Microbe of the Month: Vancomycin-Resistant Enterococci (VRE)

Enterococci bacteria grabbed the attention of public health officials in the 1980s because of its ability to survive in humans, as well as for its capability of sharing those survival tricks with other bacteria. While enterococci are not as familiar as staphylococcus (staph) or Escherichia coli (E. coli) bacteria, enterococci infections are among the most common type acquired by hospitalized patients. Enterococci, in general, are much less capable of causing disease than staph or E. coli but still can complicate and prolong hospital stays. Virtually the only people who develop illness from Enterococcus are those who are already ill, such as individuals in a hospital intensive-care unit or those who are elderly, have diabetes, have chronic kidney failure, and so forth. So, unlike other forms of resistant bacteria, there is little chance or concern among physicians of Enterococcus becoming epidemic in healthy populations.

But enterococci are of great interest because, as with many of their bacterial counterparts, they can resist and evade several forms of antibiotic therapy, including vancomycin, the antibiotic of last resort for resistant infections.

Enterococcal infections that result in human disease can be fatal, particularly those caused by strains of vancomycin-resistant enterococci (VRE). In 2004, VRE caused about one of every three infections in hospital intensive-care units, according to the Centers for Disease Control and Prevention (CDC).

In 1984, enterococci was given its own genus identity. In 1986, the first VRE strains appeared in Europe and, in 1989, the first case of VRE was reported in the United States. Between 1989 and 1993, the percentage of enterococcal tests that were positive for VRE in the United States rose from 0.3 percent to 7.9 percent. Researchers seek to develop improved therapeutics as well as gain a better understanding of VREs genetic survival characteristics and how those resistance genes are passed to other pathogens. As of 2007, the United States had reported seven cases of vancomycin-resistant Staphylococcus aureus (VRSA) infection, a serious development that has healthcare providers fearful of losing ground in their attempt to control the spread of S. aureus. In one of the cases, scientists confirmed the transfer of a key antibiotic resistance gene from Enterococcus to Staphylococcus.

Enterococci can survive for months. It primarily resides in the human digestive system and the female genital tract; the enterococci make up a significant part of the normal bacterial population of these sites in healthy people. However, colonization can progress to infection, particularly for people with certain risk factors. The infection can lead to diseases of the urinary tract, bloodstream, heart valves and brain, as well as to serious infections in open wounds.

Some of the risks for acquiring VRE infection are:

• Persons who have been previously treated with vancomycin and combinations of other antibiotics, such as penicillin and gentamicin 

• Persons who are hospitalized, particularly when they receive antibiotic treatment for long periods of time 

• Persons with weakened immune systems, such as patients in intensive-care units, cancer, or transplant wards 

• Persons who have undergone surgical procedures, such as abdominal or chest surgery 

• Persons with medical devices that stay in for some time, such as urinary catheters or central intravenous catheters 

Enterococcal infections are more common in elderly people, particularly those in long-term care facilities and skilled nursing homes, because they are more likely to experience infection risk factors, such as exposure to medical instruments. VRE is transmitted from person to person most commonly by healthcare workers whose hands have inadvertently become contaminated, either from feces, urine or blood of a person carrying the organism. It can also be spread indirectly via hand contact with open wounds or by touching contaminated environmental surfaces, where the bacterium can survive for weeks. VRE is not transmitted through the air.

Of more than a dozen forms of enterococci bacteria, two are the primary concern for human disease: E. faecium and E. faecalis. E. faecium is the most frequent species of VRE found in hospitals.

Enterococci have two types of resistance to vancomycin: acquired and intrinsic. Some types of enterococci bacteria acquire the resistance when other bacteria come in contact with enterococci and share genetic information scientists believe enterococci acquired the gene that resists vancomycin from bacteria in the digestive tract. Acquired resistance has been noted with two clinically important forms of enterococci: E. faecium and E. faecalis. Of the dozen or so types of enterococci bacteria, some, such as E. gallinarum and E. casseliflavus, have an inherent, low-level resistance to vancomycin. These are very uncommon strains, however, and are of limited clinical significance.

Most VRE infections can be treated with antibiotics other than vancomycin. Some of the antibiotics that fail to work because of intrinsic resistance include some types of penicillin, cephalosporins, clindamycin, and aminoglycosides. Treatments that are ineffective because of acquired resistance include vancomycin, some penicillins, macrolides (such as erythromycin), tetracycline, quinolones, and others. The course of treatment is determined by testing different antibiotics in the laboratory to determine which ones might be most effective against the infectious strain. If an individual develops a VRE infection and has a urinary catheter, sometimes removing the catheter will clear the infection.

Contact Precautions should be instituted for all VRE patients, regardless of colonization or infection. Handwashing, isolation/cohorting, and usage of personal protective equipment (PPE) are essential measures to prevent VRE transmission.

Source: National Institute of Allergy and Infectious Diseases (NIAID)