Citrobacter - an overview | ScienceDirect Topics (2023)

Treatment

As with other species ofEnterobacteriaciae, effective antimicrobial therapy forCitrobacter is challenged by the continuing worldwide emergence of a broad array of β-lactamases, including metallo-β-lactamases (MBL).⁵³ Many different types of β-lactamases, including a novel TEM-type (TEM-134),⁸¹ the class A β-lactamase CKO,⁸² and a VIM-1 metallo-β-lactamase, have been described in various isolates ofCitrobacter.¹¹⁸ MostC. koseri organisms are resistant to ampicillin (97% in one series)⁶⁴ and susceptible to aminoglycosides and third-generation cephalosporins.²³ A 4-year experience with neonatal septicemia caused byC. koseri has been described, however, in which all of 13 isolates were resistant to gentamicin but susceptible to third-generation cephalosporins.²⁷ The resistance patterns ofC. freundii were reported in a national surveillance study of nosocomial bloodstream infections.⁸³ Of the 23C. freundii isolates tested, resistance to piperacillin, piperacillin/tazobactam, ceftriaxone, and ceftazidime was a common (39% to 48%) finding. Isolates generally were susceptible to the aminoglycosides and ciprofloxacin (91% to 96%), and allC. freundii tested were susceptible to cefepime and imipenem.

The growing challenge of antimicrobial resistance amongstCitrobacter sp. is exemplified by a study performed in a tertiary care facility in India of bacterial isolates identified from 2010 to 2013.⁸⁶ Of 221Citrobacter isolates, 179 (81%) were found to have diminished susceptibility to carbapenems. These resistant isolates were capable of elaborating extended-spectrum-β-lactamases as well as MBLtransferable on plasmids. Additional reports have confirmed the presence ofKlebsiella pneumoniae carbapenemase-2 (KPC-2) fromCitrobacter isolates.^13,71 SomeCitrobacter isolates contain chromosomally mediated group I β-lactamases. These bacteria possess a gene that, when triggered by exposure to cephalosporins or by spontaneous mutation, produces a cephalosporinase capable of inactivating cephalosporins.⁴⁷ Clinically, it manifests as treatment failure and emergence of drug resistance to various cephalosporins despite initial susceptibility.⁷⁰ In one study, the presence of group I β-lactamases was much more common withC. freundii (9 of 22 isolates) than withC. koseri (0 of 7 isolates).⁴⁷ Resistance was associated with previous receipt of an extended-spectrum β-lactam antibiotic. Reliable estimates of the percentage ofCitrobacter strains that contain the chromosomal resistance gene are difficult to obtain because most studies lumpCitrobacter, Enterobacter, andSerratia isolates together. In one Korean study, of 152Enterobacter/Citrobacter/Serratia isolates, 45 (30%) were derepressed Amp C mutants.⁷⁹ In an in vitro study, Amp C production was inducible in eight of nine clinical isolates ofC. freundii and in one of three isolates ofC. koseri. Amp C synthesis was stably derepressed in 1/9C. freundii isolates.²⁴ In contrast, an investigation from Johns Hopkins University Hospital of isolates collected in 2010 detected Amp C β-lactamases in only 1% ofCitrobacter spp.^1,109 Cefepime seems to be less likely to induce production of these β-lactamases and more resistant to hydrolysis by them,^96,109 although a highly cefepime-resistant strain has been described.⁷ Of 3030 ceftazidime-resistant Enterobacteriaceae in a US study, 99% retained susceptibility to imipenem and 96.7% of ceftazidime-resistantCitrobacter isolates were susceptible to cefepime.⁸⁴ Quinolone resistance viaqnr genes has also been found inC. koseri⁴⁹; in one study from Korea, 29% ofC. freundii isolates were quinolone resistant. Most of these hadqnrB genes.⁴⁸ Extensively resistant isolates are more likely to be susceptible to colistin⁶⁷ and polymyxin B¹²⁴ than to tigecycline; however there is a paucity of experience in the use of these antimicrobials for treatment ofCitrobacter infections.

