CITATION:
Levy SB. 1997. Antibiotic resistance: An ecological
imbalance. In Antibiotic Resistance: Origins, Evolution,
Selection and Spread edited by DJ Chadwick, J Goode.
West Sussex, England: Wiley, Chichester (Ciba Foundation Symposium 207), pp. 1-14. (ISBN 0471 97105 7).
The symposium on Antibiotic Resistance: Origins, Evolution, Selection and Spread was held at the Ciba Foundation,
London, July 16-18 1996.
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the Table of Contents along with abstracts from this book |
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Antibiotic Resistance: An Ecological
Imbalance
Stuart B Levy, MD
Center for Adaptation Genetics & Drug Resistance, Tufts University School of Medicine, Boston, Massachusetts,
USA
Abstract
Antibiotic resistance thwarts the treatment of infectious diseases worldwide. Although a number of factors can
be identified which contribute to the problem, clearly the antibiotic as a selective agent and the resistance gene
as the vehicle of resistance are the two most important, making up a 'drug resistance equation'. Both are needed
in order for a clinical problem to arise. Given sufficient time and quantity of antibiotic, drug resistance will
eventually appear. But a public health problem is not inevitable if the two components of the drug resistance equation
are kept in check. Enhancing the emergence of resistance is the ease by which resistance determinants and resistant
bacteria can spread locally and globally, selected by widespread use of the same antibiotics in people, animal
husbandry and agriculture. Antibiotics are societal drugs. Each individual use contributes to the sum total of
society's antibiotic exposure. In a broader sense, the resistance problem is ecological. In the framework of natural
competition between susceptible and resistant bacteria, antibiotic use has encouraged growth of the resistant strains,
leading to an imbalance in prior relationships between susceptible and resistant bacteria. To restore efficacy
to earlier antibiotics and to maintain the success of new antibiotics that are introduced, we need to use antibiotics
in a way which assures an ecological balance that favors the predominance of susceptible bacterial flora.
In large part, bacteria live in harmony with other inhabitants of
the earth. Although some infections are caused by bacteria for which humans are a specific host, in most instances
the infections follow entry of bacteria into the body by chance. Over the past 50 years, the classic treatment
of bacterial infectious diseases has been antibiotics, the discovery of which vastly changed the relationship between
bacteria and people. Today we are witnessing another change, that is, among the bacteria themselves.
While diversity characterizes the microbial flora, antibiotic use
has led to a further subgrouping into those bacteria that are susceptible and those that are resistant to antibiotics.
Prior to antibiotic introduction, the large majority of commensal and infectious bacteria associated with people
were susceptible to these agents. Over the ensuing five decades, the mounting increase in the use of antibiotics,
not only in people, but also in animals and in agriculture, has delivered a selection unprecedented in the history
of evolution (Levy 1992). The powerful killing and growth inhibitory effects of antibiotics have reduced the numbers
of susceptible strains, leading to the propagation of resistant variants. These have eventually evolved into prominent
members of the microbial flora. The antibiotic susceptibility profile of bacteria on the skin of people today,
and in the environments of hospitals and homes, is very different from what it was in the pre-antibiotic era, and
even 10 years ago. Multidrug resistance is commonly found in bacteria which cause infections as well as in commensal
organisms which colonize our intestinal tract, skin and upper respiratory tracts. The resistant bacteria are the
survivors of the antibiotic selection which has been taking place within various segments of society.
Microbes circulate everywhere, and there is a continual exchange
among the different human, animal and agricultural hosts. We do not know which bacteria are resistant and which
are susceptible. As suggested, it would be very helpful if we had a system by which we could see resistant bacteria
in different colors, distinguishing them from susceptible bacterial populations (O'Brien & Stelling 1995).
We could then determine the environments needing remediation, i.e. a return of susceptible flora.
Antibiotics are unique therapeutics. They treat more than just the
individual. They treat the environment and in that way they affect society. This characteristic of antibiotics
is why today's society is facing one of its gravest public health problems; numerous infectious bacteria with resistance
to many, and in some cases to all, available antibiotics. Antibiotic resistance exemplifies par
excellence Darwinism: surviving strains have emerged under
the protection and selection by the antibiotic. Use of the same antibiotics in all parts of the world has led to
the emergence of resistant bacteria that find ready havens for propagation wherever they move.
