Вы находитесь на странице: 1из 7








Antibiotic resistance, public health


Derechos reservados, Copyright 2003:

Asociacin Mdica del American British Cowdray Hospital, AC

Otras secciones de
este sitio:

Others sections in
this web site:

ndice de este nmero

Ms revistas

Contents of this number

More journals



Trabajo de revisin


Vol. 48, Nm. 1
Ene. - Mar. 2003
pp. 42 - 47

Antibiotic resistance, public health problem

Jos Antonio Rivera-Tapia*



Antibiotic resistance has become a major clinical and public health

problem within the lifetime of most people living today. Confronted by increasing amounts of antibiotics over the past 50 years, bacteria have responded to the deluge with the propagation of progeny
no longer susceptible to them. While it is clear that antibiotics are
pivotal in the selection of bacterial resistance, the spread of resistance genes and of resistant bacteria also contributes to the problem. Selection of resistant forms can occur during or after antimicrobial treatment, antibiotic residues can be found in the environment for long periods of time after treatment. Beside antibiotics,
there is the mounting use of other agents aimed at destroying bacteria, namely the surface antibacterials now available in many household products. These too enter the environment. The stage is thus
set for an altered microbial ecology, not only in terms of resistant
versus susceptible bacteria, but also in terms of the kinds of microorganisms surviving.

La resistencia a antibiticos representa uno de los principales problemas de la poblacin en lo referente a salud pblica y a la prctica clnica. Esto es debido al incesante incremento en el uso de antibiticos durante los ltimos 50 aos, adems de que las bacterias
resistentes a los antibiticos se han propagado de forma abrumadora. Es importante sealar que hace tiempo los antibiticos participan activamente en la seleccin de bacterias resistentes sin olvidar que la velocidad con la que aparecen genes de resistencia tambin contribuyen a dicho problema. La seleccin de microorganismos resistentes puede ocurrir durante o despus de tratamientos
con antimicrobianos, los residuos de antibiticos pueden establecerse en el ambiente durante periodos de tiempo considerables
posterior al tratamiento. De forma paralela al uso de los antibiticos se presenta el empleo de otros agentes que se proponen para
eliminar bacterias, como es el uso de bactericidas ahora disponibles en algunos productos domsticos. Estos ltimos tambin tienen la capacidad de permanecer en el ambiente, participando en
la dinmica de la ecologa microbiana, en lo referente a resistencia contra la susceptibilidad bacteriana y en la supervivencia de
diversos grupos de microorganismos.

Key words: Antibiotics, public health problem,

resistant bacteria, mutation rate.

Palabras clave: Antibiticos, problema de salud pblica,

resistencia bacteriana, proporcin de mutacin.

The many mechanisms that bacteria exhibit to protect themselves from antibiotic can be classified into
four basic types. Antibiotic modification is the best
known: the resistant bacteria retain the same sensitive target as antibiotic sensitive strain, but the anti-

* Centro de Investigaciones Microbiolgicas del Instituto de Ciencias de la Benemrita Universidad Autnoma de Puebla.
Recibido para publicacin: 04/09/02. Aceptado para publicacin: 18/09/02.

biotic is prevented from reaching it. This happens,

for example, with lactamases-the lactamases enzymatically claves the four membered lactam ring,
rendering the antibiotic inactive. Most lactamases
act to some degree against both penicillins and cephalosporins, others are more specific-namely, cephalosporinasrs, for example AmpC enzyme found in
Enterobacter spp, or penicillinases for example Staphylococcus aureus penicillinase. lactamases are
widespread among many bacterial species (both
Gram positive and Gram negative) and exhibit varying degrees of inhibition by lactamase inhibitors,
such as clavulanic acid.1
Some antibiotic resistant bacteria protect the target of antibiotic action by preventing the antibiotic
from entering the cell or pumping it out faster than it


Address: Jos Antonio Rivera Tapia

Centro de Investigaciones Microbiolgicas del ICBUAP. Edifico 76
Complejo de Ciencias, Ciudad Universitaria. 72570, Puebla, Puebla, Mxico
Tel: 2 33 20 10 ext. 21. Fax: 2 33 20 10 ext. 25. E-mail: jart70@yahoo.com