CLINICAL MANIFESTATIONS

Citrobacter, similar to other neonatal pathogens, can cause early-onset and late-onset infection. In a review of 74 cases of neonatal meningitis caused by these bacteria,²¹ the mean age of onset reported for early disease was 7 days; 85 percent of patients were included in this group. Fifteen percent of cases occurred after 3 weeks of age. Twenty-three (31%) of 74 patients were younger than 36 weeks' gestational age at birth, suggesting that preterm infants are at increased risk for acquisition of Citrobacter infection. Prematurity is even more common (71%) in cases that are proven to be vertically acquired.¹³

Clinical signs and symptoms are typical of neonatal sepsis. Fever, lethargy, poor feeding, vomiting, irritability, bulging fontanelle, seizures, and jaundice are common presenting features. Umbilical infection and surgical manipulation of colonized umbilical stumps occasionally have preceded development of bacteremia and meningitis.⁵³ The white blood cell count may show leukocytosis or leukopenia. CSF fluid findings are consistent with most types of neonatal bacterial meningitis and usually show polymorphonuclear cell elevation, elevated protein, and depressed glucose; gram-negative rods may be seen on smear. Although growing the organism in culture normally is not difficult, in one reported case standard cultures were negative, but the organism was recovered by direct inoculation of CSF into a BacTec blood culture bottle.¹¹ Of Citrobacter meningitis cases in which the results of blood cultures are reported, 80 percent document concurrent bacteremia.¹³

Citrobacter is a particularly devastating cause of neonatal meningitis. The most common Citrobacter spp. causing neonatal meningitis is C. koseri, which accounts for more than 80 percent of the cases.¹³ CNS infection with this organism produces multiple brain abscesses with unusually high frequency.^{21,27,35,39,41} In the extensive reviews by Graham and Band²¹ and by Doran,¹³ three quarters of Citrobacter meningitis cases resulted in intracerebral abscesses. By comparison, the incidence of abscess formation in non-Citrobacter gram-negative meningitis is reported to be as low as 10 percent.²¹ The case-fatality rate for Citrobacter meningitis is approximately 30 percent, and at least three quarters of surviving infants have neurologic sequelae, such as mental retardation, hemiparesis, seizures, and developmental delay.¹³

The presence of brain abscess seems to contribute significantly to morbidity and mortality.^12,20,21 For unknown reasons, neonates with vertically acquired Citrobacter meningitis seem to be less likely to develop intracerebral abscesses.¹³ At least three cases in which diffuse pneumocephalus developed in association with Citrobacter meningitis have been reported; in one, gas was noted to accumulate within the brain and in the anterior chamber of the eye, a condition known as pneumatosis oculi.^1,58 Rarely, Citrobacter infection in the neonatal period may lead to focal infection not involving the CNS. A case of septic arthritis and osteomyelitis of the shoulder in a 3-week-old infant has been reported.³¹

In adults, Citrobacter is isolated most commonly from the urinary tract.^34,43,44 In earlier studies, 5 to 12 percent of bacterial isolates from urinary tract infections in adult patients were Citrobacter.^15,78 More recently, in a health maintenance organization, Citrobacter spp. accounted for only 0.8 percent of 4342 isolates from women with acute uncomplicated cystitis.²⁶ Citrobacter spp. are a similarly uncommon cause of urinary tract infection in children.¹⁷ Sputum is the second most common clinical specimen to yield Citrobacter in adults³⁴; lung abscess,¹⁸ pneumonia,^43,44 bronchitis,³² and septic arthritis⁴³ have been reported. Citrobacter is an occasional cause of bacteremia in hospitalized patients, accounting for approximately 0.5 percent of blood culture isolates.^56,64 In one series, all 45 patients had at least one underlying disease, with malignancies (particularly intra-abdominal tumors) and hepatobiliary stones being the most frequent coexisting conditions.⁶⁴ Polymicrobial bacteremia occurred in one third of patients. The case-fatality rate was 18 percent.

