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APPENDIX E
ANTIMICROBIAL RESIDUES AND RESISTANT ORGANISMS:
THEIR OCCURRENCE, SIGNIFICANCE, AND STABILITY
Stanley E. Katz1
The occurrence of antibiotic residues in animal tissue and
animal products resulting from the subtherapeutic, prophylactic,
or therapeutic use of antibiotics is a function of the antibio-
tic, the method of application, the level of treatment, and,
most importantly, adherence to withdrawal periods. Because of
the extreme variability in both the use of drugs and adherence
to withdrawal periods, it is inevitable that residues would occur
in a small but significant percent of animal products found in
the marketplace.
RESIDUE LEVELS FROM SUBTHERAPEUTIC ANTIBIOTIC USE
Huber (1971) reported the results of a 1969 survey of ani-
mals slaughtered in Illinois, Wisconsin, Iowa, and Indiana. In
27% of the swine slaughtered, there was evidence of recent treat-
ment with antibacterial substances, of which 10% were penicillin.
Because of the lack of visible evidence of injection, these resi-
dues most likely were the result of improper withdrawal times or
levels of usage. There was a 9% rate of positive findings in beef
cattle, 2% of which were attributed to penicillin. In veal, 7% of
17% of the samples in which antibiotics were found resulted from
penicillin. Some 21% of market lambs were positive for antibacter-
ial residues. Of these, 4X were positive for penicillin. The
presence of antimicrobial residues was found in 26% of the chickens,
6% of which were positive for penicillin activity. The incidence
of antibiotic residues in milk during the early 1950's was approxi-
mately 11%, dropping to 0.5% at the time of Huber's report.
Mussman (1975) reviewed the inspection and sampling proce-
dures used in the U.S. Department of Agriculture (USDA) program
to monitor levels of residues in conformance with Food and Drug
Administration (FDA) regulations and analyzed the data contained in
the USDA Biological Residue Report for 1973 (USDA, 1974~. The USDA
figures indicated that 5.3% of 529 carcass samples examined for
residues of streptomycin, tetracycline, erythromycin, neomycin,
oxytetracycline, and chlortetracycline were positive; only 17 of
Department of Biochemistry and Microbiology, Cook College - New
Jersey Agricultural Experiment Station, Rutgers University, The
State University of New Jersey, New Brunswick, N.J.
158
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159
5,301 samples, or 0.32%, were positive for penicillin; and 12
samples of 728, or 1.6%, were positive for sulfonamides. Non-
specific antimicrobial activity was found in 154 (2.9%) of 5,301
samples.
Table 1 summarizes the U.S. Department of Agriculture sampling
program for 1976-1978. Violations exist when residue levels exceed
established tolerances or when residues are found when no residue is
permitted.
If the results of the USDA sampling program are at all repre-
sentative, there is no doubt that violative residues occur in almost
all animals used for food and that residues that are not in viola-
tion occur in even higher percentages. The rates of antibiotic and
sulfonamide residue violations are lowest among poultry and cattle,
and the highest rates occur in swine and in veal calves.
The levels of residues that can be expected from feeding
subtherapeutic quantities of antibiotics vary with the degree of
absorption from the intestinal tract. Because of very poor ab-
sorption from the intestine, residues resulting from the consump-
tion of such antibiotics as streptomycin, neomycin, bacitracin,
and the bambermycins rarely, if ever, occur. Residues from the
subtherapeutic feeding of chlortetracycline (CTC) and oxytetracy-
cline (OTC) occur regularly and have been also reported for peni-
cillin. In chickens, the continuous feeding of 50 to 200 g of CTC
per ton of feed resulted in residue levels ranging from 0.036 to
0.11 fig CTC/g muscle tissue and from 0.058 to 0.199 fig CTC/g
liver tissue (Katz et al., 1972~. These residues disappeared
after 1 day of withdrawal from the medicated feed.