Rivera-Tapia JA. Antibiotic resistance

An Med Asoc Med Hosp ABC 2003; 48 (1): 42-47

can flow in (rather like a bilge pump in a boat).

lactam antibiotic in Gram negative bacteria gain access to the cell that depends on the antibiotic,
through a water filled hollow:rop
know as a porin (Figure 1). In the case of imipenem
VC ed AS,
resistant Pseudomonas
lack of the specific D2 porin confers resistance, as imipenem canarapis also seen
not penetrate the cell. This mechanism
with low level resistance to fluoroquinolones and
acidmoiB arutaretiL
efflux via an energy-requiring transport pump is a well recognized mechanism for resistance to tetracyclines and is encoded


by a wide range of related genes, such as tet(A), that

have become distributed in the enterobacteriaceae.2,3
Alterations in the primary site of action may mean
that the antibiotic penetrates the cell and reaches the
target site but is unable to inhibit the activity of the
target because of structural changes in the molecule.
Enterococci are regarded as being inherently resistant to cephalosporins because the enzymes responsible for cell wall synthesis (production of the polymer peptidoglycan) know as penicillin binding proteins have a low affinity for them and therefore are
not inhibited. Most strains of Streptococcus pneumo-

Gram positive bacteria



Cell survives

Penicillin binding

lactam fails to bind to these

proteins due to poor affinity

Cell survives

Inhibition of peptidoglycan
synthesis and activation of
autolytic enzymes

Cell death

Diffusion through

Gram negative bacteria


Slow entry





(plasmid or chromosomal)

Periplasmic space

Penicillin binding proteins

Figure 1.

Inhibition of peptidoglycan

edigraphic.comInterplay of lactam antibiotics with


Gram positive and Gram negative bacteria.


Rivera-Tapia JA. Antibiotic resistance


niae are highly susceptible to both penicillins and

cephalosporins but can acquire DNA from other bacteria, which changes the enzyme so that they develop a low affinity for penicillins.4 The altered enzyme
still syntheises peptidoglycan but in now has a different structure. Mutants of Streptococcus pyogenes
that are resistant to penicillin and express altered in
the laboratory, but they have not been seen in patients, possibly because the cell wall can no longer
bind the antiphagocytic M protein.5
The final mechanism by which bacteria may protect
themselves from antibiotic is the production of an alternative target, usually an enzyme, that is resistant to inhibition by the antibiotic while continuing to produce
the original sensitive target. This allows bacteria to survive in the face of selection: the alternative enzyme
bypasses the effect of the antibiotic. The best know
example of this mechanism is probably the altenative
penicillin binding protein (PBP2a), which is produced
in addition to the normal penicillin binding proteins by
methicillin resistant Staphylococcus aureus (MRSA).
The protein is encoded by the mecA gene, and because
PBP2a is not inhibited by antibiotics such as flucloxacillin the cell continues to synthesise peptidoglycan
and hence has a structurally sound cell wall.6 The appearance in 1987 of vancomycin resistant enterococci
has aroused much interest because the genes involved
can be transferred to Staphylococcus aureus, and this
can thus theorically result in a vancomycin resistant
MRSA. The mechanism also represents a variant of the
alternative target mechanism of resistance.7 In enterococci sensitive to vancomycin the normal target of vancomycin is a cell wall precursor that contains a pentapeptide that has a D-alanine-D-alanine terminus, to
which the vancomycin binds, preventing further cell
wall synthesis. If an enterococcus acquires the vanA
gene cluster, however, it can now make an alternative
cell wall precursor ending in D-alanine-D-lactose, to
which vancomycin does not bind.