Gastrointestinal disease occasionally has been attributed to Citrobacter, but frequent isolation of this agent from normal stools often renders this diagnosis equivocal. This genus was implicated first in an outbreak of mild gastroenteritis by Barnes and Cherry⁵ in 1946, and an outbreak of watery diarrhea in a Virginia infant care unit included two infants in whom isolates of enterotoxin-liberating Citrobacter were obtained from the stool.²⁵ Some studies have found higher incidences of Citrobacter isolation from the stool of patients with enterocolitis syndrome than from stools of control patients.⁷⁹ Shiga-toxin (verotoxin)–producing C. freundii isolated from organically grown parsley was associated with an outbreak of diarrhea and hemolytic-uremic syndrome in a daycare setting.⁷³ C. freundii has been found as a cause of appendicitis in a healthy adult,⁴³ peritonitis in adults with liver disease or pancreatitis,⁴³ neutropenic colitis following chemotherapy for breast cancer,¹⁰ and meningitis in adults as a complication of neurosurgery.⁷⁰ A case of Meleney gangrene occurring after cesarean section delivery has been reported.⁶⁹ A patient with diabetes developed necrotizing fasciitis caused by C. freundii associated with injury from a fish fin.⁹ Bone and soft tissue infections occur^8,67; 3 percent of Citrobacter pathogens were isolated from joint or bone in one adult series.⁴³

Urinary Tract Infections

The incidence of bacteriuria in term newborns is estimated to be 0.1%-1%. The incidence is higher in preterm infants and is thought to be around 2%. In the neonatal period, urinary tract infection (UTI) is more common among males than females. Over the first 6 months of life, the incidence of UTI in males decreases, whereas in females it increases. By 1 year of age or older, girls are more likely than boys to have UTIs.^54,95 Uncircumcised males are at higher risk for UTI than are circumcised males. This is owing to enhanced bacterial adherence to the foreskin and increased bacterial colonization in the urogenital tract.¹⁰³ It is rare for a UTI to occur in the first 3 days of life; therefore, urine cultures are not recommended as part of the routine early-onset sepsis evaluation.

Infection of the urinary tract in neonates is thought to be acquired either through hematogenous spread or by ascending infection, often associated with anatomic abnormalities. Hematogenous spread is thought to be the cause for many young infants with UTI, because bacteremia with the same organism is seen in approximately one-third of neonates with upper tract infection.⁹⁵ Preterm infants are at higher risk for UTI with concordant bacteremia. Associated meningitis is also seen in 1%-2% of these cases.^28,116 Urinary tract abnormalities are seen in approximately 20%-50% of infants with UTI.^51,95 The most common abnormality seen is vesicoureteral reflux (VUR). Other abnormalities that may increase the risk for UTI include ectopic ureter, duplicated collecting system, renal dysplasia, and causes of obstruction, such as posterior urethral valves and ureteropelvic junction stricture. An additional risk factor for UTI is the presence of an indwelling urinary catheter.

The most common causative pathogen for neonatal UTI isE. coli, which has been isolated in approximately 80% of cases in some large series.^51,95E. coli has several virulence factors that confer an increased ability to cause UTI. UropathogenicE. coli, the causative organism in most UTIs, can invade and replicate within uroepithelial cells. Adhesins on the bacterial surface allow for increased adhesion to urogenital tract cells, and fimbriae are important in promoting persistence of infection. Other common causative organisms include other Enterobacteriaceae:Klebsiella,Enterobacter,Proteus,Citrobacter,Salmonella, andSerratia. Gram-positive organisms are isolated much less frequently than Gram-negative organisms, although Enterococci,S. aureus, and coagulase-negative Staphylococci are also known to cause UTI in newborns.

The signs of neonatal UTI are nonspecific and may include fever, lethargy, failure to thrive, poor feeding, abdominal distention, vomiting, tachypnea, and cyanosis. The clinical presentation of preterm infants is similar to that of term infants, although preterm infants may also have hypoxia or apnea with bradycardia. Definitive diagnosis must be made by urine culture with specimens obtained by catheterization or suprapubic bladder aspiration. Samples collected into a bag are less helpful, because they are frequently contaminated. Any growth of a urinary pathogen from bladder aspirate is considered significant. Catheterization cultures with greater than 1000 colony-forming units per milliliter are considered meaningful. Urinalysis lacks specificity and sensitivity for diagnosis of UTI in neonates and is not recommended as part of the evaluation. Blood cultures should be obtained for all infants with suspected UTI, as one-third with UTI will have bacteremia with the same organism.³⁷ Clinicians should have a very low threshold for performing a lumbar puncture in neonates with UTI, because the risk for concomitant meningitis is 1%-2%.^28,116

The genus Citrobacter

The genus Citrobacter was first proposed in 1932. Citrobacter contains 11 species: C. amalonaticus, C. farmeri, C. braakii, C. freundii, C. gillenii, C. murliniae, C. sedlakii, C. werkmanii, C. youngae, C. koseri, and C. rodentium.