Messersmith et al. (1967) reported residue levels of 0.08 fig
CTC/g muscle and 0.46 fig CTC/g liver resulting from feeding 100 g
CTC/ton to swine. After a 5-day withdrawal, no detectable levels
could be measured in muscle and fat, but trace levels were de-
tected in the liver and kidney. Gale et al. (1967) reported from
0.04 to 0.08 fig CTC/g muscle and from 0.24 to 0.46 fig CTC/g liver
from the continuous feeding of lOO g/ton. After a 5-day withdrawal
period, there were no measurable levels in muscle, and only trace
levels could be found in the liver. OTC residues occurring in
poultry muscle from the continuous feeding of 25 to 200 g/ton were
approximately 1/1,000 of the feeding level. This is similar to
levels resulting from the feeding of CTC. Measurable tissue resi-
dues of CTC disappeared after a 1-day withdrawal period (Katz
_ al., 1972~. Cooking destroyed all residues of both OTC and
CTC in the muscle (Katz et al., 1972, 1973; Meredith et al., 1965~.
The only CTC residues surviving cooking were found in the liver.
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160
TABLE l
Violative Actions for Antibiotics and Sulfonamides
Ant ibiot icViola- SulfonamideViola
Species SamplesLions ~SamplesLions
1976:
Cattle 54571.347640.9
Calves 1, 37 81 188. 63 27123. 7
Sheep and goats7057.110011.0
Swine 2 4741. 61, 4 931419. 4
Chickens 15510.633110.3
Turkeys 2 5800. 06 48162. 5
Geese and ducks16010. 626500. 0
1977:
Cattle 1,739221.317542.3
Calves 1,120464.116653.0
Sheep and goats17 621.11200. 0
Swine 44961.39,4611,24213.1
Chickens 36600.0100. 0
Turkeys 45030~744540~9
Geese and ducks16110. 620610. 5
1978:
Cattle 1,769462.624320.
Calves 1,409946.721462.8
Sheep and goats21052. 440615.0
Swine 1, 399765.46, 6876489.7
Chickens 470 ~1.711910.8
Turkeys 447153.3443194.4
Geese and ducks17521.114800. 0
aFrom U. S. Department of Agriculture, 1977-1979.
1
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161
Residues of OTC and CTC in eggs were not completely destroyed
by cooking. The lower temperatures and shorter times of cooking
were responsible for the survival of the residues.
Filson _ al. (1965) found CTC levels ranging from 0.072
~g/ml in the serum of old birds to 0.330 ~g/~1 in the serum of
young birds that had received 200 g of the antibiotic per ton of
feed. They reported no muscle deposition levels, but it is gen-
erally assumed that muscle residue levels usually correspond to
serum levels.
Another potential source of tetracycline residues, especially
if the kidneys are used as the indicator organ, would be tetracy-
clines released from deposition in the bones. Bruggemann et al.
(1966) noted that the bones of all domestic animals fed tetracy-
cline contain measureable amounts of the antibiotic. After the
feeding of the tetracycline ceases, levels in the bones decrease,
indicating mobilization and excretion.
After feeding pigs lOO fig procaine penicillin per gram of feed,
Loftsgaard et al. (1968) found residues of 2 ~g/ml urine and levels
in muscle no higher than 0.005 ~g/g. Kidney levels ranged from
0.003 to 0.24 Egg, and liver levels were 0.012 agog. After feeding
the unmedicated basal ration for 24 hours, the investigators found
no measurable antibiotic activity. However, residues from intramus-
cular injections of procaine penicillin disappeared by day 6. After
injections of benzathine penicillin G. activity was detected in the
urine and kidney for as long as 14 days.
The feeding of procaine penicillin to broilers and laying hens
did not result in any penicillin activity in the blood, muscle, liver,
and kidney tissue of broilers or in the eggs of hens fed 100 g/ton.
Approximately 98% of the penicillin activity was destroyed in the
upper portion of the intestinal tract, and little or no activity
reached the small intestine (Katz et al., 1974; Messersmith et al.,
1967~. One of the major degradation products was penicilloic
acid.
McCracken (1977) reported that intramuscular injections of 3
mg penicillin and OTC per kilogram of body weight yielded muscle
residue levels less than 0.04 ~g/g and 0.20 agog, respectively.
Surprisingly, no residues in muscle were found after an intramus-
cular injection of neomycin at 100 mg/kg.
