Lelliottia amnigena is a Gram-negative, rod-shaped bacterium found in diverse environmental settings. While primarily rooted in aquatic and soil environments, its ability to persist in human-associated settings makes it an opportunistic pathogen of interest in clinical microbiology. First described in the early 1980s, its formal classification has undergone significant revision due to advancements in genomic analysis. Understanding its complete profile, from its taxonomic reclassification to its genomic blueprint, is necessary for accurate identification and managing the public health concerns related to its ecological adaptability and antibiotic resistance.
The Taxonomic Identity of Lelliottia amnigena
The formal classification of Lelliottia amnigena places it within the family Enterobacteriaceae, order Enterobacterales, and class Gammaproteobacteria. This family includes bacteria found widely in the environment and those that inhabit animal intestines. The species was originally described in 1981 as Enterobacter amnigenus, reflecting its initial grouping within the Enterobacter genus.
This classification persisted for decades until a major taxonomic overhaul based on modern molecular techniques occurred. In 2013, phylogenetic analyses, including multilocus sequence analysis (MLSA) and genomic comparisons of genes like gyrB and rpoB, revealed that E. amnigenus formed a distinct evolutionary lineage separate from the core Enterobacter group. This evidence indicated that the species warranted its own genus.
The new genus Lelliottia was established, named in honor of the British bacteriologist R.A. Lelliott. Enterobacter amnigenus was formally reclassified and renamed Lelliottia amnigena. This separation was crucial for improving identification accuracy in clinical and environmental laboratories, as misidentification can lead to incorrect assumptions about disease potential and antibiotic susceptibility. The name amnigena is derived from Latin, meaning “born in a river,” reflecting the organism’s strong association with water sources.
Morphology and Physical Characteristics
Lelliottia amnigena is a bacillus, or rod-shaped cell, typical for many bacteria within the Enterobacteriaceae family. These cells are small, generally measuring 0.6 to 1.0 micrometers in width and 1.5 to 3.0 micrometers in length. As a Gram-negative bacterium, its cell wall structure does not retain the crystal violet stain used in the Gram staining procedure.
The organism is generally motile, possessing peritrichous flagella distributed over the cell surface that enable movement. L. amnigena is facultatively anaerobic, meaning it can grow in the presence of oxygen but can also switch to anaerobic respiration when oxygen is absent. It does not form spores, distinguishing it from certain other environmental bacteria.
When grown in a laboratory setting, such as on tryptone soya agar, L. amnigena typically forms unpigmented, round, and smooth colonies. It is capable of growth on selective and differential media like MacConkey agar, often used to isolate Gram-negative bacteria. The organism is mesophilic, thriving at moderate temperatures, typically between 30 and 37 degrees Celsius.
Environmental Habitat and Ecological Role
The primary habitat for Lelliottia amnigena is the aquatic environment, consistent with its species name. It is widely prevalent in surface water, including rivers, lakes, drinking water, and sewage systems. The bacterium is also commonly isolated from soil, demonstrating high adaptability to different environmental matrices.
This species is highly resilient, capable of surviving and proliferating in diverse conditions, including nutrient-poor (oligotrophic) environments and high salinity (up to 9% sodium chloride). This adaptability allows it to persist in natural ecosystems as well as industrial and agricultural water systems.
A significant ecological behavior is its capacity to form biofilms, complex communities encased in a self-produced matrix. Biofilm formation protects the bacteria from environmental stressors, such as disinfectants, and allows persistence on surfaces like piping in water distribution networks. In agricultural settings, L. amnigena contributes to nutrient cycling, such as reducing nitrate to ammonia.
However, it is also identified as an opportunistic plant pathogen, associated with soft rot diseases in crops like potatoes, onions, carrots, and strawberries, particularly after harvest. L. amnigena is considered an opportunistic human pathogen, rarely causing illness in healthy individuals. Infections typically occur in people with underlying health conditions or those with traumatic injuries contaminated by soil or water, including urinary tract infections, sepsis, and wound infections.
Genomic Insights and Medical Significance
The genome of Lelliottia amnigena provides the molecular blueprint for its environmental resilience and infection potential. Genomic sequencing shows a typical chromosome size of 4.3 to 4.5 million base pairs, with a guanine-cytosine (GC) content around 52.85%. Whole-genome sequences are instrumental in tracking its spread and understanding its evolution, particularly concerning antibiotic resistance.
A major finding is the identification of the chromosomally encoded AmpC \(\beta\)-lactamase gene, blaLAQ, which is a core gene in the species. This enzyme is a mechanism of intrinsic resistance, conferring natural resistance to certain \(\beta\)-lactam antibiotics, such as ampicillin and first- to fourth-generation cephalosporins. This explains why L. amnigena isolates are often naturally resistant to common clinical antibiotics.
Beyond intrinsic resistance, L. amnigena can acquire additional resistance genes through horizontal gene transfer, often carried on mobile genetic elements like plasmids. Sequenced strains have carried multidrug resistance regions containing multiple resistance genes. These acquired genes confer resistance to classes such as:
- Tetracyclines (tetA)
- Quinolones (qnrS1)
- Aminoglycosides (aadA2)
The presence of these mobile resistance elements is medically significant because it enables the bacterium to resist multiple drug classes, complicating treatment for human infections. This acquired resistance profile, combined with its natural resistance, makes managing infections challenging, particularly in immunocompromised patients. Genomic sequencing is a valuable public health tool for predicting antibiotic susceptibility and monitoring the flow of resistance genes between environmental and clinical settings.