Citrobacter Species

Members of the genusCitrobacter are named for their use of citrate as their sole carbon source. Of the more than 10 recognized species,Citrobacter freundii andCitrobacter koseri (formerlyCitrobacter diversus) are the most important human pathogens. They are differentiated by their ability to convert tryptophan to indole, ferment lactose, and use malonate.⁷⁶²C. freundii produces hydrogen sulfide and therefore can be confused withSalmonella, with which it was classified at one time. The urinary tract is the most frequent site from whichCitrobacter is cultured, often in association with an indwelling catheter, andCitrobacter may be a frequent cause of nosocomial UTIs⁷⁶³ and outbreaks.⁷⁶⁴ These bacteria may also be seen in intraabdominal and pulmonary infections.^765,766 Bacteremia fromC. freundii may have an intraabdominal source more commonly than from the urinary tract, especially in patients with polymicrobial bacteremia and those with underlying malignancy.^765,768 Neonatal meningitis due toC. koseri often (approximately 75%) causes multiple brain abscesses associated with high mortality and severe neurologic sequelae in survivors.⁷⁶⁹ The predilection for CNS infection has been postulated to be due to a combination of a unique outer membrane inC. koseri and infiltration of macrophages infected withC. koseri resistant to phagolysosomal effects across the BBB.^769,770 Nosocomial outbreaks have been accompanied by high rates of intestinal colonization by the organism in infants and by carriage of the bacteria on the hands of health care workers.^764,771 A carbapenemase-expressingC. freundii outbreak was considered to have a potential foodborne source.⁷⁷²

C. freundii strains, like strains ofEnterobacter andSerratia, have inducibleAmpC genes encoding resistance to ampicillin and first-generation cephalosporins that can be produced constitutively at high levels after mutations selected by prior exposure. Furthermore, isolates ofCitrobacter may be resistant to multiple other antibiotics as a result of plasmid-encoded resistance genes. The exposure of commensalCitrobacter spp. of low virulence to repeated antibiotic courses may select for accumulation of multiple resistance genes, leading to extensive resistance profiles.⁷⁷³

CITROBACTER SPECIES

Citrobacter species are most often associated with neonatal sepsis and meningitis; species members are rare causes of sporadic pneumonia, which occurs almost exclusively in neonates and immunocompromised individuals. Citrobacter organisms were first isolated in 1932 by Werkman and Gillen,³³⁷ who proposed the generic term Citrobacter and described seven species. A bewildering array of taxonomic changes³³⁸ ended in 1977, when Brenner and associates³³⁹ designated the following species: C. freundii, C. amalonaticus, and C. diversus. Citrobacter organisms are enteric gram-negative rods that are closely related to Salmonella organisms. In humans, Citrobacter species are most often reported as a cause of meningitis in the neonate.

Most cases are sporadic, although outbreaks have been described. Once introduced into the nursery, Citrobacter species colonization may become prevalent. One study in a neonatal nursery identified 11 of 128 infants colonized with C. diversus.³⁴⁰ The umbilicus was the most frequent site of colonization.

The diagnosis is made by identifying the causative bacterium in blood, CSF, or in an older child, in sputum. Treatment is with an aminoglycoside or an extended-spectrum cephalosporin. Almost all isolates are ampicillin resistant.

The fatality rate for Citrobacter infections in newborns and older immunocompromised patients with Citrobacter pneumonia has been said to be high.^338,341 Recent data defining these rates more precisely are not available.

The complications of Citrobacter pneumonia include associated bacteremia with metastatic foci, particularly meningitis. C. diversus pneumonia may also be associated with abscess formation in the lung³⁴⁶ and with empyema.

Pathogenesis

Citrobacter spp. are often found in human faeces and may be isolated from a variety of clinical specimens. They do not often give rise to serious infections but may cause bacteraemia (Fig. 27.2).

Cit. koseri occasionally causes neonatal meningitis; in this condition there is a high mortality rate and the formation of cerebral abscesses is common.