These few reports indicate that OTC and CTC residues can and
do appear as a result of subtherapeutic and therapeutic oral feeding.
Residues attributed to the feeding of penicillin were variable, but
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162
tended to be very low or at the level of detectability of the
methodology used. Most of the analytical methods used for peni-
cillin were capable of detecting levels of 0.0125 Agog or less.
FATE OF ANTIBIOTIC RESIDUES
Residues of the tetracyclines in the muscle tissue of ani-
mals will not survive normal food preparation procedures. No
residues will enter the diet of humans unless the muscle tissue
is eaten raw or very rare. Cooking degrades CTC to isochlorte-
tracycline (Shirk _ al., 1957), and OTC is thought to be con-
verted to a- and O-apooxytetracyclines (Katz et al., 1973). The
literature contains no data to indicate that either of these
compounds has any biological significance.
Tetracycline residues remain only in products that are sub-
jected to a minimum amount of cooking. Even in chickens soaked
in CTC solutions to increase shelf life, which resulted in tissue
levels of 3 Agog or greater (Broquist et al., 1956), no residues
could be measured after cooking (Kohler et al., 1955~. However,
if residues do survive cooking, the potential for biological action
remains. After feeding raw chicken meat containing an average of
2.6 Agog CTC to dogs, Rollins et al. (1975) found an increase in
the number of resistant colifonms shed by the dogs.
The metabolic fate of tetracycline has been studied by sev-
eral investigators. Kelly and Buyske (1960) found that tetracy-
cline was metabolically unaltered in the rat and, for the most
part, in the dog. Eisner and Wulf (1963) reported that epichlor-
tetracycline and demethylchlortetracycline were metabolic products
of CTC. The formation of isochlortetracycline in the intestinal
tract of poultry was observed by Shirk et al. (1957~. Katz and
Fassbender (1967) reported that epichlortetracycline and an unknown
highly fluorescent compound was formed in feeds, but that isochlor-
tetracycline was not. These authors suggested that an equilibrium
was established between CTC and its epimer followed by a breakdown
of either the CTC or its epimer into unidentified products.
There is no apparent significance to the presence of tetra-
cycline residues if the product is adequately cooked, unless some
unknown compound forms in the animals fed the antibiotics or the
breakdown products of tetracyclines stimulate the development of
resistance in enteric organisms or the transfer of resistance
determinants between R+ Escherichia cold and recipient E. cold
or salmonellae. In Great Britain, the data indicate that the
destruction of CTC and OTC residues during cooking renders the
meat adequately safe for consumption by humans (Brander, 1970~.
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163
ALLERGIC RESPONSE
The situation is somewhat different with penic it tin resi-
dues. Huber (1971) reported that 10% of the general population
is sensitive to penicillin. McGovern et al. (1970) stated that
from 1% to 10% of the population of North America and Western
Europe may be allergic to it. Parker (1963) and Levine and
Ovary (1961) suggested that degradation products from acid and
alkaline breakdowns could be responsible for hypersensitivity
reactions. Altman and Tompkins (1978) reported that penicillin
degrades, forming benzyl penicilloic acid, a major determinant of
hypersensitivity, and some minor determinants. These degradation
products are haptens that can become antigens when they conjugate
to endogenous protein In viva. Katz et al. (1974) found that
penicilloic acid formed in the intestinal tract of chickens that
had been fed growth promotional and prophylactic levels of peni-
cillin.
Weinstein (1975) estimated that 66% of a dose of penicillin
passed into the intestine of humans where it was inactivated.
Such inactivation would undoubtedly lead to the formation of peni-
cilloic acid and the possible adsorption and deposition of this
compound into muscle tissue.
The use of injectables and the residues resulting from them
can always be a source of unwanted and potentially dangerous
residues. Perhaps the only truly documented death resulting from
residues of penicillin occurred when a butcher consumed fresh pork
that contained 0.31 Agog penicillin (Tscheyschner, 1972~.
As early as 1956 it was known that penicillin residues could
survive pasteurization at 60°C and 71°C, but not at 121°C (Shahani
_ al., 1956~. Katz et al. (1978) reported that penicillin resi-
dues in meat could survive all but the most rigorous cooking.
