosynthesis of silver nanoparticles in the presence of Ag⁺ ions, as well as the silver-resistant phenotype of the strain. Nevertheless, the acquired resistance is increasingly observed in M. morganii. According to the recent data from the SENTRY antimicrobial resistance surveillance program, M. morganii ranks 12th among the Gram-negative organisms that cause bloodstream infections. The acquired resistance of M. morganii is commonly introduced via genetic elements,^{,  ,  ,}  however, mutations in certain genes are also observed. Bacterial genetic elements consist of prophage, plasmid, transposon, inserted sequence, integron, and so on. Among them, plasmid, transposon, and integrin, are often related to antibiotic resistance, and can be transferred between homogeneous and even heterogeneous bacteria. Antibiotic resistance of M. morganii is mainly mediated by conjugative plasmids ^,  , mutation in certain genes ^,  and integrons^{,  ,  ,}  . Current genome sequence and determination with polymerase chain reaction revealed that the antibiotic-resistant genes carried by M. morganii are increasing (Table 1). Similar to Enterobacter spp. and Citrobacter freundii, M. morganii normally has an inducible AmpC (encodes–lactamase), which confers resistance to those β-lactam antibiotics (e.g., ampicillin) that induce its strong synthesis and are labile to its action. Derepression of AmpC, which is typically caused by mutation at ampD, causes constitutive β-lactamase hyperproduction and confers resistance to third-generation cephalosporins. M. morganii shows resistance to gentamycin. Aminoglycoside resistance among Morganella species is mediated by various enzyme combinations; the most frequent of these combinations is the modifying enzyme ANT(2)-I, which confers resistance to gentamicin, tobramycin, and kanamycin. A prevalence of quinolone resistance determinant exists among M. morganii. The plasmid-mediated quinolone resistant gene qnrD was first reported in 2009 in a human clinical isolate of Salmonella enteric serovar Kentucky and three Salmonella enteric serovar Bovismorbificans isolates from China. Mazzariol et al. (2011) demonstrated the presence of qnrD in isolates of P. mirabilis and M. morganii; they proposed that qnrD gene is closely linked to the bacteria of the tribe Proteeae. Carbapenems have been used in clinics as the antibiotics of last resort for the treatment of nosocomial infections caused by Enterobacteriaceae. Resistance to carbapenems is mostly driven by the production of carbapenemases, such as carbapenemase 2 (KPC-2) and New Delhi metallo-β-lactamase 1 (NDM-1). NDM-1 was first described in 2008 in Sweden from a patient who had previously been hospitalized in New Delhi, India; thereafter, a rapid worldwide dissemination of the said carbapenemase followed.^,  Multiple resistant genes (e.g., NDM-1 and qnrD1) carried in a single strain may cause M. morganii to be an XDR, which has been found in patient with sepsis. Experiences of therapeutic options for the treatment of invasive infections caused by MDR or XDR of M. morganii should be accumulated. The first bla_CTX-M-2-containing class 1 integron, termed In116, was detected in a plasmid from a cephalosporin-resistant M. morganii strain, producing CTX-M-2 β-lactamase. Other integrons were also identified in clinical M. morganii strains, e.g. In3Mor, sul1-type integron^,  , which were found to carry various resistant genes (Table 1).

Adaptive resistance has been well established in various bacterial species, including Escherichia coli, Salmonella enterica, Pseudomonas aeruginosa, and Staphylococcus aureus.^,  When these bacteria are exposed to successive steps of increasing antibiotic concentration, they will rapidly yield populations with high levels of resistance, and this resistance is highly reversible. However, no report has focused on adaptive resistance in M. morganii, and further investigation is suggested.

Virulence is a primitive character for a pathogen. Genome sequence revealed that the virulence factors of M. morganii include fimbrial adhesins, LPS, IgA protease, hemolysins, ureases, and insecticidal and apoptotic toxins, as well as proteins found in flagella, iron acquisition system, type-III secretion system (T3SS), and two-component systems (TCSs) (Table 2). Among them, fimbrial adhesins, ureases, and TCSs play an important role in M. morganii colonization and pathogenicity. In pathogenicity and colonization, adhesion is the first step in which a pathogen interacts with the host. Fimbrial adhesins help in biofilm formation and M. morganii colonization. With the exception of the genes that encode two LysR family transcriptional regulators, the organization of flagellar genes in M. morganii is similar to that of P. mirabilis. Rapid urea hydrolysis is a prominent phenotype of Proteeae organisms. The non-inducible urease gene cluster consisting of ureABCEFGD was detected in M. morganii. However, this gene cluster lacks the ureR regulatory gene that was detected in P. mirabilis HI4320 and Providen ciarettgeri DSM1131. Urease production serves as a fitness factor that facilitates bacterial growth and biofilm formation during urinary tract infections, which may explain why M. morganii mainly causes the urinary tract infection. Importantly, ureases from M. morganii urease gene cluster are required for bacterial virulence.