Pathogenic mechanisms expressed by strains of Citrobacter spp. are poorly understood. Strains of Cit. koseri express type 1 (mannose-sensitive) fimbriae and occasional strains produce a form of E. coli, Verocytotoxin type 2.

Clinical Manifestations

Citrobacter spp. are opportunistic pathogens in humans that can lead to invasive disease, including infections of the urinary tract, respiratory tract, CNS, skin, and soft tissue. The bacteria can cause osteomyelitis, suppurative arthritis, bacteremia, endocarditis, endophthalmitis, and intra-abdominal infections, particularly in neonates and immunocompromised hosts.

In infants, sepsis and meningitis are the most common clinical manifestations of infection.^12,13 Bacterial sepsis is associated with meningitis in about one half of cases. From 1969 to 1989 in Dallas, TX, Citrobacter spp. accounted for 9% of 91 cases of gram-negative enteric meningitis in infants 1 day to 2 years of age.¹⁹ C. koseri was responsible for 90% of cases, and C. freundii caused most of the remaining cases. Neonatal sepsis and meningitis can manifest as early-onset (<1 week of life) or late-onset (>1 week of life) disease and can be fulminant or insidious. In 2002 and 2003, Citrobacter spp. caused 2.9% of early-onset sepsis in very low birth weight infants.²⁰

No early features distinguish meningitis due to Citrobacter spp. from meningitis due to other gram-negative rods. Approximately 80% of infants with meningitis due to Citrobacter spp. develop one or more intracerebral abscesses (Fig. 141.1). In contrast, less than 10% of cases of infants with meningitis due to other gram-negative organisms have associated abscesses.^12,13 Neonatal brain abscesses also can occur with infections caused by Cronobacter (formerly Enterobacter) sakazakii, Proteus mirabilis, and Serratia marcescens. In a contemporary review of brain abscesses in children, intracranial abscesses caused by Citrobacter spp. were more frequent compared with historical cohorts and occurred solely in neonates.²¹ Brain abscesses can evolve any time during the acute course, including several weeks after commencement of treatment.

The prognosis for meningitis due to Citrobacter spp. in neonates is poor. Approximately 30% to 35% of infected infants die, and only 15% to 20% survive with a structurally normal brain at completion of therapy; however, data on long-term prognosis are limited.¹² Although at least 40% of survivors show some form of developmental delay or physical impairment, or both, there are reports of infants with brain abscesses due to Citrobacter spp. who develop normally.²²

Other Citrobacter spp. infections in infants occur rarely. Gastroenteritis, osteomyelitis, pyogenic arthritis, pulmonary infections, and pneumatosis intestinalis have been described.¹³ In older children, adults, and immunocompromised hosts, Citrobacter spp. is associated most commonly with urinary tract, bloodstream, intra-abdominal, skin, soft tissue, and respiratory tract infections.²³

Citrobacter spp. are the cause of urinary tract infection (UTI) in less than 3% of adults and children.^24,25 In a review of 37 pediatric cases, the mean age was 6.9 years, with a range of 1 month to 18 years.²⁴ Females predominated, and 56% of patients had underlying urinary tract or renal anomalies or neurologic impairment. Presenting symptoms were similar to those of UTI from other causes, including dysuria, fever, incontinence, frequency, flank pain, and hematuria. C. freundii accounted for 71% of cases, and C. koseri for the remainder. UTIs involving Citrobacter spp. and other enteric bacilli occurred in about 25% of patients.

In immunocompromised patients, Citrobacter spp. causes bacteremia but rarely leads to CNS disease. C. freundii caused 2.3% of bloodstream infections in the first year after lung transplantation in 190 pediatric patients.²⁶ Infections in immunocompromised patients more frequently are caused by multidrug-resistant strains.^23,27–29 Eye infections, including keratitis and endogenous and traumatic endopthalmitis caused by Citrobacter spp., have been reported.^30–32 An outbreak of severe gastroenteritis associated with several cases of hemolytic uremic syndrome occurred in a nursery school.³³ The source of this outbreak was sandwiches prepared with green butter containing a toxigenic strain of C. freundii. The butter had been made with contaminated parsley grown in an organically fertilized garden.

