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.

Klebsiella oxytoca that produced extended-spectrum beta-lactamase (ESBL) and were resistant to ceftazidime were isolated from infants in a neonatal intensive care unit (NICU). During a 30-week period, 3 infants developed infections and an additional 60 infants were colonized with these bacteria. The molecular typing data suggested transmission of a single strain of ceftazidime-resistant K. oxytoca among 48 of the 63 infants. The ESBL of 46 of the 48 similar isolates, 14 of the remaining 15 isolates, and 6 other Enterobacteriaceae appeared to be associated with a conjugative plasmid of approximately 85 kb. The ESBL gene was cloned, and DNA sequencing confirmed that the ESBL was an SHV-5. Hybridization data suggested that the SHV-5 gene was transmitted to other Enterobacteriaceae in vivo. The spread of the ESBL was reduced through adherence to infection control practices.

To study the effect of fluoroquinolone exposure on the expression of mec(A)-encoded oxacillin resistance, population analysis profiling was performed on four strains of fluoroquinolone-susceptible, mec(A)-positive, heteroresistant Staphylococcus aureus. Growth in the presence of 0.5 x MIC of a fluoroquinolone resulted in >10-fold increase in the proportion of the population that grew on agar containing oxacillin 128 mg/L. Ciprofloxacin exhibited a greater effect than moxifloxacin, levofloxacin and gatifloxacin (average 3400-, 220-, 170- and 49-fold increase in oxacillin-resistant colonies versus the control, respectively). The increase was directly proportional to the fluoroquinolone concentration and could be detected as early as 8 h after exposure to the fluoroquinolone. At 8 h, the absolute number of colonies that grew on oxacillin 128 mg/L was similar whether or not the isolate was exposed to the fluoroquinolone, but the total cfu on non-selective media decreased. The resultant oxacillin-resistant colonies also showed a 1.5- to 3-fold increase in fluoroquinolone MIC. No oxacillin resistance was observed on two similarly treated fluoroquinolone-susceptible, mec(A)-negative strains. It appears that fluoroquinolones influence oxacillin resistance by selective inhibition or killing of the more susceptible subpopulations in heteroresistant S. aureus. The surviving populations are more resistant to both oxacillin and fluoroquinolone. The mechanisms of resistance to the two agents may be unrelated but tend to be associated. This could explain in part the observed increases in fluoroquinolone-resistant MRSA.