Antibiotic Resistance of Aquatic Gram- Negative Bacteria within Metropolitan Areas: Indianapolis and the Eagle Creek Reservoir

Abstract

Antibiotic resistance has become a growing public health concern, particularly in major met-

ropolitan areas such as Indianapolis. Urban runoff and waste from hospitals and agricultural

areas can introduce resistant bacteria into natural waterways, creating opportunities for the

environmental dissemination of antibiotic resistance. Many of Indianapolis’ waterways and

reservoirs are connected through the White River watershed. Eagle Creek, a manmade reservoir

within Eagle Creek Park, is part of this watershed and receives runoff from surrounding areas.

This study assessed the antibiotic resistance profiles of Gram-negative bacteria isolated from

Eagle Creek. Water samples were collected, vacuum-filtered to remove debris and larger micro-

organisms, and cultured on rich media. Individual colonies were purified by restreaking, and

antibiotic susceptibility was determined using the Kirby-Bauer disk diffusion method with com-

parison to an Escherichia coli reference standard.

All 72 Gram-negative isolates exhibited resistance to at least one antibiotic, and 95% demon-

strated multidrug resistance. The highest susceptibility was observed with gentamicin, to which

only 5.6% of isolates were sensitive. The greatest resistance occurred against ampicillin and

amoxicillin/clavulanate, with 95.8% and 79.2% of isolates resistant, respectively. Notably,

51.4% of isolates were also resistant to imipenem, a carbapenem often reserved for severe

infections.

These findings suggest that urban aquatic environments may serve as environmental reser-

voirs of antibiotic-resistant bacteria and highlight the importance of continued surveillance of

antimicrobial resistance in local water systems. Future studies incorporating bacterial species

identification, whole-genome sequencing, and plasmid analysis are needed to better character-

ize resistance profiles and clarify the potential implications for environmental dissemination and

public health.

Keywords: Antibiotic resistance, antimicrobials, gram-negative, bacteria, multidrug resistance