Background and Etiology

Citrobacter is a genus of Gram-negative coliform bacteria of the family Enterobacteriacae (Boone et al., 2001). In the 1960s and 1970s, a disease eventually named transmissible murine colonic hyperplasia caused by coliform bacteria of the genus Citrobacter was recognized (Mundy et al., 2005; Percy and Barthold, 2007; Petty et al., 2010). Originally described as unusual biotypes of Citrobacter freundii, these organisms were eventually separated into their own species, Citrobacter rodentium, on the basis of DNA sequencing studies (Luperchio et al., 2000; Petty et al., 2010). Whether the organisms that caused severe septicemia and death in guinea pigs in 1988 (Ocholi et al., 1988) and other disease syndromes in hysterectomy-rederived guinea pigs (Boot and Walvoort, 1986) are C. freundii as originally indicated or C. rodentium is unclear.

Pathogenesis, Clinical Manifestations, and Pathology

There are two reports of disease associated with Citrobacter spp. in guinea pigs (Boot and Walvoort, 1986; Ocholi et al., 1988). First, Citrobacter spp. were isolated from germ-free guinea pigs that were exposed to defined flora from gnotobiotic mice (Boot and Walvoort, 1986). Citrobacter spp. were isolated from the middle ear, spleen, mammary gland, and kidney of some animals as well as the intestinal tract, suggesting an ability to cause varied disease in guinea pigs that are not populated with normal flora. Second, an epizootic of Citrobacter spp. septicemia in a large colony of conventional guinea pigs was reported (Ocholi et al., 1988). Animals presented with diarrhea, dyspnea, and decreased appetite. Mortality in weanlings and breeders was 115 of 1300 animals. Gross necropsy revealed lung consolidation with pleural adhesions as well as enteritis and thickening of the intestinal wall. Fibrinous pneumonia and septic thrombi in the capillaries of the lung, liver, and spleen were seen histopathologically. Citrobacter spp. were consistently isolated in pure culture from the lung, liver, spleen, and intestine.

Diagnosis

Culture is the traditional method of detecting Citrobacter spp. which grow well on normal media, are aerobic or facultative anaerobic, ferment glucose, produce catalase, but not oxidase, and are generally lactose-negative or later lactose-fermenting (Greenwood, 2007). However, PCR is more sensitive than bacterial culture for detection of C. rodentium in feces from mice (McKeel et al., 2002).

Prevention and Therapy

C. freundii strains utilize a number of methods to encode resistance to ampicillin and cephalosporins (Pfeifer et al., 2010). Enrofloxacin has been successful in treating mice infected with C. rodentium (Maggio-Price et al., 1998). Prevention would include eliminating contact with C. rodentium-infected mice and other rodents.

Classification

Citrobacter, which was first described as Bacterium freundii in 1928, was reclassified as Citrobacter in 1932 Doran (1999), Sedlak (1973). The genus Citrobacter belongs to the family Enterobacteriaceae and is subdivided into the tribe Citrobacteriaceae. Citrobacter is currently comprised of nine named species and two genomospecies (identified through DNA hybridization studies): C. amalonaticus, C. braakii, C. farmerii, C. freundii, C. koseri (diversus), C. sedlakii, C. wekmanii, C. youngae, C. gillen (C. freundiigenomospecies 10) and C. murlinae (C. freundiigenomospecies 11), and C. rodentium(not found in humans) Farmer (1999), Holmes (1998). The type species, C. freundii, is the one that is most frequently isolated.

Enterobacter is a member of the Enterobacteriaceae family and the Klebsielleaetribe, which includes Klebsiella and Serratia. Formerly known as the Bacterium aerogenes, Aerobacter aerogenes, and Cloacae,the name Enterobacter is now the accepted name of this genus, which is characterized by rod-shaped bacteria with, except for E. kobei, a positive Voges-Proskauer-test, motility, and ability to liquefy gelatin. The genus Enterobacter includes fourteen species: E. aerogenes, E. agglomerans, E. amnigenus, E. asburiae, E. cancerogenus (E. taylorae), E. cloacae, E. dissolvens, E. gergoviae, E. homaechei, E. intermedius, E. kobei, E. nimipressuralis, E. pyrinus, and E. sakazakiiFarmer (1999), Holmes (1998). There is an O-antigen typing scheme for E. cloacae that is comprised of 30 O-antigen types.