Meningoencephalitis Caused by Escherichia coli in an Adult With No Known Immunocompromising Conditions: A Case Report

Introduction

Acute bacterial meningitis is a severe and highly lethal infection. Its incidence rate varies depending on the region. In high-income countries, such as the Netherlands, it has declined from 1.72 per 100 000 in 2007–2008 to 0.94 per 100 000 in 2013–2014 [1]. The incidence of bacterial meningitis in low-income countries can reach up to 80 cases per 100 000 [2,3].

Neonatal meningitis has a significantly greater spread, especially among newborns under 90 days old. Its incidence is much higher than in adults, estimated at 0.38 per 1000 live births in the United Kingdom and Ireland [4].

The most common pathogens accounting for acute bacterial meningitis are Haemophilus influenzae type b, Streptococcus pneumoniae, and Neisseria meningitides. Several gram-negative bacilli, including Escherichia coli (E. coli), are uncommon causes of community-acquired bacterial meningitis [5]. They are estimated to be between 3% and 9% of all cases of bacterial meningitis [6]. Among all gram-negative bacilli causing bacterial meningitis, E. coli is the most common and accounts for 41.9% of all cases [7–9]. Spontaneous meningitis caused by gram-negative bacilli occurs predominantly in neonates or infants, and few studies have been performed on the clinical course and prognostic factors of gram-negative bacillary meningitis (GNBM) in adults [10,11].

The main risk factor for the development of E. coli meningitis in adults is an immunocompromised condition, including immunosuppression and altered immunity due to HIV. However, chronic alcoholism, history of cancer, and diabetes mellitus also increase susceptibility to E. coli meningitis [12,13]. Advanced age is another risk factor that increases the likelihood of adverse outcomes [3,14]. Some studies suggested that the prevalence is higher among males [15]. There are also direct risk factors. These include craniocerebral trauma, neurosurgery, and exposure to healthcare-associated infections [13]. However, the cause of E. coli meningitis remained unknown in about 40% of the cases. GNBM is thought to occur most commonly as a complication of bacteremia from a distant outbreak of infection [12]. Bacteremia (31%), urinary tract infection (24%), pneumonia (9%), septic arthritis (including one case of E. coli spondylodiscitis) (4%), and primary peritonitis (2%) have been identified as the source of infection leading to spontaneous GNBM [9]. Furthermore, patients with nontraumatic meningitis due to gram-negative bacilli were found to have a distant source of infection more frequently than patients with spontaneous meningitis due to other pathogens. This suggests that a gram-negative bacillary etiology should be suspected in adults with spontaneous acute meningitis who have a bacterial urinary tract infection [11].

E. coli is a commensal inhabitant of the human lower intestine. Outside the gut, it tends to cause infections, which are mostly gastroenteritis and urinary tract infections [12]. However, it is not expected that a bacterial infection will cross the blood-brain barrier (BBB). The mechanisms of BBB penetration include invasion of brain microvascular endothelial cells by E. coli facilitated by its outer membrane protein A and cytotoxic necrotizing factor 1 [16,17]. Moreover, E. coli uses the Caspr1 receptor of the host through its virulence factor IbeA and causes activation of focal adhesion kinase signaling, which is followed by internalization of E. coli into brain endothelial cells [18].

E. coli can disrupt the tight junctions between endothelial cells, increasing BBB permeability, which causes its breakdown [19]. In addition, E. coli triggers an inflammatory response, which contributes to BBB disruption and neuroinflammation [20]. The host immune response also contributes to higher mortality when deficiencies in MyD88 signaling or depletion of neutrophils and monocytes are present [21].

The most common clinical manifestation of E. coli meningitis in adults is fever; however, it may not be present in older adults [2]. Patients often report headache, although it may be less common in those with severe underlying conditions [2,12]. Neck stiffness is a typical sign of bacterial meningitis; however, it may be less commonly observed in older patients [22]. Altered mental status is a common sign of inflammation of the brain, and it is usually observed in older adults [22]. Nausea and vomiting are also common signs of bacterial meningitis [2]. Photophobia is also reported as a clinical manifestation of E. coli meningitis [9]. Symptoms such as seizures, cranial nerve palsy, hemiparesis, and aphasia may also be seen in patients with meningitis [3]. The classic triad of fever, neck stiffness, and altered mental status has been found to occur in only 25% of patients with E. coli meningitis [8]. Symptoms such as seizures, altered mental status, coma, and hypotension are usually predictors of fatal outcome [7].

The risk of poor prognosis could be reduced by 15% to 25% if immediate initiation of treatment with empiric antibiotics and dexamethasone when bacterial meningitis is suspected [12]. The standard empiric antibiotic regimen includes combination therapy of penicillin/ampicillin/amoxicillin with a third-generation cephalosporin (ceftriaxone or cefotaxime) as well as additional vancomycin if resistance to penicillin is expected [23,24]. Subsequently, depending on the results obtained and the pathogen isolated from the CSF culture and susceptibility test, antibiotic therapy should be updated. The most widely used antibiotic drugs for E. coli meningitis include ceftriaxone (22%), gentamicin (13%), ampicillin (11%), penicillin G (9%), amoxicillin (9%), and meropenem (7%) [9].

Bacterial meningitis is one of the leading causes of infection-related death worldwide, causing approximately 318 000 deaths per year [2]. Mortality rates are much higher in low-income countries (up to 58%) compared to high-income countries (around 10%) [2,3]. The reported in-hospital mortality ranges from 40% to 60% [7]. Patients with GNBM have a 20 times greater risk of death compared to those with other bacterial meningitides. Additionally, patients with E. coli meningitis have a higher mortality rate compared to those with other types of bacterial meningitis [7].

Older age (≥65 years), positive blood culture, inadequate initial antibiotic therapy, and neurological and systemic complications are closely related to higher mortality in patients with spontaneous bacterial meningitis [7].

This case report highlights an uncommon presentation of community-acquired E. coli meningoencephalitis in an older adult without known immunocompromising conditions. Given the rarity of this infection in immunocompetent adults or adults with no known immunocompromising conditions, its clinical presentation, diagnostic challenges, and therapeutic management remain insufficiently described in the literature. Furthermore, the case emphasizes the potential role of a possible urinary tract infection as a source of central nervous system infection. It underlines the importance of careful microbiological investigation and follow-up, particularly in the context of possible recurrent or antibiotic-resistant urinary infections. By presenting this case and reviewing the available literature, we aim to contribute to a better understanding of the clinical course, diagnostic considerations, and management strategies for this rare but potentially severe condition.

Case Report

LITERATURE REVIEW METHODOLOGY:

To contextualize our findings, we conducted a literature search to identify previously reported cases of E. coli meningitis in adults. The search was performed in the PubMed/MEDLINE and Google Scholar databases for articles published between January 1945 and March 2026. The following keywords and combinations were used: “Escherichia coli meningitis”, “E. coli meningoencephalitis”, “adult bacterial meningitis E. coli”, “community-acquired E. coli meningitis”, and “gram-negative meningitis adults”. We included case reports, case series, and observational studies describing adult patients (≥18 years) with confirmed E. coli meningitis or meningoencephalitis diagnosed by cerebrospinal fluid culture or molecular testing. Articles not written in English, reports involving neonatal cases, or cases lacking microbiological confirmation were excluded. The identified studies were reviewed to summarize patient characteristics, predisposing conditions, suspected sources of infection, treatment approaches, and clinical outcomes.

Discussion

This report describes a favorable outcome of meningoencephalitis caused by E. coli in a 69-year-old patient with no known immunocompromising conditions, with a probable previously unrecognized asymptomatic urinary tract infection as the likely preceding source. CNS infection associated with GNBM is a rare clinical entity, but it can cause severe and persistent neurological damage and high mortality rates when compared with other causative agents of bacterial meningitis [7]. Pomar et al concluded that patients with GNBM had a mortality rate of 53%, which is between the worldwide reported rates of 40% to 60% [7].

Documented cases are limited, especially in immunocompetent adult patients or patients with no known immunocompromising conditions, making this report a valuable addition to the understanding of spontaneous GNBM, its diagnosis, course, and treatment. Community-acquired bacterial meningitis with E. coli as the causative agent in immunocompetent adults is rare worldwide, and a total of 45 cases have been reported since 1945, averaging less than 1 case per year [10]. E. coli meningitis has many complications, but among these, patients die mainly due to sepsis or multiorgan failure [12].

It has been documented that spontaneous GNBM occurs more frequently in older immunocompromised patients and those with severe comorbidities such as alcoholism, cirrhosis, diabetes, or malignancy [7]. These conditions predispose individuals to infections due to a weakened immune system and impaired function. In contrast, our clinical case is unique in that the patient had no significant comorbidities, aside from well-controlled arterial hypertension, which is the most likely reason for the favorable outcome and the absence of sequelae.

GNBM cases usually start rapidly, and patients can present with fever, headache, and meningeal irritation, which are classic signs of meningitis. While our patient presented with a high fever of 40°C, fever is not always be present in other patients. Arora et al reported that fever can be absent even in the presence of widespread infections caused by pyogenic organisms such as Klebsiella [25]. Our patient was febrile at presentation with a temperature of 40°C and became afebrile after the fourth day of hospitalization following initiation of antimicrobial therapy. The absence of fever in such cases may be attributed to individual variations in immune response, the stage of infection, or the presence of an infection localized to the CNS without systemic involvement.

The presence of negative blood cultures despite confirmed E. coli infection in the cerebrospinal fluid may be explained by several factors. One possible explanation is transient bacteremia, in which bacteria enter the bloodstream briefly, seed the central nervous system, and may no longer be detectable at the time blood cultures are obtained. In addition, the timing of blood sample collection relative to fever peaks and the dynamics of bacterial dissemination can influence the likelihood of detecting bacteremia. It is also possible that the bacterial load in the bloodstream was low or intermittent, falling below the detection threshold of standard blood culture methods. Such discrepancies between CSF and blood culture results have been reported in cases of bacterial meningitis and highlight the importance of comprehensive microbiological evaluation when central nervous system infection is suspected.

GNBM occurs most commonly as a complication of bacteremia from a distant outbreak of infection [26]. Urinary tract infection is accepted as the most important independent factor associated with a higher risk of spontaneous GNBM [7]. In this context, the high prevalence of urinary tract infections and comorbidities in older patients contributes to underdiagnosis of spontaneous GNBM or GNBM.

Additionally, in immunocompetent patients or patients with no known immunocompromising conditions, the prognosis of GNBM depends on several factors, including the timeliness of diagnosis and the initiation of appropriate antimicrobial therapy. Early intervention with broad-spectrum antibiotics, followed by targeted therapy once the causative organism is identified, is crucial for improving outcomes, although significant resistance may be observed [27]. The appropriate course of antibiotics chosen for our patient was likely a major factor in a favorable outcome.

In a case of GNBM documented by Ray et al, no underlying risk factors were identified in the immunocompetent adult. However, patients have reported a recent history of urinary tract infection, although no prior urine culture results were available, and blood/urine cultures taken during hospitalization showed no growth [28]. Probably because of the preserved immunity, the patient was discharged in a clinically stable condition after a prolonged 25-day hospital stay. Moreover, she followed up at the outpatient department 1 month later, reporting that she could resume her regular activities without any medications [28].

The increasing prevalence of antimicrobial resistance among gram-negative bacilli, particularly E. coli, is a growing challenge in the management of severe infections. In recent years, the emergence and global dissemination of extended-spectrum β-lactamase (ESBL)–producing and carbapenem-resistant Enterobacterales have significantly limited the available therapeutic options. These organisms possess multiple resistance mechanisms, including β-lactamase production, efflux pumps, target modification, and decreased membrane permeability, which collectively contribute to multidrug resistance. As a result, infections caused by gram-negative pathogens are associated with increased morbidity, mortality, and healthcare costs. Recent studies emphasize that antimicrobial resistance in E. coli and other gram-negative bacteria continues to rise worldwide, highlighting the need for careful antibiotic stewardship, surveillance of resistance patterns, and development of new therapeutic strategies [29,30].

Our patient presented with E. coli meningitis without any identified predisposing risk factors, showing the potential for recurrent E. coli infections, even after initial treatment. We tested our patient’s urine and found it positive for multidrug-resistant E. coli 1 month after ICU discharge, necessitating a targeted approach with nitrofurantoin, followed by amoxicillin/clavulanic acid. Similar vigilance is crucial in monitoring for recurrent infections in patients with E. coli meningitis, especially considering the potential for antibiotic resistance. Despite the initial challenges, persistence in treatment ultimately led to a sterile urine culture and resolution of symptoms. This emphasizes the importance of tailored, persistent therapeutic strategies and regular follow-up to prevent relapse in patients with complicated infections like E. coli meningitis.

Conclusions

Spontaneous E. coli meningitis in adults without known immunocompromising conditions is extremely rare, with fewer than 1 case reported annually worldwide [6]. Nevertheless, this condition can lead to severe complications, including neurological damage, sepsis, and multiorgan failure, and is associated with high mortality if not recognized and treated promptly. The incidence of spontaneous GNBM may increase in the future due to an aging population, the rising prevalence of comorbidities such as malignancy, and the rising incidence of gram-negative bloodstream infections.

Because of the potentially severe course and the risk of underdiagnosis, clinicians should consider meningitis in older patients presenting with fever and altered mental status, particularly when a urinary tract infection or another potential gram-negative source is present, even in individuals without known immunocompromising conditions. Early recognition, prompt initiation of appropriate antimicrobial therapy, and management of the underlying infection source are essential for improving clinical outcomes.

Although GNBM is often associated with significant morbidity and mortality, favorable outcomes can be achieved when the infection is diagnosed early and treated appropriately. In the present case, the absence of significant comorbidities such as diabetes, cirrhosis, or malignancy, together with timely antimicrobial therapy, likely contributed to the patient’s full recovery without long-term neurological sequelae. This case highlights the importance of maintaining clinical suspicion for gram-negative meningitis in adults and underscores the value of early microbiological diagnosis and targeted treatment.