Acquired resistance of enterococci

Acquired resistance

β-Lactams: Enterococci, almost exclusively strains of E. faecalis, can express  β- lactamase enzymes that confer high level resistance against imipenem and against all penicillins, except those combined to β-lactamase inhibitors (sulbactam or clavulanate). β-lactamases hydrolyze the beta-lactam ring and thereby inactivate the drug. The enterococcal penicillinase gene is identical to the gene encoding staphylococcal type A penicillinase and is often found on a transferable plasmid that also encodes high-level resistance to gentamicin. The activity of the beta-lactamase of E. faecalis is reversed by the β-lactamase inhibitors clavulanate, sulbactam and tazobactam. In addition, enterococci have acquired further modified PBPs with very low affinity for all β-lactams antibiotics. Together, these two mechanisms can produce quite high resistance levels (MIC of > 256 µg/mL). The second mechanism of acquired resistance to β-lactam antibiotics in enterococci is caused by a mutations in chromosomal DNA (pbp5 gene) resulting in overproduction of a modified PBP5 with low affinity to β-lactam antibiotics.

These strains are often referred to as acquired resistance enterococci and the vast majority is E. faecium. A number of point mutations in the penicillin binding region of the pbp5 gene confer different levels of resistance to all beta-lactam antibiotics including imipenem. ARE isolates typically have ampicillin MICs of 8– 64 µg/mL, but may have MICs of >128 (and 3rd generation cephalosporin MICs of >10000). This type of beta-lactam resistance is not reversible with the beta-lactam inhibitors mentioned above. Recently, pbp5 has been described as transferable on a plasmid. Ampicillin resistance may also be linked to vancomycin resistance of the vanB-type and may be transferred between strains of E. faecium on the mobile element Tn5382. This genetic package was found in unrelated E. faecium strains from several states in the U.S.A. suggesting horizontal dissemination of these genes among enterococci in that region.

 Cephalosporins

Some studies confirmed the importance of previous extended-spectrum cephalosporin treatment in the risk of VRE acquisition. Bonten et al. studied 13 ventilated patients who acquired VRE and 25 who did not, and observed that broad-spectrum cephalosporin use predicted acquisition, whereas vancomycin use was not a significant predictor.  More recently, D’Agata et al, showed that treatment with broad spectrum cephalosporins predicted VRE acquisition among hemodialysis patients. A 52-week surveillance study of patients with hematologic malignancies substantiated the observation of an association between colonization with antibiotic resistant E. faecium and treatment with broad spectrum cephalosporins, which preceded the intestinal overgrowth with E. faecium in 93% of the patients .  

Aminoglycosides

Early studies demonstrated that two types of streptomycin resistance occur in enterococci: (i) moderate-level resistance (MIC, 62 to 500 µg/ml), because of low permeability, which can be overcome with a penicillin (which increases the cellular uptake of the aminoglycoside); and (ii) high-level resistance (MIC, ≥2,000 µg /ml), which is either ribosomally mediated or due to the production of aminoglycoside-inactivating enzymes.

Aminoglycosides act primarily by interfering with the protein synthesis of bacteria by binding to the 16S rRNA of the 30S ribosomal subunit. The intrinsic low level of resistance found among the enterococci is due to limited drug transport across the cell membrane. High-level aminoglycoside resistance in enterococci involves the acquisition of genes that are encoding aminoglycoside-modifying enzymes, like phosphotranferases, accetyltransferases or nucleotidyl transferases. The combination of an aminoglycoside with a β-lactam antibiotic results in synergistic efficacy and has long been the golden standard in enterococcal endocarditis.

The most common gene, aac (6’)-Ie-aph (2”)-Ia, is found in 90% of clinical enterococci with high-level aminoglycoside resistance, and encodes a bifunctional enzyme with both acetylating and phosphorylating activity. This gene, which is located on transposons or plasmids, mediates resistance to a broad range of aminoglycosides and has also been detected in other Gram-positive cocci like S. aureus, S. epidermidis, and Streptococcus spp.

 Fluoroquinolones

Only a few clinical studies have examined in detail the association between fluoroquinolone exposure and VRE colonization. Several studies of healthy volunteers suggest that fluoroquinolones suppress anaerobic bacteria and enterococci in the normal human intestinal microbiota only to a minor extent, whereas members of the family Enterobacteriaceae are decreased significantly. Conceivably, due to their relatively poor antianaerobic activity, fluoroquinolones such as ciprofloxacin do not promote high level colonization with VRE. In contrast, several other studies suggest that the effects of some fluoroquinolones on fecal anaerobes may be more profound in certain patient populations, such as bone marrow transplant recipients and patients undergoing gastrointestinal surgery. For instance, one study reported that aerobic and anaerobic bacteria in the fecal microbiota were markedly suppressed during surgical prophylaxis with ciprofloxacin. 

 Action of fluoroquinolones

In all Gram-positive bacteria, two proteins, DNA-gyrase and topoisomerase IV, are considered to be the main targets for the fluoroquinolones. DNA gyrase is a tetrameric enzyme with two subunits, encoded by the gyrA and gyrB genes respectively, that catalyses the negative supercoiling of DNA. Negative supercoils are important for initiation of DNA replication. Topoisomerase IV acts by separating interlocked DNA strands allowing the forming of daughter chromosomes into daughter cells. Topoisomerase IV also has two subunits, encoded by the parC and parE genes respectively. Different fluoroquinolones have different levels of action against the two enzymes. Topoisomerase IV seems to be more sensitive and is often regarded as the primary target of fluoroquinolones in Gram-positive bacteria.