trol practice, and vaccines to reduce colonization and

subsequent infection.8 Among pathogens causing hospital infections, Gram positive cocci have become
predominant over the past two decades. This trend is
related to these pathogens capacity for accumulating
antibiotic resistance determinants.9 A notable example is that of methicillin resistant strains of Staphylococcus aureus (MRSA), which emerged in the 1970s
and increased in frequency as hospital pathogens during the 1980s in many countries with the notable exception of the Scandinavian countries and Netherlands.10,11 Countries with lower incidence of MRSA
infections tend to be more restrictive in antibiotic use,
to apply strict infection control measures, and to have
better ratios of nurses to patients in their healthcare institutions. The rise in MRSA infections was initially
associated with epidemics in large teaching hospitals,
later spreading to the general hospital and nurding
homes. Control stategies, such as contact isolation
precaution and carrier decolonization with topical antimicrobials, met with varying degrees of succes but
seemed at least to slow down transmission.12,13
Enterococci, commensal inhabitants of the intestinal and genital tracts, are rising in prominence as
hospital pathogens.14 This rise is related to their natural resistance to most commonly used antibiotics
and their capacity to acquire resistance to other antibiotics either by mutation (penicillin) or by transfer
of resistance genes on plasmids and transposons
(aminoglycosides and glycopeptides).10,14
Multiple antibiotic resistance to useful classes of
antibiotic, including the penicillins, cephalosporins,
aminoglycosides and fluoroquinolones, has gradually increased among a number of Gram negative hospital pathogens, especially Klebsiella pneumoniae.14,15 Enterobacter spp,16,17 Pseudomonas aeruginosa and Acinetobacter baumannii.18,19 Epidemic
and endemic infections caused by these multiple resistant strain followed intense antibiotic use in many
hospital, particulary in intensive care units.20

Antimicrobial resistance has also stimulates the

search for new potent antimicrobials, altered but effective dosing regimens, and resistance control measures, susch as the prudent use, optimal infection con-

Antibiotic resistance can be achieved by horizontal

acquisition of resistance genes, carried by transposons or plasmids), by recombination of foreign

Rivera-Tapia JA. Antibiotic resistance


DNA into the chromosome, or by mutation in different chromosomal loci. In studies of molecular evolutionary biology the term mutation rate is applied to
estimations of the rate of mutation per nucleotide,
per locus or eventually for the whole genome, and
selective favorable, unfavorable, or neutral mutations are considered. Differing with this concept, the
frequency of mutation measures all the mutants
present in a given population, irrespective of whether the mutation events ocurred early or late during
the growth of the populations. In this respect, the frequency of mutants is a cross section of the bacterial
population at a given time and reflects not only the
mutation rate but also the history of the population
before selection is applied.21,22
In the case of antibiotic resistance, the mutation rate
is frequently defined as the in vitro frequency at which
detectable mutants arise in a bacterial population in the
presence of a given antibiotic concentration.21
The methods for distinguishing the value of the
observed frequency of mutants from the real mutation rate are not easy to apply and fluctuation test for
analysis of the presence of jackpots of preexisting
mutants in the tested populations have been developed.23,24 In the case of antibiotic resistance, the
problem is complicated by the fact that the phenotype does not always reflect the same genotypes in
all selected mutants, because mutations in different
genes can produce similar antibiotic resistance phenotypes. As an example, when a quinolone resistance mutation rate is determined, this rate is actually the result of the combination of the mutation rate
of the genes that encode the synthesis of GyrA,
GyrB, ParA, ParC and several different multidrug resistance (MDR) systems.25,26 In this respect, the calculated phenotypic mutation rate is the result of
several different genotypic mutation events. In
fact, mutations in different loci produce different
changes in MICs, and stable maintenance of heterogeneous antibiotic resistance expresion classes in
bacterial populations is a well know phenomenon.27
Mutation rates can largely change for a given antibiotic depending on its concentration during selection.28 Physiological conditions such as the availability of a given carbon source29 or, in general, bacterial
stress30,31 may regulate the mutation rate in bacteria.
Furthermore, the existence of mutations that produce


