Enterococcus

Cadmium-Triggered Metabolic Reprogramming#

Cadmium stress causes massive transcriptional reorganization in E. faecium.^{[1]Cadmium stress triggers significant metabolic reprogramming in Enterococcus faecium CX 2-6Cheng X, Yang B, Zheng J et al. · 2021Open reference 1 ↓} 1,152 differentially expressed genes (47% of the genome) under cadmium exposure.

G1 (310 genes): Downregulated—nucleotide metabolism and DNA replication inhibited (growth arrest). G2 (658 genes): Upregulated at low cadmium (Cd)—ribosome and protein translation increased (stress response machinery). G3 (184 genes): Highly upregulated at high cadmium—carbohydrate transport, anion transport, and exopolysaccharide (EPS) production.

EPS production under cadmium stress is a key defense: the extracellular polysaccharide matrix sequesters cadmium ions before they can enter the cell, analogous to siderophore-based metal chelation in Pseudomonas aeruginosa.

P-type ATPase transporters highly upregulated for active cadmium efflux.

Metal Efflux Systems#

P-type ATPases: primary cadmium and copper efflux pumps. CDF (cation diffusion facilitator) family: zinc and cadmium export. These efflux systems are shared with related genera: parallels the CzcD system in Streptococcus pneumoniae.^{[2]Metal Homeostasis in Pathogenic StreptococciAkbari MS, Doran KS, Burcham LR · 2022Open reference 2 ↓}

The 120-Year Survey#

A landmark study of 381 isolates spanning 1900-2019 reveals the co-evolution of metal and antibiotic resistance.^{[3]Diversity of metal and antibiotic resistance genes in Enterococcus spp. from the last century reflects multiple pollution and genetic exchange among phyla from overlapping ecosystemsRebelo A, Mourao J, Freitas AR et al. · 2021Open reference 3 ↓}

Metal tolerance genes surveyed: arsA (arsenic), merA (mercury), tcrB (copper). Prevalence: arsA most frequent (82% of MeT-carrying isolates); merA 97% prevalence; tcrB less common. 13 phylogenetic variants of ArsA protein and 6 variants of MerA distributed across diverse ecological contexts (human clinical, animal, food, aquatic).

Temporal acceleration: co-occurrence of metal tolerance and antibiotic resistance genes increased dramatically since the 1990s, correlating with increased antimicrobial and metal use in agriculture and medicine.

Co-Occurrence on Mobile Elements#

Metal tolerance and antibiotic resistance genes systematically co-occur on conjugative plasmids.^{[3]Diversity of metal and antibiotic resistance genes in Enterococcus spp. from the last century reflects multiple pollution and genetic exchange among phyla from overlapping ecosystemsRebelo A, Mourao J, Freitas AR et al. · 2021Open reference 3 ↓} vanA (vancomycin resistance) near mercury/arsenic tolerance regions. tet(M) (tetracycline), erm(B) (macrolide), aac6'-Ie-aph2''-Ia (aminoglycoside) co-located with MeT genes.

Flanked by IS elements (IS110, IS256, IS200/605) enabling mobilization. Associated with conjugation genes (TraC, TraE, TraG) for horizontal transfer. This means: selecting for metal resistance automatically selects for antibiotic resistance and vice versa.

Cross-Phylum Gene Exchange#

Metal resistance gene variants are shared between Enterococcus and distant taxa including Lactobacillus malefermentans, Streptococcus, and Staphylococcus.^{[3]Diversity of metal and antibiotic resistance genes in Enterococcus spp. from the last century reflects multiple pollution and genetic exchange among phyla from overlapping ecosystemsRebelo A, Mourao J, Freitas AR et al. · 2021Open reference 3 ↓} Overlapping ecosystems (gut, food production, hospital, agricultural) enable gene flow across bacterial phyla.

This has implications for Staphylococcus aureus MRSA co-resistance evolution and even for probiotic safety (probiotic Lactobacillus may acquire resistance determinants from Enterococcus).

Nutritional Immunity Context#

Enterococci are not classic "metal-dependent virulence" pathogens like Urease-producers—their metal story is about resistance and co-selection rather than metal-dependent enzymes. However, metal homeostasis is still critical: manganese is required for superoxide dismutase, and zinc for multiple metalloenzymes.

Host nutritional immunity (calprotectin-mediated zinc (Zn)/manganese (Mn) sequestration) affects Enterococcal survival at infection sites. The cadmium metabolic reprogramming demonstrates that Enterococci have sophisticated metal stress responses that likely also engage during host-imposed metal challenges.

Disease Associations#

Vancomycin-resistant Enterococcus (VRE): a top hospital-acquired infection threat; vanA on same mobile elements as metal resistance.^{[3]Diversity of metal and antibiotic resistance genes in Enterococcus spp. from the last century reflects multiple pollution and genetic exchange among phyla from overlapping ecosystemsRebelo A, Mourao J, Freitas AR et al. · 2021Open reference 3 ↓} Endocarditis: E. faecalis is a leading cause of infective endocarditis. Urinary tract infections: common hospital-acquired UTI pathogen.

Bacteremia: especially in ICU patients, post-surgical, immunocompromised. Intra-abdominal/pelvic infections: as part of polymicrobial infections. Wound infections: surgical site infections.

Connection to Environmental Metal Exposure#

Agricultural metal use: copper and zinc as growth promoters in livestock feed; arsenic historically used in poultry production. These directly select for metal-tolerant Enterococci carrying antibiotic resistance genes.^{[3]Diversity of metal and antibiotic resistance genes in Enterococcus spp. from the last century reflects multiple pollution and genetic exchange among phyla from overlapping ecosystemsRebelo A, Mourao J, Freitas AR et al. · 2021Open reference 3 ↓}

Hospital environments: copper-surfaced fittings intended to reduce hospital infections may paradoxically select for copper-tolerant (and therefore antibiotic-resistant) Enterococci. Food chain: metal-resistant Enterococci from animal production enter the human food chain, transferring resistance genes to human gut flora.

Wastewater/sewage: convergence point for antibiotic and metal residues, driving co-selection in environmental Enterococcus populations. Enterococcus is proposed as a bioindicator for metal pollution across diverse environments.^{[3]Diversity of metal and antibiotic resistance genes in Enterococcus spp. from the last century reflects multiple pollution and genetic exchange among phyla from overlapping ecosystemsRebelo A, Mourao J, Freitas AR et al. · 2021Open reference 3 ↓}