Antibiotics have also revealed the genetic fluidity of bacteria
in terms of their ability to exchange genetic traits among genera and species which are evolutionarily millennia
apart. Antibiotic resistance genes on plasmids and transposons flow to and from Gram-positive and Gram-negative
bacteria and among bacteria which inhabit vastly different ecological niches.
In assessing the antibiotic resistance problem, we can identify
a number of factors which have contributed and continue to impact on the emergence of resistance. The leading two
are the antibiotic itself and the resistance determinant. They make up what I have called the 'drug resistance
equation.' (Fig. 1) (Levy 1994) The two entities ebb and flow to affect the magnitude of the clinical drug resistance
problem. If either is absent, a drug resistance problem will not emerge; but given the presence of both the antibiotic
and a resistance trait, drug resistant bacteria will be selected and propagated. To these two factors, we can add
spread of resistant bacteria themselves and the cell to cell spread of the resistance traits. It is no wonder that
an environment can become rapidly populated with different kinds of resistant bacteria.
Antibiotics and the emergence of resistance: the selection density
Antibiotics were initially developed for the treatment of infectious diseases in people. Their miraculous effects
led to their being solicited and used for the treatment of animals and eventually plants. The same ones are being
used in all three areas. Thus, an enormous worldwide selective pressure has occurred. Antibiotics are used both
internally and externally to control bacterial problems for society, maintaining the health of people, animals
and agricultural crops. If different antibiotics had been chosen for animals and agriculture than those used in
people, we might be witnessing a lower level of resistance today. But, in fact, with each ensuing year, 4ó5%
more antibiotics have been produced, developed and used. In the USA alone, an estimated 160 million prescriptions
for antibiotics were written last year and over 50 million pounds were produced for use in people, animals and
agriculture.
There are two major effects of an antibiotic: therapeutically, it
treats the invading infectious organism, but it also eliminates other, or non-disease producing, bacteria in its
wake. The latter do, in fact, contribute to the diversity of the ecosystem and the natural balance between susceptible
and resistant strains. The consequence of antibiotic use is, therefore, the disruption of the natural microbial
ecology. This alteration may be revealed in the emergence of types of bacteria which are very different from those
previously found there, or drug resistant variants of the same ones that were already present. The dominance acquired
by these new strains in the treated environment is directly linked to the intrinsic or acquired resistance to the
antibiotics being used.
To a large extent, the reversibility of the selection process is
dependent on repopulation by the original susceptible bacteria. Their residual numbers will be related to the total
amount of selective drug used in that environment. This relationship suggests that it is the density of the antibiotic,
i.e. the total quantity applied, the number of individuals (people, animals, plants) treated, and the size of the
geographic area affected, which quantitatively and qualitatively affects microbial ecology. This concept translates
directly into a 'density' selection process which affects that ecology (Fig. 2). The introduction of an antibiotic
into an environment has the eventual effect of killing-off most, if not all, of the resident susceptible strains.
Any resistant survivors will then have a chance to propagate and take over. But adjacent to that selective environment,
and encroaching on it, are untreated, susceptible strains which are still potential competitors for the treated
area, if given the opportunity. The size of the area selected for resistance will be related to the total amount
of antibiotic used and the geographical extent of its influence. It further relies on the potential for susceptible
strains to return after the selective event. One would not expect the same ecological effect if a hundred pounds
of antibiotic were distributed among 20 animals as compared to 20 000 animals. As long as the dosage of antibiotic
is above its growth-inhibitory concentration, a greater effect will be seen in the larger numbers of animals being
treated; as they will be propagating a thousand times more resistant bacteria. Likewise, the ecological effect
of two individuals treated in one room will be different from two being treated in two different rooms or homes.
The selection of antibiotic resistance is, therefore, greatly affected by the numbers being treated as well as
the size of the treatment area and the numbers of susceptible bacteria surviving the treatment.