mutator phenotypes in bacteria32,33 and the capability

of some antibiotics to increase mutability greatly
complicate studies of the effects of population dynamics on the emergence of antibiotic resistant mutants in
bacteria. These element of variability severely challenge the possibility of predicting the real mutation
rate just by simple experimental procedures like those
frequently used in laboratory experiments.34,35
It is known from clinical trials that about 4% of infecting microorganisms (ocurring in 5.6% of all infections treated) become resistant upon therapy.36
There is also a correlation between the amount of
antibiotics used and the level of resistance. Clones of
such penicillin-resistant Streptococcus pneumoniae
have spread locally and internationally under selective pressure.37-39
The rapid increase in resistant strains of bacterial
species in the normal flora is not as commonly acknowledged, but there is increasing evidence that the
normal flora represents a pool for selection of resistance genes, wich may disseminate to other species
and genera by horizontal transfer by conjugation,
transduction or transformation.40,41
The normal flora of mounth and pharynx is exposed to antibiotic when penicillins and other oral
antibiotic are swallowed and, together with excretion
of antibiotics by salivary glands, suppresses the viridans group of streptococci and other components of
the normal flora.42,43 When bacteria are lysed by antibiotics such as penicillins, their DNA is released,
which may promote horizontal gene transfer by
transformation. Some pneumococci have become resistant to penicillin because of alterations in penicillin-binding protein 2 (PBP2), leading to decreased
affinity for penicillins. There is now compelling evidence that these pneumococcal clones originated by
importation of divergent regions in the PBP genes so
called mosaic genes that originated from Streptococcus mitis and other oral streptococci. Similar mechanisms have been found in penicillin-resistant Neisseria meningitidis and Neisseria gonorrhoeae where
the resistance genes have been derived from Neisseria flavescens and Neisseria cinerea.44



Rivera-Tapia JA. Antibiotic resistance


The normal flora of the intestine is heavily exposed to incompletely absorbed oral antibiotics and
to antibiotics excreted in the bile. These antibiotics
suppress the normal flora and lead to development of
resistance and superinfection.45
S. epidermidis and other bacteria in the normal
skin flora are exposed to antibiotics that are used for
topical treatment. Also important is the impact on
the normal skin flora of antibiotics excreted into the
sweat. This has been shown to lead to rapid and prolonged colonization of the skin of volunteers with
multiresistant S. epidermidis.46,47 This is likely to
contribute to the widespread ocurrence of multiresistant S. epidermidis in hospitals, which is associated
with the amount of antibiotics used. Susceptibility
testing is recommended to identify possible changes
in antibiotic resistance to streptococci 48-50
Application of antibiotics over the past 50 years
has resulted in an unremitting increase in the numbers of commensal and pathogenic bacteria that are
resistant to antimicrobial compounds. Although the
increases in the background levels of resistance do not
threaten control of these organisms, the result show
that bacteria, even those not regularly or directly subjected to antibiotic challenge, have changed in response to increases in the application of antibiotics
over the past several decades.
1. Livermore DM. -lactamases in laboratory and clinical resistance. Clin Microbiol Rev 1995; 8: 557-584.
2. Barriga AG, Rojas ML, Peredo LVM. Actualidades en los
patrones de resistencia a los antimicrobianos. Rev Mex Patol
Clin 2001; 48: 65-69.
3. Chopra J, Hawkey PM, Hinton M. Tetracyclines, molecular and
clinical aspects. J Antimicrob Chemother 1992; 29: 247-277.
4. Tomasz A, Munoz R. -lactam antibiotic resistance in Gram
positive bacteria pathogens of the upper respiratory ttract: a
brief overview of mechanisms. Microbiol Drug Resistance
1995; 1: 103-109.
5. Garcia-Bustos J, Tomasz A. A biological price of antibiotic
resistance: major changes in the peptidoglycan structure of
penicillin-resistant pneumococci. Proc Natl Acad Sci USA
1990; 87: 5415-5419.
6. Michel M, Gutmann L. Methicillin-resistant Staphylococcus
7. Leclercq R, Courvalin P. Resistance to glycopeptides in enterococci. Clin Infect Dis 1997; 24: 545-556.
8. Craig WA. Temas del futuro de la resistencia antimicrobiana.
Enf Infec Micro 2000; 20: 172-177.