These investigators measured only penicillin activity and provided
no information concerning the degradation products.
Breakdown Products
The literature does not provide much information regarding
the significance of breakdown products in relation to potential
hypersensitivity reactions from eating products containing resi-
dues and breakdown products of penicillin. Warrington et al.
(1978) found that both the major and minor degradation products
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164
of benzyl penicillin were useful reagents in determining hyper-
sensitivity. Hypersensitivity of one-fifth of the patients that
they tested would have been missed if only the major product had
been tested. Borrie and Barrett (1961) reported the case history
of a woman who had relapses of allergy when she consumed dairy
products. The problem disappeared when the patient received no
commercial dairy products or milk known to be free from penicillin.
Batchelor _ al. (1967) reported that a penicilloylated poly-
mer with a high molecular weight produced immunological responses
in the guinea pig.
Ferrando (1975) reported that 40 units of benzyl penicillin
was sufficient to trigger allergic responses in very sensitive
individuals. He noted that some investigators have suggested that
from 2 to 3 units of penicillin would trigger anaphylactic reac-
tions in sensitive people. From his own calculations and from
the opinions expressed by allergists in France and elsewhere in
Europe, he believes that the risk is "more theoretical than real."
The significance to human health of the residues of peni-
cillin and its breakdown products cannot be determined by review-
ing the literature. However, it is known that the breakdown
products of penicillin do have biological significance ranging
from their ability to sensitize individuals to their action as
selecting agents for the development of antibiotic resistance in
_ colt. Since up to 10% of the population is potentially sensi-
tive to penicillin and its breakdown products, the risk is too
great to be ignored. Oral feeding of subtherapeutic doses of
penicillin should be avoided, and injections should be carefully
controlled.
ROUTES OF ADMINISTRATION
-
Residues of other antibiotics resulting from injections can
be a more serious problem than residues attributed to feeding
because of the higher concentrations that can be deposited and/or
sequestered in the tissue. As was noted previously, the only
documented fatality from hypersensitivity to penicillin was caused
by a penicillin residue that resulted from an injection of the
antibiotic. Gaines _ al. (1978) pointed out that dihydrostrep-
tomycin residues of 2 and 10 Agog, which were observed in bovine
kidneys after intramuscular injection or intramammary infusion,
caused a significant increase in resistance in the enteric bacteria
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165
of animals. Residues of dihydrostreptomycin, unlike those of
tetracycline, are known to be refractory to thermal decomposi-
tion (Inglis and Katz, 1978) and are strong selectors of resistant
organisms (Huber, 1971~. Wilson (1958) reported that streptomy-
cin readily sensitized from 2% to 3% of the nurses who handled it.
Residues of neomycin have been found in serum and urine up
to 144 hours after administration of an intramuscular injection.
Residues of this antibiotic have been reported to be relatively
stable in cooked eggs (Katz and Levine , 1978) and in cooked muscle
tissue of animals (Katz and Levine, 1980, unpublished data).
Residues arising from injectables are a more serious problem
because of the high levels in which they accumulate. Therefore,
every effort must be made to minimize the use of in jectables in
animals going to market.
THE ROLE OF RESIDUES IN THE DEVELOPMENT OF RESISTANCE
It is doubtful that antibiotic residues or their degradation
products will provide any selective pressure on enteric bacteria
contaminating the carcasses of animals. Residue levels are usually
low, contact tome in relation to the number of generations rela-
tively short, the nutritional requirements too variable and usually
insufficient for sustained growth, and the temperature of storage
too low for growth. The selection process requires the inhibition
of susceptible strains, thereby allowing the more resistant strains
to grow. The transfer of resistant determinants between strains
of E. cold would be of low frequency, or nonexistent, for all the
aforementioned reasons. The resistant organisms contaminating
the carcasses come from the environment of the slaughterhouse
itself--not from the residues that may be present.
