Comparison of Antibiotic Resistance in Aeromonas Isolated
from Fish Collected from Heavy Metal Contaminated and
Uncontaminated Streams: Effect of Fish Trophic Status

J V. McArthur, D. R. Fletcher, A. Lindell and R. C. Tuckfield

Results and Discussion

All fish species in both streams had Aeromonas with varying degrees of antibiotic resistance. There were no significant differences in the proportions of antibiotic resistant Aeromonas among fish within either stream for any antibiotic (Table 1) except for Tetracycline in Meyers Branch fish and Chloramphenicol in Four Mile Fish. Highest levels of Tetracycline resistant bacteria were isolated from the bullheads and the lowest proportion from Red Breast sunfish. Highest proportion of chlorampheicol resistant bacteria were found on the bullheads again and the lowest on the bass. Multiple antibiotic resistance was observed on all fish in both streams (Tables 2 and 3). However there were no significant differences among fish species in the levels of multiple antibiotic resistant Aeromonas isolated. More fish in Four Mile Creek had visible lesions (Figure 2) than did the fish in Meyers Branch. This observation is consistent with past observations that indicated that fish in highly disturbed habitats have higher incidences of lesions. Figure 3 compares both streams and superimposes the maximum likelihood estimate of the proportion of antibiotic resistant bacteria for each fish species. From this figure it is apparent that bottom-dwelling fish species like the bullheads have much higher levels of antibiotic resistant bacteria than do the water column-dwelling species. Analysis of Aeromonas isolates collected from stream sediments showed no differences in the levels of antibiotic resistance between the two streams.

We have previously shown that sediment bacteria exposed to elevated levels of heavy metals have higher incidences o antibiotic resistance than do bacteria in more pristine sediments. We expected to see similar patterns in bacteria isolated from fish in contaminated and uncontaminated streams. However, our expectations were not met. Fish from both streams had antibiotic resistant Aeromonas on their surfaces although there were differences in which species had the highest proportions of antibiotic resistant bacteria. The most obvious pattern was seen in levels of MAR in Aeromonas collected from different fish species. These observations suggest that fish trophic status does affect the proportion of antibiotic resistant Aeromonas on their surfaces. Bottom dwelling fish, i.e., fish coming in contact with bacteria exposed to sediment levels of heavy metals, had higher proportions of MAR bacteria than did the open water dwelling fish.

Fish are highly mobile organisms capable of migrating large distances and thus exposing themselves to varying levels of stress. It is not surprising that the fish did not show differences between the two streams because the bacteria on their surfaces are an integration of all exposure/inoculations that have occurred in the fish's lifetime. Differences between species of fish must be due to potential for exposure and colonization by MAR bacteria.

These data have potential public health implications. Fisherman who catch and handle bottom-dwelling fish may have a higher probability of being infected with MAR bacteria. These data also suggest strong vectors for the dissemination and transport of AR and MAR bacteria long distances.

Table 1. Comparisons of antibiotic resistance among fish
species within a stream by antibiotic.

Table 2. Contingency test of multiple antibiotic resistance in
Aeromonas by fish species in Four Mile Creek.

Table 3. Contingency test of multiple antibiotic resistance
among fish in Meyers Branch.

Contingency Analysis of Sores By Stream Mosaic Plot

Contingency Table Stream By Sores

Oneway Analysis of Proportion (Multiple antibiotic resistance) By Stream.
Symbol represents fish species. *= bullhead