Genetics of drug resistance and spread
The emergence of resistant bacteria raises concern about the bacteria and their progeny and also the extent that
they can spread to other environments. The bacterium itself is the focus, if the resistance trait is linked solely
to that bacterium and cannot be shared by others. This is, however, not the case with most resistance traits in
the majority of bacteria. They have evolved extrachromosomal replicating genes called plasmids and their associated
transposons which allow rapid and very broad dissemination of genes (Fig. 3). Gene transfer crosses species and
genus barriers (DeFlaun & Levy 1989). Thus, resistant enterococci selected in one environment can pass resistance
genes not only to other members of their own genus and species but also to other organisms in other genera. Staphylococci
share their plasmids with Listeria; E. coli can share
genes with other members of the Enterobacteriaceae as well as the pseudomonads and Neisseria, just to mention a few. In fact, the same tetracycline resistance determinants
can be found among Gram-positive and Gram-negative bacteria as well as in the mycobacterium (Roberts 1997, this
volume). The genetic flexibility and versatility of bacteria have therefore contributed largely to the efficiency
by which antibiotic resistance has spread among bacteria and among environments globally. However, it is equally
evident that the transfer event has no consequence unless the antibiotic selection is there. Thus, the emergence
and maintenance of bacterial resistance relies on the interrelationship between the resistance determinant and
the antibiotic.
Reversal of resistance
Data on the reversal of the resistance selection offer further insights into the selection process. The fecal flora
of a volunteer, myself, taking tetracycline for five days was examined. Initially tetracycline resistance was present
at a low level; it peaked within two days of tetracycline use. After five days, tetracycline was stopped, but the
resistance frequency declined very slowly. The rate of loss did not mimic the rate of gain of resistance: it took
15 days to return to the initial pre-antibiotic level (Levy 1986). Antibiotics are so powerful that they provide
rapid selection for a new resistant breed, but when you remove the antibiotic, a reversal is slow in coming. The
resistant bacteria selected by tetracycline are no less 'fit' than the susceptible flora; hence they continue to
propagate and persist.
We did similar studies among chickens excreting E. coli with multi-resistance
plasmids. They did not lose the E. coli,
despite multiple cleanings of the cage over several months (Levy 1986). However, this was a closed environment,
and there was no easy route of entry for susceptible strains. Moreover, the resistant bacteria were clearly not
disadvantaged by bearing resistance. When the cages were relocated to different sites around the barn, the surrounding
environment was altered and the chickens' flora slowly returned to a more susceptible one (Levy 1986). In another
study, we added four chickens excreting a resistant flora to 10 other chickens excreting a susceptible flora. Resistance
was lost; the susceptible flora won out. For an immediate change in resistance frequency, the result relies on
numbers, not large differences in bacterial fitness. Moreover, there is no active counter-selective force which
propels repopulation with susceptible strains.
In the short term, the resistant bacteria were not less fit than
the susceptible ones, so we did not observe a rapid shift from resistance to susceptibility. However, in the long
term such changes have been documented in hospitals (Giamarellou & Antoniadou 1997, this volume) and on farms
(Levy et al 1976) when antibiotics have been removed. But, it takes time. In some instances, a newly gained plasmid
is not stably kept in its new host. Early on, this instability will help in reversing the resistance. However,
with time, the plasmid and bacteria may develop a synergistic relationship whereby both are needed for growth,
demonstrating a phenomenon to be discussed later in this symposium (Lenski 1997, this volume). Still, the evidence
suggests that, given a 'ready and willing' susceptible flora, a resistance predominance can be overturned if antibiotics
are removed.
The resistance reservoir
Resistance genes reside not only in disease-causing organisms, but in commensal organisms as well. These normally
harmless bacteria, such as E. coli
or enterococcus, can cause a fatal illness if the person is immunocompromised. Moreover, these bacteria harbor
resistance genes which can spread to the bacterial strains that do cause infection. Unfortunately, these reservoirs
are not being examined very much.
People today harbor many multidrug resistant bacteria. In a study
of fecal flora from an ambulatory community, we found that 40% of people on antibiotics carried two or more resistances
in 10% of their E. coli;
25% had three or more resistances, and 10% had four or more (Levy et al 1988). People excrete resistant E. coli at the 50%
level, even when not consuming antibiotics (Levy et al 1988). High carriage levels of resistant fecal flora have
been reported from Holland (Bonten et al 1992), and elsewhere (Calva et al 1992, Leistevuo et al 1996). Resistant
bacteria are plentiful in the environment, providing evidence for an environment in a state of imbalance. While
not necessarily inflicting harm, they certainly reflect a significant selection process.
One source of resistant bacteria is food. A large number of drug
resistant Gram-negative bacteria are associated with uncooked foods (Levy 1984). In the great majority of instances
these bacteria pose no health problem. But they too tell us a lot about the environmental imbalance. A study from
France assessed the contribution of food bacteria to the intestinal flora by examining the same volunteers when
eating normal or sterilized food (Corpet 1988). Tetracycline resistance in the fecal flora was high when the volunteers
were eating normal, non-sterilized food for 21 days, but dropped dramatically when the diet was shifted to sterilized
food for 17 days (Table 1).
