carbapenem-resistant enterobacteriaceae carbapenem-resistant enterobacteriaceae carbapenem-resistant enterobacteriaceae (cre) or carbapenemase-producing enterobacteriaceae (cpe) are gram-negative bacteria that are resistant to the carbapenem class of antibiotics, considered the drugs of last resort for such infections. they are resistant because they produce an enzyme called a carbapenemase that disables the drug molecule. the resistance can vary from moderate to severe. enterobacteriaceae are common commensals and infectious agents. experts fear cre as the new "superbug". the bacteria can kill up to half of patients who get bloodstream infections. tom frieden, former head of the centers for disease control and prevention has referred to cre as "nightmare bacteria". examples of enzymes found in certain types of cre are kpc (klebsiella pneumoniae carbapenemase) and ndm (new delhi metallo-beta-lactamase). kpc and ndm are enzymes that break down carbapenems and make them ineffective. both of these enzymes, as well as the enzyme vim (verona integron-mediated metallo-β-lactamase) have also been reported in pseudomonas. definition carbapenem-resistant enterobacteriaceae (cre) have been defined as carbapenem-nonsusceptible and extended-spectrum cephalosporin-resistant escherichia coli, enterobacter aerogenes, enterobacter cloacae complex, klebsiella pneumoniae, or klebsiella oxytoca. some exclude ertapenem resistance from the definition. risk factors hospitals are primary transmission sites for cre-based infections. up to 75% of hospital admissions attributed to cre were from long-term care facilities or transferred from another hospital. suboptimal maintenance practices are the largest cause of cre transmission. this includes the failure to adequately clean and disinfect medication cabinets, other surfaces in patient rooms, and portable medical equipment, such as x-ray and ultrasound machines that are used for both cre and non-cre patients. thus far, cre have primarily been nosocomial infectious agents. almost all cre infections occur in people receiving significant medical care in hospitals, long-term acute care facilities, or nursing homes. independent risk factors for cre infection include use of beta-lactam antibiotics and the use of mechanical ventilation. patients with diabetes have also been shown to be at an elevated risk for acquiring cre infections. when compared to other hospitalized patients, those admitted from long-term acute care (ltac) facilities have significantly higher incidence of colonization and infection rates. another 2012 multicenter study found that over 30% of patients with recent exposure to ltac were colonized or infected with cre. a person susceptible to cre transmission is more likely to be female, have a greater number of parenteral nutrition-days (meaning days by which the person received nutrition via the bloodstream), and to have had a significant number of days breathing through a ventilator. infections with carbapenem-resistant klebsiella pneumoniae were associated with organ/stem cell transplantation, mechanical ventilation, exposure to antimicrobials, and overall longer length of stay in hospitals. people most likely to acquire carbapenem-resistant bacteria are those already receiving medical attention. in a study carried out at sheba medical center, there was a trend toward worse charleson comorbidity scores in patients who acquired crkp during icu stay. those at highest risk are patients receiving an organ or stem cell implantation, use of mechanical ventilation, or have to have an extended stay in the hospital along with exposure to antimicrobials. a study performed in singapore, on the acquisition of ertapenem-resistant enterobacteriaceae to the acquisition of cre. revealed that exposure to antibiotics, especially fluoroquinolones and previous hospitalization dramatically increased the risk of acquisition carbapenem-resistant bacteria. this study found that carbapenem-resistant acquisition has a significantly higher mortality rate and poorer clinical response compared to that of the ertapenem-resistance acquisition. bacteruria (also known as urinary tract infection) caused by crkp and cskp have similar risk factors. these include prior antibiotic use, admittance to an icu, use of a permanent urinary catheter, and previous invasive procedures or operations. a retrospective study of patients with crkp and cskp infection asserted that the use of cephalosporins (a class of β-lactam antibiotics) used before invasive procedures was higher in patients with crkp infection, suggesting that it is a risk factor. in a three-year study, the prevalence of cre was shown to be proportional to the lengths of stays of the patients in those hospitals. policies regarding contact