Vancomycin-resistant Enterococcus (VRE), first reported in the United States in 1987, now comprises 25 percent of all Enterococcus species isolated in the clinical laboratory.1 Because of its resistance to common antimicrobial agents, VRE poses a distinct health risk both in the United States and in other countries around the world.
While Enterococcus sp. can exist as normal biota in the human body, particularly in the digestive tract, studies show that disturbances in the microbial balance caused by antimicrobial therapy can facilitate the colonization of VRE in the small and large intestine.2 The microorganism can then gain access to the bloodstream, causing a septicemia that can be fatal if left untreated.
Patients diagnosed with VRE infections have twice the mortality rate of patients with vancomycin-sensitive Enterococcus infections.1 In this article we will examine the causal link between antimicrobial therapy in hospitalized patients and the development of hospital acquired infections caused by VRE.
Enterococcus spp. are Gram-positive cocci that inhabit the gastrointestinal tract as normal biota of humans and other animals. Because they never use oxygen and derive their energy through fermentation, they are categorized as aerotolerant anaerobes. This genus was named in the 1980s, separating its members from the similar genus Streptococcus based on nucleic acid analysis.
Enterococcus spp. can withstand harsh conditions such as 6.5 percent NaCl concentration, very basic pH (9.6), and temperatures from 10 to 45º C. Currently, 14 species of Enterococcus have been identified, and five of them are known to be resistant to vancomycin.
Several genes have been identified in clinical strains that code for virulence factors. These factors include the production of hemolysins, which allow the organism to obtain nutrients such as iron from human red blood cells; the production of gelatinase to hydrolyze gelatin; and the formation of biofilms, which allows the microorganism to adhere to surfaces.
To identify Enterococcus spp. in the clinical laboratory, Gram-positive cocci are tested for catalase activity and L-pyrrolindoyl-beta-napthylamide (PYR) hydrolysis. The isolate is also plated onto selective trypticase soy agar (TSA) containing 10 µg/mL vancomycin and 8 µg/mL gentamycin. Isolates negative for catalase, positive for PYR, and positive for growth on the selective TSA are identified as a suspected Enterococcus sp.
To identify vancomycin resistance, testing is done with varying concentration of vancomycin. If the strain grows at vancomycin concentrations greater than 64 µg/mL, it is identified as VRE. The majority of VRE infections occur in hospitalized patients and are due to Enterococcus faecium and Enteroocccus faecalis.
Vancomycin prevents the cross-linking necessary to properly synthesize the bacterial cell wall. In vancomycin sensitive organisms, vancomycin binds to the two D-alanine groups at the end of each peptide chain in the peptidoglycan layer of the newly synthesizing cell wall.
However, in VRE, the last D-alanine group is replaced with a lactate molecule; this prevents binding of vancomycin to the peptide chain and renders the antibiotic ineffective.
Studies from the United States and other countries have demonstrated that antimicrobial therapy can cause increased risk of infection by VRE.
Research conducted at Sloan-Kettering Cancer Center utilized mice treated with vancomycin, ampicillin, or a combination of vancomycin, metronizadole, and neomycin.2 The mice were subsequently infected with VRE via gavage (force feeding). Both stool and small intestine were analyzed using quantitative real time polymerase chain reaction (qPCR), and the density of both intestinal microbiota and the vanA gene for vancomycin resistance was determined. Analysis found that antimicrobial therapy increased intestinal colonization by Clostridium, Enterococcus, and Enterobacteriaceae, as well as promoted intestinal colonization by VRE.
In a separate study, human stool was analyzed using PCR for genes associated with VRE.3 Five hospitals in Italy examined patients on antimicrobial therapy daily for the development of VRE over the course of 1 year. Rectal swabs were obtained from 864 inpatient subjects before beginning antimicrobial treatment and at 2, 4, 6 and 30 days after initiating therapy.
Of the 864 subjects, 10 acquired VRE during their hospital stay, with a mean acquisition rate of 6 days after initiation of antimicrobial therapy. The authors determined that carbapenam therapy carried the highest risk of colonization with VRE compared to cephalosporins and glycopeptides.
Additional studies were conducted to determine whether specific antimicrobial agents increase the likelihood of colonization with VRE more than others.
Researchers at Beth Israel Deaconess Medical Center and Harvard Medical School concluded that third generation cephalosporins and metronizadole were "highly significant independent risk factors" for VRE colonization.4
Interestingly, this study also concluded that vancomycin was not a risk factor for colonization with VRE, contradicting previous studies. The authors stated that cephalosporins act against nonenterococcal aerobic intestinal biota, suppressing competition for VRE and allowing the resistant Enterococcus spp. to populate the intestine more easily.
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