Besides selecting resistant variants, antibiotics can affect the
ecology by changing the types of organisms there. New opportunistic infectious disease agents, intrinsically resistant
to the antibiotic in use, can emerge and predominate. For instance, the use of second and third generation cephalosporins
in hospitals, introduced for Gram-negative bacteria, selected the normally harmless enterococcus, which is intrinsically
resistant to these antibiotics. The enterococci, selected by these drugs, have now become prominent members of
the hospital acquired flora. Moreover, the organism has emerged with its own multiplicity of resistances, e.g.
to aminoglycosides and vancomycin. It is a likely potential donor of vancomycin resistance to the staphylococcus.
Replacement of an endogenous flora with a new flora as a consequence of antibiotic use is an important concept
that is too often disregarded. It has a significant impact to our health. If one is thinking about using an antibiotic
to target the disease-causing organism, which, of course, is the magic of these drugs, one has to think about the
other bacteria as well. If the antibiotic's sphere of influence is large, then its ecological effect will be large.
As we widen antibiotic usage from the individual to the hospitals and the community, we see more and more effect
on the susceptible strains. Some have talked about spraying hospital rooms with susceptible commensal organisms
to replace and compete with the disease agents. It is an approach worth considering. Overall, let's focus not just
on the antibiotic, but also on the susceptible flora. Susceptible bacteria should be our teammates in confronting
and reversing the resistance problem.
Why all the current publicity?
Why has so much recent attention been given to a field that some of us in this room have been working in for decades?
Many journalists writing about it are directed by a personal experience. Many of these writers, or their editors,
have children who have, or have had, ear infections or other infections that did not respond to antibiotics. The
pneumococcus, whether the real culprit or not, has clearly brought the drug resistance issue to public awareness.
Not just the kids are suffering, but the parents, as well, because they cannot fulfill their job obligations having
to stay home with a sick child.
Besides the pneumococcus, there are other resistant bacteria confronting
society at large. The tubercle bacillus, which causes tuberculosis, is multidrug resistant and, in some patients,
incurable. The gonococcus, the agent of gonorrhoeae and a community acquired infection, is now resistant to penicillin,
tetracycline, quinolones and some strains show early signs of resistance to cephalosporins. Few if any options
remain after the cephalosporins. This is a societal problem. Imagine what's going to happen when we lose our ability
to rapidly treat this organism. The staphylococcus can only reliably be treated with vancomycin. To these can be
added Pseudomonas aeruginosa, Acinetobacter and other bacterial disease agents, all thwarting therapy by resistance.
The decade of the 1990s is unique. Resistance is no longer confined to hospital environments, but is now common
in community populations worldwide. As important, this crisis is heightened by a lack of new antibiotics developed
during the decade.
Approaches to the problem
No novel antibiotic is expected to appear soon, and an increasing
number of bacterial infectious agents bear resistance to many if not all antibiotics. We must somehow find a means
to reverse the ecological imbalance that has occurred in terms of resistant and susceptible strains. One way is
to remove or adjust the selection process so as to allow the susceptible strains to regain their former dominance.
As demonstrated above, such reversals are possible and provide the necessary optimism. There still are sufficient
susceptible bacteria in our environment which, when given a chance, can return and reestablish the susceptible
flora. The crux for reversing and curbing the resistance problem lies in restoring the susceptible microbial flora,
whether this is in the intestinal tract, the skin, or elsewhere in the environment. To do this, antibiotic use
needs to be more rational. The misconceptions and misunderstandings of antibiotics as miracle drugs without adverse
consequences have led to their inappropriate use and prescription. Education of the prescriber and the consumer
is critical.