10. Cohen ML. Epidemiology of drugs resistance: implications

for a post-antimicrobial era. Science 1992; 257: 1050-1055.
11. Vincent JL, Bihari DJ, Suter PM, Bruining HA, White J,
Nicolas-Chanoine MH et al. The prevalence of nosocomial
infection in intensive care units in Europe. JAMA 1995;
274: 639-644.
12. Vandenbroucke-Grauls C. Management of methicillin-resistant Staphylococcus aureus in the Netherlands. Rev Med Microbiol 1998; 9: 109-116.
13. Mulligan ME, Murray-Leisure KA, Ribner BS, Standiford
HC, John JF, Korvick JA et al. Methicillin-resistant Staphylococcus aureus: a consensus review of the microbiology,
pathogenesis and epidemiology with implications for prevention and management. Am J Med 1993; 94: 313-328.
14. Gold HS, Moellering RC. Antimicrobial-drug resistance. N
Engl J Med 1996; 335: 1445-1453.
15. Livermore DM, Yuan M. Antibiotic resistance and production of extended-sprectrum -lactamases amongst Klebsiella
spp from intensive care units in Europe. J Antimicrob
Chemother 1996; 38: 409-424.
16. Chow JW, Fine MJ, Shlaes DM, Quinn JP, Hooper DC,
Jonhson MP et al. Enterobacter bacteremia: clinical features
and emergence of antibiotic resistance during therapy. Ann
Intern Med 1991; 115: 585-590.
17. De Cheldre Y, Maes N, Rost F, De Ryck R,Clevenbergh P,
Vincent JL et al. Molecular epidemiology of an outbreak of
multidrug-resistant Enterobacter aerogenes infections and in
vivo emergence of imipenem resistance. J Clin Microbiol
1997; 35: 152-160.
18. Towner KJ. Clinical importance and antibiotic resistance of
Acinetobacter spp. J Med Microbiol 1997; 46: 721-746.
19. Struelens MJ, Carlier E, Maes N, Serruys E, Quint WGV,
van Belkum A. Nosocomial colonization and infection with
multiresistant Acinetobacter baumannii: Outbreak delineation using DNA macrorestriction analysis and PCR-fingerprinting. J Hosp Infect 1993; 25: 15-32.
20. McGowan JE. Antimicrobial resistance in hospital organisms and its relation to antibiotic use. J Infect Dis 1983; 5:
21. Davies JE. Origins, acquisition and dissemination of antibiotic
resistance determinants. Ciba Found Symp 1997; 207: 15-27.
22. Fluit AdC, Visser MR, Schmitz FJ. Molecular detection of antimicrobial resistance. Clin Microbiol Rev 2001; 14: 836-871.
23. Lederberg J, Lederberg EM. Replica plating and indirect selection of bacterial mutants. J Bacteriol 1952; 63: 399-406.
24. Luria SE, Delbruck M. Mutations of bacteria from virus sensitivity to virus resistance. Genetics 1943; 28: 491-511
25. Hopper DC. Mechanisms of fluoroquinolone resistance.
Drug Resis Updates 1999; 2: 38-55.
26. Martinez JL, Alonso A, Gomez-Gomez JM, Baquero F. Quinolone resistance by mutations in chromosomal gyrase
genes. Just the tip of the iceberg? J Antimicrob Chemother
1998; 42: 683-688.
27. Figueiredo AM, Ha E, Kreiswirth BN, Lencastre de H, Noel
GJ, Senterfit L, Tomasz A. In vitro stability of heterogeneous
expression classes in clinical isolates of methicillin-resistant
staphylococci. J Infect Dis 1991; 164: 883-887.
28. Kohler T, Michea-Hamzehpour M, Plesiat P, Kahr AL,
Pechere JC. Differential selection of multidrug efflux systems by quinolones in Pseudomonas aeruginosa. Antimicrob
Agents Chemother 1997; 41: 2540-2543.
29. Hughes D, Andersson DI. Carbon starvation of Salmonella
typhimurium does not cause a general increase of mutation
rates. J Bacteriol 1997; 179: 6688-6691.