A most important achievement of bacteria is its ability to adapt to the changing environmental conditions. Competition with other microorganisms has led a plethora of bacterial mechanisms to adapt to a wide variety of stress conditions rapidly. Standard virulence evolution theory assumes that virulence factors are maintained because they aid bacterial colonization, thereby increasing bacterial growth within and/or transmission between hosts. M. morganii has developed several systems to cope with the varied environment, such as TCSs, which constitute a most sensible and efficient regulatory mechanism in bacteria. TCSs generally contain paired sensor kinase and response regulator proteins and form the primary apparatus for sensing and responding to environmental cues in bacteria. Nineteen potential TCSs were identified in M. morganii. These TCSs play important roles in the virulence and fitness of M. morganii. Nevertheless, pmrA/pmrB was not detected in the genome of M. morganii and other Proteeae members. The pathogenic genes allow the usage of computation approaches to identify potential drug targets, such as the conserved proteins found in common pathogens. The presence of eut (which includes pduST) and cob-cbi operons in M. morganii but not in other Proteeae genomes studied may explain why M. morganii is more frequently associated with nosocomial bacterial infections. ArnT mediates lipid A modification. Two copies of the arnT gene were detected in the genome of M. morganii and other Proteeae members, whereas non-Proteeae bacteria have only one copy.

ICEPm1, a highly modular and highly conserved pathogenicity island (PAI), is commonly found in M. morganii strains. This 94-kb PAI (ICEPm1) encodes 91 open reading frames (ORFs) with a G+C content of 44.84%, which differs substantially from that of the M. morganii genome (51%). ICEPm1 carries several genes involved in DNA mobility, characteristic for PAIs, including an integrase, six transposases, and five plasmid-transfer related proteins. An important core segment found in ICEPm1(PMI2569 to PMI2592) shows homology to a type-IV secretion system that is important for DNA transfer of ICEHin1056. A T3SS, which comprises gene products MM0224 through MM0247 and has a low G+C content (43.7%), resides in a 20.8-kb PAI. This PAI encodes 24 ORFs and shares homologous syntenic blocks with P. mirabilis, which contains all the components needed to assemble a T3SS needle complex. Sequence comparison between M. morganii KT and the 14 members of the Enterobacteriaceae family, revealed that 459 CDSs found in M. morganii are not found in the other Proteeae species studied, and 295 CDSs found in M. morganii are not found in any of the 14 Enterobacteriaceae genomes studied. The genes specific to M. morganii include the genes in the eut operon, cob-cbi operon, eight insecticidal toxin genes, nine T3SS genes, and 17 copies of the IS4 family transposase gene. However, the evidence that M. morganii shares features with other non-Proteeae enterobacteria suggests that horizontal gene transfer has occurred between M. morganii and other intestinal bacteria.

M. morganii is an unusual opportunistic pathogen that is clinically and often isolated as a cause of nosocomial infection in adults, specifically in urinary tract or wound infections. Urinary tract is the major portal for M. morganii entry, followed by the hepatobiliary tract, skin and soft tissue, and blood. However, M. morganii has been recognized an increasingly important pathogen because of its virulence and increasing drug resistance, which has resulted in a high mortality rate in some infections. To date, a total of 136 cases with M. morganii infection have been reported. The disease spectrum associated with M. morganii infections is summarized in Table 3. The diseases caused by M. morganii are diversified; these diseases include pyelonephritis, septic shock, urinary tract infection, osteomyelitis, peritonitis, abscess, purple urine bag syndrome, joint effusions, meningitis, sepsis, necrotizing fasciitis, pericarditis, pneumonia, aortic aneurysm, hemorrhagic bullae, bacteremia, septic arthritis, endophthalmitis, Waterhouse–Friderichsen syndrome, Ludwig's angina, pancreatitis, gangrenosum, chorioamnionitis, pyomyositis, ulcer, cellulitis, and wound infection. The mortality of M. morganii infections remains high in reported cases.^{,  ,  ,  ,  ,  ,  ,  ,  ,  ,}  M. morganii mainly causes sepsis (11.0%, 15/136), abscess (9.6%, 13/136), urinary tract infection (8.1%, 11/136), bacteremia (7.4%, 10/136), purple urine bag syndrome (5.9%, 8/136), chorioamnionitis (5.9%, 8/136), cellulitis (5.9%, 8/136), and wound infection (5.9%, 8/136). Remarkably, among the M. morganii-associated sepsis cases, 11 cases are neonates. Maternal chorioamnionitis, which were found in five cases among M. morganii-associated neonatal sepsis, is the most common antenatal risk.^{,  ,  ,  ,}  All reported cases are premature, with antenatal exposure to ampicillin/amoxicillin being reported in several cases.^{,  ,  ,}  The above situation could be explained by the common practice of administering ampicillin and antenatal steroids to mothers with threatened premature delivery. Routine intrapartum antibiotic prophylaxis with ampicillin may lead to the emergence of infections because of resistant Gram-negative organisms. Antenatal steroids are beneficial in accelerating maturity of fetal lung and other organ systems; they also have an established role in the management of women with preterm rupture of membranes. However, the use of these steroids may increase the risk of infection. The use of dexamethasone and ampicillin in mothers leads to ampicillin resistance of M. morganii. This pathogen can be spread to babies by vertical transmission from the mother's genitourinary tract during delivery.