Patterns of antimicrobial resistance observed in Escherichia coli isolates obtained from domestic- and wild-animal fecal samples, human septage, and surface water.

Abstract

A repeated cross-sectional study was conducted to determine the patterns of antimicrobial resistance in 1,286 Escherichia coli strains isolated from human septage, wildlife, domestic animals, farm environments, and surface water in the Red Cedar watershed in Michigan. Isolation and identification of E. coli were done by using enrichment media, selective media, and biochemical tests. Antimicrobial susceptibility testing by the disk diffusion method was conducted for neomycin, gentamicin, streptomycin, chloramphenicol, ofloxacin, trimethoprim-sulfamethoxazole, tetracycline, ampicillin, nalidixic acid, nitrofurantoin, cephalothin, and sulfisoxazole. Resistance to at least one antimicrobial agent was demonstrated in isolates from livestock, companion animals, human septage, wildlife, and surface water. In general, E. coli isolates from domestic species showed resistance to the largest number of antimicrobial agents compared to isolates from human septage, wildlife, and surface water. The agents to which resistance was demonstrated most frequently were tetracycline, cephalothin, sulfisoxazole, and streptomycin. There were similarities in the patterns of resistance in fecal samples and farm environment samples by animal, and the levels of cephalothin-resistant isolates were higher in farm environment samples than in fecal samples. Multidrug resistance was seen in a variety of sources, and the highest levels of multidrug-resistant E. coli were observed for swine fecal samples. The fact that water sample isolates were resistant only to cephalothin may suggest that the resistance patterns for farm environment samples may be more representative of the risk of contamination of surface waters with antimicrobial agent-resistant bacteria.

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APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Mar. 2005, p. 1394–1404
0099-2240/05/$08.00?0 doi:10.1128/AEM.71.3.1394–1404.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Vol. 71, No. 3
Patterns of Antimicrobial Resistance Observed in Escherichia coli
Isolates Obtained from Domestic- and Wild-Animal Fecal
Samples, Human Septage, and Surface Water
Raida S. Sayah,1John B. Kaneene,1* Yvette Johnson,2and RoseAnn Miller1
Population Medicine Center, Michigan State University, East Lansing, Michigan,1and Lower Eastern Shore Research
and Education Center, University of Maryland, Salisbury, Maryland2
Received 13 July 2004/Accepted 4 October 2004
A repeated cross-sectional study was conducted to determine the patterns of antimicrobial resistance in
1,286 Escherichia coli strains isolated from human septage, wildlife, domestic animals, farm environments, and
surface water in the Red Cedar watershed in Michigan. Isolation and identification of E. coli were done by using
enrichment media, selective media, and biochemical tests. Antimicrobial susceptibility testing by the disk
diffusion method was conducted for neomycin, gentamicin, streptomycin, chloramphenicol, ofloxacin, tri-
methoprim-sulfamethoxazole, tetracycline, ampicillin, nalidixic acid, nitrofurantoin, cephalothin, and sulfisox-
azole. Resistance to at least one antimicrobial agent was demonstrated in isolates from livestock, companion
animals, human septage, wildlife, and surface water. In general, E. coli isolates from domestic species showed
resistance to the largest number of antimicrobial agents compared to isolates from human septage, wildlife,
and surface water. The agents to which resistance was demonstrated most frequently were tetracycline,
cephalothin, sulfisoxazole, and streptomycin. There were similarities in the patterns of resistance in fecal
samples and farm environment samples by animal, and the levels of cephalothin-resistant isolates were higher
in farm environment samples than in fecal samples. Multidrug resistance was seen in a variety of sources, and
the highest levels of multidrug-resistant E. coli were observed for swine fecal samples. The fact that water
sample isolates were resistant only to cephalothin may suggest that the resistance patterns for farm environ-
ment samples may be more representative of the risk of contamination of surface waters with antimicrobial
agent-resistant bacteria.