THE EFFECT OF REGULATION ON RES IDUES
Enforcement of FDA regulations controlling tolerances for
antibiotic residues is a function of the perceived seriousness of
the problem and the amount of money that regulatory agencies budget
for this purpose. It is both impossible and impractical to design
a program to insure that every animal going to slaughter and every
gallon of milk is free of residues. The USDA sampling program,
which results in the Biological Residue Reports (e.g., USDA, 1977-
1979) is the first line of information allowing regulatory officials
to define a problem and marshal! efforts to solve it. Practices
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166
that can result in residues, such as ignoring withdrawal times,
the use of antibiotics beyond the period of effective growth pro-
motion, the subtle sales approach used by the drug companies
suggesting that continuous usage could be an insurance policy
protecting against carcass condemnations, should be modified.
There is no doubt that strict adherence to proper usage and
adherence to withdrawal periods, which are both stimulated by
increased regulatory activity, will minimize the residue problem.
This regulatory approach was successful in lowering the incidence
of residues in milk from 11% to 0.5% (Huber, 1971) and appears
to be having similar success with the sulfonamide residues in
swine. Only through monitoring and changes in agricultural prac-
tice can the problem be minimized. State Agricultural Experiment
Stations, through their research and extension arms, should pro-
vide the farmer with the knowledge to develop proper agricultural
practices. The pharmaceutical industry should sell farmers only
effective drugs and stipulate adamantly the requirements for their
use. Considering the size and complexity of the enforcement pro-
blem and the limits of the laboratory and inspection staffs, regu-
latory efforts are reasonable.
EXCRETION OF ANTIBIOTICS
Antibiotics that are not absorbed, not metabolized or are
free or bound in a conjugated form are excreted in the feces and
urine. Antibiotics such as streptomycin, which are not readily
absorbed, are excreted in their active form (Huber, 1971~. Neo-
mycin, like streptomycin, is poorly absorbed in the gut and is
excreted unchanged in the feces (Weinstein, 1975~. Bacitracin is
poorly absorbed from the intestinal tract, but a large percentage
of the dose is destroyed in the intestinal tract (Scud) et al.,
1947).
Many different antimicrobials are absorbed in varying amounts
from the intestinal tract. Those that are most easily absorbed,
such as the tetracyclines, are excreted in both urine and feces.
Huber (1977) estimated that from 10% to 25% of an oral dose of the
tetracyclines is excreted in the feces. Elmund et al. (1971) ob-
served that 75% of the CTC ingested by yearling steers was excreted.
Alderson et al. (1975) reported that 217 of the OTC fed to sheep
was recovered from the feces. In the accumulated feces of feedlots,
the level of tetracyclines extracted was 75% of that fed (Elmund et
_ ., 1971~. Webb and Fontenot (1975) reported that the litter of
broilers contained levels of OTC ranging from 5.5 to 29.1 Agog
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167
(average, 10.9 ~g/g); CTC ranging from 0.8 to 26.3 ~g/g (average,
12.5 Agog); penicillin ranging from O to 25 ~g/g (average 12.5
Agog); bacitracin levels ranging frog 0.16 to 36.0 Agog (average,
12.3 ~g/g); and arsenic residues ranging from 1.1 to 59.7 Agog
(average 40.4 Agog). They found no residues of neomycin.
Arsenicals are excreted unchanged and can be recovered from
feces at a level of 0.1 Agog after 12 days of ingesting the com-
pound in doses of 90 g/ton (Overby and Frost, 1960~. Morrison
(1969) reported a rather constant level of arsenic in soil after
20 years of fertilization with poultry litter. This finding may
be related to metabolism and microbial conversions. Arsenicals,
such as 3-nitro-4-hydroxyphenylarsonic acid, are partially reduced
to the 3-amino product. Moreover, arsanilic acid residues can be
metabolized to arsenate in soils (Woolson, 1977~.
PERSISTENCE OF ANTIMICROBIALS IN SOILS
.
In agricultural areas, residues of antimicrobials can be
leached from soils and pastures into streams. Van Dijck and Van
de Voorde (1976) reported that 12 of 41 water samples showed in-
hibitory activity against Staphylococcus aureus, but believed that
the low activity in the water samples posed no hazard. Unfortu-
nately, they made no attempt to correlate this inhibition with the
types of antimicrobials used in the area.