In previous decades the pharmaceutical industry has been able to
identify and produce newer and more potent antibacterial agents. However, experience in the present decade indicates
that this is no longer true. Discovery has diminished, although encouraging signs are appearing once more (Service
1995). There are now renewed efforts in large pharmaceutical houses and smaller biotechnology companies to discover
truly novel drugs. These drugs would be those with no structural relationship to prior antibiotics and thus not
intrinsically subject to already existing resistances. This offers one approach towards a solution. Another is
to define sufficiently the resistance mechanism and use it to identify novel drugs which can poison or inactivate
resistance mechanisms and allow the effective antibiotic to work. This is the basis for the success of the combination
of b-lactamase blockers
and an effective b-lactam
drug, initially introduced as clavulanate and amoxicillin by Beecham Pharmaceuticals. It is this same approach
which we are using to restore efficacy to the tetracycline family. Here we are using a semi-synthetic tetracycline
to block a drug efflux, allowing a classical tetracycline to enter and stop growth (Nelson et al 1993, 1994).
The control of the antibiotic resistance problem lies in a better
understanding of how we use antibiotics. Conditions can be envisioned whereby we encourage the re-emergence of
susceptible strains following treatments and the maintenance of the normal susceptible microbial flora between
treatments. We need to restore the original microbial balance between susceptible and resistant bacteria-a balance
which has been devastatingly altered by the inappropriate and continued application of antibiotics to our environments.
References
Bonten ME, Stobberingh E, Philips J, Houben A 1992 Antibiotic resistance of Escherichia coli in fecal samples of
two different areas in an industrialized country. Infection 20:258-262
Calva JJ, Sifuentes-Osornis J, Ceron C 1992 Antimicrobial
resistance of fecal aerobic gram-negative bacilli in different age groups in a community. Antimicrob Agents Chemother
40:1699-1702
Corpet DE 1988 Antibiotic resistance from food. N Eng
J Med 318:1206-1207
DeFlaun MF, Levy SB 1989 Genes and their varied hosts.
In: Levy SB, Miller RV (eds) Gene transfer in the environment. McGraw-Hill, New York, p 1-32
Giamarellou H, Antoniadou A 1997 The effect of monitoring
of antibiotic use on decreasing antibiotic resistance in the hospital. In: Antibiotic resistance: origins, evolution,
selection and spread. Wiley, Chichester (Ciba Found Symp 207) p 76-92
Leistevuo T, Leistevuo J, Osterblad M et al 1996 Antibiotic
resistance of fecal aerobic Gram-negative bacilli in different age groups in a community. Antimicrob Agents Chemother
40:1931-1934
Lenski RE 1997 The cost of antibiotic resistance from
the perspective of a bacterium. In: Antibiotic resistance: origins, evolution, selection and spread. Wiley, Chichester
(Ciba Found Symp 207) p 131-152
Levy SB 1984 Antibiotic resistant bacteria in food of
man and animals. In: Woodbine M (ed) Antimicrobials and agriculture. Butterworths, London, p 525-531
Levy SB 1986 Ecology of antibiotic resistance determinants.
In: Levy SB, Novick RP (eds) Antibiotic resistance genes: ecology, transfer and expression. Cold Spring Hbr Press,
New York, p 17-29
Levy SB 1992 The antibiotic paradox: how miracle drugs
are destroying the miracle. Plenum, New York
Levy SB 1994 Balancing the drug resistance equation.
Trends Microb 2:341-342
Levy SB, Fitzgerald GB, Macone AB 1976 Changes in intestinal
flora of farm personnel after introduction of tetracycline-supplemented feed on a farm. N Engl J Med 295:583-588
Levy SB, Marshall B, Schluederberg S, Rowse D, Davis
J 1988 High frequency of antimicrobial resistance in human fecal flora. Antimicrob Agents Chemother 32:1801-1806
Nelson ML, Park BH, Andrews JS, Georgian VA, Thomas
RC, Levy SB 1993 Inhibition of the tetracycline efflux antiport protein by 13-thio-substituted 5-hydroxy-6-deoxy
tetracyclines. J Med Chem 36:370-377
Nelson ML, Park BH, Levy SB 1994 Molecular requirements
for the inhibition of the tetracyline antiport protein and the effect of potent inhibitors on growth of tetracycline
resistant bacteria. J Med Chem 37:1355-1361
O'Brien TF, Stelling JM 1995 WHONET: a program to monitor
local and global spread of resistance. APUA Newsletter 13/4:1-6
Roberts MC 1997 Genetic mobility and distribution of
tetracycline resistance determinants. In: Antibiotic resistance: origins, evolution, selection and spread. Wiley,
Chichester (Ciba Found Symp 207) p 206-222
Service RE 1995 Antibiotics that resist resistance.
Science 220:724-727
This article was reproduced with permission
from the Ciba Foundation.
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