Antimicrobial agent resistance has been recognized as an
emerging worldwide problem in both human and veterinary
medicine, and antimicrobial agent use is considered the most
important factor for the emergence, selection, and dissemina-
tion of antimicrobial agent-resistant bacteria (29, 51). The
principle behind the development of resistance is that bacteria
in the guts of humans and animals are subjected to different
types, concentrations, and frequencies of antimicrobial agents.
Over time, selective pressure selects resistant bacteria that
have specific fingerprints for resistance to the antimicrobial
agents that have been used (33, 45).
There are four general mechanisms of resistance, all of
which are controlled by the action of specific genes: enzymatic
inactivation or modification of antimicrobial agents, imperme-
ability of the bacteria cell wall or membrane, active expulsion
of the drug by the cell efflux pump, and alteration in target
receptors (33). Bacteria gain antimicrobial agent resistance
genes through mobile elements, such as plasmids, transposons,
and integrons (33, 37), which result in mutations in genes
responsible for antimicrobial agent uptake or binding sites (43)
or activation of portions of bacterial chromosomes (1, 17).
Once acquired, resistance genes can be transferred between
bacteria, and the ability of Escherichia coli to transfer antimi-
crobial drug resistance is well known (36).
Antimicrobial agents are used therapeutically in animals and
humans for control of bacterial infections and may be incor-
porated into commercial livestock and poultry feed at sub-
therapeutic doses for growth promotion. This practice is be-
lieved to enhance selection of resistant bacteria more than the
therapeutic use of antimicrobial agents in response to clinical
disease (47), and it may contribute to antimicrobial agent re-
sistance in humans acquired through the human food chain (2,
51). One strategy to minimize this problem that has been
recommended is to stop the use of agents needed for human
treatment as feed additives (36, 44, 52), but there is an ongoing
debate concerning whether and to what extent feed additives
contribute to the development of resistance in human bacterial
pathogens (6, 33). To approach this question, several studies
have described antimicrobial agent resistance profiles of E. coli
strains isolated from foods of animal origin, various species of
animals, and humans (4, 13, 14, 22, 23, 25, 39, 47).
In addition to the consequences for human health, concerns
have been raised about the contamination of surface water
with resistant bacteria from livestock operations and human
septage. Resistant bacteria have been isolated from a variety of
sources, including domestic sewage, drinking water, rivers, and
lakes (20, 24, 26). The levels of antimicrobial agent resistance
that have been reported range from 72% (26) up to 100 and
87% for fecal and nonfecal coliforms, respectively (24). One
study found that livestock contributed more than humans to
fecal coliform contamination of surface water and that reduc-
ing livestock access to surface water reduced the fecal coliform
levels by an average of 94% (18).
* Corresponding author. Mailing address: Population Medicine
Center, A-109 Veterinary Medical Center, Michigan State University,
East Lansing, MI 48824-1314. Phone: (517) 353-5941. Fax: (517) 432-
0976. E-mail: kaneene@cvm.msu.edu.
1394

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Resistance of a single bacterial isolate to more than one
antimicrobial drug is commonly reported. Multiple antimicro-
bial drug resistance profiles have been used to identify and
differentiate E. coli strains from different animal species (22).
This type of testing is simple, cost-effective, and suitable for
surveillance (45), and it has been used for E. coli strains col-
lected from human and animal sources (22). Recently, multiple
resistance profiles have been used to identify sources of fecal
contamination in water (15, 16, 18, 19, 20, 30, 48, 49).