There is a paucity of definitive information on the degrada-
tion of antibiotics and antimicrobials in the environment. Although
Jefferys (1952) observed that penicillin is readily inactivated in
garden soils, the FDA (1978) reported that no information was availa-
ble on the time required for the inactivation in soils or in wastes
from animals. Streptomycin forms such strong complexes with clays
that it can be irreversibly inactivated at low levels (Pinck et al.,
1961~. Soulides et al. (1961) observed that bioactive streptomycin
can be desorbed, and Pramer and Starkey (1951, 1972) reported that
it could be microbiologically decomposed in soil.
The FDA (1978) summarized the information on the stability of
tetracyclines in the environment. In soils, where as much as 5
tons of waste from cattle feedlots was applied per acre as a fer-
tilizer, no measurable CTC activity could be detected. Rums ey et
al. (1975) reported that measurable levels of CTC were not found
In most run-off water from pastures on which wastes containing that
antibiotic had been spread. Little pertinent information has been
reported for OTC. Metcalf (1976) reported that sulfamethazine was
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168
biodegradable in a 33-day terrestrial-aquatic model ecosystem,
but he provided no data on the stability of the compound in soils
and in mixtures of feces and soil.
From the data available it is difficult to make an accurate
assessment of the persistence of antibiotics in the environment.
We know that antibiotics degrade in soils, but must learn
how quickly they do and what effects they have on the bacterial
populations in the soil. Antibiotics are added to the soil in a
mixture of feces, urine, and bedding that probably does not ex-
ceed 50 tons per acre. Therefore, this mixture accounts for no
more than 5% of the top 15-cm layer of soil. Based on the aver-
age amount of antibiotics in broiler litter ~ ~ 12.5 ~g/g) (Webb
and Fontenot, 1975), the level of antibiotic in the soil would be
approximately 0.6 Agog. Because of the absorptive reactions, the
low concentrations of antibiotics, and the variety of organisms
available to act upon the antibiotic molecules, it is doubtful
that the antibiotics will persist for any great length of time
in the soil.
Will low levels of antibiotics added to the soil cause the
development of antibiotic-resistant populations? There is no
general answer. It is doubtful that penicillin will survive
sufficiently long in soil to act as a selective agent (Jefferys,
1952~. Jefferys also reported that streptomycin was absorbed
irreversibly into soil at relatively low concentrations and
became inactivated. Hence, streptomycin would be an unlikely
selective agent. The tetracyclines can survive microbial
degradation in the gut of animals and be present in fresh ma-
nure at a concentration of 14 ~g/g and at 0.34 Agog in aged
manure (Elmund et al., 1971~. The degradation of CTC in soil
fertilized with cattle feedlot manure was indicated by a find-
ing of no measurable CTC (Rumsey et al., 1975~. Although
soluble, weakly absorbed, and potentially mobile in the soil,
it is doubtful that the tetracyclines will be a strong selec-
tive agent.
SHEDDING OF RES ISTANT ORGANISMS
Animals fed antibiotics shed antibiotic-resistant organisms
(Smith, 1969; Smith and Crabb, 1957~. The longer they are fed
antibiotics, the greater the percentage of antibiotic-resistant
organisms that will be shed. For the most part, the ubiquitous
_ cold has been used as an indicator organism because it is
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171
Anderson (1974) found that the number of organisms carrying
R factors declined faster in the gut than did sensitive strains
and that they may be at an ecological disadvantage in nature in
the absence of selection pressure from antibiotics. He suggested
that the rapid decline in the resistant organisms resulted from
an impaired vitality of the organisms rather than the loss of R
factors from the cell.
Although there is a difference of opinion regarding whether
resistant strains have a selective advantage for survival in the
environment over nonresistant strains, the evidence in favor of
their not having a selective advantage is more compelling.
Anderson's observations of a decline in the number of antib~otic-
res~stant organisms in some populations, Grabow's comparison of
survival in various waters, and Smith's observations all point to
the fact that there are no easily measurable differences in sur-
vival rates in the environment between resistant and nonresistant
_ cold strains. If anything, there might be a slight advantage
to the antibiotic-sensitive E. cold because of the observations
-
that resistant populations decline and are replaced by nonresist-
ant strains over an extended period. However, there is one in-
escapable fact: because both antibiotic-resistant and sensitive
strains survive for long periods in the environment, changes of
populations will be slow.
The source of the relatively high incidence of antibiotic-
resistant E. cold in sewage, effluents, and streams can never be
established. Only where the sources of the organisms are easily
discernible can a considered judgment be made.
Smith (1970) concluded that human beings rather than animals
are the main source of antibiotic-resistant E. colt. He found
that the incidence of antibiotic-resistant organisms was generally
low in rivers and canals that flowed through rural areas and high
in rivers that flowed through urban areas. In waters with the
highest incidence of antibiotic-resistant organisms, inadequately
treated sewage was the source. No antibiotic-resistant E. cold
were found in a river before it entered predominantly urban areas.
Feary _ al. (1972) concluded that the high incidence of re-
sistance to streptomycin and tetracycline among coliforms isolated
from waters near livestock feedlots were probably a direct result
of the use of antibiotics in the feed. Hughes and Meynell (1974)
indicated that there has been an increase in antibiotic-resistant
coliforms in rivers since 1970. Popp (1974) reported that thorough
treatment of sewage failed to remove salmonellae and that there
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172
were considerable numbers of the organisms in the water into which
the treated sewage flowed. Morse and Duncan (1974) concluded that
the aquatic environment was a favorable environment for the sur-
vival of salmonellae.
The above data seem to indicate that human beings rather than
animals are the main source of antibiotic-resistant E. cold in the
environment.
CONCLUSIONS
namely:
Several conclusions can be drawn from literature reviewed,
(1) The incidence of antibiotic residues in animal products
will continue at a relatively modest rate regardless of surveys
and enforcement procedures. The current surveillance procedures
are extremely effective in identifying problem areas, but no regu-
latory system can guarantee a residue-free food supply.
(2) Residues of oxytetracycline and chlortetracycline will
usually not enter the human diet fin less the foods containing these
residues are subjected to minimal cooking times and temperatures.
Hence, such residues should not be considered a serious factor in
the development of resistance among intestinal microorganisms.
The thermal degradation products of the tetracyclines are not
known to have any biological effects.
~ 3) Penicillin residues and related breakdown products are
not innocuous and have the potential of causing hypersensitivity
reactions. Because approximately 10% of the population is potent-
ially sensitive to penicillin and its breakdown products, the sub-
therapeutic feeding of penicillin should be avoided and the use
of injectable forms carefully controlled.
(4) Antibiotic residues and/or their degradation products
are doubtful agents for the selection of antibiotic-resistant
organisms or as promoters of resistance transfer on the carcasses
of animals. Resistant organisms result from contamination by in-
testinal contents during the handling of the carcasses.
(5) Although there is a paucity of definitive information,
the available data indicate that antibiotics are not stable in
the soil and should not act as a selective agent for the develop-
ment of antibiotic resistance among microorganisms in the soil.
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173
(6) Antibiotic-resistant microorganisms shed by animals per-
sist for extended periods in the environment but do not appear to
have any superior advantage for survival as compared to sensitive
strains.
(7) Although animal agriculture contributes drug-resistant
species to the environment, especially in rural areas, human be-
ings rather than animals appear to be the main source of antibio-
tic-resistant organisms in the environment.
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174
REFERENCES
Ahart, J. G., G. C. Burton, and D. C. Blenden. 1978. The influ-
ence of antimicrobial agents on the percentage of tetracycline-
resistant bacteria in faeces of humans and animals. J. Appl.
Bacteriol. 44:183-190.
Alderson, N. E., W. Knight, R. Robinson, J. Colaianne, and P. Bradley.
1975. Ovine absorption and excretion of oxytetracycline. J.
Anim. Sci. 41:388-399 (Abstract).
Altman, L. C., and L. S. Tompkins. 1978. Toxic and allergic man~-
festations of antimicrobials. Postgrad. Med. 64:157-167.
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Representative terms from entire chapter:
antibiotic resistance
