Antibiotic exposure is known to increase the risk of selecting for an antibiotic resistant organism. However, there is limited data describing temporal relationship between antibiotic exposure and selection of resistant organisms in patients rehospitalized for the same infection. The purpose of this study is to elucidate which combinations of organisms and antibiotics are most likely to select for resistance by quantifying antibiotic exposure and timelines in common Gram-positive and Gram-negative pathogens.
This was a retrospective, observational study evaluating one hundred inpatients identified by having repeat culture sets of the same organisms, from the same source, and from separate inpatient admissions. All inpatient and outpatient antibiotic administration was recorded.
Of the one hundred patients were included in the study, 42 had Gram-positive organisms and 58 had Gram-negative organisms. Staphylococcal aureus and Enterobacteriaceae were the most common Gram-positive and Gram-negative pathogens. Resistance to any antibiotic developed in in 6/38 (16%) and 30/65 (46%) sets of cultures for S. aureus and Enterobacteriaceae, respectively (p=0.003). Resistance to an antibiotic administered developed in 4/38 (11%) and 12/65 (18%) sets of cultures for S. aureus and Enterobacteriaceae, respectively (p=0.40). The average days from antibiotic exposure to S. aureus reculture was 56 days and 61 days in those that did and did not select for resistance, respectively (p=0.87). For Enterobacteriaceae, the average time was 78 days and 92 days (p=0.46). Cephalosporins were the most common antibiotics seen in the organisms and antibiotic combinations in which resistance developed following exposure (15/22, 68%).
Enterobacteriaceae were much more likely to develop resistance to any antibiotic than S. aureus, but there was no difference in rates of resistance development to administered antibiotics. Avoiding previously administered antibiotics in a ninety day window may be a pertinent clinical opportunity for Enterobacteriaceae.
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