Risk factors associated with extended-spectrum beta-lactamase-producing organisms at a tertiary care hospital.

BACKGROUND: In 1995, beta-lactam inhibitor combinations replaced third-generation cephalosporins as empirical therapy in an effort to manage extended-spectrum beta-lactamase (ESBL) resistance. This study investigated the relationship between antibiotic usage and ESBL organisms from 1994 through 2002 using epidemiological and molecular analysis. METHODS: A case-control study of 119 patients with ESBL organisms and 132 patients with non-ESBL organisms was conducted. Demographics, co-morbidities, device utilization and antibiotic use were analysed for all patients and infected patients only (cases = 75, controls = 83). Both exposure and degree of exposure (in grams) to antibiotics were included. A dot blot hybridization technique was used to identify genes in plasmid extracts from the ESBL organisms. RESULTS: Ventilator days OR 1.1 (1.06, 1.15) P < 0.001, adult respiratory distress syndrome (ARDS) OR 3.1 (1.0, 9.7) P = 0.05, prior aminoglycoside use OR 2.7 (1.2, 6.1) P = 0.02, prior third-generation cephalosporin use OR 7.2 (2.6, 20) P < 0.001, and prior trimethoprim/sulfamethoxazole use OR 8.8 (3.1, 26) P < 0.001 were significantly associated with ESBL organisms by multivariate analysis. All models were concordant with a significant association of ventilator days, third-generation cephalosporins and trimethoprim/sulfamethoxazole with ESBL organisms. beta-Lactamase inhibitor combinations were not associated with ESBL organisms. Hybridization of plasmid extracts demonstrated that 95% of the ESBL organisms carried intI1, a mobile DNA element with a sulphonamide-resistance (R) gene and a frequent carrier of other R factors. Genes for specific types of trimethoprim-R and aminoglycoside-R were present in 26% and 40% of the extracts, respectively. CONCLUSIONS: These data indicate that, besides patient risk factors and third-generation cephalosporins, other antibiotics may provide selective pressures in maintaining ESBL organisms due to multiple resistance genes on plasmids. beta-Lactamase inhibitor combinations appear to be an acceptable substitute to third-generation cephalosporins in strategies to control ESBL organisms.

Survey of plasmid-associated genetic markers in enterobacteriaceae with reduced susceptibilities to cephalosporins.

Clinical isolates of Enterobacteriaceae with reduced susceptibilities to cephalosporins were collected from 1993 to 2000. The organisms were screened for the extended-spectrum beta-lactamase (ESBL) phenotype, and plasmid extracts were screened for genetic markers by hybridization. A bla(TEM) probe was derived from pUC19; other probes were derived from pACM1, the plasmid responsible for the first known appearance of an ESBL in our institution. These probes included bla(SHV), int, aac(3)-Ia, dfrA1, IS6100, tetA, IncM markers, and Anon 13, a marker for the Klebsiella pneumoniae chromosomal sequences that flank bla(SHV-5). There were 42 hybridization patterns among 237 isolates. Patterns designated pACM1-like occurred in 44% of the isolates (eight species) and were always associated with the clavulanic acid (CA)-susceptible ESBL phenotype. The TEM marker was not predictive of the ESBL phenotype. Mapping indicated the presence of an SHV marker and up to 7.5 kb of its flanking chromosomal sequences in three non-IncM plasmids obtained in transformation experiments. We theorize that this DNA segment spread to other plasmids from pACM1-like sources. CA insensitivity became more frequent with time and was usually associated with either the TEM marker or the absence of both bla markers. One plasmid-encoded enzyme with characteristics of an AmpC beta-lactamase was observed in a transformant lacking both TEM and SHV markers. Although SHV type ESBLs were a continuing source of reduced susceptibility to cephalosporins in our institution, organisms with different resistance mechanisms were added to the hospital microflora in later years. These changes might be related, in part, to ESBL control strategies implemented in 1995.

Risk factors associated with nosocomial methicillin-resistant Staphylococcus aureus (MRSA) infection including previous use of antimicrobials.

Methicillin-resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen worldwide. To investigate an association between antimicrobial use and MRSA, a case control study of 121 patients infected with MRSA compared with 123 patients infected with methicillin-susceptible S. aureus (MSSA) was carried out. Antimicrobial use was analysed by three different logistic regression models: all beta-lactam antibiotics, beta-lactam antibiotics grouped in classes and antimicrobial use in grammes. Patients infected with MRSA tended to have more co-morbidities, longer lengths of stay (LOS) and greater exposure to antibiotics than MSSA-infected patients. Multivariate analysis identified levofloxacin [odds ratio (OR) 8.01], macrolides (OR 4.06), previous hospitalization (OR 1.95), enteral feedings (OR 2.55), surgery (OR 2.24) and LOS before culture (OR 1.03) as independently associated with MRSA infection. All models were concordant with the exception of macrolides, which were not significant based on the number of grammes administered. There were no significant differences in the types of infection or the attributed mortality in either group. MRSA-infected patients had a significantly longer LOS before infection [18.8 +/- 18.2 compared with 8.4 +/- 6.9 (P < 0.001)] and a significantly longer post-diagnosis LOS [27.8 +/- 32.9 compared with 18.6 +/- 21 (P = 0.01)] than MSSA-infected patients.