Rivera-Tapia JA. Antibiotic resistance


30. Foster PL. Adaptive mutation: the uses of adversity. Annu

Rev Microbiol 1993; 47: 467-504.
31. Shapiro JA. Genome organization, natural genetic engineering and adaptive mutation. Trends Genet 1997; 13: 98-104.
Science 1996; 274: 1208-1211.
33. Taddei F, Radman M, Maynard-Smith J, Toupance B, Gouyon PH, Godelle B. Role of mutator alleles in adaptative evolution. Nature 1997; 387: 700-702.
34. Mamber SW, Kolek B, Brookshire KW, Bonner DP, FungTomc J. Activity of quinolones in the Ames Salmonella
TA102 mutagenicity test and other bacterial genotoxicity assays. Antimicrob Agents Chemother 1993; 37: 213-217.
35. Ren L, Rahman MS, Humayun MZ. Escherichia coli cells
exposed to streptomycin display a mutator phenotype. J Bacteriol 1999; 181: 1043-1044.
36. Fish DN, Piscitelli SC, Danzinger LH. Development of resistance during antimicrobial therapy: A review of antibiotic
classes and patient characteristics in 173 studies. Pharmacotherapy 1995; 15: 279-291.
37. Kristinsson KG, Hijalmarsdottir M, Steingrimsson O. Increasing penicillin resistance in pneumococci in Iceland.
Lancet 1992; 399: 1606-1607.
38. Ekdahl K, Cars O. Role of communicable disease control measures in affecting the spread of resistant pneumococci the Swedish model. Clin Microbiol Infect 1998; 5 (suppl 4): S48-54.
39. Tomasz A. The challenge of multiresistant Streptococcus
pneumoniae: international initiatives in day-care centers and
the use of molecular epidemiologic techniques. Clin Microbiol Infect 1998; 5, (suppl 4): S64-68.
40. Davies J. Inactivation of antibiotics and the dissemination of
resistance genes. Science 1994; 264: 375-382.


41. Houndt T, Ochman H. Long-term shifts in patterns of antibiotic resistance in enteric bacteria. Appl Envir Microbiol
2000; 66: 5406-5409.
42. Grahn E, Holm SE. Penicillin concentration in saliva and its
influence on bacterial interference. Scan J Infect Dis 1987;
19: 235-241.
43. Nord CE, Heimdahl A, Kager L. Antimicrobial induced alterations of the human oropharyngeal and intestinal microflora.
Scand J Infect Dis 1986; 49: 64-72.
44. Spratt BG. Resistance to antibiotic mediated by target alterations. Science 1994; 264: 388-393.
45. Edlund C, Nord CE. Ecological impact of antimicrobial
agents on human intestinal microflora. Alpe Adria Microbiol
J 1993; 3: 137-164.
46. Hoiby N, Johansen HK. Ciprofloxacin in sweat and antibiotic resistance. The Copenhagen Study Group on Antibiotics in
Sweat. Lancet 1995; 346: 1235.
47. Hoiby N, Jarlov JO, Kemp M, Tvede M, Bangsborg JM,
Kjerulf A et al. Excretion of ciprofloxacin in sweat and multiresistant Staphylococcus epidermidis. Lancet 1997; 349:
48. Gahm-Hansen B. Coagulase-negative staphylococci and micrococci in clinical microbiology. Danish Med Bulletin 1987;
34: 96-114.
49. Jarlov JO, Hoiby N. Coagulase-negative staphylococci in a
major Danish university hospital: diversity in antibiotic susceptibility between wards. Act Path Microbiol Immun Scan
1998; 106: 411-416.
50. Rodrguez RS, Caldern JE, Gmez BD, Espinosa ML. Caractersticas de la resistencia antimicrobiana de una coleccin
clnica de Streptococcus pyogenes. Sal Pub Mex 2000; 42: