By Nelly Escalante, Molecular and Medical Microbiology, ’23

Overview

Elizabethkingia is a family of gram-positive, aerobic bacteria that includes the species Elizabethkingia meningoseptica, Elizabethkingia miricola, Elizabethkingia anophelis, Elizabethkingia bruuniana, Elizabethkingia ursingii, and Elizabethkingia occulta [1]. E. meningoseptica and E. anophelis are the only species within the genus that have been observed to cause disease in humans. While previous research has characterized E. meningoseptica’s predominant role in infection, emerging research has revealed that E. anophelis has been responsible for most of the recent Elizabethkingia case reports. 

Elizabethkingia anophelis is an emerging pathogen first discovered in 2011. It is a symbiotic bacterium that resides in the midgut of the mosquito Anopheles gambiae, which resides in the Gambia River region in central Africa [2]. While A. gambiae is endemic to that region, outbreaks have been observed in several Asian and African countries, with the biggest outbreak so far occurring in the United States. Most cases of E. anophelis are not due to direct contact with its host, A. gambiae, but rather are community-acquired in hospitals through a yet undescribed method of transmission.

Diagnosis

Clinical Presentation

Typical symptoms of E. anophelis infection include bacteremia and meningitis. Pyrexia, chills, and dyspnea have also been observed across several case reports. E. anophelis presents the greatest bacterial burden in the blood, causing bacteremia that can lead to further complications such as sepsis and septic shock. Removal of catheters or central lines may be a necessary approach to relieve bacteremia when E. anophelis is suspected [3]. 

Most of the information known about E. anophelis has come from case reports, as an animal model has not been developed yet to examine its pathogenesis in vivo. The first identified human case of E. anophelis infection was a case of neonatal meningitis in Africa. In this case, the 8-day-old patient experienced pyrexia, seizures, and apnea. Cerebrospinal fluid (CSF) analysis revealed hypoglycorrhachia [4]. In another case, a 7-month-old patient suffered from pyrexia, ecchymotic spots on the body, respiratory failure, and hemorrhaging [5]. These symptoms, however, are not considered to be within the standard clinical presentation of an E. anophelis infection and would only be seen with an especially acute bacterial burden. In both cases, the final diagnosis of E. anophelis infection was made after positive bacterial cultures were observed.

Diagnostic Criteria

Cultures are a powerful tool in the diagnosis of bacterial infection and are grown by sampling many bodily fluids, although blood and CSF are the most common. Once cultures are grown, they can be analyzed to identify the specific bacterium or bacteria causing the infection. Common methods used to identify the different Elizabethkingia species have been unable to differentiate between E. meningoseptica and E. anophelis with great accuracy. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, one of these common methods, utilizes a laser to ionize the bacterial sample and records the time it takes the ions to travel the length of a tube. Larger ions take a longer time, thus producing a mass spectrum that is known as a peptide mass fingerprint (PMF). The PMF of the sample is compared to the over 2,000 PMF of known bacteria species in the database [6].

MALDI-TOF mostly produces accurate identification of bacteria to the genus level. In combination with the lack of PMF samples of E. anophelis, this method has caused many cases of E. anophelis to be misidentified as E. meningoseptica. However, 16S ribosomal RNA gene sequencing has been successful in identifying E. anophelis as it directly uses genomic DNA to produce the 16s rRNA gene sequence and compare it to the more then 60,000 bacterial type strains in the database [7]. This analysis has shown that E. anophelis accounts for far more infections than E. meningoseptica.

Elizabethkingia anophelis culture taken from a patient and grown on 5% sheep blood agar

Treatment

Treatment regimens for E. anophelis infections have not yet been established because the range of antibiotic resistance of the bacterium has not been completely characterized. However, studies have shown that minocycline and levofloxacin are the most effective in treatment. Minocycline belongs to the class of tetracycline antibiotics that inhibit protein synthesis in both gram-positive and gram-negative bacteria and can be given orally. This medicine, however, cannot be given safely to children under the age of 8 [8]. Given that neonates are one of the most affected, other treatments are still being explored.

Levofloxacin, on the other hand, is part of a new group of fluoroquinolones that inhibits DNA gyrase and topoisomerase IV, enzymes that are essential to bacterial DNA replication. Levofloxacin is usually not administered to children except in life-threatening infections such as one by E. anophelis [9]. Moxifloxacin, a drug in the same class of antibiotics, was successful in the treatment of the first human case of E. anophelis infection.

E. anophelis has been classified as multi-drug resistant because it is not susceptible to common antibiotics such as β-lactams and β-lactam/lactamase inhibitors. Additionally, although many cases of E. anophelis have been misidentified as E. meningoseptica, they have distinct antimicrobial susceptibilities and require different treatments [10]. Many E. anophelis strains contain variants in the catB gene that confers antibiotic resistance to phenicol drugs and antibiotic inactivation enzymes [11].

The fatality of E. anophelis infection varies greatly across case reports, but in general has been estimated to be close to 30% [12]. Incorrect antimicrobial therapy regimens are a risk factor in the mortality of patients, which means deciding on the correct antibiotics is essential to ensuring a patient’s recovery and survival [13]. For example, antibiotics that are used to treat neonatal meningitis are ineffective against E. anophelis infection, further highlighting the importance of accurate diagnosis and treatment regimens.

Bacteriophages, also known as phages, are currently being investigated as an alternative to antibiotic treatment, given the multi-drug resistant nature of E. anophelis. In Taiwan, a phage named TCUEAP1 was isolated from the wastewater of a hospital. While there is no bacteriophage specific to E. anophelis, TCEUAP1 was able to infect three strains of the bacteria and reduce the number of colony-forming units (CFU). In a mouse model, the phage was able to decrease the bacterial load in their blood from 5×105 CFU/mL to 1×105 CFU/mL. In doing so, they were able to rescue 80% of the mice that would have otherwise died due to bacteremia [14]. Phages are a promising new therapy for treating multidrug resistant bacteria because they only attack their bacterial hosts and do so with a mechanism that is distinct from drugs. 

Prevention and Future Research

As a nosocomial infection, the best prevention is good hygiene practices. Frequent hand washing by medical personnel as well as routine, thorough disinfection of surfaces may help in reducing the spread. Person-to-person transmission, either through direct or close contact with an infected individual, remains a possible infection mechanism that has yet to be confirmed by in vitro models. It has been proposed that mothers are able to vertically transmit the infection to their child during birth. The exact mechanism of how a person becomes infected by E. anophelis is unknown, but many research efforts are underway to describe its pathogenesis and route of transmission.

Recent research has shown that the bacterium has been able to evade the immune system’s defenses. Macrophages are among the first cells of the immune system to respond to an infection. They have an antibacterial polarization state known as classically activated (M1) macrophages and are activated when a pathogen is detected. In this state, they change their morphology to engulf pathogens through phagocytosis to reduce the bacterial burden. E. anophelis evades this detection and prevents M1 macrophages from activating through a yet unknown mechanism. If activated M1 macrophages are present, the bacteria are also able to avoid being engulfed, which may be due to the bacterial capsule surrounding the bacterium. This type of phagocytosis evasion using a bacterial capsule has been observed by other bacteria such as Salmonella and Mycobacterium [15]. Considering that most patients who contracted an E. anophelis infection were elderly, newborn, or immunocompromised, this type of immune system evasion may be a contributing factor to the high mortality of the infection [5].

Image taken of E. anophelis using phase contrast microscopy. Bacteria are stained with Maneval’s solution with empty space around the bacteria showing the bacterial capsule.

Overall, there are many mechanistic mysteries to Elizabethkingia anophelis that have yet to be investigated, but are nonetheless pertinent to the prevention of further outbreaks and improved patient outcomes.

References:

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