precaution for patients infected or colonized by gram-negative pathogens were also observed in hospitals reporting decreases in cre prevalence. one case study showed that patients with a compromised immune response are especially susceptible to both cre exposure and infection. in one study, an elderly patient with acute lymphoblastic leukemia being treated in a long-term care facility contracted a cre infection. her age and condition, combined with her environment and regulation by a catheter and mechanical ventilation, all contributed to a higher susceptibility. this highlights the importance of finding the source of the bacteria, as members of this class of patients are at continued risk for infection. infection control and prevention of cre should be the main focus in managing patients at high risk. another major risk factor is being in a country with unregulated antibiotic distribution. in countries where antibiotics are over-the-counter and obtainable without a prescription, the incidence and prevalence of cre infections were higher. one study from japan found that 6.4% of healthy adults carried esbl (mostly cefotaximase)-producing strains compared to 58.4% in thailand, where antibiotics are available over the counter and without prescription. an egyptian research group found that 63.3% of healthy adults were colonized. in february 2015, the fda reported about a transmission risk when people undergo a gastroenterology procedure called endoscopic retrograde cholangiopancreatography, where an endoscope enters the mouth, passes the stomach, and ends in the duodenum; if incompletely disinfected, the device can transmit cre from one patient to another. the fda's safety communication came a day after the ucla health system, los angeles, notified more than 100 patients that they may have been infected with cre during endoscopies between october 2014 and january 2015. the fda had issued its first notice about the devices in 2009. transmission by animals because the spread of cre bacteria from animals to humans may become a problem in the future, it is advised to monitor cre in livestock, as well as humans. mechanism antibiotic function the β-lactam family of antibiotic molecules consists of four groups: penicillins, cephalosporins, carbapenems (such as imipenem, ertapenem, meropenem and doripenem) and monobactams. these antibiotics share common structure and mechanism of action. they enter the periplasmic space through porins, where they then inhibit transpeptidases (which are also known as penicillin-binding proteins (pbps)), enzymes that facilitate peptide cross-links during cell wall synthesis. their binding to the pbp active site is facilitated in part by their common structure, which is similar to that of d-alanyl-d-alanine. d-alanyl-d-alanine is a residue on the nam peptide subunit involved in building peptidoglycan. carbapenem covalently binds to pbps, which causes transpeptidases to irreversibly lose their catalytic activity. inhibition of transpeptidases prevents the formation of cross-links between peptidoglycan polymers and causes a build-up of peptidoglycan precursors. newly formed peptidoglycan is weakened from the absence of cross-linkages. the continued activity of autolysins, that function like lysozymes and cleave glycosidic and peptide bonds of peptidoglycan in periplasm, weakens the cell wall and leads to osmotic bursting of the bacterial cell. a unique quality of carbapenems is their resistance to hydrolysis by bacterial plasmid and chromosomally mediated extended-spectrum β-lactamases (esbl). carbapenem resistance in general, carbapenem, a β-lactam antibiotic, targets cells by inhibiting transpeptidases (penicillin-binding proteins). this prevents synthesis of peptidoglycan, a necessary structural component, leading to cell lysis. resistance to carbapenem among gram-negative bacteria in general ( i.e. including bacteria other than enterobacteriaceae) can be acquired through several mechanisms. active transport of carbapenem drugs out of the cell, augmented drug efflux, has been observed in some resistant species. one mechanism of resistance is mutation in or loss of outer membrane porins, preventing antibiotics from entering the cells. changes within the porin protein gene cause a frameshift, altering the porin structure and function. changes in the porin protein hinder the diffusion of carbapenem and other antibiotics into the periplasm. bacteria that express plasmid-borne extended-spectrum β-lactamases (esbl) can become carbapenem-resistant if an insertion sequence or four-nucleotide duplication is present within chromosomal genes for outer membrane porin proteins. klebsiella pneumoniae has been associated with the lack of outer membrane porin proteins, ompk35 and ompk36. the loss of ompk36 porins can be attributed to point mutations that result in premature termination of translation, resulting in a truncated and consequently nonfunctional protein. these outer membrane proteins are involved in the transfer of the antimicrobial genetic material in the cell. loss of either ompk35 and ompk36 or only ompk36 leads to carbapenem resistance. in klebsiella pneumoniae, the lack of either ompk35 or ompk36 leads to carbapenem resistance, but with the lack of both proteins, a high level of resistance is present. an observed 32- to 64-fold increase in minimum inhibitory concentrations occurs for the carbapenems when both proteins are not expressed. cre produce carbapenemases, a form of β-lactamase. these enzymes cleave the β-lactam ring, an essential component of β-lactam antibiotics that are recognized by and bound to pbps. carbapenemases are divided into different classes, depending on the structure of the enzyme and the mechanism by which they hydrolyze the β-lactam ring. the two broad categories of carbapenemases are serine-carbopenemases, which contain serine at the active site, and metallocarbapenemases, which contain zinc at the active site. class a carbapenemases are serine carbapenemases and are encoded on either the chromosome of the bacteria or a plasmid. a serine at position 70 at the active site of this class of enzymes is required for hydrolysis of β-lactams to occur. class d carbapenemases, also referred to as the oxa β-lactamases, are serine β-lactamases. they are encoded on plasmids and contain a large variability in amino acid sequence. the resistance mechanism for class d carbapenemases is caused by the formation of an acyl intermediate when breaking the β-lactam ring. class b carbapenemases are metallolactamases and require a zinc at the active site for hydrolysis. a clinical isolate of e. coli from the sputum sample of a patient admitted to a beijing hospital was found to acquire resistance to carbapenem through mutations not previously observed. it involved a mutation of a regulator gene marr and the expression of a normally nontranslated membrane porin yeds; both mutations were demonstrated to have effects on the ability of this strain of e.coli to resist carbapenems. the strain lacked the outer membrane proteins ompf and ompc, and showed increased expression of a multidrug efflux pump, but did not produce carbapenemase. cr pseudomonas aeruginosa is commonly present in intensive-care units, and can lead to dangerous infections. in thai hospitals, of 261 multidrug-resistant samples collected of p. aeruginosa (not part of the enterobacteriaceae), 71.65% were carbapenem-resistant. site of cleavage by beta-lactamase transfer between bacteria gram-negative bacteria can develop and transfer β-lactam resistance (including carbapenem resistance) in many ways. they can generate new extended-spectrum β-lactamases (esbl) from the existing spectrum of plasmid-mediated β-lactamases through amino acid substitution. they can acquire genes encoding esbl from environmental bacteria. they can increase the expression of chromosome-encoded β-lactamase genes (bla genes) due to regulatory gene and promoter sequence modifications. they can mobilize bla genes through integrons or horizontal transfer of genomic islands into other gram-negative species and strains. they can disseminate plasmid-mediated carbapenemases. finally, they can lower or even inhibit the expression of porin genes. three major classes of enzymes are involved in carbapenem resistance: class a carbapenemases, class b metallo-β-lactamases (mbl), and class d β-lactamases (oxa). the four known groups of class a carbapenemases are: sme (three types associated with s. marcescens), imi (present in e. cloacae), ges (16 variants thus far found in p. aeruginosa predominantly but also found in k. pneumoniae and e. coli), and kpc (10 types of k. pneumoniae carbapenemase). at the uva medical center, a transfer mechanism of kpc-dependent carbapenem resistance was discovered in the transmission of a plasmid carrying the transposon (tn4401), which contains the kpc gene (blakpc), to several bacteria including enterobacter cloacae, klebsiella oxytoca, e. coli, and citrobacter freundii. the class b metallo-β-lactamases (mbls) are found largely in gram-negative bacteria and environmental bacteria. the subclasses of mbl enzymes are b1, b2, and b3. mbls have diverse enzymatic functions and have the ability to hydrolyze β-lactam antibiotics. the class d β-lactamases (oxa), which hydrolyze oxacillin, provide a good example of the variety of mechanisms that can be used to transfer resistance. the blaoxa genes which encode oxa β-lactamases are found on both chromosomes and plasmids, and they have their natural reservoir in environmental bacteria and