The use of antimicrobial agent resistance profiles to identify
sources of bacterial contamination is a promising and emerg-
ing procedure. One technique that has been reported to be a
useful, low-cost screening method is discriminant function
analysis of antimicrobial agent resistance profiles (48). Unfor-
tunately, little basic information is available for comparisons of
the antimicrobial agent resistance profiles of normal gut mi-
crobiota from representative samples of domestic livestock and
poultry, pets, wildlife, and humans simultaneously in the same
geographic region. If the use of antimicrobial agents is an
important factor for the development of antimicrobial agent
resistance, it could be hypothesized that the patterns of anti-
microbial agent resistance in different animal populations vary
according to the types and quantities of agents used. To test
this hypothesis, the two objectives of this study were (i) to
identify patterns of antimicrobial agent resistance of E. coli
strains obtained from human septage, domestic animals, and
wildlife living in the Red Cedar watershed in Michigan, and (ii)
to compare these antimicrobial agent resistance patterns with
those of E. coli strains obtained from surface water in the same
watershed.
MATERIALS AND METHODS
Study design. A repeated cross-sectional approach was used to collect samples
and data related to antimicrobial agent use on farms over a 12-month period,
from the winter of 2002 to the winter of 2003. Samples were collected every 3
months, and there were a total of four sampling periods during the study.
Study area. The sampling region was established by the boundaries of the Red
Cedar watershed, from which surface water samples were obtained. This region
encompasses an area of 1,186 km2in Ingham and Livingston counties in central
Michigan. The Red Cedar River arises in Cedar Lake and flows approximately 73
km to its confluence with the Grand River in the city of Lansing. Swine and dairy
cattle are the predominant forms of livestock in this watershed.
Enrollment of participating farmers. Farms were located within the Red
Cedar watershed, and county drain commissioners identified specific farms
whose premises drained into the watershed. The farmers were sent a letter
through county extension agents, inviting them to participate in the study. Re-
spondents returned a prestamped postcard to the Population Medicine Center at
Michigan State University to indicate their willingness to participate. A total of
60 farmers were asked to participate in the study. Of 60 attempted contacts, 11
no longer maintained livestock and 5 had very few or no animals on their
premises. A total of 31 of the remaining 44 farms agreed to participate, and farm
visits were arranged quarterly from winter 2002 to winter 2003.
Sample size. In order to detect at least one animal with E. coli on a farm, the
general formula used by Smith (42) was used to compute sample size (ninf): ninf
? log(?)/log(1 ? prev), where ? is the probability that none of the sampled
animals harbor E. coli and prev is expected prevalence of E. coli.
We assumed that the expected prevalence of E. coli was 10% and that the type
I error was 0.05. Using the equation and assumptions described above, we
calculated that 29 animals per species was the minimum number necessary for
testing.
Data collection. Data relating to antimicrobial agent use and numbers of
animals on the farm were collected at the time of collection of fecal samples by
using a questionnaire administered during an in-person interview. Participants
were asked about the use of antimicrobial agents for therapy, prophylaxis, and
growth promotion during the previous 60 days.
Sample collection. Fecal and farm environment samples were taken by using
culturette swabs, and 100-ml water samples were collected from specific locations
in the watershed. The water sampling sites were determined with the help of the
Ingham county drain commissioner, based on the direction of the rain flow from
every farm enrolled in the study. The water sampling bottles contained 10 mg of
sodium thiosulfate to neutralize any residual chlorine in the water. All samples
were shipped to the University of Maryland for bacterial isolation, identification,
and antimicrobial agent susceptibility testing.
(i) Animal fecal samples. Fecal samples were obtained from dairy and beef
cattle, swine, horses, sheep, goats, chickens, cats, dogs, deer, ducks, and geese.
Fecal samples from livestock (dairy cattle, beef cattle, swine, sheep, goats,
horses) and companion animals (dogs, cats) were collected rectally from indi-
vidual animals by using culturette swabs. Samples were collected from fresh
manure by using culturette swabs on feedlots where sampling of individual
animals was not feasible. Poultry samples were collected by using cloacal swabs.
Deer samples were collected from freshly voided droppings. Goose and duck
samples were collected from freshly voided droppings and by using cloacal swabs
by personnel from the Michigan Department of Natural Resources.
(ii) Farm environment samples. Samples from the manure storage facilities
(lagoons, slurry pits, and manure piles) and animal housing areas on the farms
were collected by using culturette swabs.
(iii) Septage samples. Samples representative of human fecal material were
collected from septic tanks (prior to chemical treatment) with the help of the
local septic pumping companies in the study area. Septage samples are the best
representation of human-source fecal material that is likely to affect water and
environmental quality via leakage from septic tanks or improper disposal of
pumped septic contents in the study area.
Isolation of E. coli from water samples. The membrane filtration method used
by the United States Environmental Protection Agency (46) was used to isolate
E. coli from water samples. In this procedure, water samples were filtered
through a sterile, white, grid-marked, 47-mm-diameter membrane (pore size,
0.45 ? 0.02 ?m), which retained bacteria. After filtration, the membrane con-
taining the bacteria was placed on a selective differential medium (mTEC agar)
(9, 46) and incubated at 35°C for 2 h to resuscitate the injured or stressed
bacteria and then at 44°C for 22 h. The filter was transferred from mTEC agar
to a filter pad saturated with urea substrate medium. After 15 to 20 min, yellow,
yellow-green, or yellow-brown colonies on mTEC agar were transferred to urea
substrate media; any non-E. coli colonies turned pink or purple on these media.
Identification of E. coli from surface water, fecal, and human septage samples.
Standard methods were used for the enrichment, isolation, identification, and
biochemical confirmation of E. coli isolates (8).
Upon arrival at the laboratory, culturette swabs (fecal and human septage
samples) or colonies picked from urea substrate media (surface water samples)
were placed in tubes with tryptic soy broth (TSB) and incubated at 35°C for 24 h.
Approximately 10 ?l of the turbid broth was streaked onto violet red bile agar
and incubated for 18 to 20 h at 35°C. The violet red bile agar plates were
examined for reddish purple colonies that fluoresced under black light. Selected
colonies were streaked onto MacConkey agar and incubated at 35°C for 18 to
20 h. The MacConkey agar plate was examined for red colonies that precipitated
bile and had a dark red center. One or two colonies were selected and streaked
onto tryptic soy agar and incubated for 18 h. The tryptic soy agar plate was then
examined for single colonies that were round, milk colored, and slightly convex.
A single colony was selected, placed in a tube containing TSB, and incubated for
approximately 3 to 4 h until the culture was turbid.
Bacteria from the broth were transferred into tubes for biochemical confir-
mation by indole, methyl red, Voges-Proskauer, and Simmons citrate tests (8) on
triple sugar iron (Difco, Sparks, Md.). Only the bacterial isolates that were
confirmed to be E. coli based on the results of the biochemical tests were selected
for antimicrobial agent sensitivity testing. Confirmed isolates were inoculated
into new TSB tubes and incubated until the turbidity was 0.5 McFarland standard
(approximately 2 to 3 h).
Antimicrobial agent susceptibility testing. Once a single E. coli isolate was
isolated and identified from each sample collected, the standard Kirby-Bauer
disk diffusion method was used to determine the antimicrobial agent sensitivity
profiles of the E. coli isolates (27, 28) for 12 antimicrobial agents (Table 1). These
antimicrobial agents were chosen on the basis of their importance in treating
human or animal E. coli infections and their use as feed additives to promote
growth in animals and on the basis of their ability to provide diversity for
representation of different antimicrobial agent classes (22).
A 150-mm Mueller-Hinton medium plate was swabbed with TSB inoculated
with E. coli and incubated to a turbidity of 0.5 McFarland standard. Twelve
commercially prepared antimicrobial agent disks were place on the inoculated
plates. The plates were incubated at 35°C for 18 to 20 h. The diameters (in
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millimeters) of the clear zones of growth inhibition around the antimicrobial
agent disks, including the 6-mm disk diameter, were measured by using precision
calipers (27, 28). The breakpoints used to categorize isolates as resistant or not
resistant to each antimicrobial agent were those recommended by the National
Antimicrobial Resistance Monitoring System for E. coli. E. coli ATCC 25922
(American Type Culture Collection) was used for quality control.
Data analysis. Data for the antimicrobial agent resistance of each bacterial
isolate were reported in two forms: either as the diameter of the zone of
inhibition (in millimeters) or as resistant or not resistant (based on NCCLS
breakpoints). Since these data were used to identify animal species sources of
resistant E. coli in discriminant analysis, animal species were handled in two
different ways, (i) individually by species and (ii) by groups based on animal
management and the likelihood of exposure to various antimicrobial agents, as
follows: livestock (cattle, pigs, sheep), wildlife (geese, ducks), and equines
(horses, donkeys).
Associations between livestock group and antimicrobial agent resistance (re-
sistant or not resistant) were expressed as odds ratios with 95% confidence
intervals, and the Fisher exact test was used to test for significant differences
between species groups (SAS 8.2; SAS Inc., Cary, N.C.). Differences in zones of
inhibition between species groups were assessed by using analysis of variance and
the nonparametric Wilcoxon rank sum ?2test (SAS 8.2; SAS Inc.).
RESULTS
A total of 31 farms agreed to participate in the study (Fig. 1),
including 14 cattle farms (7 dairy farms and 7 beef farms), 6
sheep farms, 5 pig farms, 2 horse farms, 2 chicken farms, and
2 deer farms. Several farms had more than one species on the
premises. A total number of 2,522 samples were collected,
from which 1,286 E. coli isolates were retrieved for antimicro-
bial agent resistance profiling (Table 2). Data for use of anti-
microbial agents, either alone or in combination with other
drugs, were collected for 448 animals from 30 farms (Table 3).
Overall, penicillin was the most commonly reported antimicro-
bial agent (86% overall), followed by chlortetracycline (30%),
sulfamethazine (16%), and oxytetracycline (14%). The most
widely used agents in food animals (dairy and beef cattle,
swine, sheep) were chlortetracycline (dairy, beef, swine,
sheep), oxytetracycline (dairy, beef, swine, sheep), and penicil-
lin (dairy, beef, swine).
Antimicrobial agent resistance was detected in all types of
samples collected (Table 4). The most frequently encountered
form of resistance in all samples was resistance to tetracycline
(27.3%), followed by resistance to cephalothin (22.7%), resis-
tance to sulfisoxazole (13.3%), and resistance to streptomycin
(13.1%). Animal fecal samples exhibited resistance to all
agents tested, while human septage and river water samples
showed resistance to three agents and one agent, respectively.
Resistance to cephalothin was present in all types of samples,
while tetracycline resistance and streptomycin resistance were
found in all types of samples except river water. When we
looked at the patterns of antimicrobial agent resistance for
fecal, farm environment, and septage samples from different
species groups (Table 5), E. coli strains from swine were resis-
tant to 10 of the 12 agents tested (there was resistance in both
fecal and farm environment samples from 9 of the 10 sources),
followed by strains from dairy cattle, poultry, and beef cattle.
Interestingly, when we compared resistance to tetracycline and
resistance to trimethoprim-sulfamethoxazole in livestock iso-
lates, we found that resistance to tetracycline was present in
both fecal and farm environment samples from all livestock
species, while resistance to trimethoprim-sulfamethoxazole
was present in both types of samples from only dairy cattle and
equids.
Disk diffusion zone sizes were also examined for differences
between types of samples collected (Table 6). Significant dif-
ferences were seen in the diffusion zone sizes for all agents
except tetracycline and sulfisoxazole. Overall, the largest dif-
fusion zones (indicating greater susceptibility) were found with
animal fecal isolates. The exceptions were the diffusion zones
for tetracycline, ampicillin, and sulfisoxazole; for these agents
the water isolates had the largest diffusion zones. Human sep-
tage isolates had the smallest diffusion zones for all agents
except neomycin, gentamicin, nitrofurantoin, and cephalothin,
for which water isolates had the smallest zones.
Animal species were also used to examine patterns of resis-
tance (Table 7). Since very little resistance to some of the
agents was observed in this study, this analysis was limited to
agents with the highest levels of resistance by type of sample:
tetracycline, cephalothin, sulfisoxazole, streptomycin, and am-
picillin. Based on the odds ratios for resistance, swine had the
greatest likelihood of harboring E. coli resistant to tetracycline,
sulfisoxazole, streptomycin, and ampicillin, while the lowest
levels of resistance were seen with isolates from wild waterfowl
and farmed deer (Table 7).
The patterns of antimicrobial agent resistance for isolates
from farm environment samples were very similar to the pat-
terns for isolates from animal fecal samples (Table 8). The only
statistically significant difference found when we compared the
rates of resistance to individual agents between all fecal sam-
ples and all farm environment samples combined was higher
levels of resistance to cephalothin in farm environment sam-
ples (Table 8). When samples were classified by the species of
animals living in the environment, significant differences were
found. Swine fecal samples had higher levels of resistance to all
TABLE 1. Concentrations and diffusion zone breakpoints for
resistance for antimicrobial agents tested in this study, sorted by
class of antimicrobial agent
Antimicrobial agent
Drug
code
Disk drug
concn (?g)
Diffusion zone
breakpoint (mm)
Aminoglycosides
Neomycin
Gentamicin
Streptomycin
N30
GM10
S10
30
10
10
?12
?12
?11
Phinicols
Chloramphenicol C30 30
?12
Quinolones and fluoroquinolones
Ofloxacin
Nalidixic acid
OFX5
NA30
5
?12
?1330
Sulfonamides and potentiated
sulfonamides
Sulfamethoxazole-trimethoprim
Sulfisoxazole
STX
G25
23.75–1.25
250
?10
?12
Tetracyclines
TetracyclineTE30 30
?14
Beta-lactams
AmpicillinAM1010
?13
Nitrofurans
Nitrofurantion F/M 300 300
?14
Cephalosporins
Cephalothin CF3030
?14
1396SAYAH ET AL.APPL. ENVIRON. MICROBIOL.

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of the antimicrobial agents except cephalothin. Cattle farm
environment samples had significantly lower levels of resis-
tance to tetracycline and sulfisoxazole than other farm envi-
ronment samples. The patterns observed for disk diffusion
zones for farm environment samples sorted by animal species
were very similar to the patterns observed for levels of antimi-
crobial agent resistance; swine isolates showed reduced sus-
ceptibility to most drugs.
We also examined the patterns of antimicrobial agent resis-
tance on farms with sufficient numbers of different animal
species to determine whether there were any common patterns
of resistance between species. Of the 31 farms used, 4 had
sufficient numbers of different species (at least 10 species).
Figure 2 shows the proportions of isolates with antimicrobial
agent resistance for a farm that housed swine and poultry. As
Fig. 2 shows, strains from both species showed high levels of
resistance to tetracycline and streptomycin and similar levels of
resistance to sulfisoxazole, and cephalothin.
Multidrug resistance was evaluated with E. coli isolates (Ta-
bles 9 and 10). The majority of E. coli isolates tested (52.33%)
were sensitive to all antimicrobial agents tested, 34% were
FIG. 1. Farm and surface water sampling locations in the Red Cedar watershed. The map was created by using USGS watershed data from the
Michigan Center for Geographic Information (http://www.mcgi.state.mi.us/mgdl/?rel?thext&action?thmname&cid?3&cat?Watersheds).
TABLE 2. Samples collected and E. coli isolates recovered
Species
Fecal samples
Farm environment
samples
No.
collected
% with
E. coli
No.
collected
% with
E. coli
Beef cattle
Dairy cattle
All cattle
351
438
789
51.57
52.28
51.96
110
131
118
56.36
42.75
48.96
Sheep
Goats
All small ruminants
266
35
301
61.13
17.43
59.77
59
6
65
52.54
83.33
55.38
Equids
Swine
Poultry
120
327
204
50.0
54.43
44.61
53
93
59
32.08
40.86
35.59
Farmed deer 6056.67 NCa
Humans
Companion animals
34
38
8.82
60.53
NC
NC
Wild geese97 56.70 NC
aNC, not collected.
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resistant to one or two antimicrobial agents, and 13% were
resistant to three or more agents. The highest levels of multi-
drug resistance were found in swine, and no multidrug resis-
tance was seen in farmed deer or wild geese (Table 9). The
majority of multidrug resistance combinations included tetra-
cycline resistance (Table 10). The combination tetracycline
resistance and sulfamethazine resistance was found in 12% of
all isolates and in more than one-half of all multidrug-resistant
isolates (Table 10).
DISCUSSION
Similar patterns of resistance of E. coli were found for ani-
mal fecal and farm environment samples classified by animal
species, suggesting that there were common sources of resis-
tant bacteria (Table 4). Livestock functioned as a reservoir of
resistant bacteria for environmental contamination, particu-
larly in cases where higher levels of resistance were seen in
fecal isolates than in farm environment isolates (all antimicro-
TABLE 3. Antimicrobial agent use reported on farms expressed as percentages of animals receiving treatments for farms reporting
treatments, by animal type
Antimicrobial agent
% of animals
Reported use
(n ? 438)
Dairy cattle
(n ? 131)
Beef cattle
(n ? 89)
Swine
(n ? 178)
Sheep
(n ? 17)
Aminoglycoside
Streptomycin 9.6032.82 0.0 0.00.0
Quinolones and fluoroquinolones
Enrofloxacin0.45 0.02.53 0.00.0
Sulfonamides and potentiated sulfonamides
Sulfamethazine
Sulfamethoxazole-trimethoprim
16.07
0.22
20.61
0.76
55.70
0.0
0.0
0.0
0.0
0.0
Tetracyclines
Tetracycline
Chlortetracycline
Oxytetracycline
13.92
30.13
14.29
12.66
21.37
16.79
0.0
55.70
16.46
0.0
30.34
8.99
0.0
41.18
58.82
Beta-lactams
Penicillin
Ampicillin
Cloxacillin
86.08
0.89
0.67
60.76
3.05
2.29
2.53
0.0
0.0
20.25
0.0
0.0
0.0
0.0
0.0
Cephalosporins
Ceftiofur6.03 19.85 1.270.0 0.0
Macrolides
Tilmicosin 8.700.76 27.858.430.0
Bacitracin
Bacitracin10.940.0 0.027.530.0
TABLE 4. Percentages of isolates exhibiting antimicrobial agent resistance, by type of sample
Antimicrobial agent
% of isolates
Animal fecal
(n ? 1,037)
Farm environment
(n ? 230)
Human septage
(n ? 3)
Surface water
(n ? 26)
Overall
(n ? 2,552)
Neomycin
Gentamicin
Streptomycin
Chloramphenicol
Ofloxacin
Sulfamethoxazole-trimethoprim
Tetracycline
Ampicillin
Nalidixic acid
Nitrofuratoin
Cephalothin
Sulfisoxazole
4.72
0.77
13.21
1.06
0.19
2.22
28.06
5.59
0.67
0.87
20.54
13.98
3.91
1.30
13.04
1.30
0.0
3.47
25.65
4.35
0.0
0.43
30.43
11.30
0.0
0.0
33.33
0.0
0.0
0.0
33.33
0.0
0.0
0.0
33.33
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
80.6
0.0
4.51
0.86
13.06
1.09
0.16
2.41
27.29
5.29
0.54
0.78
22.71
13.30
1398 SAYAH ET AL.APPL. ENVIRON. MICROBIOL.