Multidrug-Resistant Gram-Negative Bacilli (MDR-GNB)

  Gram-negative bacilli have been a source of healthcare-associated infections for many years and may be found in patients in virtually all healthcare settings as either infection or colonization. In recent years, multidrug-resistant gram-negative organisms have increased in nearly all healthcare settings. Though resistance to any class of antibiotic can occur, it occurs mainly among the extended spectrum beta-lactam antimicrobial agents. This is mainly due to the ability of these organisms to produce extended spectrum beta lactamase enzymes (ESBLs), which make them highly resistant to many of the extended spectrum beta-lactam agents such as the penicillins, cephalosporins, and monobactams. This group includes primarily, Klebsiella, E. coli, P. aeruginosa, and other Enterobacteriaceae, though numerous other drug-resistant gram-negative bacteria strains have also been reported.

 ESBL-producing gram-negative organisms and carbapenemase-producing enterobacteriaceae are a group of emerging infectious pathogens that warrant inclusion in institutional infection control policies. The HICPAC/CDC MDRO 2006 Guidelines recommend contact precautions and other tier 2, intensified control efforts when cases of MDR-GNB are identified. Two of the significant MDR-GNB include:

 1. Acinetobacter baumannii

In recent years, multidrug-resistant A. baumannii (MDRAb) has increased in prominence as a healthcare-associated pathogen. Primarily affecting hospital ICU’s, A. baumannii is associated with longer hospitalizations, greater economic cost, and increased morbidity. Infection due to MDRAb can occur sporadically, but is more commonly associated with outbreaks. MDRAb infections typically manifest as respiratory (ventilator pneumonia), urinary tract, and wound infections (including burn wounds). High rates of bacteremia have also been reported in military service members injured in the Middle East. MDRAb is an ESBL-producing gram-negative bacilli that routinely exhibit resistance to multiple classes or even all classes of antimicrobial drugs leading to greater difficulty in treatment.

A. baumannii is a ubiquitous gram-negative bacillus, found in soil, water, animals, and humans. In the clinical setting, individuals may be infected or colonized and environmental surfaces may be contaminated by A. baumannii where its ability to persist may contribute to transmission between patients, as well as long-term outbreaks. Primarily associated with acute care and long-term acute care facilities, it is now encountered in LTC facilities with increasing frequency. The epidemiology of MDRAb indicates that this is an emerging pathogen and all types of healthcare facilities should be knowledgeable of this pathogen and recommended control measures.

 2. Klebsiella pneumoniae and other Carbapenemase-Producing Enterobacteriaceae

Klebsiella pneumoniae and other gram-negative bacilli have been increasing in clinical importance. While ESBL production among the gram-negative organisms has been an infection control issue for many years, more recently strains of enteric bacilli and other gram-negative organisms have demonstrated production of carbapenemases (beta-lactamase enzymes mediating resistance to the extended spectrum cephosporins as well as carbapenem antibiotics, e.g.,impenem, ertapenem, meropenem).

 In the U.S., a type of carbapenemase referred to as KPC (Klebsiella pneumoniae carbapenemase) has been demonstrated in several species of enteric bacilli but is most commonly found in strains of Klebsiella pneumoniae. A KPC-producing strain of Klebsiella pneumoniae was first reported in North Carolina in 2001 and another was later discovered as part of an outbreak in New York that began in 2000. KPC producing strains have also been reported sporadically from various parts of the U.S., particularly the east coast,  In addition to the high level of resistance commonly found in the KPC-producing strains, the inability of most laboratories to directly detect or confirm the KPC enzyme through routine testing poses additional concern since KPC production may not be detected through standard susceptibility testing.


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